Spintronic materials

Sergio Valenzuela, Pietro Gambardella, Kevin Garello, Olivier Klein, Juan Sierra, Jairo Sinova

► To cite this version:

Sergio Valenzuela, Pietro Gambardella, Kevin Garello, Olivier Klein, Juan Sierra, et al. Spintronic materials. Encyclopedia of Condensed Matter Physics, 2024, Encyclopedia of Condensed Matter Physics, 2nd edition, 2, pp.159-176. 10.1016/B978-0-323-90800-9.00229-8. cea-05003576

HAL Id: cea-05003576

https://cea.hal.science/cea-05003576v1

Submitted on 24 Mar 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Encyclopedia of Condensed Matter Physics, 2nd edition

**ENTRY TITLE:** Spintronic Materials

**AUTHOR NAMES and AFFILIATIONS:**

**Sergio O. Valenzuela**

Institució Catalana de Recerca i Estudis Avançats (ICREA) Barcelona 08010, Spain

Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and the Barcelona Institute of
Science and Technology (BIST), Bellaterra, Barcelona, Spain

E:mail: [SOV@icrea.cat](mailto:SOV@icrea.cat)

**Pietro Gambardella**

Department of Materials, ETH Zurich, 8093 Zurich, Switzerland

E:mail: [pietro.gambardella@mat.ethz.ch](mailto:pietro.gambardella@mat.ethz.ch)

**Kevin Garello**

Universite Grenoble Alpes, CEA, CNRS, Grenoble INP, SPINTEC, 38054 Grenoble, France

E:mail: [kevin.garello@cea.fr](mailto:kevin.garello@cea.fr)

**Olivier Klein**

Universite Grenoble Alpes, CEA, CNRS, Grenoble INP, SPINTEC, 38054 Grenoble, France

E:mail: [oklein@cea.fr](mailto:oklein@cea.fr)

**Juan F. Sierra**

Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and the Barcelona Institute of
Science and Technology (BIST), Bellaterra, Barcelona, Spain

E:mail: [juan.sierra@icn2.cat](mailto:juan.sierra@icn2.cat)

**Jairo Sinova**

Institut für Physik, Johannes Gutenberg Universität Mainz, Mainz, Germany

Institute of Physics, Academy of Sciences of the Czech Republic, Staré Město, Czech Republic

E:mail: [sinova@uni-mainz.de](mailto:sinova@uni-mainz.de)

**ABSTRACT:** Progress in spintronics has been achieved with novel device concepts and the subsequent development of optimized materials and heterostructures to implement them. The heterostructures typically combine magnetic and nonmagnetic elements, which enable the integration of non-volatile magnetic memories and sensors in electrical circuits. This chapter provides an overview of spintronic devices that have found industrial applications with a focus on the materials and heterostructures being used. It further discusses selected material families that can potentially enhance device performance and/or create new application opportunities, including transition metals, magnetic insulators, antiferromagnets, altermagnets, Heusler compounds and van der Waals materials.

**KEYWORDS:** Spintronics, magnonics, magnetic random-access memory (MRAM), magnetic sensors, transition metals, Heusler compounds, van der Waals materials, antiferromagnets, altermagnets, magnetic insulators, spin-waves
KEY POINTS/OBJECTIVES BOX:

❖ Introduction to spintronics devices, magnetic memories and sensors

❖ Historical overview on spintronics concepts and material development

❖ Description of selected key phenomena and materials

* Giant magnetoresistance (GMR)

* Tunnelling magnetoresistance (TMR)

* Magnetic random-access memory (MRAM)

* Spin-transfer torque (STTT)-MRAM

* Spin-orbit torque (SOT)-MRAM

* Voltage control of the magnetization (VCMA)

* Magnonics

* Transition metals

* Heusler compounds

* 2D van der Waals materials

* Antiferromagnets and altermagnets

* Magnetic insulators

❖ Future directions
# 1. Introduction

Spintronics utilizes the electron spin degree of freedom to store and manipulate information, typically relying on material heterostructures that include magnetic and nonmagnetic elements. The interconversion of electrical and spin signals enables the seamless integration of non-volatile magnetic memories and sensors in electrical circuits, which brings substantial benefits in terms of dimensional scaling, power consumption, and data processing speed (Žutić et al., 2004). Recent advances in the understanding of charge-spin interconversion phenomena and the optimization of materials have opened new avenues to incorporate both Boolean and non-Boolean logic functions as well as high-frequency microwave devices in spintronic circuits (Sinova et al., 2015; Manchon et al., 2019; Dieny et al., 2020).

Commercial implementations of spintronic sensors and magnetic random-access memories (MRAMs) are based on the anisotropic, giant or tunnelling magnetoresistance effects (Apalkov et al., 2016; Zheng et al., 2019; Dieny et al., 2020; Khan et al., 2021). Magnetic sensors are used in automotive, consumer electronics, healthcare, and industrial applications (Zheng et al., 2019). They can detect small changes in magnetic field strength or in the orientation and position of a magnet, making them well suited for applications in hard disk drives, navigation, positioning, speed measurements and tracking. The anisotropic magnetoresistance (AMR) refers to the change in electrical resistance of a ferromagnetic material when its magnetization is rotated relative to the current direction. AMR sensors are easy to integrate, relatively small and have a low-cost compared to other magnetic sensing technologies. Giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) sensors offer higher sensitivity and resolution than AMR sensors.

The basic unit to observe the GMR and TMR effects is a trilayer FM/NM/FM consisting of two ferromagnetic layers (FM) separated by a nonmagnetic (NM) spacer (Fig. 1). The resistance of the trilayer changes with the relative alignment of the magnetization in the two FM layers. For both sensing and memory applications, the magnetization of one of the two FM layers in the FM/NM/FM stack is pinned and serves as “reference”, whereas the other is “free” to change direction under a magnetic field or a spin-polarized current. The FM layers have a preferential magnetization direction, dictated by the uniaxial magnetic anisotropy, which is induced by either crystalline, shape or interfacial effects. Additionally, a synthetic antiferromagnet (SAF) is deposited above (or below) the reference layer (RL) in order to pin its magnetization. The SAF is composed of a high-coercivity magnetic multilayer stack, which is coupled to the RL through a thin nonmagnetic spacer by the Ruderman-Yosida-Kasuya-Kittel (RKKY) interaction. Alternatively, for in-plane magnetized FM/NM/FM structures, the RL can be pinned by exchange bias to an adjacent antiferromagnetic layer (Nogués and Schuller, 1999). In magnetic memory applications, a logic bit state, 0 or 1, is encoded in the direction of the magnetization of the free layer (FL). The bit state is then read by a magnetoresistance measurement that distinguishes between the parallel (P) and antiparallel (AP) alignment between the magnetizations of the FL and RL. In sensing applications, the magnetization of the FL tilts in response to an external magnetic field, e.g., in proximity of a magnetic object, and the relative angle with the magnetization of the RL is measured by analogue variations of the GMR or TMR effects, depending on the type of device.

State-of-the-art MRAM cells rely on the TMR effect, in which the NM spacer is a thin insulator and the trilayer forms a magnetic tunnel junction (MTJ). Typical MRAM stacks are built by numerous ultrathin
layers, each of them with a distinct role in determining the memory performance (Apalkov et al., 2016; Dieny et al., 2020; Yang et al., 2022). The number and purpose of the layers and the design of the MTJ have evolved over the years following several major material and technological breakthroughs aimed at increasing the TMR, lowering the power required to control the FL magnetization, and maintaining thermal stability and data retention when downscaling the MTJs. The spin-transfer torque (STT) effect has emerged as a key technology to switch the FL magnetization using electrically generated spin currents (Chappert et al., 2007; Ralph and Stiles, 2008; Stamps et al., 2014; Apalkov et al., 2016; Sander et al., 2017; Vedmedenko et al., 2020; Na et al., 2021; Brataas et al., 2012) (Fig. 1). The current tunnelling through the tunnel barrier is spin-polarized, leading to the FL magnetization reversal owing to the transfer of angular momentum. STT-MRAM are commercialized as standalone memories and wearable electronics. They are also considered as the best candidates for embedded non-volatile applications, as replacement of embedded flash and static RAM (SRAM) (see Section 3.1).

<figure><img src="image_2.png" /><figcaption>**Figure 1.** a, The basic unit of GMR and TMR devices consist of two ferromagnetic layers (FM) separated by a nonmagnetic (NM) spacer. The NM spacer can be a metal (GMR), or an insulator (TMR) to create a magnetic tunnel junction (MTJ). The magnetization of the free layer (FL) can change direction while that of the reference layer (RL) is pinned. b, The main mechanisms to change the FL magnetization are (from left to right): the Oersted (or external magnetic) fields, spin-transfer torque (STT), the voltage control of the magnetic anisotropy (VCMA), the spin-orbit-torque (SOT) and domain-wall (DW) motion. Figure adapted from (Krizakova et al., 2022).</figcaption></figure>

The figures of merit of GMR and TMR devices depend critically on the composition and thickness of all the magnetic and nonmagnetic layers in the stack, and particularly on their structural quality, including interfacial stoichiometry and roughness. For technological applications, the fact that interfacial effects dominate the GMR and TMR is an advantage, as it helps reduce the devices volume without impairing their performances and the power required to switch them. Minimizing the device size also reduces the self-demagnetizing fields produced by both the RL and the FL. For sensors, this implies a larger sensitivity of the FL to external magnetic fields, while, for memories, this implies smaller interaction between neighbouring devices and therefore larger bit density. Reducing the variability of the magnetic and electrical parameters across devices patterned on the same wafer and from wafer to wafer is crucial for applications. This is important for MTJ devices, in which the TMR scales exponentially with the tunnel barrier thickness.
The growing demand for spintronic devices with high-density, high-speed and low-power
consumption necessitates advances in terms of materials and device architectures (Dieny et al., 2020).
This chapter starts with an historical overview of conventional and emerging materials used in
spintronics. This overview is complemented by an in-depth description of the different materials,
concepts and envisioned material advantages and bottlenecks. Requirements for further progress,
novel materials that could meet those requirements, and the opportunities that can arise with them
are also discussed. These materials include Heusler compounds, 2D van der Waals (vdW) materials,
antiferromagnets, altermagnets and magnetic insulators. **Figure 2** highlights some of the main
milestones for each of the selected material families.

The chapter ends with a summary of future directions. Given the intense research activity on
spintronics, it is not possible to cover the vast literature available in a limited space. As a complement
to this chapter, the reader is referred to comprehensive reviews, books, roadmaps and perspectives
that have been published in recent years.

## 2. Historical overview

The development of spintronics started with the discovery of the GMR effect in 1988 (Fert and Van Dau, 2019) (Fig. 2). GMR rapidly found several applications as sensors and in 1996 as read heads in hard disk drives. The discovery of material systems with TMR exceeding 100% at room temperature led to a generational change in the early 2000s and the widespread adoption of MTJs as the basic unit of magnetic storage and sensing (Parkin et al., 2003; Miao et al., 2011; Dieny et al., 2020). Although AMR and GMR effects are still being used for low-cost sensing applications, TMR is ubiquitous in the most advanced device generations. Because the density of states of FM materials are spin-split and tunnelling is a spin conserving process, the TMR depends on the product of the spin polarization of the two FM electrodes. This dependence leads to a maximum room temperature TMR ratio of about 70% in amorphous Al<sub>2</sub>O<sub>3</sub> barriers sandwiched between transition-metal electrodes such as CoFe, owing to the partial spin polarization of the conduction electrons near the Fermi level. A breakthrough occurred with the introduction of epitaxial junctions and crystalline MgO barriers, in which the electron tunnelling probability additionally depends on the symmetry of the wavefunctions penetrating the barrier. Experimentally demonstrated TMR values in CoFeB/MgO/CoFeB junctions exceed 600% at room-temperature (Miao et al., 2011). The largest TMR reported so far are obtained using Co<sub>x</sub>Fe<sub>80-x</sub>B<sub>20</sub> electrodes with x = 20-40% and 1-2 nm-thick MgO barriers.

No other materials compatible with industrial process requirements have been found to improve the
TMR and, in the last decade, applications have evolved around MgO-based MTJs. The crystalline
properties and TMR of the structures have steadily improved and suitable SAF multilayers and
interfacial perpendicular magnetic anisotropy (PMA) have been developed. However, MgO-based MTJ
devices appear to be reaching their limit. As they are made smaller, and the MgO tunnel barrier
thinner, several technological challenges are becoming difficult to overcome, including side wall re-
deposition, the increasing aspect ratio of etched structures or the relative increase of the MgO
insulator roughness.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Spintronic milestones for industrial developments and scientific advances associated to the material families discussed in this chapter.</figcaption></figure>

Besides GMR and TMR, other magnetization readout mechanisms, relying on charge spin interconversion have been proposed. They include the spin Hall magnetoresistance (SMR) and unidirectional spin Hall magnetoresistance (USMR). Their sensitivity is relatively small, and their primary use has been to determine the magnetic state of insulators in fundamental research studies.

Writing in commercialized MRAM has been achieved using a magnetic field, generated by an applied current (implemented in toggle MRAM), and by the previously mentioned STT (Fig. 1 and Fig. 2). Interfacial PMA has become crucial because perpendicularly magnetized stacks downscale better than the in-plane magnetized ones (Dieny and Chshiev, 2017). This resulted additionally in an improved trade-off between the thermal stability of the FL magnetization and the write current for the switching dynamics induced by STT. However, an enhanced PMA, required when the memory is downscaled, results in increased current densities to achieve magnetization switching, which in turn increase energy consumption and can damage the MTJ.

