High-Resolution Mapping of Cluster Magnetic Octupole Moments in Mn3Sn Nanowires for Advanced Spintronic Applications

Antiferromagnetic Weyl semimetals like Mn₃Sn exhibit remarkable anomalous transverse transport phenomena, such as the anomalous Hall effect and anomalous Nernst effect (ANE), despite having negligible net magnetization. The intriguing aspect of Mn₃Sn lies in its noncollinear spin structure within a kagome lattice. It gives rise to cluster magnetic octupole moments which are crucial as they act as macroscopic order parameters, analogous to ferromagnetic moments and significantly influence the material’s electronic properties through the momentum-space Berry curvature. Despite extensive studies on the macroscopic properties of Mn₃Sn, understanding the behavior of its cluster magnetic octupole domains at the nanoscale remains a significant challenge. Addressing this gap is critical for advancing the integration of Mn₃Sn into spintronic devices, which require precise control over magnetic domains to ensure device efficiency and stability. One of the primary challenges in studying Mn₃Sn at the nanoscale is the difficulty in directly observing and mapping the spatial distribution of the cluster magnetic octupole moments. Conventional methods, such as magneto-optical Kerr effect (MOKE) measurements and laser-induced temperature gradients, suffer from low spatial resolution, limiting their effectiveness in nanostructure investigations. Moreover, while beneficial for device applications, the negligible net magnetization of Mn₃Sn challenges detecting its magnetic properties.

To this end, a new study published in the Journal Physical Review Letters and led by Assistant Professor Hironari Isshiki, Nico Budai, Ayuko Kobayashi, in Professor YoshiChika Otani’s group in collaboration with Professor Satoru Nakatsuji’s group, including his student Ryota Uesugi, and Associate Professor Tomoya Higo at the University of Tokyo, developed a methodology that allows for high-resolution mapping of the cluster magnetic octupole moments within Mn₃Sn nanostructures. The new and innovative approach overcomes the limitations of previous methods by directly detecting the orientation of local octupole moments with spatial resolution as high as 80 nm. This method will enable us to understand the magnetic domain structures in Mn₃Sn nanowires and optimize their integration into spintronic devices.

The authors fabricated a kagome-in-plane-textured polycrystalline Mn₃Sn film using a dc magnetron sputtering method on a Si/SiO₂ substrate. The film was then annealed in vacuum to ensure the stability of its structure and capped with an AlO_x layer to prevent oxidation. This process yielded a composition within the stable range for Mn₃Sn, with crystal grain sizes estimated to be between 100 and 250 nm. Following the film preparation, they fabricated two parallel Mn₃Sn wires using electron beam lithography and Ar ion etching. One wire served as the sample, while the other acted as a heating wire. Joule heating from the heating wire increased the temperature of the sample wire by approximately 7 K. The key innovation in their experimental approach was using atomic force microscopy (AFM) to establish a tip-to-sample contact that induced a localized temperature gradient. This method enabled the researchers to measure the ANE-originated thermoelectric voltages between the wire’s ends with high spatial resolution. Initially, the researchers confirmed the behavior of the magnetic cluster octupole moments under an in-plane magnetic field by measuring the ANE in both the Mn₃Sn thin film and the fabricated nanowire at room temperature. The ANE voltages were detected, revealing a significant remanent magnetization along the wire-width direction, even without an external magnetic field. This indicated that the nanowire retained considerable magnetization, highlighting its potential for spintronic applications. To achieve high spatial resolution mapping of the cluster magnetic octupole moments, the AFM was employed to acquire simultaneously topography and voltage maps (V_2f) of the sample wire. The scanned tip in contact mode with a loading force of 50 nN generated localized out-of-plane temperature gradients. These gradients induced thermoelectric voltages that reflected the orientation of the local octupole moments. The resulting V_2f map, obtained before applying an external magnetic field, showed the presence of local ANE signals attributable to the textured Mn₃Sn sample. In other experiments, the authors applied external magnetic fields of ±2 T along the y direction to the sample. After magnetizing the sample, the researchers repositioned it back into the AFM for repeated V_2f mapping. The resulting maps revealed the distinct distribution of V_2f signals, corresponding to the remanent states of the octupole moments along the wire-width direction. They provided clear evidence of the magnetic response of Mn₃Sn to external fields and demonstrated the presence of a magnetic component in the measured signals. Numerical simulations of the temperature distribution induced by tip contact in the sample wire were performed using COMSOL Multiphysics to disentangle the magnetic and nonmagnetic contributions to the V_2f signals. The simulated temperature distribution confirmed that the out-of-plane temperature gradient was highly localized with an extent of approximately 80 nm, matching the spatial resolution of the ANE measurements. The spatial distribution of cluster magnetic octupole moments in the Mn₃Sn nanowire was successfully visualized with an unprecedented spatial resolution of 80 nm, demonstrating its potential for spintronic applications without needing external magnetic fields.

In conclusion, the research team at the University of Tokyo extended the investigation of Mn₃Sn from micrometer-scale samples to nanometer-scale structures using a novel AFM-based technique. This transition is critical as it provides detailed knowledge of the magnetic properties and behavior of Mn₃Sn at scales relevant to practical device integration. Moreover, the unprecedented spatial resolution of 80 nm was demonstrated in mapping the distribution of cluster magnetic octupole moments using AFM. This high-resolution mapping is crucial for understanding the local magnetic behavior of Mn₃Sn nanowires. The findings of Hironari Isshiki and colleagues have essential implications for advancing the integration of Mn₃Sn into spintronic devices. Additionally, efficient thermoelectric generation and heat flux sensing using ANE-based thermopiles could lead to the development of novel energy harvesting and temperature sensing technologies. According to the authors, the observations of the new study can also enhance device design and precision with tailored magnetic responses, which are critical in advanced electronics.