# Large Sign Reversal of Tunneling Magnetoresistance in an Epitaxial Fe/MgAlO x /Fe 4 N Magnetic Tunnel Junction

Tianyi Ma, Yu Zhu, Pambiang Abel Dainone, Tongxin Chen, Xavier Devaux, Caihua Wan, Sylvie Migot, Gwladys Lengaigne, Michel Vergnat, Yu Yan, et al.

► To cite this version:

Tianyi Ma, Yu Zhu, Pambiang Abel Dainone, Tongxin Chen, Xavier Devaux, et al. Large Sign Reversal of Tunneling Magnetoresistance in an Epitaxial Fe/MgAlO x /Fe 4 N Magnetic Tunnel Junction. ACS Applied Electronic Materials, 2023, 10.1021/acsaelm.3c00937. [https://doi.org/10.1021/acsaelm.3c00937](https://doi.org/10.1021/acsaelm.3c00937)

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# Large sign-reversal of tunneling magnetoresistance in epitaxial Fe/MgAlOₓ/Fe₄N magnetic tunnel junction

Tianyi Ma,<sup>1,2</sup> Yu Zhu,<sup>3</sup> Pambiang Abel Dainone,<sup>2</sup> Tongxin Chen,<sup>2</sup> Xavier Devaux,<sup>2</sup> Caihua Wan,<sup>1</sup>
Sylvie Migot,<sup>2</sup> Gwladys Lengaigne,<sup>2</sup> Michel Vergnat,<sup>2</sup> Yu Yan,<sup>3</sup> Xiufeng Han,<sup>1*</sup> Yuan Lu<sup>2*</sup>

<sup>1</sup>Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, China.

<sup>2</sup>Institut Jean Lamour, CNRS-Université de Lorraine, UMR 7198, 54011 Nancy, France

<sup>3</sup>Key Laboratory of Physics and Technology for Advanced Batteries, Department of Physics, Jilin University, Changchun 130012, China

Corresponding authors: * yuan.lu@univ-lorraine.fr; * xfhan@iphy.ac.cn

## Abstract

A sign-reversible tunneling magnetoresistance (TMR) bestows an extra control freedom to design TMR-based spintronic devices for developing spin logic applications. Here, we demonstrate a large sign-reversal of TMR in an epitaxial Fe/MgAlOₓ/Fe₄N magnetic tunnel junction (MTJ) controlled by the bias voltage. At room temperature (RT), the TMR is measured as large as -26.7% (-38% under optimistic definition) at V=+0.45V and it changes the sign to be +3.2% at V=-0.6V. The TMR sign-reversal effect is doubled compared to the Fe/MgO/Fe₄N MTJs, which is attributed to the better lattice mismatch between Fe₄N and MgAlOₓ as well as less N diffusion inside the tunneling barrier. First principle calculations reveal that the change of TMR sign originates from different symmetry-dependent tunneling channels between Fe and Fe₄N electrodes under opposite bias voltages. The Fe/MgAlOₓ/Fe₄N MTJ with voltage-controllable TMR signs and relatively large TMR ratios at RT will promote the development of versatile and reprogrammable spintronic logic applications.

**Keywords:** magnetic tunnel junction, inverse magnetoresistance, symmetry-dependent tunneling, magnetic random-access memories, spin logic
Introduction

The tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJ) is one of
cornerstones of spintronics¹⁻⁵ that has resulted in broad applications such as magnetic random-access
memories (MRAM), magnetic sensors, spin logics and neuromorphic computing devices.⁶⁻¹¹
Pioneering works has shown that a single MTJ can work as a multi-functional logic gate by properly
configurating magnetizations of both free and reference magnetic layers.¹² However, switching the
reference layer consumes redundant energy and lowers the thermal stability of devices. The
multifunctional logic gate can also be realized in magnetic heterostructures where the perpendicular
magnetization of the free layer is switched by the spin-orbit torque and controlled by an effective in-
plane field.¹³,¹⁴ However, reading out signals from these logic gates relies on the anomalous Hall
effect which is less sensitive than the TMR effect. Recently, a novel proposal¹⁵ emerges to build such
spin logic devices by utilizing MTJs with voltage-controllable and sign-reversible TMR effect.
Combining two of these MTJs and one current comparator, a real-time reprogrammable logic gate
can be built, in which all four functions: AND, OR, NAND and NOR are realized and switchable by
initializing only the free-layer magnetization of the two MTJs.¹⁵ To realize such spin logic function,
a MTJ system with large sign-reversible TMR at room temperature (RT) is indispensable.