The voltage control of magnetic anisotropy (VCMA) has been proposed as a solution to assist or directly induce magnetization switching (Dieny and Chshiev, 2017; Miwa et al., 2018; Rana and Otani, 2019; Yamamoto et al., 2022). In addition, the spin-orbit torque (SOT) is an alternative to the STT that uses a charge current flowing in a heavy metal layer to inject a spin-polarized current into the adjacent
FL through spin-orbit coupling effects such as the spin-Hall and/or Rashba-Edelstein (inverse spin galvanic) effect (Sinova et al., 2015; Manchon et al., 2019; Krizakova et al., 2022). Unlike STT technology, magnetization reversal by SOT can be achieved without passing a current through the MTJ tunnel barrier (Fig. 1). By decoupling of the write and read current paths, it circumvents write errors during read operation and reduces the risk of dielectric breakdown. Although neither VCMA nor SOT are yet sufficiently strong for implementation in commercial devices, their combination with STT is promising for improving the efficiency, reliability and speed of MRAMs (Krizakova et al., 2022).

Besides the successful magnetic sensors and MRAM, other concepts are under development such as racetrack memories and logic devices using the propagation of magnetic skyrmions or domain walls (DW), driven by either STT or SOT (Atkinson et al., 2006; Parkin and Yang, 2015; Soumyanarayanan et al., 2016; Fert et al., 2017; Hrabec et al., 2020; Dieny et al., 2020). Spintronics is also being investigated for wireless communication applications. Spin torques in nanoscale MTJs can lead not only to magnetization switching but also to self-sustained oscillations of the FL magnetization, which can be used for dc to radiofrequency (RF) conversion in microwave signal generators; the reciprocal effect can be used for microwave signal detection (Demidov et al., 2017). STT-nano-oscillators have also been investigated in neuromorphic computing architectures (Christensen et al., 2022) while THz emission has been achieved in magnetic multilayers (Vedmedenko et al., 2020). Magnonics, which has attracted significant attention in recent years, is a different approach to spin logic that encodes information in the amplitude or phase of spin waves (SW) in magnetic metals or insulators (Stamps et al., 2014; Sander et al., 2017; Brataas et al., 2020; Barman et al., 2021; Pirro et al., 2021; Chumak et al., 2022).

Regarding material processing, new protocols are required for improving the downscaling of devices (for example to avoid sidewall redeposition). Other proposals target novel families of materials while device concepts and applications are continuously put forward, which could benefit from material development. Heusler materials is one such family (Dieny et al., 2020; Hirohata and Lloyd, 2022) (Fig. 2). They comprise a vast catalogue of compounds with high degree of flexibility and physical properties, ranging from semiconductors with tuneable bandgap, superconductors, half-metallic FMs, compensated antiferromagnets, and even topological insulators. Alloys based on Heusler compounds (HC) are promising materials for spintronic applications due to their high saturation magnetization and Curie temperatures, and predicted half-metallic ferromagnetism, i.e. they can change from metallic to insulating behaviour by flipping their spin polarization. Large GMR and TMR effects have been demonstrated. However, these materials tend not to perform well at high temperatures and processing with industrial compatible methods is in development.

The atomically thin nature of 2D materials offers significant material development opportunities (Han et al., 2014; Avsar et al., 2020; Sierra et al., 2021; Yang et al., 2022). It promotes the creation of artificial quantum materials in the form of van der Waals (vdW) heterostructures, where the 2D crystals attach to each other via weak vdW interactions. This represents a new paradigm in material design that is particularly attractive for spintronic devices, which typically harness their functionalities out of thin layers of magnetic and non-magnetic materials and the interfaces between them. Given the ever-growing number of 2D materials available, it might be plausible to replace their 3D counterparts, one by one, whenever the desired functionalities are identified (Fig. 2). VdW heterostructures have the potential advantage of atomically smooth interfaces and reduced thicknesses, eventually down to the monolayer limit, which are both required for downscaling. They would also allow engineers to
seamlessly integrate multiple functionalities because the stacking of 2D materials is expected to be
less restrictive regarding (crystalline) matching between the involved layers. The 2D material
functionalities could go beyond applications in conventional MRAM and sensors and include opto-
spintronics, antiferromagnetic spintronics and magnonics. However, the main challenges of any
electronic technology based on 2D materials are upscaling and integration. 2D materials currently
suffer from reproducibility and a lack of industrially compatible growth and device fabrication
processes (Sander et al., 2017; Och et al., 2021; Yang et al., 2022).

Another highly influential research field in the last decade is the control and manipulation of magnetically ordered materials with zero net magnetization, such as antiferromagnets (Jungwirth et al., 2016; Baltz et al., 2018) and the recently coined altermagnets (Šmejkal et al., 2022b). The key aim is to exploit the advantages that come from an absence of stray magnetic fields and a natural excitation at high frequencies, in the THz regime (Gomonay et al., 2018). The compensated magnetic order results in an insensitivity to external magnetic fields and the possibility for optimal space downscaling of memory devices due to the absence of demagnetizing fields. At the same time, the high resonant frequencies, due to the exchange enhancement effect that couples the different spin sublattices, makes these materials natural candidates for optical manipulation devices that bridge the THz communication gap (Fig. 2) The main challenges are the need to develop efficient read-out and control methods of the material magnetic state.

Conventional devices rely on the delocalized electrons present in conducting materials to carry the
spin information, which induces losses by Joule heating. However, electrically insulating magnetic
materials also enable the spin information to flow between localized magnetic moments via
propagating SWs, which transmit information from an atomic site to the other without any Joule
dissipation (Fig. 2) (Barman et al., 2021). This offers insulating materials an advantage and several
research directions attempt to capitalize on it to achieve technical improvements for manipulating
and transmitting information, including non-Boolean, parallel computing, and neuromorphic
computing (Brataas et al., 2020). As in some of the other new materials discussed in this chapter,
compatibility with current industrial processes and scalability are some of the bottlenecks that need
to be resolved.

**3. Key materials: issues and research directions**

**3.1 Conventional materials and current applications**

***Giant magnetoresistance (GMR)***

The GMR originates from the spin-dependent conductivity of electrons in FM transition metals (Fe, Co, Ni and their alloys) and at FM/NM interfaces (Parkin, 1995). Exchange-biased GMR structures, also called spin valves, are entirely made of metals, typically several repetitions of Co/Cu/Co or Fe/Cr/Fe trilayers coupled to a metallic antiferromagnet with high Néel temperature such as IrMn or FeMn. The best NM spacers are metals with a long electron mean free path, such as Cu, Ag, and Au, and only 1-2 nm-thick in order to minimize current shunting and scattering of the spin-polarized electrons transmitted and reflected between the FM layers(Bass and Pratt, 2007). In the so-called current-in-plane geometry, the thickness of the FM layers must be smaller than the largest of the mean free paths of the majority and minority spin electrons in order for the current density in one FM layer to be affected by the magnetization of the next FM layer. Because interfacial spin-dependent scattering
often dominates over bulk spin-dependent scattering, the optimal FM thickness is around 1-5 nm. The in-plane GMR is further decreased if the current flows in the nonmagnetic seed layers and capping layers used to promote growth and protection, respectively. In the current-perpendicular-to-plane geometry, in contrast, the electrons necessarily flow through the entire multilayer structure and the magnetoresistance is less affected by the mean free paths of the FM layers. In either geometry, the GMR increases if electrons propagate across many FM/NM interfaces. The largest GMR ratios, defined as $(R_{AP} - R_{P})/R_{P}$, where $R_{AP(P)}$ is the resistance of the AP (P) state, are indeed found in structures with tens of repetitions of the thinnest possible FM and NM layers, and reach up to 65% in [Co(0.8 nm)/Cu(0.8 nm)]₆₀ multilayers at room temperature. Spin-valve devices, however, typically have lower GMR ratios of 25% or less(Parkin et al., 2003).

## Tunnel magnetoresistance (TMR)

Although both Fe and CoFe can be grown epitaxially with body-centred cubic (bcc) structure matching the MgO crystal lattice, exhibit the highest TMR in literature, industrial MTJs are obtained by depositing amorphous CoFeB and textured MgO(001) layers using magnetron sputtering, followed by thermal annealing between 300 and 400 °C (Miao et al., 2011). This process minimizes the formation of defects in the barrier owing to the growth of high-quality MgO on atomically smooth CoFeB. It also leads to thermally-induced recrystallization of the electrode interfaces and diffusion of B away from the crystallized CoFe layers, which in turn provides the local epitaxial order between the bcc CoFe and MgO (001) lattices that is required for symmetry filtering of the tunnelling electrons. The deposition of B-getter layers next to the FM layers, such as Ta, W, and Hf, further improves the crystallization process and TMR, provided that the getter elements do not diffuse toward the MgO barrier upon annealing, degrading both the crystalline quality of the FM and the barrier itself. In principle, half-metal electrodes with only one spin sub-band crossing the Fermi level, such as La₂/₃Sr₁/₃MnO₃, CrO₂, and some Heusler alloys (see Section 3.2), can achieve significantly higher TMR values than CoFeB. However, these materials do not perform well at high temperature because magnon and phonon excitations impair their half-metallic character.

Other useful properties that make the CoFeB/MgO/CoFeB system the basis of most MTJ designs are the resilience to thermal annealing required for compatibility with back-end-of-line (BEOL) complementary metal-oxide-semiconductor (CMOS) processes and the possibility to fabricate MTJs with FM layers magnetized in-plane or out-of-plane. Whereas in-plane magnetization is easily obtained using CoFeB layers with thickness above 2-3 nm owing to shape anisotropy, out-of-plane magnetized CoFeB are thinner and deposited next to a heavy metal such as Ta or W to improve the interfacial perpendicular magnetic anisotropy (PMA). The interface between the magnetic metals and the oxide promotes PMA due to hybridization effects between the *d*-orbitals of Co and Fe and the *2p* orbitals of oxygen(Dieny and Chshiev, 2017). The magnetic anisotropy barrier of the FL is usually expressed in terms of a uniaxial anisotropy energy constant $K= 0.8-1.6 \times 10^6 \text{ J/m}^3$ times the magnetic volume *V*, which determine the thermal stability factor $\Delta = KV/k_B T$. Although $\Delta \ge 40$ is required to ensure data retention over 10 years at room temperature, it is commonly assumed that $\Delta \ge 70$ is necessary to maintain information at operating temperatures, reaching up 150 °C for certain applications. MTJs based on FM layers with PMA, also called perpendicular MTJs, have better stability factors compared to in-plane MTJs of equal size and lower stray fields, which are important properties for good readability, size reduction, and densification. On the other hand, MTJs with large $\Delta$ require larger fields or currents for switching the FL magnetization. Typical TMR reported in MTJs for MRAM
applications are about 180% for perpendicular MTJs, and 250% for in-plane MTJs. This is mostly due to the thicker magnetic layers that can be used in in-plane MTJs, which improve the spin polarization of the tunnelling electrons. A further increase of $\Delta$ in perpendicular MTJs can be obtained by adopting a dual-FL/MgO structure, in which an extra MgO/CoFeB interface enhances the PMA(Dieny and Chshiev, 2017).

The RL is coupled to a hard SAF multilayer, typically $[Co/Pt]_N/Ru/[Co/Pt]_M$, by a thin nonmagnetic metal layer such as Ru, W or Ta in order to pin its magnetization. This metal insertion is equally important to decouple the SAF layer texture, usually (111), from the (100) CoFe texture. The SAF is used also to minimize the stray field from the RL, which is crucial to ensure symmetric switching current of the FL. In general, extensive stack engineering and optimization are required to optimize the TMR and $\Delta$ while remaining compatible with the BEOL CMOS thermal budget. The device resistance is determined by the tunnel barrier thickness, which is characterized by the resistance-area-product (RA). The optimal RA is strongly dependent on the application domain and dimensions of the device. It is tuned accordingly to the reading speed requirements, i.e., impedance matching, and the minimum current that can be detected by the peripheral electronics. The RA targets are 3-5 $\Omega$.µm$^2$ (5-10 k$\Omega$) for STT-MRAMs, ~0.5 $\Omega$.µm$^2$ for hard drive read heads, and ~1000 $\Omega$.µm$^2$ for large area sensors. Decreasing the RA below 10 $\Omega$.µm$^2$ is relevant for further miniaturization of the MRAM bit cells, but usually involves a decrease of the TMR.

## STT-MRAM

The STT strength is proportional to the spin polarization and amplitude of the tunnelling current, which makes it naturally compatible with the materials and processes used to optimize the TMR in MTJ devices (Dieny and Prejbeanu, 2017). STT allows for using two-terminal MTJs with shared reading and writing paths (Fig. 1), leading to a compact design of the MRAM bit cells scalable to 4-6F$^2$, where F is the half distance between adjacent cells. The critical STT switching current is proportional to the magnetic damping parameter, magnetic volume, and magnetic anisotropy of the FL, and thus downscales favourably with the area of the MTJ. The RA product of the MgO tunnel barrier and the critical current determine the size and voltage of the transistor used to selectively switch the MTJ. Typical RA and critical currents density for STT-MRAMs are in the range of 3-5 $\Omega$.µm$^2$ and 3-5 MA.cm$^{-2}$, respectively.

The first step of the STT-MRAM manufacturing process is the front-end-of-line (FEOL) fabrication of transistors on the semiconductor wafer followed by metallic layers (lines and via) routing their electrical contacts to a bottom electrode contact for the MTJ. The BEOL process consists of magnetic stack deposition, photolithography, etching, dielectric isolation, metal contacts and passivation (Gaidis, 2017). A chemical-mechanical polishing step is required to smooth the substrate prior to each integration module and is critical for the deposition of the magnetic stack. The root mean square substrate roughness should be of the order of 0.1 nm in order to obtain high TMR values, large breakdown voltage, and tight distribution of the electrical and magnetic parameters of the MTJs. Rough MgO interfaces should be avoided as they lead to magnetostatic coupling of the free and reference layer and pinhole formation through the barrier, and the process temperature should remain below 400° C in order to minimize interface diffusion between the different layers in the stack while remaining BEOL compatible. Subtractive patterning by ion beam etching or reactive ion etching is used to define the MTJ pillars, which requires a metallic hard mask (usually TiN) to withstand the
etchants used for the magnetic materials. The hard mask also provides a self-aligned top contact to
the MTJ device.

