Current-induced Magnetization Reversal Research Articles

Roles of Joule heating and spin-orbit torques in the direct current induced magnetization reversal

Current-induced magnetization reversal via spin-orbit torques (SOTs) has been intensively studied in heavy-metal/ferromagnetic-metal/oxide heterostructures due to its promising application in low-energy consumption logic and memory devices. Here, we systematically study the function of Joule heating and SOTs in the current-induced magnetization reversal using Pt/Co/SmOx and Pt/Co/AlOx structures with different perpendicular magnetic anisotropies (PMAs). The SOT-induced effective fields, anisotropy field, switching field and switching current density (Jc) are characterized using electric transport measurements based on the anomalous Hall effect and polar magneto-optical Kerr effect (MOKE). The results show that the current-generated Joule heating plays an assisted role in the reversal process by reducing switching field and enhancing SOT efficiency. The out-of-plane component of the damping-like-SOT effective field is responsible for the magnetization reversal. The obtained Jc for Pt/Co/SmOx and Pt/Co/AlOx structures with similar spin Hall angles and different PMAs remains roughly constant, revealing that the coherent switching model cannot fully explain the current-induced magnetization reversal. In contrast, by observing the domain wall nucleation and expansion using MOKE and comparing the damping-like-SOT effective field and switching field, we conclude that the current-induced magnetization reversal is dominated by the depinning model and Jc also immensely relies on the depinning field.

Materials for spin-transfer-torque magnetoresistive random-access memory

Abstract

Spin-torque oscillation in large size nano-magnet with perpendicular magnetic fields

DC current induced magnetization reversal and magnetization oscillation was observed in 500nm large size Co90Fe10/Cu/Ni80Fe20 pillars. A perpendicular external field enhanced the coercive field separation between the reference layer (Co90Fe10) and free layer (Ni80Fe20) in the pseudo spin valve, allowing a large window of external magnetic field for exploring the free-layer reversal. A magnetic hybrid structure was achieved for the study of spin torque oscillation by applying a perpendicular field >3kOe. The magnetization precession was manifested in terms of the multiple peaks on the differential resistance curves. Depending on the bias current and applied field, the regions of magnetic switching and magnetization precession on a dynamical stability diagram has been discussed in details. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight for synchronization of inhomogeneities in large sized device. The ability to manipulate spin-dynamics on large size devices could be proved useful for increasing the output power of the spin-transfer nano-oscillators (STNOs).

Is it possible to create magnetic semiconductors that work at room temperature?

Ferromagnetic semiconductors combine the properties of both ferromagnetic and semiconducting systems at the material level. Comparing with ferromagnetic metals, an obvious advantage of a ferromagnetic semiconductor is the great tunability of its magnetic properties via variation of its carrier density by nonmagnetic means, such as electrostatic gating or photoillumination. Over the past two decades, the ferromagnetic semiconductor has attracted much attention and developed into an important branch of condensed matter physics and materials sciences. Many kinds of important spintronic functionalities have been realized based on ferromagnetic semiconductors, like the electrical-field control of the Curie temperature and the magnetization, spin injection into nonmagnetic semiconductors, tunneling magnetoresistance, and electric current-induced magnetization reversal. Unfortunately, all these performances have been demonstrated only at low temperatures but not yet in a range warm enough for practical applications. However, room-temperature operation is a prerequisite for spintronic applications. Scientists have devoted themselves to making nonmagnetic semiconductors ferromagnetic at room temperature. Research in this direction has never been stopped. This article firstly introduces the concept of magnetic semiconductor, then provides a brief introduction of possible inside mechanisms of so-called room-temperature ferromagnetic semiconductors, such as ZnO and GaN doped with Mn, and then focuses on the recent progress of typical magnetic semiconductor (Ga, Mn)As, which is widely adopted to be with intrinsic ferromagnetism. Finally, the article provides an outlook on the possibility of creating room-temperature ferromagnetic semiconductors.

