Patent Description:
The spin-transfer torque magnetic random-access memory (or "STT-MRAM") is one of the most promising emerging non-volatile memory technologies. It combines non-volatility with a quasi-infinite write endurance, high speed, low power consumption and scalability. These properties have stimulated the commercial production of STT-MRAM for a variety of stand-alone and embedded applications, in particular NOR flash memory replacement and last-level cache static RAM (or "SRAM") replacement.

A STT-RAM is based on a magnetic tunnel junction (or "MTJ"), said MTJ comprising a reference layer and a free layer, separated by a tunnel layer. The reference layer has a magnetization with a fixed orientation and the free layer has a magnetization with a switchable orientation. MTJs with synthetic antiferromagnetic (SAF) free layer structure, comprising at least two planar magnetic layers antiferromagnetically coupled by a spacer layer, are known from <CIT> or <NPL>, for example.

Nowadays MTJ rely on out-of-plane orientation of the magnetizations in the free and the reference layers (the out-of-plane orientation being considered with respect to the plane of the free layer of MTJ). This has been shown to lead to a better scalability of the thermal stability factor Δ <MAT> where EB is the energy barrier one needs to overcome to reverse the magnetization from an out-of-plane orientation to an opposite one, kB is the Boltzmann constant and T is the operating temperature. The thermal stability factor determines the memory retention time and impacts the switching current needed to switch the magnetization from one orientation to another one.

Specific devices, called perpendicular MTJ (or "p-MTJ"), rely on the use of an interfacial magnetic anisotropy existing at the interface between a free layer, usually based on FeCoB, and a MgO layer (such as the tunnel barrier). The interfacial magnetic anisotropy tends to overcome the shape to orient the magnetization out of the plane of the free layer.

Nonetheless, further downsize scalability of these p-MTJ below sub-<NUM> diameters faces a fundamental challenge. As the diameter of the junction shrinks, there is a decrease of the thermal stability factor Δ brought by the interfacial perpendicular magnetic anisotropy. This can be understood considering that the interfacial perpendicular magnetic anisotropy energy is roughly proportional to the surface area of the free layer.

A proposal to counter this decrease of stability is to add FeCoB/MgO interfaces to increase the total surface providing the interfacial perpendicular magnetic anisotropy. Nevertheless, this approach is quite restrictive in terms of material choice, dependent on the quality of the several interfaces and has a strong dependence with temperature.

To maintain a high thermal stability factor at low technological nodes, the document [<NPL>)] suggests taking advantage of the shape anisotropy of the free layer by increasing its thickness to values of the order or larger than the free layer diameter. Thereby, the shape anisotropy of the free layer no longer promotes an easy-plane orientation but further stabilizes the magnetization along a perpendicular orientation (with respect to the plane of the free layer). This can be understood considering the magnetostatic energy of a magnetized cylinder, as expressed in [<NPL>)]: <MAT> with τ the ratio <MAT> (L being the thickness of the free layer and r its radius), <MAT> a hypergeometric function, and mz the defined orientation of the magnetization of the free layer with a saturation magnetisation M<NUM> and a vacuum permeability µ<NUM>.

The <FIG> shows an example of the magnetostatic energy of a free layer against its aspect ratio (thickness against diameter) considering two distinct orientations of the magnetization (in the plane of said free layer and perpendicular the said plane). Results are obtained considering a uniform magnetization of <NUM> MA/m. The <FIG> shows a clear crossover between the in-plane orientation (black arrow and cross-hatched region) and the out-of-plane orientation (white arrow and plain black region). Moreover, this crossover is geometry dependent, and the magnitude of the magnetization is only playing a role in the strength of the magnetostatic energy. The total energy barrier can be expanded considering the magnetostatic energy presented previously and introducing the contribution of the interfacial anisotropy, defined by a surface anisotropy term ks, with a usual value of <NUM> mJ/m<NUM>. With this approach it is possible to maintain a high enough thermal stability factor to sub-<NUM> diameters MTJ. The <FIG> shows a thermal stability factor Δ for a free layer of different aspect ratios with a magnetization saturation of <NUM> MA/m and a surface anisotropy term ks of <NUM> mJ/m<NUM> for an operation temperature of <NUM>. It shows that, by increasing the thickness of the free layer, for a fixed diameter value, it is possible to increase the thermal stability factor Δ. This increase is stronger as the different anisotropy sources are increased, opening different possibilities of engineering this free layer.

The perpendicular shape-anisotropy (also called "PSA") provided by the thick free layer is drawing attention due to its scalability and wide temperature operation range. <FIG> shows an example of a magnetoresistive device comprising a p-MTJ 1a. <FIG> shows an example of a magnetoresistive device comprising a PSA magnetic tunnel junction 1b (also noted "PSA-MTJ"). Such a PSA-MTJ is disclosed in <CIT>, for example. In both cases, the devices 1a, 1b are composed of a seed layer <NUM>, a synthetic antiferromagnet <NUM> followed by a texture breaker <NUM> and an out-of-plane magnetized reference layer <NUM>. In each magnetic junction 1a, 1b, the reference layer <NUM> is separated from a thin free layer 16a by a tunnel barrier <NUM>. Each thin free layer 16a exhibits an out-of-plane magnetization due to the interfacial effect between said thin free layer 16a and the tunnel barrier <NUM> it lies on.

The PSA-MTJ 1b of <FIG> differs from the p-MTJ 1a of <FIG> in that it comprises a ticker free layer 16b staked on top of the thin one 16a, to promote a shape anisotropy that allows for higher stability. This additional layer 16b is called "perpendicular shape anisotropy layer" or "PSA layer", while the thin free layer 16a is called "perpendicular interfacial anisotropy layer" or "interfacial anisotropy layer".

When discussing a p-MTJ, only the contribution of the interfacial anisotropy layer 16a is considered. When discussing a PSA-MTJ, both the contributions of the interfacial anisotropy layer 16a and the PSA layer 16b are considered.

Even though the perpendicular shape-anisotropy approach to engineer a free layer of a MTJ is shown as a viable candidate for a small diameter non-volatile MTJ, some challenges still need to be overcome. Indeed, it has been shown that the increased aspect-ratio of the PSA-layer 16b has an impact on the reversal mechanism driven by spin-transfer-torque, which can be avoided by considering the thickness of the PSA-layer 16b and the width of the domain-wall that can be formed in the PSA-layer 16b. The reversal mechanism is improved by keeping the aspect-ratio of the PSA-layer 16b (thickness over width) small enough, as disclosed by the document [<NPL>).

Thus, there is a need to reduce the total thickness of the free layer while keeping a stable out of plane magnetization and fast reversal times.

The invention concerns a magnetic tunnel junction comprising a synthetic antiferromagnetic free layer (or "SyAF free layer" for short), said synthetic antiferromagnetic free layer comprising a magnetic core and a magnetic shell, the magnetic core comprising a first magnetic layer having an aspect ratio comprised between <NUM> and <NUM>, the magnetic shell surrounding the magnetic core and at least a part of the thickness of the magnetic core, the magnetization of the magnetic shell being antiferromagnetically coupled with the magnetization of the magnetic core.