High-performance STT-MTJ cells are included in the first MRAMs that have been commercialized as replacement for embedded flash memory. Additionally, owing to their scalability, reduced write energy per bit, low leakage, high-speed and endurance, STT-MRAMs represent one of the most appealing non-volatile RAM technologies available for last-level cache memory replacement for internet of things (IoT) applications, always-on processors, and edge artificial-intelligence (AI) computing. Challenges for the further deployment of STT-MRAMs are of two types. First, the fabrication of dense STT-MRAMs arrays below the 14-nm technology node is hindered by the etching of magnetic films, which leads to by-products that cause sidewall redeposition, corrosion, and electrical short of the MTJ. Proposed solutions include combining reactive ion etching and ion beam etching as well as decreasing the thickness of the hard mask to reduce shadowing effects. Second, the shared reading/writing path increases the susceptibility to erroneous writing, read disturbance, and tunnel barrier degradation. These effects become prominent at high operation frequencies, which effectively limits the writing latency to > 10-20 ns. The inherent stochastic character of STT switching also affects the writing latency for low amplitude and duration of the current pulses. The speed and endurance limits of STT-MRAMs can be significantly extended by combining STT with other writing approaches, such as SOT switching and VCMA, which promote deterministic reversal of the FL and do not require passing a strong electrical current through the MTJ barrier.

SOT-MRAM

SOT-MRAM exploits the same CoFeB/MgO/CoFeB/SAF basic structure of STT-MTJs, which makes them compatible with most of the BEOL processes developed for STT-MRAMs. The distinguishing feature is the implementation of the SOT current line as a two-terminal bottom electrode (Fig. 1). The main advantage of SOT-MRAMs is the combination of high switching speed and endurance, which allows for quasi-deterministic reversal of the FL on a 100 ps timescale with sub-ns stochastic distribution of the switching times in perpendicular MTJs. Moreover, because the switching current does not flow through the tunnel barrier, its thickness and RA can be tuned for best reading efficiency and speed without affecting the writing process.

SOTs are a manifestation of the relativistic spin-orbit interaction (SOI) that couples the orbital electron motion to the electron's spin. On the microscopic level, their origin can be explained by different mechanism (Manchon et al., 2019). When a charge current is passed through a nonmagnetic conductor, the SOI mediates its conversion into a transverse spin current that propagates toward the interfaces and where it results in spin accumulation. This mechanism is known as the spin Hall effect (SHE) and is prominent in heavy metals (Sinova et al., 2015). In the presence of broken inversion symmetry, e.g., at the conductor's interface, electrons additionally experience an electric field perpendicular to the interface plane, which acts as a magnetic field in the electron's rest frame and results in non-equilibrium spin accumulation at the interface. This mechanism is known as the Rashba-Edelstein effect (REE) or inverse spin galvanic effect (ISGE) and is prominent at the interface of metals having different electronegativity, oxide interfaces, and 2D materials (Manchon et al., 2015; Gambardella and Miron, 2011). The ISGE is also responsible for charge-spin conversion in materials with spin-momentum locked electron states, such as topological insulators and Weyl semimetals (Bihlmayer et al., 2022). A combination of these effects is also possible, in conjunction with additional
phenomena due to electron scattering at the interface between magnetic and nonmagnetic conductors and the generation of spin currents within a magnetic material. Recently, orbital analogues of the SHE and ISGE have been proposed as additional mechanisms for the generation of SOT, provided that the SOI converts an orbital current into a spin current inside or outside the magnetic layer (Go et al., 2021). Overall, the multiplicity of effects that can generate the SOT allows for a wide choice of materials and optimization strategies to maximize their strength and efficiency for magnetization switching.

The most studied sources of SOT are the 5d heavy metals Pt, Ta, and W (Manchon et al., 2019). They exhibit torque efficiencies $\xi$ ranging from 0.1 to 0.5, where $\xi$ is the ratio between the spin current effectively converted into a torque and the charge current used to produce the spin current. In addition to $\xi$, other relevant figures of merit are the resistivity $\rho$, the spin Hall conductivity $\sigma_{SH} = \xi\rho^{-1}$ and the spin diffusion length $\lambda_s$. The first two parameters determine the power density dissipated in the SOT material, while the third determines its thickness. The interfacial spin accumulation decays over a distance of the order of $\lambda_s$, which is only a few nm in the 5d metals. This makes it possible to maximize the SOT produced by very thin nonmagnetic layers, thereby reducing the total current required to achieve, e.g., magnetic switching of an adjacent magnet. Alloys or multilayers containing 5d metals, such as. AuPt or [Ti/Pt]$_N$, exhibit larger $\xi$ compared to single-element systems due to enhanced electron scattering and resistivity. Owing to the combination of strong SOI and relatively small resistivity, the spin Hall conductivity in these materials is among the largest measured, reaching up to $10^5 - 10^6 (\Omega m)^{-1}$.

Metallic antiferromagnets such as MnPt and IrMn are also strong SOT sources, which additionally provide exchange bias to the magnetic layer (Manchon et al., 2019). Interface engineering by nitrification or oxidation of the heavy metal, insertion of ultrathin spacer layers, and the use of electrically insulating antiferromagnetic layers can further improve $\xi$ due to enhanced spin transmission and interfacial spin-orbit scattering. Among the 4d metals, Pd present sizable SOT in combination with PMA. The 3d metals that generate strong orbital currents can also be used as SOT sources, in particular Cr, V and Ti. These materials have similar $\xi$ and resistivity compared to the 5d metals, which makes them attractive for heavy-metal free applications. However, the orbital diffusion length, namely the orbital counterpart of $\lambda_s$, is of the order of 10 nm or larger, which requires relatively thick 3d layers to achieve a significant orbital accumulation at the interface with a ferromagnet, and the orbital current must be converted into a spin current in order to exert a torque on the magnetization. This occurs either inside the ferromagnet or in spacer layers with strong SOI, such as few nm-thick rare earth layers, for which $\xi$ of up to 0.25 has been achieved.

Topological insulators (TIs) such as the bismuth chalcogenides Bi$_2$Se$_3$, (Bi$_{1-x}$Sbx)$_2$Te$_3$, and BiSb have also drawn significant interest as SOT sources as they exhibit giant values of $\xi$, typically 1-2 orders of magnitude larger than the 5d metals and allow for room temperature SOT-induced magnetization switching at low current densities. Other materials that can be used to generate SOTs are vdW materials, discussed later, and oxide heterostructures (Trier et al., 2022). For the time being, however, integrating nonconventional materials in devices for mass production remains a challenge.

At the threshold switching current density $j_{th}$, the power density is $P_{th} = j_{th}^2 \rho \propto \xi^2 \rho_p$, where $\rho_p$ is the resistivity of the current channel and FL in parallel. Resistive materials typically have a high $\xi$.
However, if the SOT electrode is too resistive, a significant portion of the current can be shunted into the FL, decreasing the SOT efficiency. Moreover, the selector transistor of each MRAM cell is not only limited in current (depending on the transistor size and associated cell footprint), but also by voltage, which scales as $j_{th}\rho_p$. Therefore, the adoption of high-$\xi$ materials such as TIs in SOT-MTJs should be carefully considered in terms of power requirements as well as BEOL-compatibility.

A drawback is SOT-MRAM based on PMA FL magnetizations is the requirement for a symmetry-breaking mechanism to unambiguously set the FL magnetic state and ensure deterministic bipolar operation. Whereas a constant in-plane magnetic field can efficiently lift the final state degeneracy, device integration requires alternative symmetry breaking mechanisms. These include effective magnetic fields produced by an embedded in-plane magnet, exchange bias from an antiferromagnet, and Dzyaloshinskii-Moriya interaction, RKKY coupling to a ferromagnetic layer, or structural symmetry breaking such as tilted anisotropy, thickness, strain and composition gradients. Additionally, the combination of SOT and STT current pulses in 3-terminal MTJs has been shown to induce field-free switching while preserving beneficial SOT characteristics in terms of speed and write error rates (Krizakova et al., 2022).

## Voltage-control of magnetic anisotropy (VCMA)

The VCMA refers to the change of the interfacial magnetic anisotropy caused by an electric field applied perpendicularly to the interface of a magnetic layer. This effect is fully compatible with the MTJ structure, in which most of the voltage applied between the top and bottom electrodes falls across the tunnel barrier. Different microscopic phenomena can induce VCMA and can be roughly divided in two categories (Miwa et al., 2018). Electric-field induced charge accumulation at the interface of a magnetic layer has been shown to modify the electronic filling of *d* orbitals with different symmetry, leading to a change of the magnetocrystalline anisotropy energy that can be large enough to induce the transition from an out-of-plane to in-plane easy axis. Alternatively, electric-field induced oxygen migration from an oxide dielectric layer to the interface of the magnetic layer can induce redox reactions that also affect the symmetry and filling of the *d* orbitals in a reversible way. The efficiency of the VCMA process is measured by the coefficient $\eta$, namely the change of PMA energy per unit area divided by the applied electric field. The use of reducible oxides or high-ion-mobility gate dielectrics leads to values of $\eta$ of up to $10^4$ fJ/(Vm) in GdOₓ/Co layers. The activation time of the redox-induced VCMA, however, is of the order of seconds, and thus too large for memory applications. The electronic VCMA occurs on timescales that are much faster than the magnetization dynamics typically $\eta \le 10^2$ fJ/(Vm) in CoFeB/MgO layers (Miwa et al., 2018). Both effects are unipolar, i.e., the reduction of magnetic anisotropy is obtained for a given sign of the applied voltage.

The importance of VCMA is that it takes place without significant current flow across the junction of an MTJ. It is thus an energy-efficient effect to either assist or induce switching of the FL. The VCMA effect does not break time reversal symmetry and therefore does not discriminate between magnetic states with opposite magnetization. In both STT- and SOT-induced switching, the VCMA can lower the switching energy barrier, leading to a reduction of the critical switching current. In an MTJ, however, the STT and VCMA bias coincide. Because the STT switching polarity depends on the sign of the voltage, the VCMA effect inverts for parallel-to-antiparallel and antiparallel-to-parallel reversal, making it impractical to assist bipolar switching. This issue is elegantly circumvented if VCMA is used
to assist SOT switching in voltage-gate-assisted SOT-MRAMs (Krizakova et al., 2022). The application of VCMA not only reduces the critical switching current, but also the switching incubation time (Krizakova et al., 2022), and allows for selectively addressing SOT-MTJs sharing a common SOT electrode. In principle, VCMA alone can also toggle the magnetization between two opposite states, provided that an external magnetic field is applied perpendicular to the easy axis and the duration of the voltage pulses is timed to half the precessional period of the free layer (Miwa et al., 2018). This toggling technique requires, pre-read and read-verify processes, or a double write process, and a careful reduction of the variability in magnetic anisotropy, damping and pulse duration in order to minimize write errors.

Applications that exploit the VCMA effect would greatly benefit from the development of material systems with higher η, higher perpendicular magnetic anisotropy, and lower damping. In order to induce VCMA-switching of a magnetic layer of area A and thermal stability Δ, a value of η = Δk<sub>B</sub>T/(AE) is required, where E ≈ 1 V/nm is the electric field applied across the dielectric layer. Main memory applications that have thermal stability Δ > 70 thus require η ≈ 10<sup>3</sup> fJ/(Vm) of electronic origin. The most promising effort in this direction involve epitaxial Cr/ultrathin Fe/MgO junctions with in-plane anisotropy and Fe/ultrathin Ir/MgO junctions with perpendicular anisotropy, for which η of up to 370 fJ/(Vm) has been reported [Suzuki JPD 2018; Yuasa JMMM 2022]. The enhancement of the VCMA in the latter case is attributed to the large spin-orbit coupling and orbital moment of the Ir atoms dispersed in the Fe layer after annealing, which results also in an increase of the perpendicular magnetic anisotropy. Other approaches to improve η involve the introduction of ultrathin oxide layers with high dielectric constant in the MgO tunnelling barrier, such as TiO<sub>x</sub> and HfO<sub>2</sub>.

## 3.2 Heusler compounds (HC)

The predicted room temperature half-metallic behaviour in HCs providing 100% spin polarization has promoted a vast interest to implementing this class of materials in spintronic applications, particularly as spin injector and detector in GMR and TMR devices (Palmstrøm, 2016; Graf et al., 2016; Elphick et al., 2021; Hirohata and Lloyd, 2022). Additionally, the magnetic properties of HC, e.g their saturation magnetization, can be controlled by atomic substitution of the constituent elements of the alloy, a degree of freedom that is useful for sensing and MRAM technologies, where stray magnetic fields must be minimized. However, half-metallicity, low magnetic damping, and high Curie temperatures require a perfect crystalline order, which is challenging and depends on the lattice matching with substrates and seed layers. This is the case of the predicted half-metal MnNbSb, which motivated a great interest to incorporate such HC in MTJs.

Although 100% spin polarization was confirmed by polarized positron annihilation and photoemission experiments in bulk MnNbSb, thin films based on co-sputtered or epitaxial growth show much lower values. Spin-polarized tunnelling experiments in epitaxially grown MnNbSb on Al<sub>2</sub>O<sub>3</sub>/Al yielded a spin polarization of 28%, a value that depends on atomic disorder and lattice distortion (Graf et al., 2016).

