Nonlinear Damping Generated by Spin Injection in a Pseudo-spin-valve Structure
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0998-J02-04
Nonlinear Damping Generated by Spin Injection in a Pseudo-spin-valve Structure Min Ren, Lei Zhang, Jiuning Hu, Ning Deng, Hao Dong, and Peiyi Chen Institute of Microelectronics, Tsinghua University, Haidian Dist., Beijing, 100084, China, People's Republic of ABSTRACT The current induced magnetic dynamics of a nano-scale pseudo-spin-valve (PSV) structure was theoretically studied. The spin relaxation mechanisms and the influence of ferromagnetic/nonmagnetic (FM/NM) interfaces on the current polarization were investigated, and a modified magnetic dynamic equation was developed. Both the free layer's local magnetic moments and itinerant electrons' spins were regarded as a macro-spin, whose movement was resulted from two items: spin relaxation due to the magnetic damping and spin accumulation due to the polarized current. The injected current not only produces a spin transfer torque, but also alters the effective magnetic field, and thus affects the damping. Therefore, the damping is nonlinear and correlated to the current. Based on the analysis of the competition between magnetic damping and spin accumulation, the dynamic behaviors of magnetization switching and oscillation can be explained. INTRODUCTION The magnetization configuration of a PSV is switched conventionally by an external magnetic field. An alternative mechanism to switch the magnetization of a nano-scale PSV by a spin-polarized current was proposed by Slonczewski [1] and Berger [2] in 1996, which provided a novel proposal of magnetic random access memories (MRAM) operated only by current. A large interest in this effect has emerged since then [3-11], due to its potential for practical applications. Slonczewski’s early theory [1] believed that current induced magnetization switching (CIMS) effect originated from the spin dependent scattering and reflection at the FM/NM interface and the absorption of the spin current’s transverse component by the FM layer. Later, Heide et al. [3] realized the contribution of the spin relaxtation in FM layers. Recent experiments [7, 8] have demonstrated that both of the two factors contribute to CIMS effect, which calls for a unified theory taking both of them into account. Actually, recent theoretical studies are making efforts toward this direction.[9-11] Guo et al. [9] proposed a spin transport model which combines ballistic spin scattering across the interfaces and diffusive spin relaxation within the FM layer. We developed a macroscopic phenomenological model, taking into account the two factors, similarly. The magnetic movements of local electrons and itinerant electrons in the free layer were investigated, and then an equivalent macro-spin method was proposed. The effects of spin dependent scattering at the interface and the spin relaxtation in the bulk are included in the spin accumulation term of the macro-spin’s magnetic dynamic equation.
THEORY The system is a nano-scale PSV composed of two ferromagnetic layers separated by a nonmagnetic layer. The fixed layer and free layer are magnetized in an i
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