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Along with the progress of spin science and spintronics research, the flow of electron spins, (i.e. spin current), has attracted interest. New phenomena and electronic states were explained in succession using the concept of spin current. Moreover, as many of the conventionally known spintronics phenomena became well organized based on spin current, it has rapidly been recognized as an essential concept in a wide range of condensed matter physics. In this article, we focus on recent developments in the physics of spin, spin current, and their related phenomena, where the conversion between spin angular momentum and different forms of angular momentum plays an essential role. Starting with an introduction to spin current, we first discuss the recent progress in spintronic phenomena driven by spin-exchange coupling: spin pumping, topological Hall torque, and emergent inductor. We, then, extend our discussion to the interaction/interconversion of spins with heat, lattice vibrations, and charge current and address recent progress and perspectives on the spin Seebeck and Peltier effects. Next, we review the interaction between mechanical motion and electron/nuclear spins and argue the difference between the Barnett field and rotational Doppler effect. We show that the Barnett effect reveals the angular momentum compensation temperature, at which the net angular momentum is quenched in ferrimagnets., 23 pages, 19 figures

We develop the in situ magnetization measurement apparatus for observing the Barnett effect consisting of a fluxgate sensor, a high speed rotor with frequencies of up to 1.5 kHz, and a magnetic shield at room temperature. The effective magnetic field (Barnett field) in a sample arising from rotation magnetizes the sample and is proportional to the rotational frequency. The gyroscopic g factor, g ′, of rare earth metals, in particular, Gd, Tb, and Dy, was estimated to be 2.00 ± 0.08, 1.53 ± 0.17, and 1.15 ± 0.32, respectively, from the slopes of the rotation dependence of the Barnett field. This study provides a technique to determine the g ′ factor even in samples where the spectroscopic method may not be available.

We present spin pumping using a Heusler alloy Co2MnSi/Pt bilayer film. A spin current is produced by a ferromagnetic resonance (FMR) technique. The pure spin current injected into the Pt layer from the Co2MnSi layer is detected by the inverse spin-Hall effect (ISHE), which converts the spin current into an electric current. We estimated a damping constant of the Co2MnSi/Pt bilayer film from an angular dependence of FMR spectra. Using the damping constant efficiency of spin pumping from the Co2MnSi layer is evaluated. We found that a mixing conductance at the Co2MnSi/Pt interface is comparable to that at a permalloy/Pt interface.