The aspect ratio is equal to the thickness of a layer divided by the width said layer (or its diameter when it is a circular layer). The thickness is measured perpendicularly to a plane which correspond to the plane of the layers (or the plane of the substrate). The width is measured parallel to said plane.

Magnetizations antiferromagnetically coupled means that said magnetizations tend to orient antiparallel.

Due to the high aspect ratio of the first magnetic layer, it exhibits a magnetization spontaneously pointing out of the plane.

The high aspect ratio also helps to rigidly couple the magnetic core and the magnetic shell. Then, due to the additional magnetic volume of the SyAF free layer, brought by the magnetic shell, there is an increase in the stability of said SyAF free layer. Thus, the thickness of the magnetic core can be reduced to achieve suitable stability values for, for example, memory or sensor requirements.

The switching reversal of the magnetic core is improved, avoiding non-coherent switching reversals, leading to faster commutation times and lower switching voltages for a specified stability.

This approach shows viable for a magnetic tunnel junction used as a storage means or as a sensor.

The antiferromagnetic coupling also provides a reduced stray field of the SyAF free layer. As the magnetic material of the magnetic shell can be chosen to reduce the total footprint of the junction, it can also offer a significant improvement to integrate narrower magnetic tunnel junctions.

Beneficially, the magnetic shell and the magnetic core are separated from each other by a non-magnetic material. It can be a dielectric and non-magnetic material, such as air, ZnO, HfO<NUM>, Al2O<NUM> or TiN, or a non-magnetic material providing exchange coupling between magnetic shell and the magnetic core, such as Ru.

Beneficially, the magnetic shell surrounds exclusively the magnetic core. The magnetic shell does not surround a magnetic core belonging to another magnetic tunnel junction otherwise it would complexify the switching reversal of the magnetic core (it became a three bodies reversal mechanism) and reduce both the stability and the commutation speed. This exclusive surrounding also prevents from extending the magnetic shell along the junction. It could reach other non-free magnetic layers, such as the reference layer, which could interfere with the reversal of the SyAF free layer and also reduce the stability of said non-free magnetic layers. By non-free magnetic layer, one means a magnetic layer with a fixed orientation of its magnetization.

Beneficially, the magnetic core has a cylindrical shape extending in a direction, said "normal direction", the magnetic core having an outer surface, radially oriented with respect to the normal direction, the magnetic shell having an inner surface facing at least a part of the outer surface of the magnetic core. The inner surface of the magnetic shell and the outer surface of the magnetic core can be separated from each other by the dielectric and non-magnetic material or by the non-magnetic material providing exchange coupling.

Beneficially, the magnetic shell is centered with respect to the thickness of the magnetic core.

Beneficially, the magnetic core has a first magnetic volume and the magnetic shell has a second magnetic volume greater than <NUM> % of the first magnetic volume and lower than <NUM> % of the first magnetic volume.

Beneficially, the magnetic shell exhibits an aspect ratio Rshell, computed as <MAT> with Zshell a thickness of the magnetic shell, rinner an inner radius of the magnetic shell and rext an external radius of the magnetic shell, the aspect ratio Rshell of the magnetic shell being higher than <NUM>.

Beneficially, the aspect ratio of the first magnetic layer is comprised between <NUM> and <NUM>.

The first magnetic layer can be made of a ferromagnetic material, such as Fe, Ni, Co or an alloy thereof. The first magnetic layer can also comprise a magnetic multilayer, such as [Co/Pt]n multilayer or [FeCo(B)/MgO]n multilayer, providing uniaxial or interfacial anisotropy.

Beneficially, the magnetic core comprises a second magnetic layer, the second magnetic layer comprising a ferromagnetic material, such as Fe, the second magnetic layer lying on a tunnel barrier layer, such as an MgO layer, a perpendicular magnetic anisotropy of the second magnetic layer being induced by the interface between the second magnetic layer and the tunnel barrier layer. By tunnel barrier layer, one means a non-magnetic layer configured to allow spin transfer torque between the second magnetic layer and a non-free magnetic layer.

Beneficially, the magnetic core comprises a structural transition layer lying against the second magnetic layer and separating the first magnetic layer from the second magnetic layer, the second magnetic layer comprising an amorphising element, such as B, and the structural transition layer comprising an amorphising-getter material such as Ta, Mo, W of Hf.

An amorphising element prevents crystallization of the second magnetic layer during its deposition. During annealing the second magnetic layer crystallizes in the same structure as the tunnel barrier as the amorphising element migrates from the interface due to the amorphising getter in the transition.

Beneficially, the magnetic material of the magnetic shell has a high damping value, for example greater or equal to <NUM>.

Another aspect of the invention concerns a method for producing a magnetic tunnel junction, the method comprising the following steps:.

Another aspect of the invention concerns an array of magnetic tunnel junctions, at least one magnetic tunnel junction of the array of magnetic tunnel junctions being a magnetic tunnel junction according to the invention.

Beneficially, the magnetic tunnel junctions of the array of magnetic tunnel junctions are separated from each other by a predefined distance and by an insulator.

The invention and its various applications will be better understood with the following description and the accompanying figures.

The figures are shown for information only and are not limitative of the invention. Unless otherwise specified, the same element appearing on different figures has a unique reference.

The invention concerns a magnetic tunnel junction <NUM> (also called "junction") providing an improved reversal mechanism and a good thermal stability. The junction <NUM> is intended to be used as a storage or a magnetic field probe. It also provides a side effect by reducing or even cancelling the magnetic field it emits.

The junction <NUM> is also relevant as it can be obtained from a magnetic junction according to a prior art, such as the one shown in <FIG>. Indeed, the junction <NUM> comprises a first magnetic free layer <NUM> with a large aspect ratio. The first magnetic free layer <NUM> is similar to the one 16b shown in <FIG>.

The <FIG> show an embodiment of a junction <NUM> according to the invention. The common elements with the <FIG> are shown using the same reference sign.

The junction <NUM> comprises a synthetic antiferromagnetic free layer <NUM>, also called "SyAF free layer" for short. Said SyAF <NUM> comprises a magnetic core <NUM> (also called "core" for short) and a magnetic shell <NUM> (also called "shell" for short). Both the core <NUM> and the shell <NUM> are coupled in an antiferromagnetic way. In other words, the shell <NUM> is arranged about the core <NUM> to align its magnetization ms antiparallel to the magnetization mc of the core <NUM>. In this embodiment, the shell <NUM> surrounds the core <NUM> and along the full thickness of the core <NUM>.