Cobalt-based HC are especially appealing due to their high prospects for magnetoresistive devices, PMA, spin injection into bulk semiconductors, and more recently into 2D materials such as graphene(Hirohata and Lloyd, 2022). The list of potential candidates is long with many different compounds already tested, e.g. Co<sub>2</sub>FeSi, Co<sub>2</sub>MnSi, Co<sub>2</sub>MnGe, Co<sub>2</sub>Fe<sub>0.5</sub>Mn<sub>0.5</sub>Si, and Co<sub>2</sub>FeAl<sub>0.5</sub>Si<sub>0.5</sub>. The
first family of Heusler-based MTJs structures utilized AlOₓ tunnel barrier, yielding room temperature TMR values from 15% for Co₂Cr₀.₆Fe₀.₄Al to 70% for Co₂MnSi (Elphick et al., 2021). Besides atomic order within the HC film, the surface roughness and the interface between HCs and tunnel barriers have a strong influence on the TMR. As a result, the combination of HC magnetic electrodes with epitaxial MgO barriers have significantly improved the device performance. Room temperature TMR values of 217%, 340% and 365% were obtained for Co₂MnSi/MgO/CoFe, Co₂FeAl/MgO/CoFe and Co₂MnSi/MgO/Co₂MnSi epitaxial MTJs, respectively, suggesting a coherent tunnelling process. With partial substitution of Mn with Fe to form Co₂Mn₀.₇₃Fe₀.₂₇Si, TMR ratios increased to 430% (Elphick et al., 2021).

Co-based HC has been also incorporated in current-perpendicular-to plane (CPP)-GMR devices (Elphick et al., 2021). Room temperature GMR values of 2.5% and 36.5% using Cr and Ag metal spacers have been reported with Co₂MnSi HC top and bottom electrodes (Graf et al., 2016). The choice of the metal spacer becomes essential since long λₛ materials with minimal lattice mismatch with the HC electrode are required. Despite the large lattice mismatch, GMR values at room temperature of 8.5% have been obtained using Co₂MnSi layers sandwiched by a Cu metal spacer. Other approaches include full Heusler CPP-GMR devices using Ru₂CuSn spacers with GMR values of 7% at room temperature. All these considerations have led to the choice of Ag as the most optimal metal spacer. Another important aspect of Co-based HC is the possibility to induce PMA by varying the Co concentration. Memory STT-MRAM cells based on magnetic HC with PMA and low magnetic damping have been already developed which reflects the potential of HCs in spintronic technologies (Hirohata and Lloyd, 2022).

The approach to use Co-based HC for spin injection would –in principle—provide highly efficient spin polarization in some spin logics or interconnect realizations (Elphick et al., 2021). Spin injection into semiconductors using HC is promising due to their good crystal lattice matching with most semiconductors and their high Curie temperature. However, they have failed to fulfil the expectations mostly due to atomic disorder and interdiffusion of magnetic impurities at the interface with the semiconductor spin channel. Spin injection in CoMnGe/GaAs heterostructures yielded spin polarization efficiencies of 25%, which is smaller than the 40% efficiency reached in GaAs using Fe as spin injector. Lateral spin devices with Cu channels and Co₂FeSi spin injectors and detectors yielded spin injection efficiencies of 27%, the highest value reported to date in these hybrid devices. In order to enhance the spin injection efficiency, the intercalation of a MgO tunnel barrier between the semiconductor and the magnetic HC has been proposed (Farshchi and Ramsteiner, 2013).

From a practical point of view, fully ordered atomic arrangements in HCs are technically feasible but cumbersome. The growing condition of highly ordered HC requires elevated temperatures, which is an obstacle for efficient device implementation. Steady advances in thin film growth have led to improved conditions. Layer-by-layer growth has been demonstrated to reduce the crystallization energy of the materials, reducing the annealing temperatures to 225 °C, which avoids interfacial material intermixing (Dieny et al., 2020). A further reduction has been achieved when depositing HCs on a W seed layer. In this regard, the improvement in the growth conditions as well as the choice of appropriate substrates becomes essential to successfully incorporate Heusler alloy films into spintronic devices.
### 3.3 Two-dimensional (2D) van der Waals materials

A large variety of vdW materials are being investigated for spintronics, targeting different purposes even beyond the conventional applications described in the previous sections (Table 1). The materials that have attracted more attention are (multilayer) graphene, transition metal dichalcogenides (TMDCs) ($MX_2$ with M = Mo, W, ... and X = S, Se, Te), hexagonal boron nitride (hBN), TIs of the $Bi_2Te_3$ family, and several magnetic materials, which belong to the most recent class of 2D materials discovered (Han et al., 2014; Soumyanarayanan et al., 2016; Avsar et al., 2020; Gibertini et al., 2019; Sierra et al., 2021; Yang et al., 2022; Wang et al., 2022).

Graphene and bilayer graphene present excellent spin transport properties, exhibiting very long spin diffusion length (up to ~30 um), and are considered as a keystone for spin communications and, potentially, spin logic devices (Han et al., 2014; Avsar et al., 2020). The spin-current propagation has been controlled using electric drift and an thermal currents (Avsar et al., 2020; Sierra et al., 2021). However, the weak-SOI enabling slow spin relaxation in graphene prevents efficient spin manipulation and charge-to-spin interconversion. Control of spin flow in graphene by means of an electric gate was achieved with $MoS_2$, which acted as a spin sink of variable strength. As the Schottky barrier between n-doped $MoS_2$ and graphene is suppressed with the gate, the $MoS_2$ shunts and absorbs the spin current, which can no longer propagate in graphene (Avsar et al., 2020; Vedmedenko et al., 2020; Sierra et al., 2021). A key element for these experiments is efficient spin injection into graphene. To circumvent the conductivity mismatch problem, resistive barriers are used (Avsar et al., 2020; Sierra et al., 2021), which are based on oxides such as $AlO_x$, $MgO$, $TiO_x$, $YO_x$ and $SrO$ or alternatives such as hBN, fluorinated and hydrogenated graphene and amorphous carbon.

Proximity SOI can be imprinted in graphene with large SOI materials such as some TMDC (Žutić et al., 2019). Spin relaxation anisotropy measurements in TMDC-graphene bilayers, using oblique spin precession, demonstrated a much larger spin lifetime for out-of-plane than for in-plane spins at room temperature (Sierra et al., 2021). The anisotropy, observed even when the Fermi level lies within the TMDC energy gap, is ascribed to valley Zeeman SOI and intervalley scattering induced in graphene by the TMDC (Garcia et al., 2018; Sierra et al., 2021). Subsequent experiments, using non-local spin Hall devices, further supported this conclusion. Room-temperature tuneable SHE, related to the valley Zeeman SOI, and ISGE, associated to induced Rashba SOI, were both observed in $WS_2$-graphene. The strength of the SOI deduced from the SHE measurements is in good agreement with the values extracted by density functional theory. Spin relaxation anisotropy has been also found in bilayer graphene and attributed to intrinsic spin-orbit splitting (Avsar et al., 2020; Vedmedenko et al., 2020). The SOI is small (about 24 uV) but enough to induce an out-of-plane spin orbit field, which for high-quality devices reduces significantly the in-plane spin lifetime, as in the case of TMDC-graphene. The spin-relaxation anisotropy is strongly gate and temperature dependent, being maximum near the charge neutrality point and only observed at low temperatures (about 100 K).
<table><caption>**Table1.** Functionalities of representative van der Waals materials</caption><thead><tr><td>Electronic/Magnetic Property</td><td>Representative vdW Materials</td><td>Applications</td></tr></thead><tbody><tr><td>Insulator</td><td>hBN</td><td>MTJ barrier, diffusion/intermixing barrier</td></tr><tr><td>Semiconductor</td><td>Black-Phosphorous, WS<sub>2</sub>, WSe<sub>2</sub>, MoSe<sub>2</sub>, MoS<sub>2</sub>, WSSe, SnS<sub>2</sub>, ReSe<sub>2</sub>, Bi<sub>2</sub>Te<sub>3</sub>, etc</td><td>Spin transport (BP), SOT, optical spin injection, MTJ barrier, charge-spin conversion, SOI proximity</td></tr><tr><td>Semimetal</td><td>C, Si, Ge, WTe<sub>2</sub>, PtSe<sub>2</sub>, etc</td><td>MTJ barrier, spin-transport and diffusion/intermixing barrier, PMA (C), SOT, unconventional SOTs (e.g. 1T-WTe<sub>2</sub>), charge-spin conversion</td></tr><tr><td>Metal</td><td>MoTe<sub>2</sub>, NbSe<sub>2</sub>, PdTe<sub>2</sub>, PtTe<sub>2</sub>, etc</td><td>SOT, charge-spin conversion</td></tr><tr><td>Ferromagnet</td><td>Fe<sub>5.3</sub>GeTe<sub>2</sub> (metal), CrTe<sub>2</sub> (metal), Cr<sub>2</sub>Ge<sub>2</sub>Te<sub>6</sub> (semiconductor), MnSb<sub>2</sub>Te<sub>4</sub>, CrCl<sub>3</sub>, CrBr<sub>3</sub>, etc</td><td>Spin injector, MTJ FM layers, magnetic proximity</td></tr><tr><td>Antiferromagnet</td><td>Crl<sub>3</sub>, CrCl<sub>3</sub>, MnPS<sub>3</sub>, FePSe<sub>3</sub>, CrSBr (semiconductor), MnBi<sub>2</sub>Te<sub>4</sub>, etc</td><td>TMR: insulating AFM (e.g. Crl<sub>3</sub>) tunnel barrier, charge-spin conversion, magnetic proximity</td></tr></tbody></table>

Besides SOI, it is also possible to induce a magnetic-exchange field in graphene (Žutić et al., 2019; Vedmedenko et al., 2020; Sierra et al., 2021). Arguably, the most compelling evidence of proximity-induced exchange is reported in CrSBr-(bilayer) graphene heterostructures. CrSBr is a vdW semiconductor with a bandgap of about 1.5 eV and presents interlayer antiferromagnetic coupling with a Néel temperature of about 130 K. Within each layer, the material shows in-plane ferromagnetic order. When in contact to CrSBr, the bilayer graphene acquires a magnetization that is collinear to the magnetization of the closest CrSBr layer. By characterizing the nonlocal transport under an applied magnetic field, a large proximity exchange field of 170 T was estimated, which promoted an effective spin polarization in bilayer graphene of 14%. Additional evidence of magnetic proximity effects was found, for instance, in YIG-graphene, Cr<sub>2</sub>Ge<sub>2</sub>Te<sub>6</sub>-graphene, CrBr<sub>3</sub>-graphene, and CrSe-graphene structures.

Graphene could play a significant role in MRAM, specifically, it has been shown to act as a highly efficient diffusion barrier, able to unlock atomic layer deposition processes or to provide encapsulation or large TMR values, which can be tunable (Piquemal-Banci et al., 2017; Avsar et al., 2020; Dayen et al., 2020; Vedmedenko et al., 2020; Yang et al., 2022; Och et al., 2021). The latter are comparable to those reported in early generation AlO<sub>x</sub>-based MTJs, but still not competitive with CoFeB/MgO based MTJs. FM junctions have successfully been fabricated with other 2D barriers such as hBN and TMDCs (Dayen et al., 2020; Och et al., 2021). In all these cases, the coupling between the 2D material and the FM plays a crucial role in determining the spin polarization and (magnetic) proximity effects. Other remarkable phenomena involving graphene, important for MRAM, include the PMA enhancement in Co-graphene and the possibility of creating artificial AFMs out of graphene-Co/graphene/Co multilayers.

Relevant TMDCs for spintronics exhibit semiconducting, metallic or semi-metallic behaviours. Among those materials belonging to the first group, WS<sub>2</sub> and MoS<sub>2</sub> have demonstrated significant spin-filtering with TMR approaching 100% and predictions orders of magnitude larger (Dayen et al., 2020). TMDCs are also potentially useful SOT materials. The reduced crystal symmetries of some TMDCs, such as semi-metallic WTe<sub>2</sub>, can lead to unconventional torque components that are required for field-free switching in perpendicularly magnetized MRAMs (Yang et al., 2022). The SOT has been investigated using MoS<sub>2</sub>, WSe<sub>2</sub>, and WTe<sub>2</sub> as SOT materials both in hybrid 2D/3D and all-2D systems (Liu and Shao, 2020; Hidding and Guimarães, 2020; Kurebayashi et al., 2022). Magnetization switching has been observed although in a regime of domain wall motion (Yang et al., 2022).
Beyond TMDCs, efficient SOTs have been reported with vdW TIs of the $(Bi_{1-x}Sb_x)_2(Te_{1-y}Se_y)_3$ family of materials, as already discussed in Section 3.1 (Yang et al., 2022). The fact that processing tools for physical vapor deposition, which are standard to deposit MTJ stacks, can be used for TI layers with high SOT efficiency, despite the polycrystalline nature and chemical disorder, is encouraging toward the realization of TI-based MRAM. Open questions remain concerning the presence of strong intermixing between conventional FMs and the TI, damage due to the 400 °C heat treatment required by CMOS processing, and topological arguments against bulk origin of the spin current responsible for SOT (Galceran et al., 2021).

