Symmetry and magnitude of spin-orbit torques in ferromagnetic heterostructures (1301.3573v2)

Abstract: Current-induced spin torques are of great interest to manipulate the orientation of nanomagnets without applying external magnetic fields. They find direct application in non-volatile data storage and logic devices, and provide insight into fundamental processes related to the interdependence between charge and spin transport. Recent demonstrations of magnetization switching induced by in-plane current injection in ferromagnetic heterostructures have drawn attention to a class of spin torques based on orbital-to-spin momentum transfer, which is alternative to pure spin transfer torque (STT) between noncollinear magnetic layers and amenable to more diversified device functions. Due to the limited number of studies, however, there is still no consensus on the symmetry, magnitude, and origin of spin-orbit torques (SOTs). Here we report on the quantitative vector measurement of SOTs in Pt/Co/AlO trilayers using harmonic analysis of the anomalous and planar Hall effects as a function of the applied current and magnetization direction. We provide an all-purpose scheme to measure the amplitude and direction of SOTs for any arbitrary orientation of the magnetization, including corrections due to the interplay of Hall and thermoelectric effects. Based on general space and time inversion symmetry arguments, we show that asymmetric heterostructures allow for two different SOTs having odd and even behavior with respect to magnetization reversal. Our results reveal a scenario that goes beyond established models of the Rashba and spin Hall contributions to SOTs. The even SOT is STT-like but stronger than expected from the spin Hall effect in Pt. The odd SOT is composed of a constant field-like term and an additional component, which is strongly anisotropic and does not correspond to a simple Rashba field.

• The paper introduces a novel three-dimensional measurement technique using harmonic analysis of Hall effects to quantify spin-orbit torques.
• It reveals that asymmetric heterostructures yield distinct field-like and spin-transfer torque components influenced by annealing and heavy metal composition.
• The study underscores that interfacial phenomena beyond traditional spin Hall and Rashba effects are critical for tuning spintronic device performance.

Analyzing Spin-Orbit Torques in Ferromagnetic Heterostructures

The paper under discussion explores a critical analysis of spin-orbit torques (SOTs) observed in ferromagnetic heterostructures, focusing on their symmetry, magnitude, and underlying mechanisms. It addresses the ongoing debate surrounding the origin of SOTs, employing an experimental strategy that emphasizes harmonic analysis of the anomalous and planar Hall effects. The paper employs a comprehensive method to measure the amplitude and direction of SOTs using trilayers such as AlO_x/Co/Pt and MgO/CoFeB/Ta for various magnetization orientations.

Core Findings

1. Three-Dimensional Vector Measurement: The researchers developed a method to measure SOTs as three-dimensional vectors. This is achieved by using harmonic analysis, which accounts for both anomalous Hall effect (AHE) and planar Hall effect (PHE) contributions, while correcting for Hall and thermoelectric impacts.
2. Symmetry and Diverse Mechanisms: The paper reveals that asymmetric heterostructures generate two distinct SOTs with contrasting symmetries relative to magnetization inversion. This characteristic manifests as strongly anisotropic field-like and spin-transfer like components, significantly affected by factors like annealing conditions and the stratified composition of heavy metal layers.
3. Alternative Models Beyond Traditional Effects: The findings emphasize the need for SOT models that transcend conventional spin Hall and Rashba effects, highlighting significant contributions from interfacial phenomena.
4. Dependence on Material Parameters: A critical insight from the research is the sensitivity of both T_⊥ and T_∥ to interface characteristics and the type of heavy metal employed, mediating effects like diffusion and magnetic anisotropy, especially evident in annealed samples.

Experimental Methodology

The paper conducts its experiments by patterning Hall crosses and varying the orientation of applied magnetic fields. Using an ac current model, the authors derive effective fields from the second harmonic contributions of Hall measurements. The employed techniques are validated through macrospin simulations that confirm the exactitude of the vector measurement approach.

Implications and Future Directions

The implications of these findings are significant for both theoretical advancement and practical applications. The ability to predict or engineer SOT characteristics based on material and process parameters can significantly influence the design and optimization of spintronic devices — devices that increasingly require precise control over magnetic states for functions such as data storage and processing.

Theoretically, the insights highlight the necessity for expanding beyond traditional SOT models, urging future work to include more complex interfacial and anisotropic effects when analyzing magnetic heterostructures. Moreover, the characteristic sensitivity of such torques to fabrication and material variations underscores an avenue for fine-tuning spintronic device properties through controlled layer deposition and thermal treatments.

In essence, this paper provides a robust framework for exploring the comprehensive influence of SOTs in ferromagnetic heterostructures, with profound implications for the future development of spintronic technologies. Although technical challenges remain, particularly in modeling complex interfacial interactions, the paper offers a foundational step towards resolving the discrepancies and harnessing the full potential of spin-orbit interactions in nanoscale devices.