Perpendicular magnetization reversal in Pt/[Co/Ni]3/Al multilayers via the spin Hall effect of Pt

We experimentally investigate the current-induced magnetization reversal in Pt/[Co/Ni]3/Al multilayers combining the anomalous Hall effect and magneto-optical Kerr effect techniques in crossbar geometry. The magnetization reversal occurs through nucleation and propagation of a domain of opposite polarity for a current density of the order of 3 × 1011 A/m2. In these experiments, we demonstrate a full control of each stage: (i) the Ørsted field controls the domain nucleation and (ii) domain-wall propagation occurs by spin torque from the Pt spin Hall effect. This scenario requires an in-plane magnetic field to tune the domain wall center orientation along the current for efficient domain wall propagation. Indeed, as nucleated, domain walls are chiral and Néel-like due to the interfacial Dzyaloshinskii-Moriya interaction.

Perpendicular magnetic anisotropy in Mn2CoAl thin film

Heusler compound Mn2CoAl (MCA) is attracting more attentions due to many novel properties, such as high resistance, semiconducting behavior and suggestion as a spin-gapless material with a low magnetic moment. In this work, Mn2CoAl epitaxial thin film was prepared on MgO(100) substrate by magnetron sputtering. The transport property of the film exhibits a semiconducting-like behavior. Moreover, our research reveals that perpendicular magnetic anisotropy (PMA) can be induced in very thin Mn2CoAl films resulting from Mn-O and Co-O bonding at Mn2CoAl/MgO interface, which coincides with a recent theoretical prediction. PMA and low saturation magnetic moment could lead to large spin-transfer torque with low current density in principle, and thus our work may bring some unanticipated Heusler compounds into spintronics topics such as the domain wall motion and the current-induced magnetization reversal.

Spin-Transfer Torque Switching at Ultra Low Current Densities

The influence of the tantalum buffer layer on the magnetic anisotropy of perpendicular Co-Fe-B/MgO based magnetic tunnel junctions is studied using magneto-optical Kerr-spectroscopy. Samples without a tantalum buffer are found to exhibit no perpendicular magnetization. The transport of Boron into the tantalum buffer is considered to play an important role on the switching currents of those devices. With the optimized layer stack of a perpendicular tunnel junction, a minimal critical switching current density of only 9.2 kA/cm$^2$ is observed. As of today, this value is the lowest reported value for current-induced magnetization reversal.

Strain-assisted current-induced magnetization reversal in magnetic tunnel junctions: A micromagnetic study with phase-field microelasticity

Effect of substrate misfit strain on current-induced in-plane magnetization reversal in CoFeB-MgO based magnetic tunnel junctions is investigated by combining micromagnetic simulations with phase-field microelasticity theory. It is found that the critical current density for in-plane magnetization reversal decreases dramatically with an increasing substrate strain, since the effective elastic field can drag the magnetization to one of the four in-plane diagonal directions. A potential strain-assisted multilevel bit spin transfer magnetization switching device using substrate misfit strain is also proposed.

Improvement of Spin Transfer Torque in Asymmetric Graphene Devices

A graphene lateral spin valve structure with asymmetric contacts is presented for the first time, with enhancement of spin angular momentum absorption in its receiving magnet. The asymmetric device with tunneling barrier only at the injector magnet shows a comparable spin valve signal but lower electrical noises compared to the device with two tunneling barriers. We also report experimental measurements of spin transfer torque. Assisted by an external magnetic field of 2.5 mT, spin diffusion current-induced magnetization reversal occurs at a nonlocal charge current density of 33 mA/μm(2), smaller than that needed in devices with two tunneling barriers.

Magnetization reversal induced by in-plane current in Ta/CoFeB/MgO structures with perpendicular magnetic easy axis

We investigate in-plane current-induced magnetization reversal under an in-plane magnetic field in Hall bar shaped devices composed of Ta/CoFeB/MgO structures with perpendicular magnetic easy axis. The observed relationship between the directions of current and magnetization switching and Ta thickness dependence of magnetization switching current are accordance with those for magnetization reversal by spin transfer torque originated from the spin Hall effect in the Ta layer.

