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Significance
Antiferromagnetic materials with non-collinear spin structures have garnered significant attention in the field of spintronics due to their unique properties and potential for applications. One of the intriguing phenomena observed in these materials is the anomalous Hall effect (AHE), which occurs despite the absence of a significant magnetization. Additionally, they exhibit a spin Hall effect (SHE) with spin polarization directions that deviate from the conventional perpendicular orientation. These characteristics make non-collinear antiferromagnets (NCAFs) particularly interesting for spin-orbitronic applications and high-density memory devices.
In a new paper research on antiferromagnetic compounds Mn3SnN and Mn3GaN, focusing on the structural symmetry lowering induced by large displacements of magnetic manganese atoms. The research paper is published in Advanced Materials and led by Prof. Dr. Stuart Parkin from Max Planck Institute in Germany. These displacements play a crucial role in enabling spin canting and the observation of intrinsic transport effects such as the AHE and SHE. It is worth noting that these effects can only be observed when the sample predominantly resides in a single antiferromagnetic domain state, necessitating the presence of weak moments due to spin canting for external domain control.
The authors provided an overview of NCAFs and their distinctive transport phenomena. NCAFs exhibit the AHE and SHE due to the non-zero Berry curvature of their electronic bands. Unlike traditional heavy metals, NCAFs allow for spin currents with components in various directions, offering potential for novel spin-orbitronic applications. Compounds such as Mn3Z and Mn3ZN, composed of stacked layers of Mn3Z with kagome lattices, have been extensively studied for their NCAF properties. The frustrated spin textures in these compounds give rise to the AHE and SHE.
To observe the AHE, an imbalance in the volume fraction of antiferromagnetic domains is required, similar to ferromagnets. In hexagonal compounds like Mn3Ge and Mn3Sn, weak in-plane canted moments induced by single Mn ion anisotropy allow for domain structure control via a magnetic field. However, in cubic systems, an additional mechanism is needed to generate spin canting, traditionally attributed to tetragonal lattice strain induced by the substrate. These strains alleviate the stress caused by lattice mismatch, resulting in inequivalent inter-atomic distances within the kagome plane and the appearance of net moments.
The authors reveal that the AHE can be observed in thin films of Mn3SnN despite their seemingly cubic structure as determined by conventional X-ray diffraction (XRD) characterization. They demonstrate that previously unexplored displacements of Mn atoms away from high-symmetry positions significantly lower the symmetry of the kagome arrangement, inducing spin canting in the otherwise fully compensated NCAF spin texture. These displacements enable the observation of the AHE and provide a mechanism for domain structure control.
The researchers also discover Mn displacements in Mn3GaN thin films grown through molecular beam epitaxy (MBE), suggesting that Mn displacements and associated symmetry lowering are common in specific antiperovskite nitrides. These hidden displacements are typically missed when only probing the lattice metric, emphasizing the need to examine atomic positions using a large set of scattering vectors.
The findings presented in the new work shed light on the importance of atomic displacements and symmetry lowering in antiferromagnetic materials, particularly in thin film form. These displacements offer a potential mechanism for stress relaxation and may be involved in the experimental observation of various intrinsic transport effects beyond the AHE and SHE. This includes phenomena like the anomalous Nernst effect, magneto-optical Kerr effect, and longitudinal spin current polarization. The authors suggest that structural symmetry lowering induced by atomic displacements may provide new opportunities for controlling the domain structure of antiferromagnets, paving the way for their integration into spintronic devices using established architectures.
In conclusion, recent research has uncovered the role of hidden symmetry lowering induced by atomic displacements in enabling spin canting and the observation of intrinsic transport effects in antiferromagnetic thin films. The study highlights the importance of these displacements in NCAF materials and their potential impact on the field of spintronics. By understanding and harnessing the effects of symmetry lowering, researchers can advance the development of antiferromagnetic-based devices and explore their unique properties for future technological applications.
Reference
Rimmler BH, Hazra BK, Pal B, Mohseni K, Taylor JM, Bedoya-Pinto A, Deniz H, Tangi M, Kostanovskiy I, Luo C, Neumann RR, Ernst A, Radu F, Mertig I, Meyerheim HL, Parkin SSP. Atomic Displacements Enabling the Observation of the Anomalous Hall Effect in a Non-Collinear Antiferromagnet. Adv Mater. 2023;35(23):e2209616.
