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Journal of Magnetism and Magnetic Materials, Volume 330, March 2013, Pages 16-20.
H.B. Huang, X.Q. Ma, Z.H. Liu, F.Y. Meng, S.Q. Shi, L.Q. Chen
Department of Physics, University of Science and Technology Beijing, Beijing 100083, China
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
Abstract
We investigated the critical current density of spin transfer torque switching in a full-Heusler Co2FeAl0.5Si0.5alloy spin-valve nanopillar through micromagnetic simulations. The simulations explain the experimental results on the resistance versus external magnetic field and yield good agreement with the measured switching behavior. It is shown that different magnitudes of current densities and directions of external magnetic fields give rise to a shift of resistance hysteretic loop and a variable range of switching. We demonstrated that three critical current densities have different slopes with Gilbert damping constant {Alpha} and spin polarization constant η, indicating that {Alpha} and η have different contributions to the critical current densities. Furthermore, we found that the area of resistance–current hysteretic loop decreases as the nanopillar size decreases. The domain structures indicated that the magnetization reversals have different switching processes between small and large sizes of pillars.
Additional Information
Spin transfer torque (STT) switching has attracted considerable attention due to its application in high density magnetic random access memory. However, the critical current density Jc required to induce the STT-based magnetization dynamics in the spin-valves is as high as 106-108A/cm2, and it is challenging to reduce Jc to achieve the compatibility with highly scaled complementary metal-oxide-semiconductor technology while maintaining the thermal stability.
Hesuler alloys with lower saturation magnetization Ms, smaller Gilbert damping a and higher spin polarization constant η are demonstrated to be the excellent candidates for reducing Jc compared to the normal metal and metallic alloy. We investigated the critical current density of spin transfer torque switching in a full-Heusler Co2FeAl0.5Si0.5 alloy spin-valve nanopillar through numerical simulations. The simulation explains the experimental result on the resistance versus external magnetic field and yields good agreement with the measured switching behavior. It is shown that different magnitudes of current densities and directions of external magnetic fields give rise to a shift of resistance hysteretic loop and a variable range of switching. We demonstrated that the critical current densities decrease with different slopes as the decrease of Gilbert damping constant α and reciprocal of spin polarization constant 1/η, in agreement with the macrospin approximation model. Finally, the critical densities decease with the device area except for sizes below a critical dimension at which the magnetization structure is a single domain.
During spin transfer two-step switching of current-resistance loop, we found two possible directions, -90° state and 90° state, for the intermediate state, which could not be distinguished due to the same resistance in the experiment. As a matter of fact, we found the magnetization flips randomly to -90° state or 90° state with equal probability within the current range of -4.0×106A/cm2<J<-8.0×106A/cm2 because switching to two states should overcome the same energy barrier of magnetic anisotropy. The degeneracy of the two states can be lifted by adding a small external magnetic field perturbation along +y or –y axis. Recently, we reported a spin-transfer multi-step magnetization switching through the coordinate transformation magnetic anisotropy and obtained the unsymmetrical hysteresis loops. The results may be utilized in designing four state magnetic memories driven by spin transfer torques.