The sign-reversible TMR phenomena have been reported in several systems due to various
mechanisms. For instance, the TMR ratio of the NiFe/composite Ta<sub>2</sub>O<sub>5</sub>/Al<sub>2</sub>O<sub>3</sub>/NiFe MTJs varies from
+1% to -4% with different bias at RT.<sup>16</sup> Its sign inversion is resulted from different densities of states
(DOS) at two electrode/barrier interfaces. In Ni/NiO/Co nanowires, the TMR ratio can be inverted by
voltage due to the matching of the energy of localized states in NiO, which is sensitive to the quality
of the barrier, with the Fermi energy of ferromagnetic electrodes.<sup>17</sup> In
La<sub>0.67</sub>Sr<sub>0.33</sub>MnO<sub>3</sub>(LSMO)/SrTiO<sub>3</sub>(STO)/SrO/Co MTJs, their TMR ratio varies from +8% to -15%
with applied voltage at 85 K.¹⁸ The sign of TMR ratio is found to be very sensitive to the interface spin polarization at SrTiO₃/Co interface with one atomic insertion layer of TiO₂ or SrO as well as the oxygen content in the barrier. In addition, MTJs with Fe₃O₄ electrode and MgO barrier also show a reversal TMR sign effect. Depending on the materials used for the second electrode (Co¹⁹, CoFeB²⁰ and Fe²¹), the maximum negative TMR have been demonstrated to be -8.5%, -12% and -9.6% at RT, respectively. Recently, the multiferroic tunnel junctions (MFTJs) combining the ferroelectric tunnel barrier with ferromagnetic electrodes attract intensive interests. In such systems, TMR and tunneling electroresistance (TER) coexist and the sign of TMR can be modulated by changing the ferroelectric polarization of ferroelectric tunnel barrier. In Co/PbZr₀.₂Ti₀.₈O₃(PZT)/LSMO MFTJ, the TMR ratio varies from +4% to -3% with a ±3 V polarizing voltage pulse at 50 K.²² In LSMO/ Polyvinylidene fluoride(PVDF) or P(VDF-TrFE)/Co MFTJs, the TMR ratio varies from +8.3% to -12.5% with a ±1.5 V polarizing voltage pulse at 10 K.²³,²⁴ Note that LSMO is usually chosen as the ferromagnetic electrode to obtain high spin polarization and well-defined ferromagnetic/ferroelectric interfaces. However, the low Curie temperature of LSMO hampers its room-temperature applications. Another issue for the MFTJ is that the TMR will be suppressed if the ferroelectric barrier is too thick, since the transport mechanisms are different from direct tunnelling.²⁵ Thus, a trade-off relating to an optimized thickness of the ferroelectric barrier must be taken into account to keep both effects (TMR and TER) sizable.²⁴

Although many systems have been demonstrated for the sign-reversal TMR effect, the amplitude of TMR is small and operation temperature is low. Recently, Fe₄N has been demonstrated to have high negative spin-polarization,²⁶⁻²⁸ unique magnetic structure and low critical switching current density.²⁹,³⁰ These merits co-make Fe₄N MTJs suitable for spintronic applications and also a potential candidate to achieve the sign-reversible TMR effect. In this work, we have epitaxially grown
Fe/MgAlOₓ/Fe₄N MTJ by molecular beam epitaxy (MBE). We have observed a voltage-dependent TMR ratio which can be modulated from +3.2% to -26.7% at RT, showing a superior performance than previous reported results. From first principle calculations, the sign-reversible TMR phenomenon is found to be relevant to the symmetry-dependent tunneling process between Fe and Fe₄N electrodes. The voltage-controllable TMR signs and relatively large TMR ratios, especially at RT will contribute to develop the real-time reprogrammable spin logic gates for different spintronic applications.

