Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal

Abstract: When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties as well as for various applications including magneto-optical recording. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of $M \sim$0.002 $\mu_{\rm B}$/Mn, the non-collinear AF metal Mn$_3$Sn exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Neel state may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.

• The paper demonstrates that antiferromagnetic Mn₃Sn exhibits a substantial 20 milli-degree Kerr effect driven by magnetic octupole ordering rather than net magnetization.
• Experimental observations paired with first-principles calculations confirm MOKE imaging of octupole domains, challenging traditional magnetization-based mechanisms.
• The findings pave the way for antiferromagnetic spintronics, offering new avenues for high-density, ultrafast memory devices and advanced magnetic applications.

Large Magneto-Optical Kerr Effect in Antiferromagnetic Metal Mn$_3$Sn

The paper presents an in-depth examination of the magneto-optical Kerr effect (MOKE) observed in a non-collinear antiferromagnetic metal, Mn$_3$Sn, at room temperature. This advance provides new insights into spintronics applications, spurred by Mn$_3$Sn’s ability to exhibit substantial MOKE of 20 milli-degrees, despite its minimal magnetization comparable only to 0.002 $\mu_{\rm B}$/Mn. The paper elucidates the mechanism behind this significant MOKE without relying on spin magnetization, attributing it to the ferroic ordering of magnetic octupoles within the non-collinear Néel state. This contrasts with traditional views, which usually link MOKE occurrence with magnetization in ferromagnets.

The experimental observations were corroborated by first-principles calculations, which supported the hypothesis that magnetic octupoles significantly contribute to the MOKE in Mn$_3$Sn. The findings indicate that while the magnetization remains largely unchanged upon increasing the external field beyond the point required for domain switching, the MOKE signal remains robust. This suggests a decoupling between the magnetic octupole order and the small ferromagnetic component, implying alternate mechanisms underlying the magneto-optical properties in this antiferromagnet.

The detection of a large MOKE in Mn$_3$Sn provides an invaluable tool for imaging magnetic octupole domains, an area previously challenging to explore with antiferromagnets (AFs) due to their negligible net magnetic moment and, thus, the absence of conventional magnetic contrast. The MOKE imaging vividly demonstrates octupole domain dynamics through changes in contrast in magnetic domain configurations under the influence of an external field.

Implications and Future Directions

The implications of this research are paramount for developing AF-based spintronics, especially for the design of high-density, ultrafast memory devices. AFs, generally characterized by minimal stray fields and faster spin dynamics, now gain attractiveness due to the capability of optical probing through MOKE. The unique structure of Mn$_3$Sn, particularly its hexagonal lattice allowing six domain types, opens avenues for complex domain interactions that could be manipulated for technological applications.

From a theoretical perspective, the work challenges the established notion that a net magnetization is a prerequisite for MOKE, presenting a novel class of magnetic materials driven by octupole and topological order. This finding prompts revisiting the fundamental interactions in AFs and also encourages the application of Berry curvature-driven mechanisms to predict and enhance optical and spintronic properties in other AF systems.

In terms of future investigations, exploring the current-induced domain dynamics and the potential for manipulation of topological defects in Mn$_3$Sn could further the understanding of unconventional magnetic interactions. Such investigations are anticipated to reveal new physics related to topological states, especially relevant in the context of the magnetic Weyl semimetal phase, potentially influencing the MOKE signal via novel bulk-edge correspondence phenomena.

The study hence contributes not just incrementally but significantly to the field of magnetism by expanding the functional repertoire of antiferromagnetic materials, heralding a new understanding of magneto-optical effects sans magnetization. As momentum builds around AF materials in practical applications, Mn$_3$Sn stands out as a beacon pointing toward advanced, spintronic-enabled technologies.