Spin orbit torque based magnetic random access memory (SOT-MRAM) are non-volatile memory devices which make use of spin based transport electronics named as spintronics. In such devices, capacitors in conventional memory devices are replaced by a trilayer structure called magnetic tunnel junction (MTJ). As spin is non-volatile, it overcomes the charge leakage associated with conventional memory devices. Unlike STT-RAM, SOT doesn’t require a high current passing through the MTJ during the write operation. Read operation is achieved through the tunnel magnetoresistance and write operation is achieved through the in-plane current passing through an adjacent heavy metal layer. This decoupled read and write path is the major advantage of SOT MRAM as compared to that of STT-MRAM.
Spin-orbit Torque (SOT)
Fig. 1 A current flowing through the heavy metal (blue arrow) generates current of spins (Red arrows) which causes the magnetization of the ferromagnet to change direction from Perpendicular to parallel
Likewise, the ordinary torque, Spin-orbit torque (SOT) is also a jolt, but it is usually experienced by the magnetization of the magnetic layer in a bilayer system composed of heavy metal and a ferromagnet. It often happens when an electric current passes through this bilayer system. A current flowing through the heavy metal generates current of spins which causes the magnetization of the ferromagnet to change direction from perpendicular to parallel as demonstrated in the schematic (Fig.1) or vice versa. This current induced SOT mediates the transfer of angular momentum from the lattice to the spin system and causes sustained magnetic oscillations or switching of the ferromagnetic or antiferromagnetic spins. SOT relies on the conversion of electric current to spin. This particular torque is named as SOT in order to underline its link to spin orbit interaction.
Spin-orbit Coupling (SOC)
Spin-orbit coupling is the interaction between the spin of electron usually understood with spin angular momentum quantum number $\vec{S}$ and its orbital motion around the nucleus which is described by the orbital angular momentum quantum number $\vec{L}$.
Electrons are found at a certain energy level inside the atom called shells. The electrons in the shells have angular momentum. Suppose, if the electron is not spinning or traveling around the nucleus, if we examine it, we can observe some angular momentum. This implies the electron still behaves as if it has angular momentum. This angular momentum electron possesses regardless of its state of rotation or rest is called spin. That is, spin is the inherent angular momentum. In the spin orbit interaction term $\vec{L}.\vec{S}$. The orbital angular momentum part $\vec{L}$ is very much depends on which shell the electron is present whereas the spin angular momentum S is just the spin of the electron.
Reading Data in SOT-MRAM
Reading operation in SOT-MRAM is achieved through a phenomenon called tunnel magnetoresistance (TMR). It is possible to switch the magnetization of two magnetization states of ferromagnets such as parallel and antiparallel states by applying an external magnetic field. If the magnetization direction of the two ferromagnetic layers shown as the free layer and fixed layer (title figure ) are aligned parallel, it is more likely that the electron will tunnel through the insulating layer as compared to that of the antiparallel alignment of spins in free layer with respect to the fixed layer. As a consequence of this easy tunneling associated with parallel alignment and difficulty in tunneling associated with antiparallel (opposite direction of spin states) alignment, the junction will experience a low resistance state and high resistance state respectively. Read operation is realized with logic states of 1s and 0s corresponding to high resistance state and low resistance state.
Writing Data in SOT-MRAM
Writing of data is achieved in SOT-MRAM by an in-plane current passing through an adjacent heavy-metal/magnetic heterostructure.
The parallel and antiparallel configuration of spins in MTJ can be switched either with the influence of an in-plane current passing through the heavy metal or by applying an external magnetic field to the bilayer structure composed of heavy metal and magnetic material.
The fundamental mechanisms which play a vital role in achieving write operation in SOT- MRAM are spin Hall effect (SHE) and inverse spin Galvanic effect (iSGE).
The spin Hall effect is a transport mechanism in which spin accumulation occurs on the lateral surfaces of a sample carrying electric current. The boundaries of opposing surfaces possess spins of opposite signs. The carriers with opposite spin prefer to diffuse to different directions when scattered from the impurities in the material. The carrier trajectories will be distorted as a result of spin orbit interaction and a net accumulation of spin density occurs at the lateral surface of the heterostructure system. The polarization of spin currents at the boundaries remain proportional to the charge current and altering in the alignment happens when the direction of charge current is changed.
The coupling between charge and spin current due to spin orbit interaction is the root cause of SHE.
In SHE, the conversion of charge current to spin current occurs whereas iSGE is understood as an inverse mechanism where a spin polarization at non equilibrium state is converted into transverse charge current.
In either case the read and write operation in SOT-MRAM are realized with logic 0 and 1 state of MTJ corresponds to the switching of parallel and antiparallel state realized by either applying a current in suitable direction or with the help of an external magnetic field.