## Results and discussions

*Structure and chemistry study by STEM combined with EELS*

The magnetic tunnel junction stack consisting of MgO(001) substrate||MgO(10 nm)/Fe(45 nm)/MgAlOₓ(0.6 nm)/Fe(7 nm)/MgAlOₓ(2.5 nm)/Fe₄N(3 nm)/Co(20 nm)/Au(10 nm) was grown in a MBE system. The epitaxial growth and annealing process were controlled by high-energy electron diffraction (RHEED). The detailed growth condition can be found in **Methods** and **Supporting Information (SI) Notes 1 and 2**. For comparison, another sample with MgO tunneling barrier was grown consisting of stack structures: MgO(001) substrate||MgO(10 nm)/Fe(45 nm)/MgO(2.5 nm)/Fe₄N(3 nm)/Co(20 nm)/Au(10 nm). High-resolution scanning transmission electron microscope (HR-STEM) characterizations were performed to verify the structure of the MTJs. Figs. 1(a,b) shows the high-angle annular dark-field (HAADF) images for MgAlOₓ MTJ in a large and magnified scale, respectively. In Fig. 1(a), one can observe sharp interfaces between MgAlOₓ (in black contrast) and Fe or Fe₄N layers (in grey contrast). The MgAlOₓ barrier layer appears quite flat and continuous, indicating a high quality of MTJ structure without any pinholes. The magnified image in Fig. 1(b) further proves the epitaxial growth characters with well-aligned atomic arrays between Fe, MgAlOₓ
and Fe₄N layers. Bulk Fe₄N has a cubic antiperovskite structure, and the lattice constant is reported
to be 3.795 Å in experiment.³¹ Despite the large lattice mismatch of 6.5% between Fe₄N and MgAl₂O₄
(lattice constant 4.212Å³¹), the epitaxial crystalline relationship can be determined to be
Fe[110]//MgAlOₓ[100]//Fe₄N[100] combined with RHEED analysis (see **SI Note 1** and **3**). However,
Co on Fe₄N exhibits large disorientated polycrystalline features. For MTJ with MgO barrier, one can
also observe an epitaxial growth of Fe₄N on MgO (see **SI Note 2** and **4**), however the intensity of
RHEED appears weaker and more crystalline deformation can be found in the Fe₄N layer (see **SI
Figs. S2** and **S3(b))**, which could be attributed to the larger lattice mismatch (11%) between Fe₄N
and MgO (see more discussion in **SI Note 3**).

To characterize the interfacial chemistry structure of MTJ multilayers, the element distribution
analyses were performed by STEM combined with spatially resolved electron energy loss
spectroscopy (EELS) characterizations. Figs. 1(d-i) show semi-quantitative chemical maps drawn by
processing the EELS spectrum images. A sharp chemical interface is found between Fe and MgAlOₓ
layers. However, from the element mapping of N (Fig. 1(d)), a partial N diffusion towards MgAlOₓ
barrier can be detected at MgAlOₓ/Fe₄N interface. This N diffusion can be also clearly evidenced by
a larger peak width (red line) compared to the Fe peak (blue line) in the element intensity profiles, as
shown in Fig. 1(c). Here, a background signal of Al can be found, which is mainly due to the artefact
during the signal processing of EELS peaks. For MTJs with MgO barrier, the situation of N diffusion
is more pronounced. In SI Figs. S3(c,d) (see SI Note 4), one can observe that N can penetrate MgO
barrier and even reach the bottom Fe/MgO interface. The diffusion of N could be attributed to the
oxygen vacancy in the MgO and MgAlOₓ barriers. Since the bond energy for Fe-N (350 kJ/mol)<
Mg-O (380 kJ/mol) < Mg-N (400 kJ/mol) < Al-O (510 kJ/mol) < Al-N (650 kJ/mol),³² ionic N species
tend to diffuse into MgO or MgAlOₓ barrier to occupy oxygen vacancy position during Fe₄N growth.
For MgAlOₓ barrier, the formation of Al-N bonds can help to stop further diffusion of N inside the barrier. Nevertheless, the N diffusion can easily penetrate MgO barrier because of the low bond energy of Mg-N, which is comparable to Fe-N bond, thus resulting in an accumulation of N at the bottom Fe/MgO interface. The element concentration in the MgAlOₓ barrier is found to be Mg:Al:O=1:0.2:1.3, far from the stoichiometric condition for spinel MgAl₂O₄ structure (Mg:Al:O=1:2:4), indicating possible Al and O vacancies in our MgAlOₓ barrier. In addition, the ratio of Fe and N in the Fe₄N layer (5.2:1) slightly deviate from the stoichiometric condition of 4:1.

