Unidirectional spin Hall magnetoresistance in ferromagnet/normal metal bilayers (1502.06898v2)

Abstract: Magnetoresistive effects are usually invariant upon inversion of the magnetization direction. In noncentrosymmetric conductors, however, nonlinear resistive terms can give rise to a current dependence that is quadratic in the applied voltage and linear in the magnetization. Here we demonstrate that such conditions are realized in simple bilayer metal films where the spin-orbit interaction and spin-dependent scattering couple the current-induced spin accumulation to the electrical conductivity. We show that the longitudinal resistance of Ta|Co and Pt|Co bilayers changes when reversing the polarity of the current or the sign of the magnetization. This unidirectional magnetoresistance scales linearly with current density and has opposite sign in Ta and Pt, which we associate with the modification of the interface scattering potential induced by the spin Hall effect in these materials. Our results suggest a route to control the resistance and detect magnetization switching in spintronic devices using a two-terminal geometry, which applies also to heterostructures including topological insulators.

• The paper demonstrates that FM/NM bilayers exhibit nonlinear unidirectional magnetoresistance via spin accumulation induced by the spin Hall effect.
• It shows that the resistance scales linearly with current density and varies oppositely in Ta|Co and Pt|Co systems due to differing spin Hall angles.
• The findings suggest promising applications for spintronic devices, offering enhanced directional sensing and control through interfacial engineering.

An Overview of Unidirectional Spin Hall Magnetoresistance in Ferromagnet/Normal Metal Bilayers

The paper "Unidirectional spin Hall magnetoresistance in ferromagnet/normal metal bilayers" explores the magnetoresistive behavior of bilayer films composed of ferromagnetic (FM) and normal metal (NM) materials. This paper primarily focuses on the unique magnetoresistive characteristics introduced by the spin Hall effect (SHE) in such bilayer systems, particularly exploring the nonlinear resistive terms that arise when the directions of current and magnetization are manipulated.

Key Findings

1. Unidirectional Magnetoresistance: This work demonstrates the existence of a unidirectional magnetoresistance in FM|NM bilayers, notably in Ta|Co and Pt|Co films. Unlike conventional anisotropic magnetoresistance (AMR) or giant magnetoresistance (GMR), this effect introduces a nonlinear dependence on the current direction and magnetization, whereby reversing either significantly alters the resistance.
2. Interfacial Spin Accumulation: The observed effect is attributed to the coupling between current-induced spin accumulation and spin-orbit interaction at the FM|NM interface. This accumulation modulates the interface scattering potential, with the spin Hall effect in Pt and Ta contributing oppositely to the resistive behavior.
3. Dependence on Spin Hall Angle: The paper finds a linear scaling of resistance with current density, with opposite trends in Ta and Pt, directly linked to their differing spin Hall angles. This underscores the role of SHE in generating significant interfacial spin accumulations sufficient to switch magnetization orientations in thin FM layers.

Implications and Future Prospects

• Spintronic Device Applications: A significant implication of this research is the potential application of the observed effect in spintronic devices. The ability to detect magnetization changes using unidirectional spin Hall magnetoresistance offers a promising tool for non-volatile memory devices and spintronic sensors.
• New Directions for Sensor Technologies: The unidirectional nature of the effect could lead to more sensitive AMR sensors, offering enhanced 360° directional capability crucial for various sensing technologies.
• Material and Heterostructure Engineering: The paper suggests potential enhancements in USMR magnitude through material optimization. Future exploration into topological insulators and other heavy metals with large spin Hall angles could yield even greater spin accumulation effects.

Experimental and Numerical Consistency

The rigorous experimental design and data analysis employed in this paper provide a comprehensive understanding of both linear and nonlinear magnetoresistive phenomena in FM|NM systems. The authors support their findings with harmonic measurements of resistance and detailed angular analysis of magnetic field alignments. Future theoretical work involving more robust Boltzmann equation approaches could refine the quantitative understanding of these effects.

Conclusion

Overall, the paper contributes significantly to the understanding of magnetoresistive effects in spintronic materials, showcasing a previously unobserved facet of SHE-driven resistivity changes. It opens new pathways for utilizing interfacial phenomena in practical applications, presenting a valuable addition to the field's understanding of spin-dependent transport in ferromagnetic systems. The potential to extend these findings across various material systems through engineering and experimental modulation is an exciting prospect for future research endeavors in spintronics and related areas.