Published papers

We develop a hydrodynamic theory for an electron system exhibiting the anomalous Hall effect, and show that an additional anomalous Hall effect is induced by a vorticity generated near boundaries. We calculate the momentum flux and force proportional to the electric field using linear response theory. The hydrodynamic equation is obtained by replacing the local electric field with the electric current, focusing on a scale that is sufficiently larger than the mean-free path. It is demonstrated that there is a coupling between a vorticity of an electric current and a magnetization which generates a pressure from nonuniform vorticity. Taking into account Hall viscosity and relaxation forces, a nonuniform flow near a boundary and an additional Hall force are calculated. The additional anomalous Hall force is opposite to a conventional anomalous Hall force, resulting in a sign reversal in thin systems. An antisymmetric viscosity turns out to arise from the side-jump process due to the anomalous velocity.

We develop a microscopic theory of tunneling spin transport at the magnetic interface between a ferromagnetic insulator (FI) and a ferromagnetic superconductor (FSC) driven by ferromagnetic resonance. We show that the spin susceptibilities of the FSC can be extracted from the spin currents by tuning the easy axis of the FI, and thus the spin currents can be a probe for the symmetries of the spin-triplet Cooper pairing. Our results will offer a route to exploiting the synergy of magnetism and superconductivities for spin devices.

Magnon drag effect induced by an applied electric field in ferromagnetic metals is theoretically studied by a microscopic calculation of the force on magnons arising from magnon emission and absorption and scattering due to driven electrons. It is shown that magnon scattering contribution dominates over the emission and absorption ones in a wide temperature regime in good metals with long elastic lifetime τ, as the latter has a relative suppression factor of (Δτ)-2 due to the electron spin flip by the magnon, where Δ is the sd exchange interaction energy. Spin-transfer efficiency is discussed including the magnon drag effect.

We explore the theoretical possibility of P03 neutron superfluid in dilute spin-polarized neutron matter, which may be relevant to the crust region of a magnetized neutron star. In such a dilute regime where the neutron Fermi energy is less than 1 MeV, the S01 neutron superfluid can be suppressed by a strong magnetic field of the compact star. In the low-energy limit relevant for dilute neutron matter, the P03 interaction is stronger than the P23 one which is believed to induce the triplet superfluid in the core. We present the ground-state phase diagram of dilute neutron matter with respect to the magnetic field and numerically estimate the critical temperature of the P03 neutron superfluid, which is found to exceed 107 K.

We present a hydrodynamic theory to describe a chiral electron system with a Weyl spin-orbit interaction on a field-theoretic basis. Evaluating the momentum flux density as a linear response to a driving electric field, we derive an equation of motion for the orbital angular momentum. It is shown that the chiral nature leads to a dynamic bulk angular momentum generation by inducing a global torque as a response to the applied field. The steady-state angular momentum is calculated taking into account rotational viscosity.

Motivated by the recent experiment done by Yu et al. [X. Z. Yu, et. al., Sci. Adv., 6, 25, eaaz9744 (2020)] on magnetic skyrmion creation and annihilation by electric currents in FeGe devices with a notch structure, we examine the interaction between the electric current vorticity near the notch and localized spins, and obtain three effects of the current vorticity on the localized spins. The first one is an effective Zeeman field induced by the current vorticity. This effective Zeeman field is responsible for skyrmion creation. The second effect is a vorticity-induced Dzyaloshinskii-Moriya (DM) interaction with the direction of the DM vector being parallel to the vorticity direction and is different from the vector of the intrinsic DM interaction. This vorticity-induced DM interaction makes the skyrmions distorted. The third effect is a spin current due to the gradient of the vorticity and the spin current drives the magnetic skyrmions far from the notch. Our theory opens a new path to design the magnetic textures by using structural settings.