RELATED APPLICATION

This application claims the benefit of European Application No. EP19315030, filed on May 6, 2019. The entire content of this application is hereby incorporated by reference.

FIELD

The present invention concerns a magnetic memory cell having deterministic switching, high data retention and high immunity to magnetic field disturbance.

DESCRIPTION OF RELATED ART

A non-volatile magnetic random-access memory (MRAM) cell includes a magnetic tunnel junction (MTJ) having a magnetically hard layer (i.e., a “reference” layer) and a magnetically soft layer (i.e., the “free” or “storage” layer) and a tunnel barrier layer between the reference layer and the storage layer. Conventional MRAM cells can be written by passing current through current leads that are formed on either side of each memory element in order to create a local induced magnetic field, which sets the direction of the soft layer magnetization.

Spin transfer torque (STT) MRAM cells are similar to conventional MRAM cells except that the write current path passes perpendicularly through the magnetic layers of the MTJ and through the tunnel barrier of the MTJ. The storage layer is set via the spin transfer torque from the spin polarized current induced by the reference magnetic layer.

Recently, an alternative way to switch the magnetization of a thin ferromagnetic layer using an electric current has been demonstrated based on the concept of the spin-orbit torque (SOT). In this scheme, the magnetic storage layer is in contact with a SOT-generating stripe (layer), typically Pt, Ta, W, Hf, IrMn or PtMn. When injecting a current in the nonmagnetic SOT generating layer, the spin-orbit coupling leads either to a perpendicular spin current inside the SOT generating layer as well as a spin accumulation at the interface with the magnetic material. Both effects lead to a transfer of angular momentum to the magnetization of the ferromagnetic layer, creating spin orbit torques and inducing magnetization reversal in the ferromagnetic layer. The spin current generation is referred to as spin Hall effect while the spin reorientation is commonly linked to the Rashba effect. Their relative contribution and more generally the precise physical mechanism is currently under debate.

A key advantage of the SOT-MRAM cell is that the write and read operations are decoupled due to the different current paths, which naturally solves the problems related to endurance and read disturbance of the STT-MRAM. Moreover, the TMR can be tuned independently of writing constraints, relaxing the RA requirements (typically in STT-MRAM RA<10 Ωμm2) in order to achieve high TMR (>200%).

A spin-orbit torque switching for various SOT-MRAM cell configurations is schematically illustrated inFIGS. 1a-c. In the figures, the SOT-MRAM cell is only represented by the storage layer11adjacent to a bottom electrode5(SOT-generating stripe). A current20is passed in the bottom electrode5.

InFIG. 1a, the bit storage layer11comprises an out-of-plane storage magnetization110, i.e., the storage magnetization110is oriented substantially perpendicular to the plane of the storage layer11.

The storage layer11further presents a strong magnetic anisotropy such that sufficient data retention (at least a few seconds) is also achieved.

However, the spin orbit torques generated by the current20do not allow for a deterministic switching as both magnetic orientations (up and down) of the storage magnetization110are symmetrical for these torques and none of these two magnetic orientations are favored. In order to establish a determined switched orientation of the storage magnetization110, an additional symmetry-breaking factor is required, such as an additional external magnetic field (not shown) being at least partly aligned in the direction of the current20.

FIG. 1bshows a configuration of the SOT-MRAM cell1where the storage layer11comprises an in-plane storage magnetization110, i.e., the storage magnetization110is oriented substantially parallel to the plane of the storage layer11. The storage magnetization110is oriented substantially collinear to the current direction so that the current induced SOT is maximized and a fast switching can be achieved. As the spin orbit torques induced by the current20act symmetrically on the two orientations of the storage magnetization110(parallel and antiparallel to the current direction), the current20does not allow for a deterministic switching unless an additional symmetry breaking factor is provided, such as an additional external magnetic field (not shown) being at least partly oriented substantially perpendicular to the plane of the storage layer11.

InFIG. 1c, the storage layer11of the SOT-MRAM cell1comprises an in-plane storage magnetization110oriented substantially perpendicular to the direction of the current20. In this configuration, the action of the spin orbit torques is not symmetrical for the two orientations of the storage magnetization110, such that a deterministic switching of the storage magnetization110can be achieved without any additional symmetry-breaking factor.

The configurations ofFIGS. 1band 1cwith the in-plane storage magnetization110typically rely on shape anisotropy and do not present a coercivity that is large enough (>500 Oe) to ensure immunity against magnetic disturbances.

The configurations ofFIGS. 1band 1cwith the in-plane storage magnetization110show a weak magnetic anisotropy so that sufficient data retention cannot be achieved in small sized cells, i.e., typically with largest dimension being below 60 nm.

