Current-induced magnetization switching in MgO barrier magnetic tunnel junctions with CoFeB based synthetic ferrimagnetic free layers

We investigated the effect of using a synthetic ferrimagnetic (SyF) free layer in MgO-based magnetic tunnel junctions (MTJs) on current-induced magnetization switching (CIMS), particularly for application to spin-transfer torque random access memory (SPRAM). The employed SyF free layer had a Co40Fe40B20/ Ru/ Co40Fe40B20 and Co20Fe60B20/Ru/Co20Fe60B20 structures, and the MTJs(100x(150-300) nm^2) were annealed at 300oC. The use of SyF free layer resulted in low intrinsic critical current density (Jc0) without degrading the thermal-stability factor (E/kBT, where E, kB, and T are the energy potential, the Boltzmann constant, and temperature,respectively). When the two CoFeB layers of a strongly antiferromagnetically coupled SyF free layer had the same thickness, Jc0 was reduced to 2-4x10^6 A/cm^2. This low Jc0 may be due to the decreased effective volume under the large spin accumulation at the CoFeB/Ru. The E/kBT was over 60, resulting in a retention time of over ten years and suppression of the write current dispersion for SPRAM. The use of the SyF free layer also resulted in a bistable (parallel/antiparallel) magnetization configuration at zero field, enabling the realization of CIMS without the need to apply external fields to compensate for the offset field.Comment: 6 page

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

The role of current induced effective magnetic field in ultrathin magnetic heterostructures is increasingly gaining interest since it can provide efficient ways of manipulating magnetization electrically. 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, quantitative understanding of the effective field, including its direction with respect to the current flow, is lacking. Here we show vector measurements of the current induced effective field in Ta|CoFeB|MgO heterostructrures. The effective field shows significant dependence on the Ta and CoFeB layers' thickness. In particular, 1 nm thickness variation of the Ta layer can result in nearly two orders of magnitude difference in the effective field. Moreover, its sign changes when the Ta layer thickness is reduced, indicating that there are two competing effects that contribute to the effective field. The relative size of the effective field vector components, directed transverse and parallel to the current flow, varies as the Ta thickness is changed. Our results illustrate the profound characteristics of just a few atomic layer thick metals and their influence on magnetization dynamics

Spin-Orbit Torque in Structures With Magnetization-Compensated MnGa/Co2MnSi Bilayer

We have systematically investigated the spin-orbit torque (SOT)-induced effective magnetic field in a structure consisting of a Ta heavy metal layer and an antiferromagnetically coupled Mn1.8Ga1/Co2MnSi (CMS) bilayer around the magnetization compensation point by varying CMS film thickness. The efficiency of SOT generation takes the maximum around the compensation point, and it is approximately six times as large compared with that in the devices with a MnGa single structure. The enhancement of SOT efficiency can be explained mainly by the reduction in saturation magnetic moment around the compensation point. Moreover, a significant enhancement of the effective spin Hall angle was observed around the compensation point because of the inversion of the magnetization configuration before and after the compensation point

Current-induced effective magnetic field in La0.67Sr0.33MnO3/LaAlO3/SrTiO3 structures

We investigate the current-induced effective magnetic field H-eff in La0.67Sr0.33MnO3 (18)/LaAlO3(0,2,6)/SrTiO3 (LSMO/LAO/STO) structures by using the planar Hall effect (PHE), where numbers in parentheses give the nominal thickness of the given layer in unit cells. In all the structures, applying an in-plane current creates an in-plane H-eff orthogonal to the current direction, and the direction of H-eff for the LSMO/LAO(6)/STO structure is opposite to that for the LSMO/LAO(0,2)/STO structures. At low temperature, the sign of the PHE coefficient originating from the spin-orbit interaction (SOI) for the LSMO/LAO(6)/STO structure is also opposite to that for the LSMO/LAO(0,2)/STO structures, which suggests that the SOI in the LSMO layer is modified by the LAO(6)/STO structure. The direction of H-eff is consistent with that induced by the SOI at the interface of the LSMO layer, and thus the opposite-polarity H-eff detected in the LSMO/LAO(6)/STO structure can be related to the SOI variation induced by the LAO(6)/STO structure

Peculiar magnetotransport properties in La0.67Sr0.33MnO3/LaAlO3/SrTiO3

We have investigated the planar Hall effect (PHE) and the anisotropic magnetoresistance (AMR) in La0.67Sr0.33MnO3/LaAlO3/SrTiO3 (LSMO/LAO/STO) and LSMO/STO structures, where the LSMO (LAO) thickness was 13 unit cells (u.c.) and 18 u.c. (8 u.c. and 6 u.c.). The LAO/STO structures under the LSMO layers are conductive, which is consistent with the formation of a two-dimensional electron system (2DES) at the LAO/STO interface. The magnetotransport measurements show that the sign of the PHE coefficient for the LSMO/LAO/STO structures is opposite to that for the LSMO/STO structures, whereas the AMR coefficient, which is generally correlated with the PHE coefficient, is negative for both structures at temperatures below ∼175 K. The LAO/STO structures show no apparent PHE or AMR, and the coefficients obtained thus originate from the LSMO. These results suggest that the 2DES and/or the LSMO/LAO interface affect the PHE in the LSMO layer