Nanowire Spin Torque Oscillator Driven by Spin Orbit Torques (1404.7262v2)

Abstract: Spin torque from spin current applied to a nanoscale region of a ferromagnet can act as negative magnetic damping and thereby excite self-oscillations of its magnetization. In contrast, spin torque uniformly applied to the magnetization of an extended ferromagnetic film does not generate self-oscillatory magnetic dynamics but leads to reduction of the saturation magnetization. Here we report studies of the effect of spin torque on a system of intermediate dimensionality - a ferromagnetic nanowire. We observe coherent self-oscillations of magnetization in a ferromagnetic nanowire serving as the active region of a spin torque oscillator driven by spin orbit torques. Our work demonstrates that magnetization self-oscillations can be excited in a one-dimensional magnetic system and that dimensions of the active region of spin torque oscillators can be extended beyond the nanometer length scale.

Nanowire Spin Torque Oscillator Driven by Spin Orbit Torques

This paper explores the magnetization dynamics in ferromagnetic nanowires subjected to spin orbit torques (SOTs), specifically examining the activation of self-oscillations within these structures. As the dimensionality of magnetic systems plays a crucial role in spin torque effects, the paper provides insights into one-dimensional (1D) ferromagnetic systems through empirically sustained self-oscillations in Pt/Py (Permalloy) bilayer nanowires.

Summary of Findings

The research delineates that spin currents, typically understood to influence nanoscale ferromagnetic contacts, also effectively induce magnetization self-oscillations in 1D systems, exceeding the constraints of nanoscale interactions. Experimental results affirm that SOT-driven self-oscillations occur over a 1.8 μm extended active region in nanowires, which considerably enlarges the operational scale beyond nanoscale constraints of previous models and devices. The paper elucidates that these oscillations comprise two distinct types of modes: bulk and edge modes, each with inherent spatial and frequency characteristics.

Experimental Framework

Utilizing nanowires of Pt(5 nm)/Py(5 nm), the experiments involved electrical measurements of microwave signals emitted due to magnetization oscillations influenced by SOT. A critical current threshold was established beyond which coherent microwave emissions were observed, attributed to magneto-resistance oscillations. Investigation of electronic and thermal contributions to mode formation was supplemented by micromagnetic simulations to map and verify the spin wave eigenmodes excited in the experimental setup.

Numerical and Observational Results

Key observations demonstrate that nanowires exhibit oscillation amplitudes reaching up to 15% of the anisotropic magneto-resistance (AMR) amplitude of the device. The work further classifies the significant disparity in linewidth between bulk and edge mode oscillations, highlighting temperature-dependent transitions in linewidth attributes.

Implications and Future Directions

The implications of this research are substantial for both practical applications and theoretical advancements. The demonstrated ability of STOs to engage over wider dimensions has evident implications for the improvement and scalability of spintronic devices. Moreover, by alleviating the constraints of nonlinear magnon scattering via geometric confinement, the paper introduces potential for enhanced functionality in magnetic sensors and devices leveraging spin-torque phenomena. Future developments might focus on exploring complementary materials or configurations to further expand the dimensional and functional capabilities of STO systems.

Conclusion

In summary, the paper effectively expands on the understanding and functional deployment of spin torque-induced phenomena in 1D magnetic systems. By validating the role of dimensional confinement in nanostructured systems, it reveals opportunities for novel, larger-scale applications of magnetization oscillations driven by spin torques. This paper stands as a foundational work that may inspire further investigations into the dynamics of nanowire-based spintronic devices, potentially fostering advancements in high-frequency device technology and the exploration of unconventional magnetic states.