Thermally assisted current-induced magnetization reversal in SrRuO3

We inject a sequence of 1 ms current pulses into uniformly magnetized patterns of the itinerant ferromagnet SrRuO3 until a magnetization reversal is detected. We detect the effective temperature during the pulse and find that the cumulative pulse time required to induce magnetization reversal depends exponentially on 1/T. In addition, we find that the cumulative pulse time also depends exponentially on the current amplitude. These observations indicate current-induced magnetization reversal assisted by thermal fluctuations.

Spin-torque efficiency enhanced by Rashba spin splitting in three dimensions

We examine a spin torque induced by the Rashba spin-orbit coupling in three dimensions within the Boltzmann transport theory. We analytically calculate the spin torque and show how its behavior is related with the spin topology in the Fermi surfaces by studying the Fermi-energy dependence of the spin torque. Moreover we discuss the spin-torque efficiency which is the spin torque divided by the applied electric current in association with the current-induced magnetization reversal. It is found that high spin-torque efficiency is achieved when the Fermi energy lies on only the lower band and there exists an optimal value for the Rashba parameter, where the spin-torque efficiency becomes maximum.

Current-induced magnetization reversal in terms of power dissipation

Magnetization excitation and reversal induced by spin-transfer torques is described in terms of power received or dissipated in a macrospin system. This approach provides a clear and intuitive understanding of the effect of both applied magnetic fields and injected spin-polarized currents on magnetization reversal. It is illustrated by solving the case of magnetization reversal in a nanopillar spin valve with perpendicular magnetizations although the approach can be applied more generally. The appearance of critical currents below which spin-transfer torque is no longer efficient is explained by a break in the uniaxial symmetry of such structures.

Current-induced magnetization reversal on the surface of a topological insulator

We study dynamics of the magnetization coupled to the surface Dirac fermions of a three di- mensional topological insulator. By solving the Landau-Lifshitz-Gilbert equation in the presence of charge current, we find current induced magnetization dynamics and discuss the possibility of mag- netization reversal. The torque from the current injection depends on the transmission probability through the ferromagnet and shows nontrivial dependence on the exchange coupling. The mag- netization dynamics is a direct manifestation of the inverse spin-galvanic effect and hence another ferromagnet is unnecessary to induce spin transfer torque in contrast to the conventional setup.

짧은 전류 펄스를 이용한 전류 유도 자화 반전에서 에너지 장벽 분포의 효과

본 논문에서는 짧은 전류 펄스를 이용한 미소자기 소자에서의 전류 유도 자화 반전에 대한 매크로 스핀 시뮬레이션 연구를 수행하였다. 자화 반전 전류 분포에 있어서 에너지 장벽이 미치는 효과에 특별히 주목하였다. 자화 반전 전류의 크기 및 분포는 전류 펄스 폭의 감소에 따라 증가했다. 여기서 긴 전류 펄스 폭 영역에서는 에너지 장벽과 자화 반전 전류 분포 사이의 관계가 아레니우스-닐 법칙에 의해 서술된다. 하지만 짧은 전류 펄스 폭의 영역에서는 이 관계가 풀리지 않은 채로 남아있다. 이는 짧은 전류 펄스로 인한 자화 반전이 열적 활성화에 의해서가 아닌 세차 운동에 의해 좌우되기 때문이며, 이를 해결하는 데에 있어서 어려움이 발생한다. 그러므로 포커-플랑크 방정식을 풀어서 짧은 전류 펄스 영역에서의 자화 반전에 대한 정확한 공식을 얻어내는 것이 필요하며 이를 통해 짧은 전류 펄스 영역에서의 자화 반전 양상을 이해 할 수 있을 것으로 본다. We performed macro-spin simulation studies of the current-induced magnetization reversal of nanomagnetic elements with short current pulses. A special attention was paid to the effect of the energy barrier on the switching current distribution. The switching current and its distribution increase with decreasing the current pulse-width. The relationship between the energy barrier and switching current distribution is described by the Arrhenius-N<TEX>$\'{e}$</TEX>el law at a long pulse-width regime. At a regime of short pulse-width, however, the relationship is left unaddressed. The difficulty to address this issue arises because the magnetization switching with a short current pulse is governed not by the thermal activation but by the precession motion. Therefore, an exact formulation for the short pulse regime by solving the Fokker-Plank equation is needed to understand the result.