Further opportunities are present in opto-spintronics devices. In the monolayer limit, semiconducting TMDCs become direct band-gap semiconductors with the valley states at the *K* and *K'* points in reciprocal space being strongly coupled to the spin degree of freedom (Manzeli et al., 2017; Avsar et al., 2020; Vedmedenko et al., 2020). Circularly polarized light can excite a selected valley. Using time resolved Kerr rotation, spin-valley lifetimes in the microsecond scale have been observed at cryogenic temperatures. The suppression of the exciton-valley depolarization channel in $WS_2-WSe_2$ heterostructures has led to hole spin-valley lifetimes in $WSe_2$ in excess of 20 µs and propagation lengths of over 20 µm. Notably, optical generation of spin-valley polarized carriers in the TMDC can be used to inject spins into a neighbouring graphene layer, providing an alternative to spin injection from FM electrodes. This has been demonstrated in $MoS_2$- and $WSe_2$-graphene heterostructures (Avsar et al., 2020; Vedmedenko et al., 2020; Sierra et al., 2021).

Magnetic proximity effects are also intensively studied in TMDCs and in TIs (Sierra et al., 2021). In TMDCs, the exchange splitting has been quantified with the reflection and photoluminescence spectra of circularly polarized light. The exchange field in $WSe_2$- and $WS_2-EuS$ has been estimated in the scale of 10 and 100 T, respectively. In all-2D structures, such as $WSe_2-CrI_3$, the proximity effect has been found to be layered-dependent, with the largest contribution to the magnetic exchange originating from the $CrI_3$ interfacial layer. Some compelling signatures of magnetism in TIs have also been reported, in proximity with YIG, EuS, and other magnetic insulators, including some vdW materials. However, most of these results rely on transport experiments that are currently not fully understood.

The most recent and arguably fastest growing class of 2D materials relevant for spintronics are vdW magnets, including FMs, AFMs and, possibly, alternative magnets (see Section 3.4) (Gibertini et al., 2019; Och et al., 2021; Wang et al., 2022; Kurebayashi et al., 2022). Reports on monolayer materials of $CrSiTe_3$ and of the $MPX_3$ family ($M = Fe, Mn, Ni, Cd; X = S, Se$) started to appear in 2016. The following year, conclusive evidence of magnetism was observed in $CrI_3$ and $Cr_2Ge_2Te_6$. Their respective Curie temperatures $T_c$ were well below 50 K in the thinnest layers investigated (monolayer $CrI_3$ and bilayer $Cr_2Ge_2Te_6$). Nevertheless, these results triggered intense research on 2D magnets, and a frantic search for 2D magnetism at room temperature. Possible room-temperature magnetism was reported in $VSe_2$ and $MnSe_2$. One advantage of thin 2D materials is their potential gate tunability and the likely maximization of VCMA (Gibertini et al., 2019). Using ion gating increased the $T_c$ of $Cr_2Ge_2Te_6$ from 60 K to nearly 200 K. A promising material with PMA is metallic $Fe_3GeTe_2$, with a $T_c$ ~230 K. Changing the Fe content $x$ from 3 to 5 increased the $T_c$ to just above room temperature, although results indicate a transition to in-plane anisotropy. An even larger $T_c$ has been reported in $Fe_3GaTe_2$, when Fe is substituted with Co in $Fe_xGeTe_2$ and in $Bi_2Te_3/Fe_3GeTe_2$. The latter results, originally attributed to proximity effects, are not yet well understood. Another system of interest is $Cr_xTe_y$, which has
significant complexity because of the large number of phases with different Cr and Te stoichiometry, some of them not truly vdW materials. Room temperature magnetism has been reported in CrTe$_2$, Cr$_2$Te$_3$ and in V-doped WSe$_2$.

Large TMR (160%) has been observed in all-2D Fe$_3$GeTe$_2$/hBN/Fe$_3$GeTe$_2$, with even larger values predicted, including other barrier materials (Yang et al., 2022). A notable TMR exceeding 19,000% was observed in tunnel junctions with four layers of CrI$_3$. The coercivity in WTe$_2$/Fe$_3$GeTe$_2$ can be modulated with current, while SOT switching has been achieved up to 200 K in this system. Another area of intense research involves magnetic skyrmions, which have been observed in several pristine 2D magnets and 2D-magnet based heterostructures.

## 3.4 Antiferromagnets and altermagnets

### Antiferromagnets

The recently established field of antiferromagnetic spintronics takes AFMs as the central active elements of a device-focused system (Jungwirth et al., 2016; Baltz et al., 2018; Jungwirth et al., 2018). It aims at demonstrating a range of functionalities, such as memory devices based on the electrical and optical control of the Néel magnetic order, and at exploring noncollinear AFMs, hybrid AFM/FM systems, artificial realizations of AFMs in multilayer structures, and the interplay of AFMs with topological phenomena. The approach promises important advantages. The compensated magnetic order in AFMs– both collinear, noncollinear, and non-coplanar with zero magnetization – makes them insensible to external magnetic fields and is favourable for memory device densification. The high resonant frequencies of AFMs - typically of the order of THz due to the exchange enhancement effect that couples the different spin sublattices - makes them natural candidates for optical manipulation devices bridging the THz communication gap.

A challenge to integrate AFMs into present technological memory devices is the fact that their electronic structure is spin-degenerate -- in the absence of SOI -- and often electrically inactive, i.e. insulating. From symmetry considerations, GMR/TMR and STT effects are excluded for readout or manipulation. Therefore, in many aspects, conventional AFMs are akin to non-magnetic materials, and invisible to the macroscopic electrical or optical probes commonly used in FMs. The lack of time-reversal symmetry breaking in reciprocal space further implies that the effects associated with the direct manipulation of AFMs magnetic order must arise from SOI (relativistic) origins, which are relatively weak compared to exchange-mediated effects. An example of this manipulation is the Néel SOT where a current-induced spin accumulation couples directly to the Néel order parameter and can rotate it (Železný et al., 2018; Manchon et al., 2019). This can occur in a co-linear AFM with two sublattices possessing opposite inversion symmetry breaking. Two of the key materials fulfilling this requirement are Mn$_2$Au and CuMnAs.

From the symmetry perspective, any response that is proportional to the orientation of the order parameter will be observable in the electronic transport and optical regimes. For transport, this is the case for AMR, which has been observed in many AFMs. The AMR indicates up to a 90 degree rotation of the Néel vector, which can be measured either by electron tunnelling or dc-transport (Železný et al., 2018). The tunnelling AMR in AFMs was first demonstrated experimentally in a NiFe/IrMn/MgO/Pt tunnelling junction, while AMR was exploited in FeRu in one of the first AFM memory prototypes. A
more direct visualization of the Néel vector can be achieved by X-ray magnetic linear dichroism (XMLD). Indeed, XMLD - photoemission electron microscopy (XMLD-PEEM) in CuMnAs confirmed the control of the Néel vector with Néel SOT (observed with AMR) when changing the polarity and direction of the applied current (Němec et al., 2018). While current-induced manipulation of the AFM order requires metallic materials, functional AFMs can be insulating. In this context, current induced switching has also been demonstrated in NiO/Pt bilayer systems by exploiting the spin-Hall magnetoresistance effect in combination with XMLD-PEEM imaging.

AFMs, as many other materials, offer the possibility to generate spin Hall effects and related spin-
dependent transport, which are associated primarily with paramagnets. This is due to their spin-
degenerate band structure, which is only lifted by SOI effects. There are many materials investigated
in this context, including MnX, X = Fe, Pd, Ir and Pt. The magnitude of the SOTs generated by the spin
Hall currents in these materials can compete with that in heavy metals (Manchon et al., 2019).
Switching of FMs with AFMs has been demonstrated while the exchange coupling felt by the FM
enables field-free switching.

Other active areas are the study of the topology in AFMs and AFM opto-spintronics (Němec et al., 2018; Šmejkal et al., 2018). Soon after the discovery of graphene, layered AFMs of the SrMnBi₂-type were reported to show massive Dirac quasiparticles near the Fermi level and quasi-two-dimensional quantum transport. The AFM order of Eu moments in EuMnBi₂ was found to change the interlayer conduction of quasi-2D Dirac carriers, tuning the transport in the quantum Hall regime. At the same time, CuMnAs was predicted to be a topological Dirac semimetal in its orthorhombic phase with the quasiparticle protection arising from non-symmetric symmetries. A Weyl semimetal state could also emerge, which was first proposed in pyrochlore AFMs. Nontrivial surface states (the Fermi arcs connecting surface projections of the Weyl points) were predicted for the chiral non-collinear AFM Mn₃Ge. Weyl semimetal states could also be realized in the paramagnetic and AFM phase of GdPtBi. The presence of Weyl points in the pyrochlore Eu₂Ir₂O₇ was inferred from optical experiments. However, despite an extensive effort, a clear material candidate for AFM Weyl semimetal remains elusive.

Light has been shown to be an efficient tool to control and detect spins and their ultrafast dynamics in AFM materials (Němec et al., 2018). As mentioned already, XMLD has proven vital in visualizing AFM domain structures. As with AMR, the quadratic dependence of the permittivity on the AFM order parameter is revealed in the magneto-refraction and magneto-absorption. The effects were observed as an intensity change of the diffracted X-rays in La0.5Sr1.5MnO4, transmitted or reflected light in EuTe, FeRh and Cr2O3. A key aspect of AFM opto-spintronics is the ultrafast modification of the magnetic order. Femto-second lasers have made this field thrive. The first prediction focused on NiO, however, a key observation was the AFM to paramagnetic optically induced transition in the canted antiferromagnet iron borate FeBO3, arising from an increase in magnon temperature transferred from the lattice. The ultrafast laser-induced reorientation of spins (Kirilyuk et al., 2010) in AFM was shown in the dielectric orthoferrite TmFeO3 where a temperature-dependent magnetic anisotropy is observed at 80–91 K. Related effects are reviewed extensively in Ref. (Němec et al., 2018).
## Altermagnets

As discussed above, the compensated magnetic order of AFMs makes them insensitive to moderate magnetic fields. At the same time, because their electronic band structure is spin degenerate – as mandated by non-relativistic symmetries – manipulation by electrical means must arise from relativistic SOI effects. This entails that, in order to control them electrically, we must circumvent the fact that their spin-responses are relatively weak, as compared to the strong spin-responses of FMs.

A direct solution to this problem comes in the discovery of a novel class of magnetic order with zero net magnetization termed altermagnetism (Šmejkal et al., 2022a, 2022b). Altermagnets exhibit broken time-symmetry responses to an electric field akin to FMs, e.g. spin-polarized currents, and have unique properties, such as strongly asymmetric spin-polarized current response, where, for instance, a change of the current direction by 90 degrees can reverse the polarization of the current. The three collinear magnetic order classes – ferromagnetism, antiferromagnetism, and altermagnetism – are illustrated in **Fig. 3**. Conventional FMs are ordered in space and have principally isotropic (s-wave) spin-polarization order parameter in the electronic structure, **Fig. 3** (left panel). AFMs are spin ordered in direct space but have no spin order, i.e., they are spin degenerate, in reciprocal space, **Fig. 3** (middle). In contrast, altermagnets have an anisotropic (d-, g-, or i-wave) spin-polarization order, **Fig. 3** (right), the naming arising from the alternating spin splitting in reciprocal space (Šmejkal et al., 2022b).

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Illustrative models of collinear ferromagnetism (left), antiferromagnetism (middle) and altermagnetism (right) showing the crystal-structure in direct space and non-relativistic electronic-structure in reciprocal space.</figcaption></figure>

Shortly after the theoretical predictions of altermagnets, supporting evidence was shown by experiments detecting the AHE and spin-polarized currents in *d*-wave RuO$_2$ and Mn$_5$Si$_3$ and the anomalous Hall current in a *g*-wave MnTe. Besides these initial results, the predictions of GMR, TMR, or STT in altermagnetic multi-layer devices has opened a new avenue for spintronics, without the limiting downscaling of non-zero magnetization materials and allowing for charge-spin conversion without relying directly on SOI. This is the case for RuO$_2$, where its *d*-wave anisotropy of the spin-split Fermi surface allows for a pure spin currents transverse to the charge current, but without any connection to the SOI and, therefore, a potentially large $\lambda_s$. The propagation angle between spin-up and spin-down currents has been calculated to reach 34%, larger than in any known non-magnetic charge-spin conversion materials. A further unique effect is that the spin-polarized current in *d*-wave magnetic materials is the switching of the current's spin polarization when the current direction rotates by 90 degrees. There is no counterpart of this effect in the *s*-wave FMs, where polarization reversal only arises from a switching of the magnetization direction.
The basic steps to determine the alternative spin group of a material are to identify: i) the crystallographic group, ii) the crystallographic group of the spin-sublattice (in the case of bipartite lattice the spin sublattice point group corresponds directly to Wyckoff position point group), and iii) the crystallographic rotation transformation connecting the opposite-spin sublattices. For a full discussion of the symmetry considerations and classifications of alternatives, we refer the reader to Refs. (Šmejkal et al., 2022b). Here, we give an overview of the predicted range of material types, illustrated with specific examples. Material candidates for alternativeism include both 3D and 2D crystals, in different structural or chemistry types, and can span the entire conduction spectrum from insulators, semiconductors and semimetals, to metals and superconductors. They can occur in materials with common light elements, high magnetic ordering temperatures and strong spin-coherence. They include quasi-2D oxide insulator $V_2Se_2O$, semimetal $Cr_2O$, 3D rutile fluoride or oxide insulators $FeF_2$, $MnF_2$, $MnO_2$ and metal $RuO_2$, perovskite oxide insulators $LaMnO_3$, $CaCrO_3$ and parent cuprate of high-Tc superconductor $La_2CuO_4$, ferrite insulator $Fe_2O_3$, pnictide with metal-insulator transition $FeSb_2$ and metal $CrSb$, chalcogenide semiconductor $MnTe$ and (semi)metal $VNb_3S_6$, $CoNb_3S_6$, silicide metal $Mn_5Si_3$, and organic insulator $\kappa$-Cl. One should also note that alternativeism can occur in structures with inversion symmetry (e.g. rutiles), or without inversion symmetry (e.g. $VNbS_6$ or $CoNb_3S_6$).

