Spin Transfer Torque Effect Research Articles

(Invited) Materials and Physics Challenges for Spin Transfer Torque Magnetic Random Access Memories

Magnetic random access memories utilizing the spin transfer torque effect for writing information are a strong contender for non-volatile memories scalable to the 20 nm node, and perhaps beyond. I will here examine how these devices behave as the device size is scaled down from 70 nm size to 20 nm. As device sizes go below ~50 nm, the size becomes comparable to intrinsic magnetic length scales and the device behavior does not simply scale with size. This has implications for the device design and puts additional constraints on the materials in the device.

Small Angle Neutron Scattering Studies of Skyrmion Lattices

Skyrmion lattices in chiral magnets can be regarded as macroscopic lattices formed by topological entities with particle like properties where the particle-like character of the Skyrmions is reflected in the integer winding number of their magnetization [1]. Caused by their topology, Skyrmions cannot be destroyed or created by a smooth deformation of a trivial spin texture. Skyrmion lines can be seen as magnetic whirls – sharing strong similarities with vortices of type II superconductors. Analogous to the large variety of different phases of superconducting vortex matter, Skyrmion lattice melting transitions, Skyrmion liquids and Skyrmion glass phases are expected to exist in the various B20 materials. The structure of Skyrmion matter thereby sensitively reflects the underlying physical properties of the material carrying the Skyrmions. Moreover, due to their topology, Skyrmion lattices in chiral magnets provide an excellent showcase for the investigation of topological stability and phase conversion [2]. A further consequence of the special topological properties of Skyrmions is called emergent electrodynamics: While moving through the Skyrmion lattice, conduction electrons collect a Berry´s phase in real space which leads to a very efficient coupling of transport current and magnetic structure. This leads to considerable spin transfer torque effects at current densities as low as 10^6 A/m2 [3]. Small angle neutron scattering (SANS) has played a central role in the investigation of Skyrmion lattices. We show how SANS can be used for a detailed investigation of the structure, stability, transport properties and topological decay of Skyrmion lattices in B20 compounds. *Work in collaboration with: T. Adams, A. Bauer, H. Berger, P. Böni, S. Buhrandt, A. Chacon, S. Dunsiger, L. Eng, K. Everschor-Sitte, M. Garst, R. Georgii, M. Janoschek, F. Jonietz, J. Kindervater, P. Milde, A. Neubauer, C. Pfleiderer, A. Rosch, C. Schütte, J. Seidel, J. Waizner, W. Wünzer

Colossal spin transfer torque effect on skyrmion along the edge.

We study by the micromagnetic simulations the skyrmion motion along the edge driven by the current transverse to it. We found that (i) the velocity is enhanced by the factor of ∼ 1/α (α: the Gilbert damping) with the maximum value determined only by the confining force from the edge, (ii) the inertia appear due to the confining potential with the coordinate perpendicular to the edge playing the role of the kinetic momentum, and (iii) the collision between the two skyrmions is almost elastic without causing any internal distortions.

自旋纳米振荡器

Spin transfer nano-oscillator (STO), a new class of nano-sized microwave device based on giant magnetoresistance and spin transfer torque effect, has aroused tremendous and continuous research interest due to its potential applications in wireless communications system as either microwave source or detectors. The key features of STOs are simple structure, nanoscale size, frequency tunability, broad working temperature, low operating voltage and easy integration with standard silicon technology. In this review paper, we give an overview of the working principle of STOs, recent developments and breakthroughs. Challenges and perspectives of the STOs that might be productive topics for future research are also briefly discussed.

Voltage induced magnetostrictive switching of nanomagnets: Strain assisted strain transfer torque random access memory

A spintronic device, called the “strain assisted spin transfer torque (STT) random access memory (RAM),” is proposed by combining the magnetostriction effect and the spin transfer torque effect which can result in a dramatic improvement in the energy dissipation relative to a conventional STT-RAM. Magnetization switching in the device which is a piezoelectric-ferromagnetic heterostructure via the combined magnetostriction and STT effect is simulated by solving the Landau-Lifshitz-Gilbert equation incorporating the influence of thermal noise. The simulations show that, in such a device, each of these two mechanisms (magnetostriction and spin transfer torque) provides in a 90° rotation of the magnetization leading a deterministic 180° switching with a critical current significantly smaller than that required for spin torque alone. Such a scheme is an attractive option for writing magnetic RAM cells.

Current-induced domain wall motion in nanostrip–nanobars system

Transverse domain wall motion in strips with a series of nanobars placed perpendicularly on either side has been investigated systematically by micromagnetic simulation. Domain wall motion in strips is driven by electric current owing to the spin transfer torque effect. In a strip in contact with nanobars, a domain wall can withstand a high current density (above A/cm2) to maintain structural stability, but the domain wall velocity cannot be increased markedly. Domain wall motion in strips not in contact with nanobars can reach a higher speed under a relatively low current density. To examine the effects of nanobars on domain wall motion, the DW depinning energy and the Walker limit uw at different parameters are discussed systematically.