## Magneto-transport study of MTJs

The magneto-transport measurements have been performed by a dc two-probe configuration, where the negative bias corresponds to the electrons tunneling from the top to bottom electrode (Fig. 2(a)). Although there are two MgAlOₓ insulating layers in the structure, the junction resistance is mainly dominated by the thicker one. Figs. 2(b,c) show the TMR curves measured at RT and 10K for bias voltages *V*=+0.45V and *V*=-0.6V, respectively. It is interesting to find that the TMR sign can be reversed with different voltages. At 300K, the TMR can be modulated from -27% to +3%. At 10K, the TMR sign reversal effect is enhanced from -40% to +5%. Here, both positive and negative TMR has been calculated by the definition of TMR ratio=(R<sub>AP</sub>-R<sub>P</sub>)/R<sub>P</sub>×100%, where R<sub>P</sub> and R<sub>AP</sub> are the junction resistance when the magnetizations of both electrodes beside the tunneling barrier are in parallel (P) and antiparallel (AP) configurations, respectively. This definition limits the negative TMR ratio up to 100%. Under optimistic definition (TMR ratio=(R<sub>AP</sub>-R<sub>P</sub>)/R<sub>AP</sub>×100%)), our maximum negative TMR is -70% at 10K and -38% at 300K, which is approaching the reported highest negative TMR ratios (-75%) in CoFeB/MgO/Fe₄N MTJs.<sup>33,34</sup> However, no TMR sign change has been observed in those systems.<sup>33,34</sup> In the TMR curves at 10K, it is found that the AP plateaus are not well established with an asymmetric form at positive and negative magnetic field. This could be due to the
complicated magnetic structure in Fe₄N, which has a partial out-of-plane magnetic component²⁹ (see **SI Note 5**). **Fig. 2(d)** shows the resistance in P and AP states as a function of temperature for *V*=+0.45V and *V*=-0.6V. The corresponding variation of TMR as a function of temperature is shown in **Fig. 2(e)** (see more details in **SI Note 6**). It is found that the resistance of junction gradually reduces with increase of temperature. However, the variation of resistance is lower than 25% for the largest case, which is an important signature to exclude either pinhole dominated tunneling for thin barrier (increased resistance with *T*)³⁵ or hopping dominated tunneling through homogenous localized defect states in thick barrier (exponentially reduced resistance with increasing *T*).²⁵ Although inelastic tunneling through inhomogeneous defects may result in a small temperature dependence,¹⁷ it should be significantly reduced at low temperature. Therefore, our TMR at low temperature is mainly dominated by coherent direct tunneling or elastic tunneling.

To have more insight on the TMR sign change for the Fe/MgAlOₓ/Fe₄N MTJ, we measured TMR values at different bias at 10K, as shown in the blue dots in **Fig. 2(f)** (see more details in **SI Note 7**). It is clearly that TMR changes its sign at about -0.2 V. Below this voltage, TMR almost keeps constant with positive TMR around +5%. Above -0.2 V, the TMR becomes negative and the value varies a lot with a maximum at +0.5V. For comparison, we have also plotted the TMR values from a Fe/MgO/Fe₄N MTJ (red dots in **Fig. 2(f)**). Both samples show the same trend of TMR sign change, but the negative TMR ratio of MTJ with MgO barrier is only half of that with MgAlOₓ barrier. Please find more characterizations of MTJs with MgO barrier in **SI Note 8**. The superior performance of MgAlOₓ based MTJ could be attributed to the better lattice mismatch between Fe₄N and MgAlOₓ as well as less N diffusion inside the barrier which have been observed in the above STEM characterizations.