## 3.5 Magnetic insulators

The spintronic applications described so far typically depend on conducting materials where the spin information is carried by delocalized electrons. However, spin information can be generated, transported and manipulated through SWs, or their quanta magnons, a field that has been coined as magnonics (Stamps et al., 2014; Sander et al., 2017; Demidov et al., 2017; Rana and Otani, 2019; Barman et al., 2021; Pirro et al., 2021; Chumak et al., 2022). In this regard, spin transport can be activated between localized electrons. When considering only magnetic insulators, the term spin insulatronics has also been used (Brataas et al., 2020). As a matter of fact, the best spin conductors are insulators. The crucial distinction in the information carrier can lead to ground-breaking changes in device designs and new applications. This necessarily introduces novel research directions with its own challenges and opportunities as described below.

Magnetic insulators are typically governed by indirect antiferromagnetic superexchange, which results in either antiferromagnetic or ferrimagnetic order. In the former, the magnetizations of the sublattices are fully compensated, leading to overall zero magnetization, while in the latter the compensation is not complete, leading to a net magnetization. It is however possible to modify the properties of one or both sublattices of an AFM, for example by doping, to achieve a net magnetization. In addition, their characteristic SW dispersion relation can be tuned with external magnetic field; the SW frequencies and wavelengths vary from GHz to THz and from $\mu$m to nm, respectively (Gurevich and Melkov, 2020; Chumak et al., 2022). As already mentioned in Section 3.4 the fast dynamics of AFMs represent an advantage for fast spintronic devices.

Among magnetic insulators, the material that has been investigated the most is yttrium iron garnet $Y_3Fe_5O_{12}$ (YIG), which is a ferrimagnetic insulator. YIG has the lowest known magnetic dissipation in nature and is characterized by an extremely small magnetic damping constant. Other materials include $NiO$, $CoO$, $\alpha-Fe_2O_3$, $Cr_2O_3$, $BiFeO_3$ and other garnets such as $Tm_3Fe_5O_{12}$ (TmIG). Long-range spin
transport has been reported in α-Fe₂O₃. Domain wall motion and the observation of skyrmions have been investigated in TmIG/Pt bilayers at room temperature, and there are reports of skyrmions in multiferroic Cu₂OSeO₃ at low temperature. In this section, several research directions are introduced that attempt to capitalize on the characteristics of magnetic insulators to achieve significant technical improvements.

Magnons as carriers of information

Spin devices with magnetic insulators capitalize on the transport properties of magnons. Here best performances are achieved in materials supporting long propagation distance for SWs. The propagation distance is here relative to their group velocity, which turns to be slow for low-energy spin excitations (of the order of sound velocity) allowing the development of compact solutions. The concept is available in the form of i) analogic devices such as filters or delay lines or ii) digital devices, which exploits the ondulatory character of magnons. The later effort aims at developing Boolean functions based on wave interference to calculate the output with much smaller footprint compared to their semiconductor counterparts. More recent topics that have emerged in the field of magnonics apply to neuromorphic (Christensen et al., 2022) or bio-inspired computations, which rely on the coupling between long wavelength SWs to solve efficiently certain class of problems such as pattern recognition, which are famously difficult for algorithmic.

Among the key advantages offered by magnons are their very large energy tunability via an external magnetic field enabling a wide control range of operating frequencies spanning a spectral window from GHz to THz (Brataas et al., 2020). Another key advantage is their scalability down to the nanometre level and their wide tolerance to external settings such as the temperature of the device, which can operate from cryogenics values to hundreds of Celsius or under irradiation. The current research topics in this area are advanced computational methods such as logic or artificial intelligence (Barman et al., 2021; Chumak et al., 2022). For example, using magnonics, one can envision a neural-network implementation that is qualitatively different from all existing proposals. Instead of designing and structuring individual nonlinear elements (neurons) and their interconnections (synapses) in real space, one can propose a different paradigm in which these elements are constructed in reciprocal or k-space of large dimensionality. One can then imagine realizing the concept of hyper-connectivity using SWs as neurons. The key property of magnetization dynamics is its strong nonlinearity, which, when coupled with geometrical effects and external stimuli like applied fields and currents, translates into two useful features for neural networks: i) nonlinear interactions through exchange and dipole-dipole interactions couple potentially all SW modes together, thereby creating high connectivity; ii) the strength of the coupling depends on the population of each k mode, thereby allowing for synaptic weights to be modified dynamically. The breakthrough concept is that real-space structuration is not necessary to achieve hyper-connectivity or reconfigurable synaptic weights. One of the major roadblocks for SW computing is the difficulty to efficiently couple electrically to a magnon in an insulator. For short wavelength SWs, the inductive coupling introduces some significant insertion losses that are critically detrimental to the overall energy efficiency of the device in particular at high frequencies. The solution seems to be the exploitation of an interconversion process between the spin and charge degrees of freedom, which enables the electrical control of SWs amplitude in insulators, via the spin transfer effect from an adjacent metal electrode.
## Non-linear magnonic devices

A second research direction is the exploitation of the strong propensity of magnons to behave non-linearly, which could be exploited to create spin diodes, spin amplifiers or spin rectifiers of microwave signals (Brataas et al., 2020). Reducing the magnetic damping in a magnetic material allows for a drastic decrease of the non-linear threshold for parametric instabilities. In materials with ultra-low damping, a 0.1% change in thermal occupation is usually enough to drive the magnetization dynamics into the nonlinear regime. These non-linear effects can be very useful for various nonlinear applications such as rectification, amplification, asymmetric transport (spin diode effect), mixing or generation of chaos.

Again, the spin transfer effect enables the electrical control of $\mu_M$, the magnon chemical potential, via an electrical current in an adjacent metal electrode. The anticipated benefit is the realization of a new form of solid state spin diode obtained by shifting $\mu_M$ relative to $E_g$, the energy gap in the magnons' band diagram. The level-arm is set here by the magnetic damping parameter, $\alpha_{LLG}$: the smaller is the damping the larger are the shifts of $\mu_M$ produced electrically.

In this respect, the ferrimagnet YIG, with the lowest known magnetic damping, is expected to lead to the highest benchmark in nonlinear transport performance. While the spin diode effect in laterally confined geometries is rather well understood, its generalization to extended thin films has so far been elusive. The corresponding difficulty is the collective dynamics that emerges inside the magnon continuum, when the level splitting between eigen-modes becomes smaller than their linewidth.

## Interaction of magnons with other carriers of information

A third research direction addresses the coupling between SWs to other waveforms (Lachance-Quirion et al., 2019; Li et al., 2020; Zare Rameshti et al., 2022). The aim is to achieve strong coupling to preserve the coherence, which means that the phase information is preserved during the interconversion process. Such an interconversion process is desirable to extend the quantum functionalities of the spin state. The figure of merit is set here by the cooperativity, which defines when the coupling strength becomes greater than the geometric mean of the coherence time of the different waveforms. In this regard, the insulating magnetic garnets are the best choice with respect to the magnon lifetime.

While optimizing the interaction by maximizing the overlap integral of two spatio-temporal patterns is primarily a design issue, diminishing the dissipation in a magnet is a complex task, which requires know-how in material research and growth technology. YIG combines extremely low magnetic damping, excellent acoustic attenuation (10× better than quartz), and optical transparency. YIG thus provides the best possible coherent interlink between magnons, phonons, and both microwave and optical photons (Lachance-Quirion et al., 2019; Li et al., 2020). So far experiments performed on millimetre-size YIG single crystals have demonstrated strong coupling at microwave frequencies between magnon, photon and phonons. This encompasses the extension to the quantum regime of the magnon-photon coupling at low temperature (Lachance-Quirion et al., 2019). Devices coupling a YIG sphere to a superconducting quantum circuit have demonstrated the ability to detect a single magnon at ultra-low temperature (Zare Rameshti et al., 2022). It has been also reported that high cooperativity between two macrospins can be achieved due to magnon-phonon coupling. Acoustic phonons in garnets are likely the best waveform to couple coherently distant spins. This is because mechanical vibrations at microwave frequencies can benefit from ultralow damping in crystals.
Interestingly one can control the parity of the magnon-phonon coupling in these systems, where resonances with odd/even phonon modes correspond to out-of-phase/in-phase lattice displacements at the position of the macrospin, leading to bright/dark states in response to uniform microwave magnetic fields, respectively. While such experiments have been performed at room temperature, quantum information exchange and distant entanglement of magnons, phonons, and microwave photons could occur at low temperature. All these are crucial steps in the quest of developing integrated solutions in quantum technologies, in which long-range transport of coherent information between distant quantum gates is a key technological challenge.

These findings establish fruitful connections between the fields of magnetism and quantum information. The next step is to establish this connection with much smaller objects (ideally in the micron-size). The expected difficulties are a degradation of the damping once the size of the sample shrinks. Additional relaxation processes are expected to emerge due to the increased influence of various crystal imperfections and surface defects. In the case of elastic modes, it might be necessary to suspend the oscillating body to isolated from the substrate. However, the reasons for the deterioration of the damping in small structures are still poorly understood. Experiments with magnetic garnets of decreasing size, composition and growth/patterning methods are required to optimize magnonic devices in which large coherence times are preserved.

## 4. Summary and future directions

Spintronics is a very lively field, which has experienced many major advances in the last 35 years thanks to the synergy between industry and academic laboratories, where a significant proportion of novel materials and devices has been developed. Several generations of hard disk drives, sensors and non-volatile memories have adopted fundamental concepts such as GMR, TMR, spin filtering and STT in short time spans (Fig. 2). STT-MRAM products are already in the market and have proven potential for replacing various existing memories, where non-volatility, endurance and speed are required. The discovery of the SHE in metals has triggered fundamental research leading to the development of SOT-MRAM, which could be suitable for fast caches and embedded memories. VCMA, optical switching, also in combination with STT, SOT and orbital phenomena, could expand the technological opportunities for beyond-CMOS memories and devices. AFMs and altermagnets could add functionalities for dense memories and high-frequency electronics bridging the THz gap, while insulator spintronics may open the way for low-power information transport and logics.

This chapter has discussed the main device concepts and applications from a material perspective. Although, it cannot cover the entire field, it provides an overview of some of the most relevant families of practical materials including several that are being intensively investigated to overcome future technological challenges such as transition-metal alloys, Heusler compounds, 2D materials, antiferromagnets and altermagnets, and insulators. However, this list is not exhaustive, other materials such as organic magnets or semiconductors, which have been an active area of research in recent years, are not discussed. Materials for quantum information processing using single electron spins or magnetoelectric interactions for magnetic logics are also not addressed.

Research on these emerging topics and materials for spintronics will only grow in the future. Many technical challenges remain for applications, but a vast number of opportunities are envisioned.
Synchronized MTJ devices can provide novel solutions for neuromorphic computing. AFM/FM bilayers present a memristor behaviour like the multilevel SOT switching in bulk AFMs, which could also be relevant for neuromorphic computing. This behaviour is attributed to a progressive current-induced switching of different magnetic domains in the FM, which are pinned by the exchange coupling with the AFM. Considering that magnons behave as bosons, the predicted emergence of new condensed magnon phases in insulators, such as a superfluid phase produced by a Bose-Einstein condensate, could lead to novel coherent transport phenomena.

Progress will continue to be achieved by developing novel functional materials, heterostructures and device architectures, followed by the implementation of compatible industrial processes. For example, for any technology using vdW materials to be successful, large-scale production and subsequent integration into CMOS-compatible production lines will be required. Approaches relying on chemical vapour and physical deposition are being developed for growth, which likely must be combined with reliable transfer techniques. Optimizing these processes for each material and application is crucial and must be accompanied with the development of dedicated process monitoring and metrology tools. The emergence of new material simulation methods, including machine learning and artificial intelligence, to search and identify specific functionalities could significantly accelerate the adoption of fundamental concepts into novel technologies.

