  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.