To further understand the bias-dependent TMR, we set the MTJ into P and AP states respectively
by applying appropriate in-plane field, and measured the bias dependence of differential conductance $dI/dV$ at different temperatures (Figs. 3 (a,b)). All $dI/dV$ curves in both states have been normalized by $G_p$ ($V=0, T=10$ K). Fig. 3(c) shows the normalized differential conductance for both states at 10K. When $V<-0.1$V, $G_p$ is always higher than $G_{ap}$, resulting a positive TMR. However, when $0V<V<0.3$V $G_{ap}$ increases faster than $G_p$, resulting a large negative TMR. At $V>0.3$V, the increase of $G_p$ becomes more important than $G_{ap}$, which effectively reduces the negative TMR. Fig. 3(d) displays the bias dependence of TMR deduced by the $I-V$ curves ($R=V/I$) at different temperatures, showing a good agreement with the bias dependent TMR ratios measured directly from the $R-H$ curves in Fig. 2(f). The large attenuation of negative TMR with temperature can be found, which is mainly due to the large increase of $G_p$ while $G_{ap}$ almost keeps constant with the temperature, as shown in Figs. 3(a,b).

It is noticed that there are some small conductance oscillations appearing when $V<0$V in Fig. 3 (a-c) in both P and AP states. These oscillations can be explained by the electron tunneling in the quantum well (QW) states formed in the MgAlOx/Fe/MgAlOx structure. However, the oscillation amplitudes are much smaller than that observed in our previous studied single or double QW MTJs with all Fe electrodes³⁶,³⁷ (see SI Note 9). This can be understood that the top Fe₄N electrode has no majority electrons with $\Delta_1$ symmetry at the Fermi level ($E_F$) to match the bottom Fe QW states (formed by majority $\Delta_1$ bands).³⁶,³⁷ The main origin of TMR sign change should not be related to the QW states since similar variation of conductance in P and AP states can also be evidenced in MgO based MTJs where there is no QW structure (see SI Note 8).

In addition, it is well-known that for MBE samples the feature in $dI/dV$ at -0.2 V could be due to the interface resonance state (IRS) at Fe/MgO interface.³⁸ To clarify if IRS plays a role on the transport properties, we have conducted the inelastic electron tunneling spectroscopy (IETS) experiments through the derivative of the dynamic conductance ($d²I/dV²$). However, no IRS signature
can be found at the MgO/Fe₄N, MgAlOₓ/Fe₄N, or Fe/MgAlOₓ interfaces, thus excluding the influence
of IRS on TMR reversal (see **SI Note 10**).

Since N diffusion inside the barrier could lead to the creation of inhomogeneous defects, we
should also consider the possibility of TMR reversal due to the inelastic tunneling through these
defects. Indeed, the inelastic tunneling through inhomogeneous localized states inside the barrier
could potentially lead to a similar TMR sign reversal effect.¹⁷,³⁹ However, the observed TMR ratio
and TMR sign reversal effect are highly device-specific, given that the defects introduced in the
barrier during growth are highly random. Moreover, to observe such inversion of TMR due to the
localized defect states, the junction area must be sufficiently small, displaying two-level fluctuations
of electric current and indicating impurity/defect-driven transport.⁴⁰ On the contrary, for large-size
junctions (ranging from a fraction of µm² to a few mm²), the resultant conductance is the sum over a
large number of local disorder configurations. On average, this leads to a reduction in TMR, and the
TMR sign reversal is no longer observed.⁴¹ In our case, our junction has a relatively large area (10×10
to 40×40 µm²) with high structural quality and homogeneity. Additionally, MTJs with different
barriers (MgO or MgAlOₓ) exhibit almost identical TMR reversal behavior. Therefore, the
mechanism involving inelastic tunneling through inhomogeneous defects should be excluded.