Publication “Evaluation of Ultrahigh-speed Magnetic Memories Using Field-Free Spin-Orbit Torque”, by Wang Zhaohao et al, evaluates the potential of two types of SOT-MRAMs, whose data can be ultrafast written based on known field-free SOT mechanisms. A cross-layer analysis is presented involving device modeling, circuit-level optimization, and architecture-level evaluation. First, the principle of the magnetization switching is analyzed with the macrospin simulation. Then, the optimization strategies at the circuit level are generalized through the SPICE-type simulation. Finally, the memory architecture with the SOT-MRAMs, STT-MRAM, and static RAM is built. Their read/write performances are evaluated with NVSim software. It is demonstrated that one of the studied SOT-MRAMs shows the promising prospect in the non-volatile memory, especially suitable for high-capacity cache.

Publication “Micromagnetic Simulation of Spin-Orbit Torque Induced Ultrafast Switching of In-Plane Magnetization” by Li Zuwei et al, teaches that, through micromagnetic simulation, the in-plane magnetization can be ultrafast and reliably switched by two methods. First, enhancing the field-like torque can eliminate the incubation delay to accelerate the magnetization switching. Second, tilting the easy-axis can strengthen the initial spin torque to decrease the switching delay. These two switching mechanisms are analyzed in this publication based on micromagnetic simulation results.

US2017/125078 discloses a method and apparatus for deterministically switching a free layer in a spin orbit torque magnetoresistive random access memory (SOT-MRAM) cell is disclosed herein. In one embodiment, an SOT-MRAM memory cell is provided. The SOT-MRAM memory cell includes a magnetic tunnel junction, a ferromagnetic bias layer, and an antiferromagnetic layer. The magnetic tunnel junction includes a free layer having primarily two bi-stable magnetization directions, a reference layer having a fixed magnetization direction, and an insulating tunnel barrier layer positioned between the free layer and the reference layer. The ferromagnetic bias layer is configured to provide spin orbit torque via anomalous Hall effect and simultaneously configured to provide a magnetic bias field on the free layer to achieve deterministic switching. The antiferromagnetic layer is positioned below the ferromagnetic bias layer and is configured to pin a magnetization direction of the ferromagnetic bias layer in a predetermined direction.

US2011/170339 discloses a method of operating a magnetoresistive device. The device comprises a ferromagnetic region configured to exhibit magnetic anisotropy and to allow magnetisation thereof to be switched between at least first and second orientations and a gate capacitively coupled to the ferromagnetic region. The method comprises applying an electric field pulse to the ferromagnetic region so as to cause orientation of magnetic anisotropy to change for switching magnetisation between the first and second orientations.

US 2012/314488 discloses a magnetic random access memory (MRAM) cell which includes a storage layer, a sense layer, and a spacer layer between the storage layer and the sense layer. A field line is magnetically coupled to the MRAM cell to induce a magnetic field along a magnetic field axis, and at least one of the storage layer and the sense layer has a magnetic anisotropy axis that is tilted relative to the magnetic field axis. During a write operation, a storage magnetization direction is switchable between m directions to store data corresponding to one of m logic states, with m>2, where at least one of the m directions is aligned relative to the magnetic anisotropy axis, and at least another one of the m directions is aligned relative to the magnetic field axis. During a read operation, a sense magnetization direction is varied, relative to the storage magnetization direction, to determine the data stored by the storage layer.

WO 2017/044132 discloses an apparatus which comprises, a substrate, and a magnet formed with one or more materials or a stack of materials with perpendicular magnetic anisotropy (PMA), wherein a magnetic moment of the magnet points mainly in a plane of the substrate. There is also disclosed a method comprising: defining a pitch grating pattern; transferring the pitch grating pattern into a backbone layer to form a patterned backbone layer; and depositing a layer of magnetic spacer, formed of one or more materials or a stack of materials of different metals or oxides, on the patterned backbone layer.

Therefore, there is still a need for a MRAM cell design providing all of the following features: fast switching, deterministic switching, sufficient data retention and large coercivity for sufficient immunity to magnetic field disturbance.

SUMMARY

The present disclosure concerns a magnetic memory (MRAM) cell, comprising: a first electrode formed from a substantially conductive layer, wherein the first electrode has a surface arranged to accumulate spin when a write current is flowed through the substantially conductive layer, in the plane of the first electrode; and a magnetic tunnel junction (MTJ) stack formed over the first electrode, wherein the MTJ stack comprises: a ferromagnetic reference layer having an in-plane reference magnetization; a tunnel barrier layer; and a ferromagnetic storage layer between the tunnel barrier layer and the first electrode, the storage layer having an in-plane storage magnetization; wherein the MTJ stack comprises an arrangement for providing an in-plane uniaxial anisotropy in the storage layer; wherein said in-plane uniaxial anisotropy makes an angle with the direction of the write current that is between 5° and 90°, and wherein said in-plane uniaxial anisotropy has an energy between 40 and 200 kBT and a coercive field above 1000 Oe.

The MRAM cell design provides all of the following features: fast switching, deterministic switching, sufficient data retention and large coercivity without an additional symmetry-breaking factor.

