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Current-induced magnetization dynamics in magnetic nanostructures

Authors Yan, Peng
Issue Date 2011
Summary This thesis deals with the study of current-induced magnetization dynamics for both macrospins and magnetic domain walls, driven by the so-called spin transfer torque effect. It consists of three distinct components: (1) a study of spin transfer torque enhancement in dual spin valves in the ballistic regime, (2) a proposal of domain wall propagation due to the synchronization with circularly polarized microwaves, which can be mapped to uniform spin current driven case, and (3) a derivation of an optimal temporally and spatially varying spin current pattern for fast domain wall propagation along nanowires. The spin transfer torque in all-metal dual spin valves, in which two antiparallelly aligned pinned ferromagnetic layers are on the two sides of a free ferromagnetic layer with two thin normal metal spacers in between, is studied in the ballistic regime. It is argued that, similar to the results in the diffusive regime, the spin transfer torque is dramatically enhanced in comparison to that in a conventional spin valve in the ballistic regime. Within the Slonczewski approach, an analytical expression of the torque on the free magnetic layer is obtained, which may serve as a theoretical model for the micromagnetic simulation of the spin dynamics in dual spin valve. Depending on the orientation of free layer and the degree of electron polarization, the spin transfer torque enhancement could be tens of times. The general cases when transmission and reflection probabilities of free layer are different from zero or one are also numerically calculated. Finding a new control parameter for magnetic domain wall motion is important in general and in particular for the spintronics applications. Here, we show that a circularly polarized magnetic field (CPMF) at gigahertz frequency (microwave) can efficiently drive a domain wall to propagate along a magnetic nanowire. Two motion modes are identified: rigid-domain wall propagation at low frequency and oscillatory propagation at high frequency. Moreover, domain wall motion under a CPMF is equivalent to the domain wall motion under a uniform spin current in the current perpendicular to the plane magnetic configuration proposed recently by Khvalkovskiy et al. [Phys. Rev. Lett. 102, 067206 (2009)], and the CPMF frequency plays the role of the current. How to lower the current needed for a technologically useful domain wall propagation speed is also an important issue in nanomagnetism. Based on the modified Landau-Lifshitz-Gilbert equation with both Slonczewski spin-transfer torque and the field-like torque, we derive an optimal temporally and spatially varying spin current pattern for fast domain wall propagation along nanowires. Under such conditions, the domain wall velocity in biaxial wires can be enhanced as much as tens of times higher than that achieved in experiments so far. Moreover, the fast variation of spin polarization can efficiently help domain wall depinning. Possible experimental realizations are discussed.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2011
Language English
Format Thesis
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