James M. Taylor
The field of antiferromagnetic spintronics is based on recent developments in the manipulation and detection of antiferromagnetic properties using electrical methods, opening up the possibility of these materials evolving from passive to active components of spintronic devices. Doing so offers a number of advantages, such as improved stability, reduction in stray fields and increased speed of dynamics. However, changes in the orientation of typical antiferromagnets’ Néel vectors do not produce read-out signals of the size required for applications. Topological antiferromagnets may offer the solution.
In this talk, we focus on the particular example of noncollinear antiferromagnets of the type Mn3X, whose chiral spin textures break time- and inversion-symmetries, leading to novel magnetotransport properties driven by momentum-space Berry curvature. These include a large intrinsic anomalous Hall effect [1], anomalous Nernst effect [2], and both intrinsic- [3] and magnetic-spin Hall effects [4].
However, for the utilization of these Berry curvature generated phenomena in topological antiferromagnetic spintronic devices, further exploration of the behavior of Mn3X materials in thin film form is required. We therefore present results from our recent work, where we grow high-quality thin films of such noncollinear antiferromagnets with different crystal structures by exploiting epitaxial engineering. Specifically, we demonstrate the thin film deposition of two distinct varieties of noncollinear antiferromagnet: Mn3Ir, with a cubic structure [5], and Mn3Sn, with a hexagonal structure [6].
The crystal structure of the films is characterized using a combination of x-ray diffraction and transmission electron microscopy, whilst their magnetic properties are studied using vibrating sample magnetometry and x-ray magnetic circular dichroism experiments. In doing so, we illuminate the important role played by uncompensated moments in both materials, exploring how these are affected by sample microstructure and how, in turn, they affect antiferromagnetic domain distribution.
Such chiral domains play a key role in governing topological magnetotransport in these compounds. We elucidate this by measuring the Hall effect in lithographically patterned samples of both Mn3Ir and Mn3Sn, and find very different behavior in both cases. Whilst Mn3Ir shows a small conventional anomalous Hall effect [7], we observe a large anomalous Hall effect in Mn3Sn. Films down to 30 nm in thickness demonstrate an anomalous Hall conductivity of σxy (µ0H = 0 T) = 21 Ω-1cm-1, which we attribute to a Berry curvature mechanism [8]. Following cooling of Mn3Sn below its transition temperature into a glassy ferromagnetic state, we identify a change in transport behavior from anomalous to topological Hall effects.
Finally, we bring noncollinear antiferromagnets closer to functionality by moving to investigate the generation and interaction of spin currents in our thin films. Specifically, we use spin-torque ferromagnetic resonance measurements in Mn3X / NiFe bilayers to quantify their spin Hall angle. Significant charge-to-spin current conversion is identified, which depends intimately on epitaxial growth conditions, thin film magnetic state, and chiral domain structure. We conclude by discussing the origin of these different phenomena, and the potential for Mn3X materials to be used in chiralitronic devices.
[2] H. Reichlová et al., Nature Communications 10, 5459 (2019)
[3] W. Zhang, W. Han, S. H. Yang, Y. Sun, Y. Zhang, B. Yan, and S. S. P. Parkin, Science Advances 2, e1600759 (2016)
[4] M. Kimata et al., Nature 565, 627 (2019)
[5] J. M. Taylor et al., Physical Review Materials 3, 074409 (2019)
[6] A. Markou, J. M. Taylor, A. Kalache, P. Werner, S. S. P. Parkin, and C. Felser, Physical Review Materials 2, 051001(R) (2018)
[7] J. M. Taylor et al., Applied Physics Letters 115, 062403 (2019)
[8] J. M. Taylor, A. Markou, E. Lesne, P. K. Sivakumar, C. Luo, F. Radu, P. Werner, C. Felser, and S. S. P. Parkin, Physical Review B 101, 094404 (2020)