The SyAF free layer <NUM> shows an improved reversal behaviour, compared with the isolated core <NUM>, thanks to the shell <NUM>. The shell <NUM> coupled to the core <NUM> improves the thermal stability of the core <NUM>. It allows to reduce the thickness of the magnetic core <NUM> to improve the reversal behaviour. For example, the magnetization mc of the core <NUM> behave like a unique magnetic momentum (also called "macrospin") exhibiting a coherent reversal.

The core <NUM> comprises at least a first magnetic layer <NUM>. This first layer <NUM> is remarkable as it is a thick ferromagnetic layer. It can be made of ferromagnetic material or ferromagnetic alloy based on Ni, Fe and Co, as well as others ferromagnetic materials. The thickness of the first layer <NUM> is such that the first layer <NUM> has an aspect ratio comprised between <NUM> and <NUM>. Such aspect ratio adds a shape anisotropy perpendicular to the plane of the layers which tends to orient spontaneously the magnetization ms of the first layer <NUM> out of the plane of the layers. For this reason, the first layer <NUM> may also be called "perpendicular shape anisotropy layer" or "shape anisotropy layer" or "PSA layer". The thickness of the PSA layer <NUM> is preferably chosen so it has an aspect ratio that allows for a sufficiently high stability after consideration of the magnetostatic coupling with the magnetic shell <NUM>. A high stability is greater than or equal to Δ ≈ <NUM>. The aspect ratio corresponds to the thickness of said PSA layer <NUM>, measured perpendicular to the plane of the layers, divided by its diameter (or its width), measured parallel to the plane of the layers. An aspect ratio closer to <NUM> would be preferred as it improves the reversal behaviour. For example, it is preferably comprised between <NUM> and <NUM>.

The magnetic core <NUM> can also comprise a second magnetic layer <NUM>. Said second layer <NUM> can be made of a ferromagnetic material such as Fe or an alloy of Fe and Co (for example CoFe or CoFeB). The second layer <NUM> is preferably in contact with a layer comprising O, such as MgO. The Fe-O coupling at the interface create a perpendicular magnetic anisotropy which tends to orient spontaneously the magnetization of the second layer <NUM> out of the plane of the layers. For this reason, the second layer <NUM> can also be called "perpendicular interfacial anisotropy layer" or "interfacial anisotropy layer" or "IA layer". The IA layer <NUM> contribute to perpendicular anisotropy of the magnetic core <NUM> and reinforce the out-of-plane orientation of the magnetization ms of the core <NUM>.

The IA layer <NUM> is preferably thinner than the PSA layer <NUM> as the anisotropy only come from the interface with the O-based layer. It can exhibit a thickness comprised between <NUM> and <NUM>.

The magnetic core <NUM> can also comprise a structural transition layer <NUM> (also known as "B-getter layer" because it tends to trap B element from the interfacial anisotropy layer <NUM>). The B-getter layer <NUM> can lie directly on the IA layer <NUM>. It can be made of W, Ta, Mo or Hf. Such B-getter layer <NUM> has preferably a thickness comprised between <NUM> and <NUM>.

The PSA layer <NUM> can also comprise a ferromagnetic multilayer that induce a perpendicular anisotropy, such as a [Co/Pt]n multilayer (where n is the number of Co/Pt layers stacked) or a [FeCo(B)/MgO]n multilayer. For example, a [FeCo(B)/MgO]n with an aspect ratio of <NUM> exhibits a strong anisotropy as the shape anisotropy (induced by the high aspect ratio) is reinforced by the interfacial anisotropy at each FeCo(B)/MgO interfaces. The total anisotropy obtained can be equal to a unique thick layer of magnetic material, for example Fe, with an aspect ratio greater or equal to <NUM>.

The IA layer <NUM> and the PSA layer <NUM> can be spaced from each other by the B-getter layer <NUM>. Thanks to the thickness of the B-getter layer <NUM>, the magnetization of the IA and PSA layer <NUM>, <NUM> can be coupled to form a total magnetization ms which behaves as a unique magnetic momentum. Moreover, the perpendicular anisotropies brought by each layer <NUM>, <NUM> tends to spontaneously orient the total magnetization out of the plane of the layers.

The material composing the B-getter layer <NUM> may have a coupling effect between the magnetizations of the two layers <NUM>, <NUM>, such as an exchange coupling. However, the magnetostatic coupling between each magnetization is sufficient.

The magnetic core <NUM> comprises preferably every free layer of the junction <NUM>. More specifically, every free layer <NUM>, <NUM> couple to a defined tunnel barrier <NUM> belongs to the same magnetic core <NUM>. Considering a magnetic pillar comprising a plurality of magnetic tunnel junction, each magnetic core is associated with one of the tunnel barrier and comprise every magnetic free layer coupled to this tunnel barrier.

The magnetic core <NUM> exhibits a magnetization which is the sum of the magnetizations of the free layers <NUM>, <NUM> belonging to said core <NUM>.

The magnetic core <NUM> defined a volume comprising a plurality of free layers. This volume preferably also comprises the non-magnetic layers lying between these free layers <NUM>, <NUM> (such as the B-getter layer <NUM>).

To form a magnetic tunnel junction, the junction <NUM> may also comprise layers such as the ones shown in <FIG>. For example, it may comprise at least a magnetic reference layer <NUM> separated from the core <NUM> by a tunnel layer <NUM>.

The junction <NUM> may preferably comprise a top electrode and a bottom electrode (not shown), allowing an electrical current to flow throughout the junction, in a direction close to perpendicular to a plane of the layers.

The tunnel layer <NUM> is made of a non-magnetic material allowing the electrical current to cross this layer <NUM> in a tunnelling regime. The tunnel layer <NUM> is preferably made of a dielectric material such as an oxide, for example TiOx, AlOx or MgO, or a nitride layer, for example TiN. The MgO oxide is preferred as it provides the highest tunnel magnetoresistance signal. The tunnel layer <NUM> has usually a thickness comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>. It gives a resistance times area product (known as "RA product") of the junction <NUM> preferably lower than <NUM>Ω. In order to have a low RA product and a high tunnel magnetoresistance signal (known as "TMR signal"), for example higher than <NUM>% or preferably higher than <NUM>%, the tunnel layer <NUM> and the two electrodes (layers <NUM> and <NUM>) should have the same crystallographic BCC structure with a (<NUM>) texture.

The reference layer <NUM> is a ferromagnetic layer having a fixed magnetization orientation, preferably perpendicular to the plane of the layers. The reference layer <NUM> can be made of an Fe, Co or alloy thereof, such as FeCo(B) alloy. The orientation of the magnetization of the reference layer 14a is kept fixed thanks to a strong perpendicular anisotropy. Such perpendicular anisotropy can be brough by the interfacial magnetic anisotropy between a Fe alloy and a MgO layer (such as the tunnel layer <NUM>). It can also be brough by an exchange-like coupling with an out-of-plane magnetized multilayer <NUM> called "pinning layer". The pinning layer can comprise one or more [Co/Pt]n multilayers (however not belonging to the core <NUM>). The [Co/Pt]n multilayers can be antiferromagnetically coupled to provide a fixed magnetization orientation.