*First principle calculations of the band structure*

To understand the origin of the *V* dependence of TMR ratio, first principle calculations have
been performed to investigate the coherent tunneling in MTJ closely linked with the spin and
symmetry of the electrons in both electrodes. Since Fe and Fe₄N have different crystalline structures,
we have defined the Γ-X in the band structure for the transport direction (*k*//=0) in both cases for the
reason of uniformity. **Fig. 4(a)** shows the symmetry and spin-resolved band structures along transport
direction (Γ-X) for bulk Fe and Fe₄N. For bcc Fe, the majority-spin Δ₁ band crosses the Fermi level
while the minority-spin $\Delta_1$ band situates well above $E_F$, which reveals a half-metal nature in terms of $\Delta_1$ state along the transport direction.⁴² For Fe₄N, the minority $\Delta_1$ and $\Delta_5$ bands cross the Fermi level, while no majority bands cross $E_F$ along the transport direction, which is in a good agreement with the calculations reported by Yang *et al.*²⁸ This indicates that Fe₄N also reveals a half-metal nature in terms of both $\Delta_1$ and $\Delta_5$ states. Since in single crystal Fe(001)/MgO/Fe⁴³ or Fe(001)/MgAl₂O₄/Fe⁴⁴ MTJs, MgO and MgAl₂O₄ tunneling barriers can efficiently filter electrons with other symmetries than $\Delta_1$ and $\Delta_5$ bands, it is reasonable to consider that only the $\Delta_1$ and $\Delta_5$ bands are mainly involved in the tunneling process. Previous theoretical work in Fe₄N/MgO/Fe₄N MTJ also reveals that not only the $\Delta_1$ band but also $\Delta_5$ band electrons play an important role for the symmetry dependent coherent transmission.²⁸

To make the analysis clear, the symmetry and spin-resolved DOS for $k_\parallel=0$ of Fe and Fe₄N is plotted in **Figs. 4(b-e)** as a function of the energy. Generally, the conductance is created by the electrons tunneling from an occupied band of one electrode to the unoccupied band of the other electrode. Therefore, the conductance of the P state is determined by: Majority (↑) to Majority (↑) and Minority (↓) to Minority (↓) band tunneling while the conductance of the AP state is determined by: Majority (↑) to Minority (↓) and Minority (↓) to Majority (↑) band tunneling.

**Figs. 4(b,c)** show the tunneling through $\Delta_1$ and $\Delta_5$ bands under positive bias, respectively. Since there are no DOS near the Fermi energy for the majority bands of Fe₄N, the tunneling channels when $V>0$V are mainly from: (1) Fe $\Delta_1$↑ to Fe₄N $\Delta_1$↓, (2) Fe $\Delta_5$↑ to Fe₄N $\Delta_5$↓ for AP conductance and (3) Fe $\Delta_5$↓ to Fe₄N $\Delta_5$↓ for P conductance. From **Fig. 3(c)**, we know that the normalized $dI/dV$ in the AP state increases fast first when $0<V<0.2$ V, then slows down when $0.2<V<0.4$ V, and increases fast again when $V>0.4$ V. This fast-slow-fast increase of conductance is well attributed to the strong Fe₄N $\Delta_1$↓ peak around 0.2 V in the AP tunneling channel (1), as shown in **Fig. 4(b)**. Notice that the channel
(2) is less important since the DOS of Fe₄N Δ₅↓ near E_F is smaller than that of Fe₄N Δ₁↓. The low DOS intensity in Fe₄N Δ₅↓ also limits the P conductance channel (3). This can well explain the observed large negative TMR ratio. In Figs. 3(a,b), the temperature dependence of conductance shows that AP conductance almost keeps constant, while P conductance increases with temperature at positive bias. This results in the rapid decrease of negative TMR with increase of T, which can be understood as following. Since the thermal effect induces the phonon scattering, the distribution of DOS as a function of the energy will be smeared. Therefore, the conductance in channel (1) reduces while that in channels (2) and (3) increases, so that channels (1) and (2) compensate together resulting in a small variation of the AP conductance while the P conductance increases with the increase of conductance in channel (3).

Under negative bias as shown in Figs. 4(d,e), the tunneling channels are mainly from: (4) Fe₄N Δ₁↓ to Fe Δ₁↑ and (5) Fe₄N Δ₅↓ to Fe Δ₅↑ for AP conductance and (6) Fe₄N Δ₅↓ to Fe Δ₅↓ for P conductance. From Fig. 3(c), the changes of the normalized dI/dV under negative bias are similar in the AP and P states, and the normalized dI/dV in the P state is slightly higher than that in the AP state. From Figs. 4(d,e), the AP conductance is mainly determined by the channel (4) since the DOS of Fe Δ₅↑ in channel (5) decreases rapidly with higher energy. By comparison of channels (4) and (6) for AP and P conductance, the DOS of the Fe Δ₅↓ in (6) is larger than that of Fe Δ₁↑ in (4). This can explain the higher P conductance observed in normalized dI/dV curves in Fig. 3(c), which results in the positive TMR ratio. However, since there is no distinguished DOS peak in Fe Δ₅↓, the positive TMR ratio is less pronounced than the negative TMR ratio. Here, we can qualitatively explain the experimental observation by the simple analyses based on the band structure of bulk Fe₄N and Fe. A more rigorous model should be taken into account the electrodes, barrier and interfaces together, which will give more insight of the transport process.
## Conclusion