Low switching current in a modified exchange-biased spin valve via antiferromagnetic spin transfer torque

We analyze the current-induced spin transfer torque and magnetization reversal properties in an exchanged-biased spin valve (EBSV) structure FM2/NM/FM1/AFM, taking into consideration the exchange interaction between the ferromagnetic (FM) and the antiferromagnetic (AFM) layers. The passage of the spin current above a certain threshold value causes the magnetization to switch in some parts of the AFM layer. This in turn leads to a change in the magnitude and direction of the exchange-bias field, which can subsequently assist or hinder the magnetization switching of the adjacent FM layer and results in so-called inverse current-induced magnetization switching for a weakly-biased EBSV structure. The requisite critical current density to switch the AFM layer is theoretically found to be lower than that for the FM layer, which provides us a potential method to substantially reduce the critical current density for the spin transfer switching in EBSV-based devices.

Field and current-induced magnetization reversal studied through spatially resolved point-contacts

We present results from scanning tunneling microscopy based point-contact measurements of the local resistance in octagon shaped, Co(20 nm)/Cu(5 nm)/Fe19Ni81(2.5 nm) spin-valve rings. Through this technique one can detect the magnetoresistance with spatial resolution, and link it to magnetic domain wall motion within the ring. Measurements with varying currents indicate current-induced effects leading to offsets in the magnetic fields required for magnetic switching. The offsets can be attributed to current-induced spin-transfer torque effects for the thin Fe19Ni81 layer and to Oersted field effects for the thick Co layer.

Inverse Spin-Galvanic Effect in the Interface between a Topological Insulator and a Ferromagnet

When a ferromagnet is deposited on the surface of a topological insulator the topologically protected surface state develops a gap and becomes a two-dimensional quantum Hall liquid. We demonstrate that the Hall current in such a liquid, induced by an external electric field, can have a dramatic effect on the magnetization dynamics of the ferromagnet by changing the effective anisotropy field. This change is dissipationless and may be substantial even in weakly spin-orbit coupled ferromagnets. We study the possibility of dissipationless current-induced magnetization reversal in monolayer-thin, insulating ferromagnets with a soft perpendicular anisotropy and discuss possible applications of this effect.

Effect of interlayer coupling in CoFeB/Ta/NiFe free layers on the critical switching current of MgO-based magnetic tunnel junctions

This paper reports the current-induced magnetization reversal characteristics of MgO-based magnetic tunnel junctions (MTJs) with CoFeB/Ta/NiFe composite free layers designed for spin-transfer-torque magnetoresistive random access memory. As the Ta spacer thickness (≤8 Å) was increased, the MTJs embedded into nanoscale integrated circuits demonstrated not only higher tunneling magnetoresistance ratios but also lower intrinsic critical switching currents. This suggests that promoting weak interlayer exchange coupling between CoFeB and NiFe is desirable for reducing the intrinsic critical switching current of CoFeB/Ta/NiFe. While the energy barrier was also reduced with a thicker Ta spacer, it was maintained at an adequate level (∼57kBT) even for the thickest Ta (8 Å) of this work.

Nonequilibrium thermodynamic study of magnetization dynamics in the presence of spin-transfer torque

The dynamics of magnetization in the presence of spin-transfer torque was studied. We derived the equation for the motion of magnetization in the presence of a spin current by using the local equilibrium assumption in nonequilibrium thermodynamics. We show that, in the resultant equation, the ratio of the Gilbert damping constant, $\ensuremath{\alpha}$, and the coefficient, $\ensuremath{\beta}$, of the current-induced torque called nonadiabatic torque, depends on the relaxation time of the fluctuating field ${\ensuremath{\tau}}_{c}$. The equality $\ensuremath{\alpha}=\ensuremath{\beta}$ holds when ${\ensuremath{\tau}}_{c}$ is very short compared to the time scale of magnetization dynamics. We apply our theory to current-induced magnetization reversal in magnetic multilayers and show that the switching time is a decreasing function of ${\ensuremath{\tau}}_{c}$.