## REFERENCES:

Apalkov, D., Dieny, B., Slaughter, J.M., 2016. Magnetoresistive Random Access Memory. Proceedings of the IEEE 104, 1796–1830. https://doi.org/10.1109/JPROC.2016.2590142

Atkinson, D., Faulkner, C.C., Allwood, D.A., Cowburn, R.P., 2006. Domain-Wall Dynamicsin Magnetic Logic Devices, in: Hillebrands, B., Thiaville, A. (Eds.), Spin Dynamics in Confined Magnetic Structures III, Topics in Applied Physics. Springer, Berlin, Heidelberg, pp. 207–223.
https://doi.org/10.1007/10938171_6

Avsar, A., Ochoa, H., Guinea, F., Özyilmaz, B., van Wees, B.J., Vera-Marun, I.J., 2020. Colloquium: Spintronics in graphene and other two-dimensional materials. Rev. Mod. Phys. 92, 021003.
https://doi.org/10.1103/RevModPhys.92.021003

Baltz, V., Manchon, A., Tsoi, M., Moriyama, T., Ono, T., Tserkovnyak, Y., 2018. Antiferromagnetic spintronics. Rev. Mod. Phys. 90, 015005. https://doi.org/10.1103/RevModPhys.90.015005

Barman, A., Gubbiotti, G., Ladak, S., Adeyeye, A.O., Krawczyk, M., Gräfe, J., Adelmann, C., Cotofana, S., Naeemi, A., Vasyuchka, V.I., Hillebrands, B., Nikitov, S.A., Yu, H., Grundler, D., Sadovnikov, A.V., Grachev, A.A., Sheshukova, S.E., Duquesne, J.-Y., Marangolo, M., Csaba, G., Porod, W., Demidov, V.E., Urazhdin, S., Demokritov, S.O., Albisetti, E., Petti, D., Bertacco, R., Schultheiss, H., Kruglyak, V.V., Poimanov, V.D., Sahoo, S., Sinha, J., Yang, H., Münzenberg, M., Moriyama, T., Mizukami, S., Landeros, P., Gallardo, R.A., Carlotti, G., Kim, J.-V., Stamps, R.L., Camley, R.E., Rana, B., Otani, Y., Yu, W., Yu, T., Bauer, G.E.W., Back, C., Uhrig, G.S., Dobrovolskiy, O.V., Budinska, B., Qin, H., Dijken, S. van, Chumak, A.V., Khitun, A., Nikonov, D.E., Young, I.A.,
Zingsem, B.W., Winklhofer, M., 2021. The 2021 Magnonics Roadmap. J. Phys.: Condens. Matter 33, 413001. https://doi.org/10.1088/1361-648X/abec1a

Bass, J., Pratt, W.P., 2007. Spin-diffusion lengths in metals and alloys, and spin-flipping at metal/metal interfaces: an experimentalist's critical review. J. Phys.: Condens. Matter 19, 183201. https://doi.org/10.1088/0953-8984/19/18/183201

Bihlmayer, G., Noël, P., Vyalikh, D.V., Chulkov, E.V., Manchon, A., 2022. Rashba-like physics in condensed matter. Nat Rev Phys 4, 642–659. https://doi.org/10.1038/s42254-022-00490-y

Brataas, A., Kent, A.D., Ohno, H., 2012. Current-induced torques in magnetic materials. Nature Mater 11, 372–381. https://doi.org/10.1038/nmat3311

Brataas, A., van Wees, B., Klein, O., de Loubens, G., Viret, M., 2020. Spin insulatronics. Physics Reports, Spin Insulatronics 885, 1–27. https://doi.org/10.1016/j.physrep.2020.08.006

Chappert, C., Fert, A., Van Dau, F.N., 2007. The emergence of spin electronics in data storage. Nature Mater 6, 813–823. https://doi.org/10.1038/nmat2024

Christensen, D.V., Dittmann, R., Linares-Barranco, B., Sebastian, A., Le Gallo, M., Redaelli, A., Slesazeck, S., Mikolajick, T., Spiga, S., Menzel, S., Valov, I., Milano, G., Ricciardi, C., Liang, S.-J., Miao, F., Lanza, M., Quill, T.J., Keene, S.T., Salleo, A., Grollier, J., Marković, D., Mizrahi, A., Yao, P., Yang, J.J., Indiveri, G., Strachan, J.P., Datta, S., Vianello, E., Valentian, A., Feldmann, J., Li, X., Pernice, W.H.P., Bhaskaran, H., Furber, S., Neftci, E., Scherr, F., Maass, W., Ramaswamy, S., Tapson, J., Panda, P., Kim, Y., Tanaka, G., Thorpe, S., Bartolozzi, C., Cleland, T.A., Posch, C., Liu, S., Panuccio, G., Mahmud, M., Mazumder, A.N., Hosseini, M., Mohsenin, T., Donati, E., Tolu, S., Galeazzi, R., Christensen, M.E., Holm, S., Ielmini, D., Pryds, N., 2022. 2022 roadmap on neuromorphic computing and engineering. Neuromorph. Comput. Eng. 2, 022501. https://doi.org/10.1088/2634-4386/ac4a83

Chumak, A.V., Kabos, P., Wu, M., Abert, C., Adelmann, C., Adeyeye, A.O., Åkerman, J., Aliev, F.G., Anane, A., Awad, A., Back, C.H., Barman, A., Bauer, G.E.W., Becherer, M., Beginin, E.N., Bittencourt, V.A.S.V., Blanter, Y.M., Bortolotti, P., Boventer, I., Bozhko, D.A., Bunyaev, S.A., Carmiggelt, J.J., Cheenikundil, R.R., Ciubotaru, F., Cotofana, S., Csaba, G., Dobrovolskiy, O.V., Dubs, C., Elyasi, M., Fripp, K.G., Fulara, H., Golovchanskiy, I.A., Gonzalez-Ballestero, C., Graczyk, P., Grundler, D., Gruszecki, P., Gubbiotti, G., Guslienko, K., Haldar, A., Hamdioui, S., Hertel, R., Hillebrands, B., Hioki, T., Houshang, A., Hu, C.-M., Huebl, H., Huth, M., Iacocca, E., Jungfleisch, M.B., Kakazei, G.N., Khitun, A., Khymyn, R., Kikkawa, T., Kläui, M., Klein, O., Kłos, J.W., Knauer, S., Koraltan, S., Kostylev, M., Krawczyk, M., Krivorotov, I.N., Kruglyak, V.V., Lachance-Quirion, D., Ladak, S., Lebrun, R., Li, Y., Lindner, M., Macêdo, R., Mayr, S., Melkov, G.A., Mieszczak, S., Nakamura, Y., Nembach, H.T., Nikitin, A.A., Nikitov, S.A., Novosad, V., Otálora, J.A., Otani, Y., Papp, A., Pigeau, B., Pirro, P., Porod, W., Porrati, F., Qin, H., Rana, B., Reimann, T., Riente, F., Romero-Isart, O., Ross, A., Sadovnikov, A.V., Safin, A.R., Saitoh, E., Schmidt, G., Schultheiss, H., Schultheiss, K., Serga, A.A., Sharma, S., Shaw, J.M., Suess, D., Surzhenko, O., Szulc, K., Taniguchi, T., Urbánek, M., Usami, K., Ustinov, A.B., van der Sar, T., van Dijken, S., Vasyuchka, V.I., Verba, R., Kusminskiy, S.V., Wang, Q., Weides, M., Weiler, M.,
Wintz, S., Wolski, S.P., Zhang, X., 2022. Advances in Magnetics Roadmap on Spin-Wave Computing. IEEE Transactions on Magnetics 58, 1–72.
https://doi.org/10.1109/TMAG.2022.3149664

Dayen, J.-F., Ray, S.J., Karis, O., Vera-Marun, I.J., Kamalakar, M.V., 2020. Two-dimensional van der Waals spininterfaces and magnetic-interfaces. Applied Physics Reviews 7, 011303.
https://doi.org/10.1063/1.5112171

Demidov, V.E., Urazhdin, S., de Loubens, G., Klein, O., Cros, V., Anane, A., Demokritov, S.O., 2017. Magnetization oscillations and waves driven by pure spin currents. Physics Reports, Magnetization oscillations and waves driven by pure spin currents 673, 1–31.
https://doi.org/10.1016/j.physrep.2017.01.001

Dieny, B., Chshiev, M., 2017. Perpendicular magnetic anisotropy at transition metal/oxide interfaces and applications. Rev. Mod. Phys. 89, 025008.
https://doi.org/10.1103/RevModPhys.89.025008

Dieny, B., Prejbeanu, I.L., 2017. Magnetic Random-Access Memory, in: Introduction to Magnetic Random-Access Memory. John Wiley & Sons, Ltd, pp. 101–164.
https://doi.org/10.1002/9781119079415.ch5

Dieny, B., Prejbeanu, I.L., Garello, K., Gambardella, P., Freitas, P., Lehndorff, R., Raberg, W., Ebels, U., Demokritov, S.O., Akerman, J., Deac, A., Pirro, P., Adelmann, C., Anane, A., Chumak, A.V., Hirohata, A., Mangin, S., Valenzuela, S.O., Onbaşlı, M.C., d’Aquino, M., Prenat, G., Finocchio, G., Lopez-Diaz, L., Chantrell, R., Chubykalo-Fesenko, O., Bortolotti, P., 2020. Opportunities and challenges for spintronics in the microelectronics industry. Nature Electronics 3, 446–459.
https://doi.org/10.1038/s41928-020-0461-5

Elphick, K., Frost, W., Samiepour, M., Kubota, T., Takanashi, K., Sukegawa, H., Mitani, S., Hirohata, A., 2021. Heusler alloys for spintronic devices: review on recent development and future perspectives. Science and Technology of Advanced Materials 22, 235–271.
https://doi.org/10.1080/14686996.2020.1812364

Farshchi, R., Ramsteiner, M., 2013. Spin injection from Heusler alloys into semiconductors: A materials perspective. Journal of Applied Physics 113, 191101.
https://doi.org/10.1063/1.4802504

Fert, A., Reyren, N., Cros, V., 2017. Magnetic skyrmions: advances in physics and potential applications. Nat Rev Mater 2, 1–15.
https://doi.org/10.1038/natrevmats.2017.31

Fert, A., Van Dau, F.N., 2019. Spintronics, from giant magnetoresistance to magnetic skyrmions and topological insulators. Comptes Rendus Physique, La science en mouvement 2 : de 1940 aux premières années 1980 – Avancées en physique 20, 817–831.
https://doi.org/10.1016/j.crhy.2019.05.020
Gaidis, M.C., 2017. Magnetic Back-End Technology, in: Introduction to Magnetic Random-Access Memory. John Wiley & Sons, Ltd, pp. 165–198. https://doi.org/10.1002/9781119079415.ch6

Galceran, R., Tian, B., Li, J., Bonell, F., Jamet, M., Vergnaud, C., Marty, A., García, J.H., Sierra, J.F., Costache, M.V., Roche, S., Valenzuela, S.O., Manchon, A., Zhang, X., Schwingenschlögl, U., 2021. Control of spin-charge conversion in van der Waals heterostructures. APL Materials 9, 100901. https://doi.org/10.1063/5.0054865

Gambardella, P., Miron, I.M., 2011. Current-induced spin-orbit torques. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 369, 3175–3197. https://doi.org/10.1098/rsta.2010.0336

Garcia, J.H., Vila, M., Cummings, A.W., Roche, S., 2018. Spin transport in graphene/transition metal dichalcogenide heterostructures. Chem. Soc. Rev. 47, 3359–3379. https://doi.org/10.1039/C7CS00864C

Gibertini, M., Koperski, M., Morpurgo, A.F., Novoselov, K.S., 2019. Magnetic 2D materials and heterostructures. Nat. Nanotechnol. 14, 408–419. https://doi.org/10.1038/s41565-019-0438-6

Go, D., Jo, D., Lee, H.-W., Kläui, M., Mokrousov, Y., 2021. Orbitronics: Orbital currents in solids. EPL 135, 37001. https://doi.org/10.1209/0295-5075/ac2653

Gomonay, O., Baltz, V., Brataas, A., Tserkovnyak, Y., 2018. Antiferromagnetic spin textures and dynamics. Nature Phys 14, 213–216. https://doi.org/10.1038/s41567-018-0049-4

Graf, T., Felser, C., Parkin, S.S.P., 2016. Heusler Compounds: Applications in Spintronics, in: Xu, Y., Awschalom, D.D., Nitta, J. (Eds.), Handbook of Spintronics. Springer Netherlands, Dordrecht, pp. 335–364. https://doi.org/10.1007/978-94-007-6892-5_17

Gurevich, A.G., Melkov, G.A., 2020. Magnetization Oscillations and Waves. CRC Press, London. https://doi.org/10.1201/9780138748487

Han, W., Kawakami, R.K., Gmitra, M., Fabian, J., 2014. Graphene spintronics. Nature Nanotech 9, 794–807. https://doi.org/10.1038/nnano.2014.214

Hidding, J., Guimarães, M.H.D., 2020. Spin-Orbit Torques in Transition Metal Dichalcogenide/Ferromagnet Heterostructures. Frontiers in Materials 7.

Hirohata, A., Lloyd, D.C., 2022. Heusler alloys for metal spintronics. MRS Bulletin 47, 593–599. https://doi.org/10.1557/s43577-022-00350-1
Hrabec, A., Luo, Z., Heyderman, L.J., Gambardella, P., 2020. Synthetic chiral magnets promoted by the Dzyaloshinskii–Moriya interaction. Appl. Phys. Lett. 117, 130503.
https://doi.org/10.1063/5.0021184

Jungwirth, T., Marti, X., Wadley, P., Wunderlich, J., 2016. Antiferromagnetic spintronics. Nature Nanotech 11, 231–241. https://doi.org/10.1038/nnano.2016.18

Jungwirth, T., Sinova, J., Manchon, A., Marti, X., Wunderlich, J., Felser, C., 2018. The multiple directions of antiferromagnetic spintronics. Nature Phys 14, 200–203.
https://doi.org/10.1038/s41567-018-0063-6

Khan, M.A., Sun, J., Li, B., Przybysz, A., Kosel, J., 2021. Magnetic sensors-A review and recent technologies. Eng. Res. Express 3, 022005. https://doi.org/10.1088/2631-8695/ac0838

Kirilyuk, A., Kimel, A.V., Rasing, T., 2010. Ultrafast optical manipulation of magnetic order. Rev. Mod. Phys. 82, 2731–2784. https://doi.org/10.1103/RevModPhys.82.2731

Krizakova, V., Perumkunnil, M., Couet, S., Gambardella, P., Garello, K., 2022. Spin-orbit torque switching of magnetic tunnel junctions for memory applications. Journal of Magnetism and Magnetic Materials 562, 169692. https://doi.org/10.1016/j.jmmm.2022.169692

Kurebayashi, H., Garcia, J.H., Khan, S., Sinova, J., Roche, S., 2022. Magnetism, symmetry and spin transport in van der Waals layered systems. Nat Rev Phys 4, 150–166.
https://doi.org/10.1038/s42254-021-00403-5

Lachance-Guirion, D., Tabuchi, Y., Gloppe, A., Usami, K., Nakamura, Y., 2019. Hybrid quantum systems based on magnonics. Appl. Phys. Express 12, 070101. https://doi.org/10.7567/1882-0786/ab248d