A thin texture breaker layer can be placed between the reference layer <NUM> and the pinning layer <NUM>. It allows a transition between the FCC structure of the pinning layer <NUM> to the BCC structure of the reference layer <NUM>. This texture breaker layer can be made of W, Ta, Mo or Hf.

The pinning layer <NUM> can also reduce the stray field that the reference layer <NUM> exerts on the magnetic core <NUM>.

The junction <NUM> can also comprise a seed layer <NUM>, made of Ta or Pt, lying on a substrate or the bottom electrode (to allow the circulation of an electrical current) which allowed the pinning layer <NUM> growth. Said substrate can be made of silicon. Said bottom electrode can be made of a metallic material. The plane P of the layers is preferably considered as the plane of the substrate or the plane of the bottom electrode.

The magnetic shell <NUM> is also made of a ferromagnetic material. For example, it can be made of a low moment magnetic material or a high moment magnetic material, such as a Ni alloy or a Fe alloy. For example, the magnetic shell <NUM> is made of Ni<NUM>Fe<NUM> which is a low moment material, which can be called "mu metal". Other example, the magnetic shell <NUM> is made of Ni<NUM>Fe<NUM> which is a high moment material.

The magnetic shell <NUM> surrounds the magnetic core <NUM>. In the embodiment of the <FIG>, it exhibits a hollowed shape surrounding the cylindrical shape of the magnetic core <NUM>. The gap can be filled by a non-magnetic material <NUM>. It ensures that only magnetostatic interactions act on both the core <NUM> and the shell <NUM>. Here the non-magnetic material <NUM> is air which also act as an insulator.

The gap distance between the core <NUM> and the shell <NUM> is set so the magnetizations ms, mc of the shell <NUM> and the core <NUM> align antiferromagnetically to form a synthetic antiferromagnet. Aligning antiferromagnetically means that the magnetizations align spontaneously antiparallel and behave as a unique body. It means that, when the magnetization mc of the core <NUM> commute from one orientation, for example up, to another, for example down, the magnetization ms of shell <NUM> also commute from down to up.

The <FIG> shows a cut view of an embodiment of the SyAF free layer <NUM> according to the invention. In this embodiment, the magnetic core <NUM> shows a cylindrical shape, extending perpendicularly to the plane of the layers (parallel to the radial direction eR in a cylindrical coordinate system). In the figure, the core <NUM> extends along a normal direction ez, perpendicular to the plane of the layers (and thus the radial direction eR). The magnetic core <NUM> has a height Zcore, also called "thickness", measured along the normal direction ez. Because of its cylindrical shape, the magnetic core <NUM> has a radius router, measured along the radial direction eR. The width (or diameter here) is therefore equal to 2router.

Due to its cylindrical shape, the magnetic core <NUM> has an outer surface <NUM>, radially oriented. "Radially oriented" means that the surface is parallel to the normal direction ez. In other words, its orientation (defined by a vector perpendicular to the outer surface <NUM>) is oriented along the radial direction eR.

In this embodiment, the shell <NUM> exhibits a hollowed cylindrical shape, extending along the normal direction ez. It comprises a through-hole wherein the magnetic core <NUM> is located. The shell <NUM> exhibits a thickness Zshell, measured along ez. In this embodiment, the thickness Zshell of the shell <NUM> is smaller than the thickness Zcore of the magnetic core <NUM>. The shell <NUM> and the core <NUM> are located at the same height, measured along the normal direction ez. In other words, the shell <NUM> is centered with respect to magnetic core <NUM>, along the normal direction ez. It means that the shell <NUM> surrounds the outer surface <NUM> of the magnetic core <NUM> on a part of its thickness (or height). The shell <NUM> preferably surrounds at least a part of the thickness of the PSA layer <NUM> and more preferably the whole thickness of the magnetic core <NUM> (as shown in <FIG>).

The magnetic shell <NUM> comprises an inner surface <NUM>, in the through-hole. The inner surface <NUM> is also radially oriented, as the outer surface <NUM> of the core <NUM>, however, its orientation (along -eR) is opposed to the orientation of the outer surface <NUM> of the core <NUM>. Therefore, the inner surface <NUM> of the shell <NUM> faces at least a part of the outer surface <NUM> of the core <NUM>. In the embodiment of the <FIG>, the inner surface <NUM> of the shell <NUM> faces the whole outer surface <NUM> of the core <NUM>.

The shell <NUM> and the core <NUM> are coupled in a way that the magnetization ms of the shell <NUM> spontaneously aligns antiparallel to the magnetization mc of the magnetic core <NUM>. To do so, the magnetic shell <NUM> is spaced from the magnetic core <NUM>. Here a gap between the ferromagnetic materials ensures the magnetization ms of the shell <NUM> will orient antiparallel with respect to the magnetization mc of the core <NUM>.

If, however, the magnetic shell <NUM> is in direct contact with the magnetic core <NUM>, the inner surface <NUM> of the shell <NUM> being in a direct contact with the outer surface <NUM> of the core <NUM>, therefore the magnetization ms of the shell <NUM> will tend to align parallel to the magnetization mc of the magnetic core <NUM>. It will enhance the stray field in the vicinity of the junction <NUM>, which may not be desired.

To prevent any magnetic coupling between the magnetizations ms, mc which could align the magnetizations ms, mc parallel, such as a direct coupling, a non-magnetic material <NUM> is located in the gap between the magnetic shell <NUM> and the magnetic core <NUM>. It preferably embeds the outer surface <NUM> of the core <NUM> and the inner surface <NUM> of the shell <NUM>.

For example, the non-magnetic material <NUM> can be an insulator such as air, vacuum. It can also be made of an oxide material such as ZnO, HfO<NUM>, Al<NUM>O<NUM> or TiN. Alternatively, the non-magnetic material <NUM> can be a metallic material, such as Ru, providing exchange coupling between magnetic shell <NUM> and the magnetic core <NUM>. Whatever the non-magnetic material <NUM>, the thickness dgap of the gap is adjusted so the coupling between the shell <NUM> and the core <NUM> is antiferromagnetic. The thickness dgap of the gap is, for example, comprised between <NUM> and <NUM>.

The <FIG> shows a top view of three embodiments of the SyAF free layer <NUM>. In every embodiment, the magnetic core <NUM> is cylindrical. The first embodiment, on the left, shows a cylindrical magnetic shell <NUM> surrounding the magnetic core <NUM>. It corresponds to the embodiment shown in the <FIG>. The second and third embodiment (in the middle and on the right of the figure) show that the magnetic shell <NUM> can exhibit a non-cylindrical shape. The shape is parallelepipedal with, respectively, a square base (in the middle) or a diamond base (on the right). It can also exhibit different shape such as an ellipsoidal base, with a larger size in a direction and a smaller one in the perpendicular direction.