Magnetization switching in a CoFeB/MgO magnetic tunnel junction by combining spin-transfer torque and electric field-effect

We propose and demonstrate a scheme for magnetization switching in magnetic tunnel junctions, in which two successive voltage pulses are applied to utilize both spin-transfer torque and electric field effect. Under this switching scheme, a CoFeB/MgO magnetic tunnel junction with perpendicular magnetic easy axis is shown to switch faster than by spin-transfer torque alone and more reliably than that by electric fields alone.

Optical characterization of nonlocal spin transfer torque acting on a single nanomagnet

Time-resolved scanning Kerr microscopy (TRSKM) has been used to examine the effect of nonlocal spin transfer torque (NL-STT) within a two-terminal current perpendicular-to-plane nonlocal spin valve. A combination of Oersted field and NL-STT was used to excite and control the magnetization dynamics. By comparison with a macrospin model, the strength of the NL-STT was quantified and found to be comparable to that achieved by direct injection of spin-polarized current. The sensitivity of the magneto-optical probe to small out-of-plane deflections of the magnetization also allows the NL-STT to be quantified in quasi-dc measurements, greatly simplifying the investigation of STT phenomena.

Efficient spin transfer torque in La2/3Sr1/3MnO3 nanostructures

We carry out low temperature magnetotransport measurements on nanostructured La2/3Sr1/3MnO3 wires to study the interaction between spin-polarized current and magnetization in this half metallic material. We selectively position domain walls by applying external fields. The domain wall resistance is found to be positive, in contrast to conventional 3d metals. The depinning field is reduced when current pulses are injected into the wire. By comparing measurements for both current polarities, we can disentangle heating and spin transfer torque effects. The determined spin transfer torque efficiency is of the order of 4 × 10−14 Tm2/A, which is significantly higher than in permalloy.

Current-induced switching of magnetic tunnel junctions: Effects of field-like spin-transfer torque, pinned-layer magnetization orientation, and temperature

We study current-induced switching in magnetic tunnel junctions in the presence of a field-like spin-transfer torque and titled pinned-layer magnetization in the high current limit at finite temperature. We consider both the Slonczewski and field-like torques with coefficients aJ and bJ, respectively. At finite temperatures, σ=bJ/aJ=±1 leads to a smaller mean switching time compared that with σ=0. The reduction of switching time in the presence of the field-like term is due to the alignment effect (for σ>0) and the initial torque effect.

Dynamical properties of three terminal magnetic tunnel junctions: Spintronics meets spin-orbitronics

This Letter introduces a micromagnetic model able to characterize the magnetization dynamics in three terminal magnetic tunnel junctions, where the effects of spin-transfer torque and spin-orbit torque are taken into account. Our results predict that the possibility to separate electrically those two torque sources is very promising from a technological point of view for both next generation of nanoscale spintronic oscillators and microwave detectors. A scalable synchronization scheme based on the parallel connection of those three terminal devices is also proposed.

Current-induced modification of spin wave mode interference

We have studied the effect of adiabatic spin-transfer torque on mode interference of spin waves. The mode interference generates amplitude-localized spots at special positions which do not move with time. When applying current, the wavevector of spin wave is modified, resulting in current-dependent displacement of amplitude-localized spots. This current-dependent change in the mode interference may allow to probe current-induced spin wave Doppler shift in space-domain. In favorable situations, it can be used to estimate the intrinsic properties of magnetic materials such as spin polarization.

ChemInform Abstract: MgO(001) Barrier Based Magnetic Tunnel Junctions and Their Device Applications

AbstractReview: 210 refs.

Oscillation Stability of a Small Size Spin Torque Oscillator for MAMR

Oscillation behavior of a spin torque oscillator (STO) less than 40 nm in size was investigated by using micromagnetic simulations for microwave-assisted magnetic recording (MAMR) over 5 Tb/in2. Decreasing the STO size from 40 to 20 nm deteriorates the stability of a reference layer (REF) and it shrinks the stable oscillation region of a field generation layer (FGL). Enhancing the magnetic anisotropy energy of the REF Ku_REF stabilizes the magnetization motion of the REF against the spin transfer torque (STT) effect and the external magnetic field Hz, and the stable oscillation region of the FGL expands within a relative lower applied voltage Vsto region, but random oscillation with fluctuating elevation angles appears at a higher Hz and Vsto. Similar phenomenon is observed when increasing the thickness of the REF tREF. Enhancing both Ku_REF × tREF and the thickness of FGL are found to stabilize oscillation of the FGL against the STT effect from a stable REF at a larger strength of Hz and a higher Vsto without the appearance of random oscillation, making it possible to apply about 20-nm-square STO to a magnetic head for over 5 Tb/in2.