Li, Y., Zhang, W., Tyberkevych, V., Kwok, W.-K., Hoffmann, A., Novosad, V., 2020. Hybrid magnonics: Physics, circuits, and applications for coherent information processing. Journal of Applied Physics 128, 130902. https://doi.org/10.1063/5.0020277

Liu, Y., Shao, Q., 2020. Two-Dimensional Materials for Energy-Efficient Spin-Orbit Torque Devices. ACS Nano 14, 9389–9407. https://doi.org/10.1021/acsnano.0c04403

Manchon, A., Koo, H.C., Nitta, J., Frolov, S.M., Duine, R.A., 2015. New perspectives for Rashba spin-orbit coupling. Nature Mater 14, 871–882. https://doi.org/10.1038/nmat4360

Manchon, A., Železný, J., Miron, I.M., Jungwirth, T., Sinova, J., Thiaville, A., Garello, K., Gambardella, P., 2019. Current-induced spin-orbit torques in ferromagnetic and antiferromagnetic systems. Rev. Mod. Phys. 91, 035004. https://doi.org/10.1103/RevModPhys.91.035004
Manzeli, S., Ovchinnikov, D., Pasquier, D., Yazyev, O.V., Kis, A., 2017. 2D transition metal dichalcogenides. Nat Rev Mater 2, 1–15. https://doi.org/10.1038/natrevmats.2017.33

Miao, G.-X., Münzenberg, M., Moodera, J.S., 2011. Tunneling path toward spintronics. Rep. Prog. Phys. 74, 036501. https://doi.org/10.1088/0034-4885/74/3/036501

Miwa, S., Suzuki, M., Tsujikawa, M., Nozaki, T., Nakamura, T., Shirai, M., Yuasa, S., Suzuki, Y., 2018. Perpendicular magnetic anisotropy and its electric-field-induced change at metal-dielectric interfaces. J. Phys. D: Appl. Phys. 52, 063001. https://doi.org/10.1088/1361-6463/aaef18

Na, T., Kang, S.H., Jung, S.-O., 2021. STT-MRAM Sensing: A Review. IEEE Transactions on Circuits and Systems II: Express Briefs 68, 12–18. https://doi.org/10.1109/TCSII.2020.3040425

Němec, P., Fiebig, M., Kampfrath, T., Kimel, A.V., 2018. Antiferromagnetic opto-spintronics. Nature Phys 14, 229–241. https://doi.org/10.1038/s41567-018-0051-x

Nogués, J., Schuller, I.K., 1999. Exchange bias. Journal of Magnetism and Magnetic Materials 192, 203–232. https://doi.org/10.1016/S0304-8853(98)00266-2

Och, M., Martin, M.-B., Dlubak, B., Seneor, P., Mattevi, C., 2021. Synthesis of emerging 2D layered magnetic materials. Nanoscale 13, 2157–2180. https://doi.org/10.1039/D0NR07867K

Palmstrøm, C.J., 2016. Heusler compounds and spintronics. Progress in Crystal Growth and Characterization of Materials, Special Issue: Recent Progress on Fundamentals and Applications of Crystal Growth; Proceedings of the 16th International Summer School on Crystal Growth (ISSCG-16) 62, 371–397. https://doi.org/10.1016/j.pcrysgrow.2016.04.020

Parkin, S., Jiang, X., Kaiser, C., Panchula, A., Roche, K., Samant, M., 2003. Magnetically engineered spintronic sensors and memory. Proceedings of the IEEE 91, 661–680. https://doi.org/10.1109/JPROC.2003.811807

Parkin, S., Yang, S.-H., 2015. Memory on the racetrack. Nature Nanotech 10, 195–198. https://doi.org/10.1038/nnano.2015.41

Parkin, S.S.P., 1995. Giant Magnetoresistance in Magnetic Nanostructures. Annual Review of Materials Science 25, 357–388. https://doi.org/10.1146/annurev.ms.25.080195.002041

Piquemal-Banci, M., Galceran, R., Martin, M.-B., Godel, F., Anane, A., Petroff, F., Dlubak, B., Seneor, P., 2017. 2D-MTJs: introducing 2D materials in magnetic tunnel junctions. J. Phys. D: Appl. Phys. 50, 203002. https://doi.org/10.1088/1361-6463/aa650f
Pirro, P., Vasyuchka, V.I., Serga, A.A., Hillebrands, B., 2021. Advances in coherent magnonics. Nat Rev Mater 6, 1114–1135. https://doi.org/10.1038/s41578-021-00332-w

Ralph, D.C., Stiles, M.D., 2008. Spin transfer torques. Journal of Magnetism and Magnetic Materials 320, 1190–1216. https://doi.org/10.1016/j.jmmm.2007.12.019

Rana, B., Otani, Y., 2019. Towards magnonic devices based on voltage-controlled magnetic anisotropy. Commun Phys 2, 1–12. https://doi.org/10.1038/s42005-019-0189-6

Sander, D., Valenzuela, S.O., Makarov, D., Marrows, C.H., Fullerton, E.E., Fischer, P., McCord, J., Vavassori, P., Mangin, S., Pirro, P., Hillebrands, B., Kent, A.D., Jungwirth, T., Gutfleisch, O., Kim, C.G., Berger, A., 2017. The 2017 Magnetism Roadmap. J. Phys. D: Appl. Phys. 50, 363001. https://doi.org/10.1088/1361-6463/aa81a1

Sierra, J.F., Fabian, J., Kawakami, R.K., Roche, S., Valenzuela, S.O., 2021. Van der Waals heterostructures for spintronics and opto-spintronics. Nat. Nanotechnol. 16, 856–868. https://doi.org/10.1038/s41565-021-00936-x

Sinova, J., Valenzuela, S.O., Wunderlich, J., Back, C.H., Jungwirth, T., 2015. Spin Hall effects. Rev. Mod. Phys. 87, 1213–1260. https://doi.org/10.1103/RevModPhys.87.1213

Šmejkal, L., Hellenes, A.B., González-Hernández, R., Sinova, J., Jungwirth, T., 2022a. Giant and Tunneling Magnetoresistance in Unconventional Collinear Antiferromagnets with Nonrelativistic Spin-Momentum Coupling. Phys. Rev. X 12, 011028. https://doi.org/10.1103/PhysRevX.12.011028

Šmejkal, L., Mokrousov, Y., Yan, B., MacDonald, A.H., 2018. Topological antiferromagnetic spintronics. Nature Phys 14, 242–251. https://doi.org/10.1038/s41567-018-0064-5

Šmejkal, L., Sinova, J., Jungwirth, T., 2022b. Emerging Research Landscape of Altermagnetism. Phys. Rev. X 12, 040501. https://doi.org/10.1103/PhysRevX.12.040501

Soumyanarayanan, A., Reyren, N., Fert, A., Panagopoulos, C., 2016. Emergent phenomena induced by spin-orbit coupling at surfaces and interfaces. Nature 539, 509–517. https://doi.org/10.1038/nature19820

Stamps, R.L., Breitkreutz, S., Åkerman, J., Chumak, A.V., Otani, Y., Bauer, G.E.W., Thiele, J.-U., Bowen, M., Majetich, S.A., Kläui, M., Prejbeanu, I.L., Dieny, B., Dempsey, N.M., Hillebrands, B., 2014. The 2014 Magnetism Roadmap. J. Phys. D: Appl. Phys. 47, 333001. https://doi.org/10.1088/0022-3727/47/33/333001
Trier, F., Noël, P., Kim, J.-V., Attané, J.-P., Vila, L., Bibes, M., 2022. Oxide spin-orbitronics: spin-charge interconversion and topological spin textures. Nat Rev Mater 7, 258–274.
https://doi.org/10.1038/s41578-021-00395-9

Vedmedenko, E.Y., Kawakami, R.K., Sheka, D.D., Gambardella, P., Kirilyuk, A., Hirohata, A., Binek, C., Chubykalo-Fesenko, O., Sanvito, S., Kirby, B.J., Grollier, J., Everschor-Sitte, K., Kampfrath, T., You, C.-Y., Berger, A., 2020. The 2020 magnetism roadmap. J. Phys. D: Appl. Phys. 53, 453001. https://doi.org/10.1088/1361-6463/ab9d98

Wang, Q.H., Bedoya-Pinto, A., Blei, M., Dismukes, A.H., Hamo, A., Jenkins, S., Koperski, M., Liu, Y., Sun, Q.-C., Telford, E.J., Kim, H.H., Augustin, M., Vool, U., Yin, J.-X., Li, L.H., Falin, A., Dean, C.R., Casanova, F., Evans, R.F.L., Chshiev, M., Mishchenko, A., Petrovic, C., He, R., Zhao, L., Tsen, A.W., Gerardot, B.D., Brotons-Gisbert, M., Guguchia, Z., Roy, X., Tongay, S., Wang, Z., Hasan, M.Z., Wrachtrup, J., Yacoby, A., Fert, A., Parkin, S., Novoselov, K.S., Dai, P., Balicas, L., Santos, E.J.G., 2022. The Magnetic Genome of Two-Dimensional van der Waals Materials. ACS Nano 16, 6960–7079. https://doi.org/10.1021/acsnano.1c09150

Yamamoto, T., Matsumoto, R., Nozaki, T., Imamura, H., Yuasa, S., 2022. Developments in voltage-controlled subnanosecond magnetization switching. Journal of Magnetism and Magnetic Materials 560, 169637. https://doi.org/10.1016/j.jmmm.2022.169637

Yang, H., Valenzuela, S.O., Chshiev, M., Couet, S., Dieny, B., Dlubak, B., Fert, A., Garello, K., Jamet, M., Jeong, D.-E., Lee, K., Lee, T., Martin, M.-B., Kar, G.S., Sénéor, P., Shin, H.-J., Roche, S., 2022. Two-dimensional materials prospects for non-volatile spintronic memories. Nature 606, 663–673. https://doi.org/10.1038/s41586-022-04768-0

Zare Rameshti, B., Viola Kusminskiy, S., Haigh, J.A., Usami, K., Lachance-Quirion, D., Nakamura, Y., Hu, C.-M., Tang, H.X., Bauer, G.E.W., Blanter, Y.M., 2022. Cavity magnonics. Physics Reports, Cavity Magnonics 979, 1–61. https://doi.org/10.1016/j.physrep.2022.06.001

Železný, J., Wadley, P., Olejník, K., Hoffmann, A., Ohno, H., 2018. Spin transport and spin torque in antiferromagnetic devices. Nature Phys 14, 220–228. https://doi.org/10.1038/s41567-018-0062-7

Zheng, C., Zhu, K., Cardoso de Freitas, S., Chang, J.-Y., Davies, J.E., Eames, P., Freitas, P.P., Kazakova, O., Kim, C., Leung, C.-W., Liou, S.-H., Ognev, A., Piramanayagam, S.N., Ripka, P., Samardak, A., Shin, K.-H., Tong, S.-Y., Tung, M.-J., Wang, S.X., Xue, S., Yin, X., Pong, P.W.T., 2019. Magnetoresistive Sensor Development Roadmap (Non-Recording Applications). IEEE Transactions on Magnetics 55, 1–30. https://doi.org/10.1109/TMAG.2019.2896036

Žutić, I., Fabian, J., Das Sarma, S., 2004. Spintronics: Fundamentals and applications. Rev. Mod. Phys. 76, 323–410. https://doi.org/10.1103/RevModPhys.76.323
Žutić, I., Matos-Abiague, A., Scharf, B., Dery, H., Belashchenko, K., 2019. Proximitized materials. Materials Today 22, 85–107. https://doi.org/10.1016/j.mattod.2018.05.003

**Optional Elements**

**FIGURES/TABLES:** Embedded in the manuscript

**GLOSSARY:** Abbreviations used in the text:

AFM antiferromagnet

AMR anisotropic magnetoresistance

BEOL, FEOL back-end-of-line, front-end-of-line

BP black phosphorous

CMOS complementary metal-oxide semiconductor

CPP current-perpendicular-to-plane

DW Domain wall

FL free layer

GMR giant magnetoresistance

HC Heusler compound

ISGE inverse spin galvanic effect

IP in-plane

MTJ magnetic tunnel junction

MRAM magnetoresistive random access memory

PMA perpendicular magnetic anisotropy

OOP out of plane

RA resistance-area

REE Rashba-Edelstein effect

RL reference layer

SAF synthetic antiferromagnet

SHE spin Hall effect

SMR spin Hall magnetoresistance

SOI spin-orbit interaction

SOT spin-orbit torque

SRAM static random-access memory

STT spin transfer torque

SW spin wave

TMDC transition metal dichalcogenide

TMR tunnel magnetoresistance

TI topological insulator

USMR unidirectional spin Hall magnetoresistance

VCMA voltage control of magnetic anisotropy

vdW van der Waals

YIG yttrium iron garnet

**RELEVANT WEBSITES:**
- https://www.mram-info.com/

- IEEE Magnetic society: https://ieeemagnetics.org/

- EMA: https://magnetism.eu/

## ACKNOWLEDGEMENTS

SOV and JFS acknowledge support from the Spanish Ministry of Science and Innovation (MCIN) and the Spanish Research Agency (AEI/10.13039/501100011033) through grants PID2019-111773RB-I00, RYC2019-028368-I and Severo Ochoa CEX2021-001214-S. SOV and OK acknowledge support from the European Innovation Council (EIC), call HORIZON-EIC-2021-PATHFINDEROPEN-01 under GA 101046630 PALANTIRI. PG acknowledges support from ETH Zurich and the Swiss National Science Foundation through grant 200020_200465. KG acknowledges support from France 2030 government grant managed by the French National Research Agency ANR-22-PEEL-0008.

## CONFLICT OF INTEREST

N/A