The magnetic core <NUM> can also exhibits another shape such as a parallelepiped with a square base or a diamond base as well. Whatever the shape of the core <NUM> and the shell <NUM>, they are spaced from each other, ensuring an antiferromagnetic coupling.

A positive side effect of the SyAF free layer <NUM> is to mitigate the stray field, as the antiparallel magnetizations compensate each other. To mitigate efficiently the stray field, the magnetic volume of the magnetic shell <NUM> should be as close as possible to the magnetic volume of the magnetic core <NUM>. The magnetic volume of the magnetic core <NUM> is equal to the volume of the magnetic layers (such as the free layers) belonging to the core <NUM>, times the magnetization saturation of each layer. The magnetic volume of the magnetic shell <NUM> is equal to the volume of the magnetic shell times its magnetization saturation. Considering a magnetic core <NUM> comprising two layers, a PSA-layer <NUM> made of Fe and a IA-layer <NUM> made of CoFe(B) and a magnetic shell <NUM> also made of Fe. The magnetic volume MVcore of the magnetic core <NUM> is equal to <MAT> With MFe and MCoFe(B) the magnetization saturations of Fe and CoFe(B), VPSA the volume of the PSA-layer, VIA the volume of the IA-layer <NUM>. The magnetic volume MVshell of the magnetic shell <NUM> is equal to <MAT> With Vshell the volume of the magnetic shell <NUM>.

Following this example, the stray field is optimally mitigated when the volume of the shell is equal to the volume of the volumes of the two magnetic layers belonging to the magnetic core <NUM>. The magnetic volume of the magnetic shell <NUM> are preferably at least equal to the magnetic volume of the magnetic core <NUM> within <NUM> %, and more preferably within <NUM> %.

Indeed, when the magnetic volume of the magnetic shell <NUM> is lower than <NUM> % of the magnetic volume of the magnetic core <NUM>, the stray field is not entirely compensated, and the mitigation is not efficient. When the magnetic volume of the magnetic shell <NUM> is larger than <NUM> % of the magnetic volume of the magnetic core <NUM>, the stray field of the magnetic core <NUM> is largely overcompensated. However, a field strayed by the junction <NUM> remains. This remaining field is strayed by the shell <NUM> instead of the core <NUM>.

The inner radius of the shell may not be modified to fit the magnetic volume of the magnetic shell <NUM> to the magnetic volume of the core <NUM>. Indeed, the antiparallel coupling between the magnetizations of the core <NUM> and the shell <NUM> is set regarding the gap distance dgap between each surface <NUM>, <NUM> (and the non-magnetic material <NUM> used). Modifying this gap distance dgap may weaken the coupling strength, especially in a presence of a strong external field. It may also reverse the shell's magnetization <NUM> which would enhance the total stray field.

The magnetic volume of the shell <NUM> may be adjusted by modifying its thickness Zshell and/or its external radius rext.

For example, the thickness Zshell of the shell <NUM> can be extend up to the thickness of the magnetic core <NUM>. In this configuration, the shell <NUM> surrounds the entire outer surface <NUM> of the magnetic core <NUM>. It improves the coupling strength between the core <NUM> and the shell <NUM> as the surfaces facing each other are larger. It also improves field mitigation. Especially because the shell <NUM> surrounds every magnetic layer <NUM>, <NUM> of the magnetic core <NUM>.

As another example, the external radius rext can be increased to increase the magnetic volume of the shell <NUM>. Indeed, the thickness Zshell of the shell <NUM> should not be too large. It should be about the same thickness Zcore of the core <NUM> (for example the same thickness Zcore) to ensure a rigid coupling between the core <NUM> and the shell <NUM>. This way the reversal is optimized.

In an embodiment, the shell <NUM> has a thickness Zshell equal to the thickness Zcore of the magnetic core <NUM> and the external radius rext of the shell is adjusted so the magnetic volumes of the core <NUM> and the shell <NUM> matches. In this embodiment, the shell <NUM> looks like an hollow cylinder. In another embodiment, the shell <NUM> has a thickness Zshell much smaller than the thickness Zcore of the magnetic core <NUM>. However, the external radius rext of the shell <NUM> is increased to keep the magnetic volume of the shell <NUM> constant. In this embodiment, the shell <NUM> looks like a disc surrounding the core.

The strength of the antiferromagnetic coupling between the core <NUM> and the shell <NUM> depends, among others, of the thicknesses of the core <NUM> and/or the shell <NUM> and more specifically the surfaces of the shell <NUM> and the core <NUM> facing each other. The higher the surfaces, the higher the coupling strength. The high aspect ratio of the magnetic core <NUM> (comprising at least the PSA-layer <NUM>) provides a large thickness of the core <NUM>. To ensure a high enough thickness of the shell <NUM>, the shell's thickness Zshell is preferably greater or equal to <NUM>% of the core's thickness Zcore.

In another example to ensure a high enough thickness of the shell <NUM>, the shell <NUM> aspect ratio Rshell, is greater than <NUM> and preferably greater than <NUM>. The aspect ratio Rshell of the shell <NUM> is defined as : <MAT>.

Moreover, a high aspect ratio Rshell also induces a perpendicular shape anisotropy (align along eZ) to the shell <NUM> in a way the magnetization ms of the shell spontaneously orients out of the plane of the layers. This out-of-plane orientation may improve the core/shell coupling and the core's magnetization stability. In contrary, a low aspect ratio Rshell induce an in-plane shape anisotropy (parallel to the plane of the layers). Therefore, the magnetization ms of the shell <NUM> would spontaneously orient parallel to the plane and the core's magnetization stability would be greatly reduced.

The perpendicular shape anisotropy of the magnetic shell <NUM> can reinforce the stability of the magnetic core <NUM>, as their magnetizations ms, mc are coupled. Therefore, the increase in stability of the magnetic core <NUM> can be compensated by decreasing its thickness (especially by reducing the thickness of the shape anisotropy layer), which avoid non-uniform reversal of its magnetization mc. Moreover, the lateral size of the junction <NUM> can be further reduced, allowing for further downsize scalability.

The magnetic volume of the shell <NUM> may also be adjusted by modifying the magnetic material composing said shell <NUM>. For example, the magnetic core <NUM> can be made of CoFeB, having a saturation magnetization of <NUM> MA/m, instead of Ni, which has a saturation magnetization of <NUM> MA/m. Therefore the external radius rext does not need to be increased to match the magnetic volume of the core <NUM>. In contrary, a softer material, such as Ni, can provide a better control over the magnetic volume of the shell <NUM> by modifying its external radius rext. Using a softer magnetic material, compared to Fe, also makes the magnetization switching easier as the dipolar interaction is weaker.

The <FIG> show two cut views of an embodiment comprising three magnetic tunnel junctions <NUM> according to the invention. As the junction <NUM> shown in <FIG>, the reference layers <NUM> are placed under the magnetic core <NUM> (and separated from said core <NUM> by the tunnel barrier <NUM>). This arrangement is called "bottom-pinned" junctions. However, the junction of <FIG> could also be lying on their caping layers <NUM> which would lie on the substrate <NUM>. This upside-down arrangement is called "top-pinned" as the reference layers <NUM> are places on the magnetic core <NUM>.