In summary, we have fabricated epitaxial Fe/MgAlOₓ/Fe₄N MTJs by MBE growth. The voltage-controlled and sign-reversible TMR effect is demonstrated from 10 K to 300 K. At RT, the positive TMR ratio of +3.2% at V=-0.6 V can be changed to the negative TMR ratio of -26.7% (-38% under the optimistic definition) at V=+0.45 V. The voltage-dependence of TMR ratio can be well explained by a simple model taking account of the symmetry and spin-resolved band structures of Fe and Fe₄N. This new freedom of TMR turnability paves a way for the development of new spintronic devices for real-time reprogrammable spin logic gate applications.

## Methods

### Sample preparation

The main stack MgO(001)‖MgO(10 nm)/Fe(45 nm)/MgAlOₓ(0.6 nm)/Fe(7 nm)/MgAlOₓ(2.5 nm)/Fe₄N(3 nm)/Co(20 nm)/Au(10 nm) was deposited in a MBE system with a base pressure better than 1×10⁻¹⁰ torr. Firstly, the substrate was annealed at 700°C for 60 mins and 10 nm MgO seed layer was deposited. Then the first 45 nm thick Fe layer was deposited at RT by e-beam evaporation and annealed *in situ* at 600°C for 30 mins to smooth the surface. The first 0.6 nm thick MgAlOₓ barrier was grown at about 70°C and the growth process was monitored by reflection high-energy electron diffraction (RHEED, see **SI Note 1**). Two-dimensional layer-by-layer growth of MgAlOₓ was observed by RHEED intensity oscillation. The second 7 nm thick Fe layer was deposited at about 70°C and annealed *in situ* at 500°C for 20 mins followed by a deposition of 2.5 nm thick MgAlOₓ barrier. During the growth of Fe₄N films on MgAlOₓ, the Fe source was heated to 1250°C by Knudsen cell, and the substrate temperature was kept at 400°C with a nitrogen partial pressure of 2×10⁻⁵ Torr in the MBE chamber. The PCS-ECR (plasma cracker source - electron cyclotron resonance) source
was used with the following characteristics: $V_{anode}=400$ V, $V_{extractor}=-200$ V, $I_{magnetron}=40$ mA. Finally, 20 nm thick Co layer was deposited above the Fe₄N layer to enhance the coercivity and 10 nm thick Au was used to prevent the films from oxidation. A control stack MgO(001)//MgO(10 nm)/Fe(45 nm)/MgO(2.5 nm)/Fe₄N(3 nm)/Co(20 nm)/Au(10 nm) was also deposited through a similar procedure. More detailed MTJ growth can be found elsewhere.³⁶,³⁷,⁴³

The multilayers were then patterned into junctions with the area size varying from 10×10 to 40×40µm² by standard ultraviolet lithography combined with Ar⁺ ion etching. The transport measurements were carried out in a cryostat (cooled by liquid helium) with electrical magnets. A Keithley 2450 was used to apply the bias voltage to the MTJ and probe the current accordingly.

## First principle calculations

The first principle calculations were performed using the projector-augmented-wave (PAW) method⁴⁵ as implemented in the Vienna *ab initio* simulation package (VASP).⁴⁶⁻⁴⁸ An energy cutoff of 500 eV and Perdew-Burke-Ernzerhof generalized-gradient approximation (GGA-PBE)⁴⁹ for the exchange-correlation function were used throughout. A 21×21×21 k-point mesh are performed for structural relaxations and self-consistent calculations for Fe₄N until the force on each atom is smaller than 0.01 eV/Å and the total energy is converged to be less than 1×10⁻⁵ eV. The bulk Fe₄N has a cubic antiperovskite structure and the experimental lattice constant is 3.795 Å. We also calculate the bcc Fe to analyze the tunneling between these two electrodes.