A Method for Compensating the Joule-Heating Effects in Current-Induced Domain Wall Motion

We propose here a method for compensating the Joule-heating effects in the current-induced domain wall motion (CIDWM). In CIDWM experiments, the current induces not only the spin-transfer torque (STT) effects but also the Joule-heating effects, and both effects influence the domain wall (DW) motion. It is thus desired to develop a way to compensate the Joule-heating effects, in order to determine the pure STT effects on the DW motion. Up to now, in studies of DW creeping motions, such Joule-heating effects have been eliminated based on the Arrhenius law by assuming the temperature-independent creep scaling constants. However, here we find that such scaling constants are sensitive to the temperature, from the DW creeping experiment in Pt/Co/Pt wires with temperature control in a cryostat. By accounting the temperature dependence of the scaling constants, we demonstrate that all the DW speeds with various temperatures are exactly collapsed onto a single universal curve, which enables us to examine the pure STT effects on the DW motion.

Ferromagnetic resonance driven by an ac current: A brief review

Excitation of ferromagnetic resonance (FMR) by an ac current has been observed in macroscopic ferromagnetic films for decades and typically relies on the ac Oersted field of the current to drive magnetic moments into precession and classical rectification of ac signals to detect the resonance. Recently, current-driven ferromagnetic resonances have attracted renewed attention with the discovery of the spin-transfer torque (STT) effect due to its potential applications in magnetic memory and microwave technologies. Here the STT associated with the ac current is used to drive magnetodynamics on the nanoscale that enables FMR studies in sample volumes smaller by a factor of 1000 compared to conventional resonance techniques. In this paper, we briefly review the basics of STT–FMR technique and the results of various STT–FMR experiments.

Analytic Model of Spin-Torque Oscillators (STO) for Circuit-Level Simulation

Spin-torque oscillators (STO) is a new device that can be used as a tunable microwave source in various wireless devices. Spin-transfer torque effect in magnetic multilayered nanostructure can induce precession of magnetization when bias current and external magnetic field are properly applied, and a microwave signal is generated from that precession. We proposed a semi-empirical circuit-level model of an STO in previous work. In this paper, we present a refined STO model which gives more accuracy by considering physical phenomena in the calculation of effective field. Characteristics of the STO are expressed as functions of external magnetic field and bias current in Verilog-A HDL such that they can be simulated with circuit-level simulators such as Hspice. The simulation results are in good agreement with the experimental data.

A New Circuit Model for Spin-Torque Oscillator Including Perpendicular Torque of Magnetic Tunnel Junction

Spin-torque oscillator (STO) is a promising new technology for the future RF oscillators, which is based on the spin-transfer torque (STT) effect in magnetic multilayered nanostructure. It is expected to provide a larger tunability, smaller size, lower power consumption, and higher level of integration than the semiconductor-based oscillators. In our previous work, a circuit-level model of the giant magnetoresistance (GMR) STO was proposed. In this paper, we present a physics-based circuit-level model of the magnetic tunnel junction (MTJ)-based STO. MTJ-STO model includes the effect of perpendicular torque that has been ignored in the GMR-STO model. The variations of three major characteristics, generation frequency, mean oscillation power, and generation linewidth of an MTJ-STO with respect to the amount of perpendicular torque, are investigated, and the results are applied to our model. The operation of the model was verified by HSPICE simulation, and the results show an excellent agreement with the experimental data. The results also prove that a full circuit-level simulation with MJT-STO devices can be made with our proposed model.

Micromagnetic modeling of magnetization dynamics driven by spin-transfer torque in magnetic nanostructures

Magnetization orientation of a nanoscale ferromagnet can be manipulated by an electric current via spin-transfer torque (STT) effect, which holds great promise in the applications of non-volatile magnetic random access memory (MRAM) and spintorque oscillators. We review the fundamental mechanism and experimental progress of the STT effect. Then, different formula of STT torque has been classified, which can be added to the conventional Landau-Lifshitz-Gilbert equation. After that, we show some simulation results that mainly concern the STT-driven vortex dynamics, magnetization oscillations excited by a perpendicular polarizer, and the detail dynamics by in-plane and out-of-plane dual spin polarizers.

MgO(001) barrier based magnetic tunnel junctions and their device applications

Spintronics has received a great attention and significant interest within the past decades, and provided considerable and remarked applications in industry and electronic information etc. In spintronics, the MgO based magnetic tunnel junction (MTJ) is an important research advancement because of its physical properties and excellent performance, such as the high TMR ratio in MgO based MTJs. We present an overview of more than a decade development in MgO based MTJs. The review contains three main sections. (1) Research of several types of MgO based MTJs, including single-crystal MgO barrier based-MTJs, double barrier MTJs, MgO based MTJs with interlayer, novel electrode material MTJs based on MgO, novel barrier based MTJs, novel barrier MTJs based on MgO, and perpendicular MTJs. (2) Some typical physical effects in MgO based MTJs, which include six observed physical effects in MgO based MTJs, namely spin transfer torque (STT) effect, Coulomb blockade magnetoresistance (CBMR) effect, oscillatory magnetoresistance, quantum-well resonance tunneling effect, electric field assisted magnetization switching effect, and spincaloric effect. (3) In the last section, a brief introduction of some important device applications of MgO based MTJs, such as GMR & TMR read heads and magneto-sensitive sensors, both field and current switching MRAM, spin nano oscillators, and spin logic devices, have been provided.