The <FIG> shows examples of energy barriers computed for a junction <NUM>, <NUM> according to the prior art and according to the invention, the energy barriers being computed as a function of a dimension of the junction. The energy barrier corresponds to the energy to overcome to switch the magnetization of the core <NUM> from one configuration (for example up) to another configuration (for example down). The junction is considered within an array of junctions of the same kind. The barrier is shown against a reaction coordinate which is proportional to an angle of the core's magnetization with respect to a normal direction (such as the ez direction in the <FIG>). An illustration of the reaction coordinate is also given in the <FIG>. For a reaction coordinate of <NUM>, the magnetization of the core <NUM> is oriented in a up direction while the magnetization of the shell <NUM> is oriented in a down direction. For a reaction coordinate of <NUM>, the orientation is flipped, the magnetization of the core <NUM> is oriented in a down direction while the magnetization of the shell <NUM> is oriented in a up direction. For a reaction coordinate of <NUM>, the magnetizations of the core <NUM> and the shell <NUM> are in plane.

Six examples of energy barrier are given. The black dots show the energy barrier for junction without a magnetic shell <NUM>. The five other examples show energy barriers of a junction <NUM> with a magnetic shell <NUM>, for five different core thicknesses Zcore. A variation of the energy barrier induced a variation in the thermal stability factor Δ of the junction (which is computed thanks to the maximum energy of the barrier). The six energy barrier examples are also associated with the corresponding thermal stability factor Δ.

The six energy barrier examples shown in this <FIG> has been calculated using a finite differences 3D solver (called "micro3D") with a cubic computational cell of <NUM>. The magnetic core <NUM> considered is made of FeCo(B) alloy with a saturation magnetization of <NUM> MA/m and an exchange stiffness of <NUM> pJ/m. The magnetic shell <NUM> is considered to have magnetization oriented in an opposite direction with respect to the magnetization of the magnetic core <NUM>. The magnetic shell <NUM> has a saturation magnetization of <NUM> MA/m and an exchange stiffness of <NUM> pJ/m. The interfacial perpendicular magnetic anisotropy is added in the simulation as a bulk anisotropy that exponentially decays as <MAT> where λiPMA is the decay length (usually some Angstroms). K<NUM> is calculated considering that the incremental sum of the areal value of this bulk contribution results in a macrospin surface anisotropy, taken as <NUM> mJ/m<NUM>. The stability Δ of the SyAF free layer <NUM> with and without the contribution of the magnetic shell <NUM> is computed considering the defined anisotropy. This is done by computing the minimum energy that the magnetic system (and especially the SyAF free layer <NUM>) follows to reverse the magnetization of the magnetic core <NUM> between two stable states (up state and down state considered in this example).

It is observed in <FIG> that the magnetic shell is, indeed, increasing the stability of the SyAF free layer <NUM>. The curve with white dots shows that adding a shell <NUM> antiferromagnetically coupled with the magnetic core <NUM> improves the thermal stability factor of the SyAF free layer <NUM> to Δ = <NUM> from Δ = <NUM> (Δ = <NUM> corresponding to the single core <NUM>).

It is then possible to reduce the total thickness Zcore of the magnetic core <NUM> (and also the magnetic shell <NUM>) while still matching thermal stability factor expected in working devices (Δ ≈ <NUM>). A magnetic core <NUM> having a thickness Zcore of <NUM> matches the thermal stability (Δ = <NUM>) of a junction without shell <NUM> and thickness Zcore of <NUM>. A magnetic core <NUM> having a thickness Zcore of <NUM> matches the minimal expected thermal stability (Δ = <NUM>).

A smaller thickness Zcore of the core <NUM> improves the reversal of its magnetization as the magnetization behave as a unique giant spin (model called "macrospin") and the energy barrier to overcome is lower. Therefore, adding a shell <NUM> improves the reversal of the junction <NUM>, compared to a higher core <NUM> without shell <NUM>.

The <FIG> shows calculations of the dynamics of the magnetization of the magnetic core <NUM> not coupled to a shell <NUM> (said "unshelled core") and coupled to the shell <NUM>. The junction <NUM> has similar parameters as <FIG>. The magnetic core <NUM> has a diameter equal to <NUM> and a thickness of <NUM>. The amplitude of voltage applied to commute the magnetization is equal to <NUM> V.

The calculation has been performed considering that the polarization of the spin current occurs near the interface with the tunnel barrier. An evanescent decay of the spin transfer torque pre-factor all (so called "STT pre-factor") is considered as <MAT> where λSTT is the decay length along the normal direction ez and a0 is the STT pre-factor at the interface. Considering values like ones of the document [<NPL>)], one can obtain the temporal dependency of the magnetization under a DC voltage pulse, as shown in the <FIG>.

The contribution for the magnetic shell <NUM> and the magnetic core <NUM> are separated. Two different damping values of the magnetic shell are considered: a low damping regime with αshell = <NUM>; and a high damping regime with αshell = <NUM>. For both these situations, the reversal of the magnetic core starts <NUM> sooner than the unshelled core (with dots), with a faster reversal between up (+<NUM>) to down (-<NUM>) magnetization orientation. The αshell is related to this. For a low damping regime, it is observed that the reversal of the magnetic core starts quicker, the higher the damping, the slower the initiation. In both situations, it is seen that the shell starts reversing when the magnetic core <NUM> is in-plane (〈mz〉 ≈ <NUM>). In a low damping regime, the shell will take a significant amount of time to relax. However, for higher damping regime, this relaxation is faster, as the energy of the system tends to go to the ground state faster.

The <FIG> shows a three-dimensional snapshot of the magnetizations of the magnetic core and the magnetic shell against the time, considering a damping value of <NUM>. The magnetization behaves like a macrospin, without a long relaxation time.

The <FIG> show an example of field strayed from the magnetic core <NUM> and the magnetic shell <NUM>, each considered alone. In the same way, the <FIG> show the field strayed by the magnetic core <NUM> and the magnetic shell <NUM>, alone. However, they also show a dashed line correspond to a <NUM> mT amplitude field. In this embodiment, the magnetic core <NUM> comprises only a PSA-layer <NUM> with a <NUM> diameter and <NUM> thickness. The magnetic shell <NUM> has an inner diameter of <NUM> and thickness of <NUM>. Both the magnetic core <NUM> and the magnetic shell <NUM> strayed a field which can reach <NUM> mT of more at <NUM> from the centre of the shell/core, along the X axis. The <FIG> shows that the field lines of the magnetic core <NUM> loop in a direction while the field lines of the magnetic shell <NUM> loop in an opposite direction.

