  We investigate the spin injection and the spin transport in paramagnetic
insulators described by simple Heisenberg interactions using auxiliary particle
methods. Some of these methods allow access to both paramagnetic states above
magnetic transition temperatures and magnetic states at low temperatures. It is
predicted that the spin injection at an interface with a normal metal is rather
insensitive to temperatures above the magnetic transition temperature. On the
other hand below the transition temperature, it decreases monotonically and
disappears at zero temperature. We also analyze the bulk spin conductance. It
is shown that the conductance becomes zero at zero temperature as predicted by
linear spin wave theory but increases with temperature and is maximized around
the magnetic transition temperature. These findings suggest that the compromise
between the two effects determines the optimal temperature for spintronics
applications utilizing magnetic insulators.
