https://ralphgroup.lassp.cornell.edu/papers/JMMM-320-1190-2008.pdf

Spin transfer torques

Current-induced effective magnetic fields can provide efficient ways of electrically manipulating the magnetization of ultrathin magnetic heterostructures. Two effects, known as the Rashba spin orbit field and the spin Hall spin torque, have been reported to be responsible for the generation of the effective field. However, a quantitative understanding of the effective field, including its direction with respect to the current flow, is lacking. Here we describe vector measurements of the current-induced effective field in Ta|CoFeB|MgO heterostructrures. The effective field exhibits a significant dependence on the Ta and CoFeB layer thicknesses. In particular, a 1 nm thickness variation of the Ta layer can change the magnitude of the effective field by nearly two orders of magnitude. Moreover, its sign changes when the Ta layer thickness is reduced, indicating that there are two competing effects contributing to it. Our results illustrate that the presence of atomically thin metals can profoundly change the landscape for controlling magnetic moments in magnetic heterostructures electrically.

Layer thickness dependence of the current-induced effective field vector in Ta|CoFeB|MgO

https://dr.ntu.edu.sg/bitstream/10356/146755/2/Published_Spintronics%20based%20random%20access_a%20review.pdf

Spintronics based random access memory: a review

Memristive devices are novel devices, which can be used in applications ranging from memory and logic to neuromorphic systems. A memristive device offers several advantages: nonvolatility, good scalability, effectively no leakage current, and compatibility with CMOS technology, both electrically and in terms of manufacturing. Several models for memristive devices have been developed and are discussed in this paper. Digital applications such as memory and logic require a model that is highly nonlinear, simple for calculations, and sufficiently accurate. In this paper, a new memristive device model is presented-TEAM, ThrEshold Adaptive Memristor model. This model is flexible and can be fit to any practical memristive device. Previously published models are compared in this paper to the proposed TEAM model. It is shown that the proposed model is reasonably accurate and computationally efficient, and is more appropriate for circuit simulation than previously published models.

TEAM: ThrEshold Adaptive Memristor Model

A magnetoresistive element according to an embodiment includes: a first ferromagnetic layer having an axis of easy magnetization in a direction perpendicular to a film plane; a second ferromagnetic layer having an axis of easy magnetization in a direction perpendicular to a film plane; a nonmagnetic layer placed between the first ferromagnetic layer and the second ferromagnetic layer; a first interfacial magnetic layer placed between the first ferromagnetic layer and the nonmagnetic layer; and a second interfacial magnetic layer placed between the second ferromagnetic layer and the nonmagnetic layer The first interfacial magnetic layer includes a first interfacial magnetic film, a second interfacial magnetic film placed between the first interfacial magnetic film and the nonmagnetic layer and having a different composition from that of the first interfacial magnetic film, and a first nonmagnetic film placed between the first interfacial magnetic film and the second interfacial magnetic film

Magnetoresistive element and magnetic memory

We present experimental and numerical results of current-driven magnetization switching in magnetic tunnel junctions. The experiments show that, for MgO-based magnetic tunnelling junctions, the tunnelling magnetoresistance ratio is as large as 155% and the intrinsic switching current density is as low as 1.1 ? 106?A?cm?2. The thermal effect and current pulse width on spin-transfer magnetization switching are explored based on the analytical and numerical calculations. Three distinct switching modes, thermal activation, dynamic reversal, and precessional process, are identified within the experimental parameter space. The switching current distribution, write error, and read disturb are discussed based on device design considerations. The challenges and requirements for the successful application of spin-transfer torque as the write scheme in random access memory are addressed.

https://iopscience.iop.org/article/10.1088/0953-8984/19/16/165209/pdf

Spin-transfer torque switching in magnetic tunnel junctions and spin-transfer torque random access memory

Dual magnetic tunnel junction (MTJ) structures consisting of two MgO insulating barriers of different resistances, two pinned reference layers aligned antiparallel to one another, and a free layer embedded between the two insulating barriers have been developed. The electron transport and spin dependent resistances in the dual MTJ structures are accounted for by sequential tunneling with some spin-flip relaxation in the central electrode (the free layer). With a tunneling magnetoresistance ratio of 70%, a switching current density Jc (at 30ms) of 0.52MA∕cm2 is obtained, corresponding to an intrinsic value of Jc0 (at 1ns) of 1.0MA∕cm2. This value of Jc0 is 2–3 times smaller than that of a single MgO insulating barrier MTJ structure and results from improvements in the spin-transfer torque efficiency. The asymmetry between JcAP→P and JcP→AP is significantly improved, which widens the read-write margin for memory device design. In addition, the experimental results show that the switching current density can be further reduced when an external field is applied along the hard axis of the free layer.

Spin transfer switching in dual MgO magnetic tunnel junctions

We quantitatively determine a perpendicular spin torque in magnetic tunnel junctions by measuring the room-temperature critical switching current at various magnetic fields and current pulse widths. We find that the magnitude of the torque is proportional to the product of the current density and the bias voltage, and the direction of the torque reverses as the polarity of the voltage changes. By taking into account the energy-dependent inelastic scattering of tunnel electrons, we formulate the bias dependence of the perpendicular spin torque which is in qualitative agreement with the experimental results.

Perpendicular spin torques in magnetic tunnel junctions.

A method and system for providing a magnetic element is described. The magnetic element includes a first pinned layer, a first spacer layer, a free layer, a second spacer layer, and a second pinned layer. The first and second pinned layers have first and magnetizations oriented in first and second directions, respectively. The first and second spacer layers are nonferromagnetic. The first and second spacer layers are between the free layer and the first and second pinned layers, respectively. The magnetic element is configured either to allow the free layer to be switched to each of multiple states when both a unidirectional write current is passed through the magnetic element and the magnetic element is subjected to a magnetic field corresponding to the each states or to allow the free layer to be switched to each of the plurality of states utilizing a write current and an additional magnetic field that is applied from at least one of the first pinned layer and the second pinned layer substantially only if the write current is also applied.

Method and system for providing a spin transfer device with improved switching characteristics

Dynamic switching behavior of antiferromagnetically coupled single-domain bilayers at 0 and 300 K has been investigated using Landau-Lifshitz (LL) equation. The operating field windows for toggle-magnetic random access memory (MRAM) have been found narrower than the ones previously obtained by analytic calculations, even at 0 K, with the general trend that the longer the duration and rise/fall times are, the wider the window is. The simulation at 300 K showed that the thermal effect is substantial in degradation of the operating field window, especially due to the increase of the error rate in the higher operating field region, although the enhancement of the damping factor in nanomagnetic devices helps reduce the window reduction considerably

Zhanjie Li

Papers

Spin-transfer torque switching in magnetic tunnel junctions and spin-transfer torque random access memory

Spin transfer switching in dual MgO magnetic tunnel junctions

Perpendicular spin torques in magnetic tunnel junctions.

Method and system for providing a spin transfer device with improved switching characteristics

Dynamic Simulation of Toggle Mode MRAM Operating Field Margin