The <FIG> and <FIG> show the effect of the magnetic shell <NUM> on the field strayed from itself and from the magnetic core <NUM>. The magnetic core <NUM> and the magnetic shell <NUM> are put together and spaced by a <NUM> non-magnetic and nonconductive material <NUM> (such as air of vacuum). The field lines from the magnetic core <NUM> loop through the magnetic shell <NUM>. The effect on the amplitude of the strayed field is shown in <FIG>. The amplitude is reduced. For example, the field amplitude perceived at <NUM> from the centre of the shell/core, along the direction X, is lower than <NUM> mT.

The <FIG> show a 5x5 array of magnetic tunnel junctions <NUM> according to the prior art with different magnetic configurations. Said junctions do not comprise a shell and are, for this reason, named "unshelled junctions". The array of unshelled junctions is arranged following a square lattice. The diameters of the unshelled junctions <NUM> are <NUM> (discussed in <FIG>) or <NUM> (discussed in <FIG>). The different magnetic bit configurations are, respectively: A) all unshelled neighbors in an up configuration; B) all unshelled neighbors in a down configuration; C) the first nearest unshelled neighbors in a down configuration; and D) the second nearest unshelled neighbors in a up configuration. Up magnetization is shown with a black junction and down magnetization is shown with a grey junction. Only a part of each unshelled junction is shown, comprising a magnetic free layer and a tunnel barrier (in grey color at the bottom of each free layer).

The <FIG> show the field felt by a central unshelled junction and its thermal stability factor, in each array configuration of <FIG>. The <FIG> show the normal component of the field Hz felt by a central unshelled junction and its thermal stability factor, in an array of <NUM> diameter junctions <NUM>. The <FIG> show the normal component of the field Hz felt by a central unshelled junction and its thermal stability factor, in an array of <NUM> diameter junctions.

The individual effect of each of the neighbouring junctions is added as a total contribution to the central unshelled junction <NUM>. It is taken into consideration four different magnetic configuration following the <FIG>. In addition to the averaged stray field in the central junction, the values of the deviation of the thermal stability factor is also given considering that the outside stray field Hstray: <MAT> where Δ<NUM> is the stability factor of the central unshelled junction without external contributions, Hstray is the averaged stray field made by the squared array and Hk is the magnetic anisotropy field of the free layer of the central junction. The closer the unshelled junctions <NUM>, the higher is the field felt and its impact on the thermal stability. When the first nearest neighbours are parallel to the magnetization of the central junction <NUM>, the thermal stability is reduced. However, when the first nearest neighbours are antiparallel to the considered junction <NUM>, the thermal stability is increased. The more drastic configurations are obtained when all the first and second neighbours are aligned antiparallel with respect to the central junction (<FIG>).

It is observed that, for a <NUM> diameter junction (following the prior art, it means without any shell <NUM>) the stray field is very high when the junctions are in contact with each other. For a center-to-center distance <NUM> times the diameter of the junction, one still obtains a significant variation in stray field from the two worst configurations (<FIG>). The stray field contribution is discarded for center-to-center distances higher than <NUM> times the diameter of the junction. For the case of a <NUM> diameter array of junction (<FIG>), the decay of the stray field is slower than the decay for larger diameter junctions (<FIG>).

The <FIG> show a 5x5 array <NUM> of magnetic tunnel junctions <NUM> according to an embodiment of the invention. The array is arranged following a square lattice the same way as the <FIG>. Each junction <NUM> of the array <NUM> comprises a SyAF free layer <NUM> comprising a magnetic core <NUM> and a magnetic shell <NUM>. For this reason, the junctions <NUM> are said "shelled". The shell <NUM> is spaced from the core it surrounds by <NUM>. Two diameters of the magnetic cores <NUM> are considered: a diameter of <NUM> (discussed in <FIG>) of diameters of <NUM> (discussed in <FIG>). The same magnetic configurations are studied, respectively: A) all neighbours in a up configuration (the core's magnetization defining the orientation) ; B) all neighbours in a down configuration; C) the first nearest neighbours in a down configuration; and D) the second nearest neighbours in a up configuration. Up magnetization is shown with a black core <NUM> or shell <NUM> while the down magnetization is shown with a grey core <NUM> or shell <NUM>. Each shelled junction <NUM> is set to exhibits an antiparallel configuration between the magnetization of the core <NUM> and the magnetization of the shell <NUM>. For this reason, every shelled junction <NUM> shows a black shell and a grey core or a grey shell and a black core.

The <FIG> compare the field felt by a central shelled junction and a central unshelled junction, in the worst configuration (every neighbour aligned antiparallel of <FIG> and <FIG>) within the <NUM> x <NUM> array of junctions (respectively shelled and unshelled). The field felt by the shelled junction <NUM> (in the array <NUM> of <FIG>) is plotted using white markers while the field felt by the unshelled junction <NUM> (in the array of <FIG>) is plotted using black markers. Whatever the diameter of the junctions (<FIG>), the field felt by the shelled junction <NUM> is smaller than the one felt by the unshelled junction, even for the smaller pitch (when the shell <NUM> of each junction are in contact). The inset in the <FIG> show, in log scale, that the decay of the field felt by the shelled junction <NUM> decreases faster than the field felt by the unshelled junction <NUM>.

The <FIG> shows a side view of the SyAF free layers <NUM> of the <NUM> x <NUM> array of <FIG>. Thanks to this decrease in stray field, one can increase the areal density of the array <NUM>. It is possible to estimate a density of magnetic junctions <NUM> per squared millimetre following that, for a certain allowed stray field, the distance Ls between <NUM> different SyAF free layer <NUM> is given by <MAT> where δsh is the distance between to nearest shells <NUM>.

Following the allowed stray field as a variation in the anisotropy field of each magnetic core <NUM> (without consideration for SyAF free layer <NUM>). Knowing that the anisotropy field HK for a <NUM> diameter and <NUM> thick magnetic core is about <NUM> mT and allowing variations of <NUM>% and <NUM> % of the stability factor Δ (corresponding to an increase/reduce of the anisotropy field), one can determine the density of the Table <NUM>:.

The same applies for a <NUM> diameter and <NUM> thick magnetic core which exhibits an anisotropy field HK about <NUM> mT. One can then determine the density of the Table <NUM>:.

It is observed a substantial increase in the areal density (in Gbit/mm<NUM>) when the junctions are shelled. The highest density is obtained using <NUM> diameter shelled junctions. It should be also considered that the increase in the anisotropy field HK due to the interaction with the magnetic shell is not considered. This interaction would increase the anisotropy field HK, allowing for a smaller pitch between the junctions and therefore a higher density. The calculation of an optimal value of the external radius of the magnetic shell <NUM> for a predetermined pitch value may be given by: <MAT> where HZ(r) is the field, along the normal direction ez, felt at the coordinate r.

The SyAF free layer <NUM> can be dimensioned to reduce the stray field at the certain position instead of providing a global reduction in every direction, even if it increases the stray field at another position.

