Switching of perpendicular magnetization by spin-orbit torques in the absence of external magnetic fields (1311.0929v1)

Abstract: Magnetization switching by current-induced spin-orbit torques (SOTs) is of great interest due to its potential applications for ultralow-power memory and logic devices. In order to be of technological interest, SOT effects need to switch ferromagnets with a perpendicular (out-of-plane) magnetization. Currently, however, this typically requires the presence of an in-plane external magnetic field, which is a major obstacle for practical applications. Here we report for the first time on SOT-induced switching of out-of-plane magnetized Ta/Co20Fe60B20/TaOx structures without the need for any external magnetic fields, driven by in-plane currents. This is achieved by introducing a lateral structural asymmetry into our devices during fabrication. The results show that a new field-like SOT is induced by in-plane currents in such asymmetric structures. The direction of the current-induced effective field corresponding to this new field-like SOT is out-of-plane, which facilitates switching of perpendicular magnets. This work thus provides a pathway towards bias-field-free SOT devices.

• The paper introduces a novel method that achieves bias-field-free switching of perpendicular magnetization by engineering lateral asymmetries in Ta/Co20B60 structures.
• It employs a symmetry-based analysis and experimental techniques like extraordinary Hall effect and second-harmonic measurements to validate the induced field-like spin-orbit torque.
• The findings pave the way for scalable, energy-efficient spintronic memory devices by eliminating the reliance on external magnetic fields.

Spin-Orbit Torque-Induced Switching of Perpendicular Magnetization Without External Magnetic Fields

The paper presents a significant advancement in the domain of spintronic devices, specifically addressing the challenge of switching perpendicular magnetization via spin-orbit torques (SOTs) without the application of external magnetic fields. This is achieved by exploiting structural asymmetries at the nanoscale level, which lead to the introduction of a perpendicular effective field induced by lateral symmetry-breaking. The work provides a pathway towards the development of more power-efficient spintronic memory devices, such as spin-orbit torque magnetic random access memory (SOT-MRAM), potentially enhancing their scalability and thermal stability.

A key finding is the demonstration of bias-field-free SOT switching in Ta/Co$_{20}$B$_{60}$20 structures, facilitated by engineering lateral structural asymmetries during device fabrication. These asymmetries enable in-plane currents to induce a novel field-like SOT that provides an out-of-plane effective field. The device structure effectively eliminates the need for external magnetic fields for deterministic switching, as evidenced by experiments showing repeatable current-induced magnetization switching in the absence of external fields.

Results and Symmetry-Based Analysis

The paper conducts a thorough symmetry-based analysis to elucidate the origins of the novel field-like torque facilitating current-induced switching. The authors systematically break structural symmetry, thus allowing a particular direction of current to uniquely determine the magnetization's z-component. Their approach hinges on breaking mirror symmetry both along the growth direction (z axis) and laterally (y axis), which allows for the induction of effective field-like SOTs that are pivotal for bias-field-free switching of perpendicular magnetization.

Experimental results highlight the influence of the engineered asymmetry on device performance. Using sputter-deposited layers and meticulous oxygen concentration control, the authors demonstrated varied coercivity and effective anisotropy fields across the sample. Importantly, the measurements indicated that the current-induced effective field correlates with the gradient of perpendicular magnetic anisotropy, verified through both extraordinary Hall effect (EHE) measurements and corroborative second-harmonic measurements.

Implications and Future Directions

This research constitutes a substantial stride towards efficient, scalable spintronic devices, aligning with the growing demand for low-power, high-density memory solutions. By overcoming the limitations associated with the need for external magnetic fields, this paper opens avenues for implementing SOT effects in commercial applications more feasibly.

The proposed design paradigm could stimulate further investigations into alternative structural symmetries and material systems to achieve similar functionality. Future research might focus on diverse techniques for inducing lateral asymmetry, such as localized strain or electronic potential application, to facilitate integration on a larger scale. Moreover, understanding the detailed microscopic mechanisms, perhaps via first-principles calculations, could refine the description of the SOT phenomena and enhance the control over device performance parameters significantly.

Conclusion

This paper offers a well-rounded framework for understanding and applying spin-orbit torques in spintronic devices without relying on external magnetic fields. This advancement potentially streamlines the architecture of future SOT-based memory technologies, thereby laying the groundwork for innovations in ultralow-power and high-performance computing hardware. With continued research and development, the methodologies and insights discussed herein could foster significant technological advancement in spintronics and related fields.