Effect of spin current on magnetization dynamics Open Access
Zholud, Andrei (Fall 2017)
Abstract
The main subject of this work is the magnetization dynamics excited in magnetic nanostructures by spin polarized electrical or pure spin current. This research is important for the development of spintronic devices - devices that in addition to the electron charge exploit the spin degree of freedom for information storage, transmission, processing, and/or for sensing. The research presented in this thesis addresses three relevant problems: 1) Enhancing the efficiency of spin current-driven spintronic devices by optimizing their geometry; 2) Enhancing the dynamical characteristics of spin current-driven devices via their interaction with external signals; 3) The role of quantum magnetization fluctuations in the interaction between the magnetization and the spin-polarized current.
An emerging promising type of spintronic devices for microwave applications is the Spin Hall nanooscillator (SHNO). In SHNO, microwave-frequency magnetization dynamics is excited in an “active” nanomagnet by pure spin current generated by the spin Hall effect. In this work, it is shown that spectral, thermal and electrical properties of SHNO can be enhanced by optimizing the geometry of the nanodevice. In particular, increased current concentration in a small region, achieved by nanopatterning the spin-Hall material (the source of spin current), reduces the current required for the device operation. Moreover, the reduced area of interface between the spin Hall material and the active nanomagnet improves the spectral properties of the device. In addition to modifying geometry of spin Hall layer to modify properties of spin Hall nanooscillator. In addition, a new type of SHNO is experimentally demonstrated in this work. It is based on a bilayer of spin Hall material and active layer nanopatterned into a bow-tie nanoconstriction. Theoretical analysis and micromagnetic simulations performed in this work demonstrate the importance of nonlinear dynamical mechanisms, dipolar magnetic fields, and the Oersted field of the current for the spatial and spectral characteristics of the studied structures.
The presented work also addresses the dynamical stability and coherence of SHNO, by studying their interaction with external microwave signals. It is shown that strong dynamical nonlinearity of SHNO is responsible both for their limited coherence and their ability to efficiently synchronize with external microwave signals. The synchronization is shown to dramatically improve the spectral characteristics of SHNO, and is possible in a wide range of temperature and frequencies. The demonstrated synchronization of SHNO opens a way for the development of arrays of mutually synchronized oscillators with improved microwave generation characteristics.
The last part of this work addresses the fundamental mechanisms of interaction between the magnetization and the spin polarized currents. The present understanding of the underlying mechanism, called the spin transfer effect, is based on the classical approximation for the magnetization. In this work, the theory of spin transfer is extended to include the quantum-mechanical description of magnetization. The central result of the presented work is the prediction of spin transfer due to quantum magnetization fluctuation, and its experimental demonstration in a nanomagnetic system. Both the analysis and the presented measurements demonstrate that quantum fluctuations provide the dominant contribution to spin transfer at cryogenic temperatures, and their role remains significant even at room temperatures. Multiple consequences for the magnetoelectronic phenomena in ferromagnetic and antiferromagnetic systems are predicted based on these results.
Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . 14
1.0.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.1 Magnetization dynamics and spin waves . . . . . . . . . . . . . . . . . . . . . . . . 16
1.2 Electron spin and spin current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.2.1 Magnetic order and magnetization dynamics . . . . . . . . . . . . . . . . . . . .17
1.2.2 Magnetization dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.2.3 Spin Hall effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.3 Spin transfer torque . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.4 Anisotropic magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
1.5 Giant magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
1.6 Experimental techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
1.6.1 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2 Electronic and spectral properties of SHNO . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.1 Geometry and operation of SHNO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.1.1 SHNO with patterned spin injector . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.1.2 Nanoconstriction based SHNO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
3 Synchronization of SHNO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.1 Synchronization of SHNO to external signals 1 . . . . . . . . . . . . . . . . . . . 49
3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
3.3 Effect of the external microwave signals . . . . . . . . . . . . . . . . . . . . . . . 53
3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4 Prediction of quantum spin transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.1 Limitation of semi-classical spin transfer model . . . . . . . . . . . . . . . . . . .60
4.2 Quantum model of spin transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.3 Time evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.3.1 Relation between magnon population and magnetoresistance of GMR spinvalve nanopillar . . .75
4.4 Numerical calculation and results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5 Experimental demonstration of quantum spin transfer . . . . . . . . . . . . . . . . . . 79
5.1 Dependence of resistance of spin valve nanopillar on current at low temperatures . 79
5.2 GMR nanopillar fabrication details . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
5.3 Magnetoeletronic measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81
6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
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