<FIG> shows an embodiment of a method <NUM> to produce a magnetic tunnel junction <NUM> following the invention. An example of process flow to produce the junction <NUM> is shown in <FIG>. Certain elements commonly used for the usual CMOS integration are not shown for simplicity.

Firstly, the method <NUM> comprises a step of forming <NUM> a magnetic stack, which can be usual magnetic tunnel junction according to the prior art. The formation <NUM> step comprises, for example, the sub-step of depositing <NUM> multiple layers material on an electrode <NUM>. Said stack of layers comprising, from the electrode <NUM>, a first layer <NUM> of a magnetic material intended to form a reference layer <NUM>, a second layer <NUM> of a dielectric material, intended to form a tunnel barrier <NUM> and at least a third layer <NUM> of a magnetic material, intended to form the magnetic core <NUM>, it means at least the PSA-layer as discussed previously.

A magnetic tunnel junction may comprise different crystallographic structures: BCC (body centered cubic) close to the tunnel barrier <NUM> and (face centered cubic) in the bottom and top part of the junction. The stack of layers is preferably deposited in a way that the reference layer <NUM> exhibits a BCC structure. Additional layers, intended to form a synthetic antiferromagnet, can be deposited to exhibit a FCC structure. A thin texture breaker layer can be added between these layers to allow the growth of the reference layer following a BCC structure.

The resistance × area product (known as RA product) of the junction to produce should be smaller than <NUM>Ω. µm<NUM> to achieve high TMR signal (above <NUM>%, even though it is preferable above <NUM>%). To do so, the tunnel barrier <NUM> and the two magnetic layers <NUM>, <NUM> (intended to form the reference layer <NUM> and the magnetic core <NUM>) should have the same BCC structure with a (<NUM>) texture. This can be achieved by using a metallic alloy : which comprises an amorphising element such as B; and which crystallizes in the same structure as the material of the tunnel barrier, such as MgO. FeCo(B) alloy can be a good candidate. The crystallization can be performed upon annealing at a temperature usually ranging from <NUM> to <NUM>. To help the crystallization, the B element (or any amorphising element present in the alloy), should be removed from the vicinity of the tunnel barrier. Thus, a thin layer, composed of a material which can act as a amorphising-getter (usually named as "B-getter"), is deposited adjacent to each FeCo(B) alloy layer, opposite to the interface with the tunnel barrier <NUM>. Such amorphising-getter can be made of W, Ta, Mo or Hf.

The deposition sub-step <NUM> can also comprise a deposition of a metallic layer on top of the upper free layer <NUM> (or the B-getter if applicable) and its etching to a form metallic hard-mask <NUM>. The etching can be done by electron beam etching.

A sub-step of vertical etching <NUM> is then performed through the metallic hard-mask <NUM> to etch the previously deposited layers <NUM>, <NUM>, <NUM> and form a magnetic junction under the hard-mask <NUM>. The surface of the junction, measured parallel to the plane of the layers, is preferably chosen to allow the magnetization of the magnetic core <NUM> to spontaneously points out of the plane of the layers. The etching of the third layer <NUM> of a magnetic material is, for example, performed to form a PSA-layer <NUM> having an aspect ratio comprised between <NUM> and <NUM>.

Afterwards, the magnetic stacks are encapsulated <NUM> in a first dielectric material <NUM>. The first dielectric material <NUM> is then etched <NUM> down to the bottom of the third layer <NUM> and preferably to the bottom of the magnetic core <NUM>. This etching <NUM> can be done by reactive ion-etching or physical ion-etching. This etching <NUM> defines the lower limit of the magnetic shell <NUM> and also breaks the magnetic contact between adjacent junctions.

The method <NUM> then comprises a step of forming <NUM> the SyAF free layer <NUM>. It comprises the formation of a magnetic shell <NUM> surrounding the magnetic core <NUM> located under the hard-mask <NUM>. First, it comprises a sub-step of depositing <NUM> a first conformal layer <NUM> of a non-magnetic material <NUM> on the previously formed magnetic stacks. Said material <NUM> is, for example, a second dielectric material such as ZnO, HfO<NUM>, Al<NUM>O<NUM> and TiN. The thickness of the first conformal layer <NUM> is set to allow the magnetization of the magnetic shell <NUM> (which is formed later) to align antiparallel with the magnetization of the magnetic core <NUM> located under the hard-mask <NUM>. The thickness of the first conformal layer <NUM> is comprises between <NUM> to <NUM>, and preferably from <NUM> to <NUM>, for example equal to <NUM>. The thickness of the first conformal layer <NUM> should not be reduced below a certain threshold thickness because it will cause the magnetization of the magnetic shell <NUM> to spontaneously align parallel with respect to the magnetization of the magnetic core. The first conformal layer can be deposited <NUM> using conformal deposition techniques, such as atomic layer-deposition.

The method <NUM> then comprises then comprises a sub-step of depositing <NUM> a second conformal layer <NUM> of a magnetic material on the first conformal layer <NUM>. Said magnetic material can be a Fe or Ni alloy. The second conformal layer <NUM> can also be deposited <NUM> using conformal deposition techniques, such as atomic layer-deposition.

The external radius for the second conformal layer <NUM>, measured parallel to the plane of the layers, depends on the saturation magnetisation of the materials of the magnetic core <NUM> and the second conformal layer <NUM>, as discussed before, and defines the outer radius of the SyAF free layer <NUM>.

A sub-step of etching <NUM> the first and second conformal layers <NUM>, <NUM> is performed until the magnetic material of the second conformal layer <NUM> surrounds the magnetic core <NUM>. This etching <NUM> is preferably performed to expose the metallic hard-mask <NUM> to define a superior limit of the resulting magnetic shell <NUM>. However, the etching is stopped such that magnetic shell <NUM> surrounds at least a part of the thickness (measured perpendicular to the plane of the layers) of the magnetic core <NUM>. The etching can be done resorting ion beam-etching.

The subs-step of etching <NUM> the first and second conformal layers is preferably performed to the magnetic shells <NUM> of adjacent junctions <NUM>. It is performed until there is no magnetic material in between said adjacent junctions <NUM>. This way, the reversal mechanism of the SyAF free layer <NUM> belonging to different junctions <NUM> behave as a unique magnetization. Magnetic material in between two adjacent SyAF free layers <NUM> could complexify the reversal mechanism, reduce the thermal stability and increase the commutation time.

Claim 1:
Magnetic tunnel junction (<NUM>) comprising a synthetic antiferromagnetic free layer (<NUM>), characterised in that said synthetic antiferromagnetic free layer (<NUM>) comprises a magnetic core (<NUM>) and a magnetic shell (<NUM>), the magnetic core comprising a first magnetic layer (<NUM>) having an aspect ratio comprised between <NUM> and <NUM>, the magnetic shell surrounding the magnetic core and at least a part of the thickness (Zcore) of the magnetic core, the magnetization (ms) of the magnetic shell being antiferromagnetically coupled with the magnetization (mc) of the magnetic core.