Anomalous temperature dependence of current-induced torques inCoFeB/MgOheterostructures with Ta-based underlayers

Abstract

We have studied the underlayer thickness and temperature dependences of the current induced effective field in CoFeB|MgO heterostructures with Ta based underlayers. The underlayer thickness at which the effective field saturates is found to be different between the two orthogonal components of the effective field, i.e. the damping-like term tends to saturate at smaller underlayer thickness than the field-like term. For large underlayer thickness films in which the effective field saturates, we find that the temperature significantly influences the size of the effective field. A striking difference is found in the temperature dependence of the two components: the damping-like term decreases whereas the field-like term increases with increasing temperature. Using a simple spin diffusion-spin transfer model, we find that all of these results can be accounted for provided the real and imaginary parts of an effective spin mixing conductance are negative. These results imply that either spin transport in this system is different from conventional metallic interfaces or effects other spin diffusion into the magnetic layer need to be taken account in order to model the system accurately.*Email: hayashi.masamitsu@nims.go.jp 2 Materials and/or interfaces with large spin orbit coupling are attracting considerable interest since they can generate significant amount of spin current or accumulate spins to manipulate magnetic moments 1, 2 . The spin Hall effect 3-5 and current induced spin polarization (i.e. the Rashba-Edelstein effect) are considered to be the major sources that enable spin current generation and spin accumulation, respectively, in ultrathin magnetic heterostructures. The non-equilibrium spins can act on the magnetic moments via spin transfer torque 6, 7 or exchange torque 8-10 to trigger magnetization switching. These torques are termed the "spin orbit torques" which are to be distinguished from the conventional spin transfer torque since the system requires strong spin orbit coupling to generate the spins.The action of spins on the magnetic moments can be evaluated by measuring the "effective magnetic field" which reflects the size and direction of the spin orbit torque. Recently, it has been shown 11-14 that the spin orbit torque also possesses a damping-like torque 6, 7 and a fieldlike torque 15 , analogous to the spin transfer torque in magnetic tunnel junctions [16][17][18][19][20] . In many systems, however, the field-like torque is much larger than the damping-like torque 11,14 , and its direction is pointing opposite to the incoming spin direction of the electrons (assuming the spin source is the spin Hall effect) 11,14, 21 . Moreover, the size and direction of both components of the torque vary depending on the materials and the film structures [11][12][13][14] . Up to date, many of these results cannot be accounted for with existing theories that consider spin transfer and/or exchange torques 22, 23 . It is thus essential to understand the origin of spin orbit torques in order to apply them for p...

Electrical manipulation of ferromagnetic NiFe by antiferromagnetic IrMn

“…IrMn is an alloy, and thus its resistivity decreases less with temperature compared to Ru, resulting in a smaller proportion of current flowing through IrMn, and thus smaller h z at lower temperatures. The ratio can also change monotonously with temperature if there are additional temperature dependent contributions to h y [39]. Nevertheless, as one can see the monotonic trend is broken below 50 K, coinciding with the abrupt increase in the exchange bias and decrease in the coercivity [ Fig.…”

Temperature dependence of spin-orbit torques in W/CoFeB bilayers

“…12,13,20 A slightly higher value is expected for thicker W, as t W ¼ 2 nm used here is smaller than the spin diffusion length of W. 20 The temperature dependence of the longitudinal spin-orbit-torque is similar to Ta. 18 On the other hand, the magnitude of the transverse effective field decreases with increasing temperature, which is qualitatively different compared to Ta buffers. In order to explain this discrepancy and to shed some light on the large spin Hall angles that are reported in W/CoFeB bilayers, we studied the interface between these two layers using STEM and XRR analysis.…”

“…17 In this letter, we report on effective magnetic fields arising from spin-orbit interactions in W/CoFeB/MgO system. Transverse and longitudinal torques are measured in a temperature range from 19 to 300 K. Contrary to Ta buffers, 18 we find that both transverse and longitudinal torque magnitudes increase with decreasing temperature, with the longitudinal torque reaching an efficiency of 0.55 at 19 K. Considerable mixing between W and CoFeB is measured by scanning transmission electron microscopy (STEM) and X-ray reflectivity (XRR) analysis, which may be responsible for strong spinorbit interactions in W/CoFeB bilayers.…”

Spin–Orbit Effects in CoFeB/MgO Heterostructures with Heavy Metal Underlayers

“…Identifying the origin of the¯eld-like torque as well as proper understanding of the spin mixing conductance remain one of the important issues that need to be addressed. [99][100][101][102] Finally we note that there remains an uncertainty in obtaining the current induced e®ective¯eld using the harmonic Hall voltage measurements, which has been discussed in Ref. 29.…”