Yoshinobu NAKATANI (仲谷 栄伸)

Abstract

This study investigates spin-splitter torque (SST) for magnetization switching in nanoscale magnets with perpendicular anisotropy, aiming to reduce current requirements in magnetic memory devices. A micromagnetic model shows that optimizing the spin polar angle ( ) can lower the threshold current density by 1.6–27 times compared to the traditional spin–orbit torque method. A above ∼134° ensures high switching probability, while tailoring the current pulse shape cuts switching error rates by about 200 times. The findings highlight SST’s promise for energy-efficient and reliable switching in future magnetic random-access memory (MRAM) technologies.

The determination of material parameters is significantly important in material science, which is often a challenging task. Recently, advancements have shown that magnetic parameters, such as the Dzyaloshinskii–Moriya interaction (DMI), can be estimated from a magnetic domain image using machine learning (ML). This development suggests a potential shift in how magnetic parameters are determined, moving away from traditional measurement techniques to more innovative methods involving image-based inputs processed by ML. In previous studies, the test images used for estimation always matched the training images in size. However, since the image size is contingent on the microscopy technique used, the ability to accurately estimate parameters from images of varying sizes is essential. Here, we investigated the feasibility of estimating the DMI constant and saturation magnetization from magnetic domain images of different sizes using ML. We demonstrated that it is possible to estimate these parameters even when the imaging sizes differ between training and test datasets. Additionally, our comparison of the estimation accuracy for the DMI constant and saturation magnetization revealed that the tolerance for differences in the image size varies depending on the specific parameter being estimated. These findings could have a significant impact on the future methods of determining magnetic parameters.

Abstract

Antiferromagnets (AFMs) have the natural advantages of terahertz spin dynamics and negligible stray fields, thus appealing for use in domain-wall applications. However, their insensitive magneto-electric responses make controlling them in domain-wall devices challenging. Recent research on noncollinear chiral AFMs Mn₃X (X = Sn, Ge) enabled us to detect and manipulate their magnetic octupole domain states. Here, we demonstrate a current-driven fast magnetic octupole domain-wall (MODW) motion in Mn₃X. The magneto-optical Kerr observation reveals the Néel-like MODW of Mn₃Ge can be accelerated up to 750 m s^-1 with a current density of only 7.56 × 10¹⁰ A m^-2 without external magnetic fields. The MODWs show extremely high mobility with a small critical current density. We theoretically extend the spin-torque phenomenology for domain-wall dynamics from collinear to noncollinear magnetic systems. Our study opens a new route for antiferromagnetic domain-wall-based applications.

Abstract

We report current-induced magnetization switching (j_sw) with a nanosecond-duration pulse current (t_p) in a perpendicularly magnetized nanomagnet under the Dzyaloshinskii–Moriya interaction (DMI) and investigate the effect of the Gilbert damping constant (α), t_p, and DMI value (D) on j_sw via micromagnetic simulations. When α is sufficiently small at t_p = 1 ns, j_sw for D = 1.0 erg cm⁻² decreases by 42% compared to that for D = 0 erg cm⁻². Further, j_sw can be reduced under the DMI when α is small and t_p is short, which is attributed to the twisted magnetization and increased initial magnetization angle.

Antiferromagnets with the intrinsic advantages of terahertz spin dynamics and negligible stray fields have been extensively studied for spintronic applications. In particular, spintronic research on antiferromagnets has expanded its focus from collinear to noncollinear Weyl antiferromagnets and discovered that Mn3X (X = Sn, Ge) produces substantial magneto-electric responses. Therefore, noncollinear antiferromagnets could be an ideal spintronic platform. Exploring the domain-wall features in Mn3X is, on the other hand, essential for spintronic device engineering. Here, we report an in-depth study on magnetic octupole domain evolution and domain-wall structure with a choice of Mn3Ge single crystal. Our magneto-optical imaging and the anomalous Hall measurements elucidate the nontrivial magnetic octupole domain nucleation, domain-wall propagation, and pinning behaviors. Moreover, combining the micromagnetic simulation, we reveal that Bloch- and Néel-like walls coexist in bulk with comparable sizes and energy densities. Our findings promote understanding the magnetic octupole domain-wall physics and designing domain-wall-based spintronic devices.