## STEM-EELS characterization

HR-STEM combined with spatially resolved EELS was performed by using a probe-corrected microscope JEOL ARM200F (cold FEG) equipped with a GATAN GIF quantum energy filter to
reveal the structure and element distribution. The microscope was operated at 200 kV. High angle
annular dark-field (HAADF), annular dark-field (ADF) and bright-field (BF) images were
simultaneously recorded for investigating the heterostructure while only HAADF signal was recorded
during EELS mapping. EELS spectrum images were recorded with a probe current of about 50 pA.
Two EELS spectrum images were simultaneously recorded: one for the low-loss part containing the
zero-loss, the other for the core loss, which allows advanced data post processing (correction of
energy drift, multiple scattering corrections). A multivariate statistical analysis software (temDM
MSA) was used to improve the quality of the STEM-EELS data by de-noising the core-loss spectrum
images before its processing to draw quantitative chemical maps.⁵⁰ Thin lamellas were extracted by
focused ion beam (FIB) milling using an FEI Helios Nanolab 600i dual beam.

Acknowledgement

We acknowledge Ludovic Pasquier for the support of MBE growth. This work is supported by French National Research Agency (ANR) SOTspinLED (No. ANR-22-CE24-0006-01) project. Experiments were performed using equipments from the platform TUBE-Davm and the platform CCMEM both funded by FEDER (EU), ANR, the Region Lorraine and Grand Nancy.

**Supporting Information**

RHEED patterns of the MTJ with MgAlOₓ barrier; RHEED patterns of the MTJ with MgO barrier; Lattice mismatch of Fe₄N/MgO and Fe₄N/MgAlOₓ; Structure characterization of the MTJ with MgO barrier; Magnetic properties of Fe₄N layer; Temperature dependence of the TMR ratio of the MTJ with MgAlOₓ barrier; Bias dependence of the TMR measurements of the MTJ with MgAlOₓ barrier; Bias dependence of TMR of the MTJ with MgO barrier; Elimination of the QW structure's influence on TMR enhancement; Elimination of the interface resonant states' influence on TMR
reversal.
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<figure><img src="image_2.png" /><figcaption>**Figure 1:** Interfacial structures and chemical maps of MTJ stack. STEM-HAADF images with different scales are shown in (a) and (b). Normalized elements density profiles and elements maps extracted from STEM-EELS are shown in (c) and (d-i), respectively.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Figure 2:** TMR measurement results of the Fe/MgAlOₓ/Fe₄N MTJ. (a) Schematic of the measurement setup. (b,c) TMR curves measured at RT and 10 K for bias voltage (b) *V*=+0.45 V and (c) *V*=-0.60 V, respectively. (d) Temperature dependence of the P and AP state resistance when *V*=+0.45 V and -0.60 V. (e) Temperature dependence of the TMR ratios when *V*=+0.45 V and -0.60 V. (f) Bias dependence of TMR ratios of Fe₄N MTJs with MgAlOₓ and MgO barriers at 10 K.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 3:** Bias dependence of the differential conductance *dI/dV* for Fe/MgAlOx/FeN MTJ. (a,b) Bias dependence of normalized *dI/dV* (divided by *G*<sub>p</sub>(0V, 10K)) at different temperatures in (a) P and (b) AP states, respectively. (c) Bias dependence of normalized *dI/dV* for both states measured at 10 K. (d) Bias dependence of the TMR ratio at different temperatures deduced by the *I-V* measurements.</figcaption></figure>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

**Figure 4:** First principle calculations of band structures of bulk Fe and Fe₄N. (a) Symmetry and spin-resolved band structures of bulk Fe and Fe₄N along the transport direction (Γ-X). (b-e) Symmetry and spin-resolved DOS for *k*//=0 of bulk Fe and Fe₄N as a function of the energy; (b,c) correspond to the tunneling under the positive bias, and (d,e) correspond to the tunneling under the negative bias. Dash lines represent the tunneling channels.
TOC figure:

<figure><img src="image_11.png" /></figure>