  Pure spin currents, i.e. the transport of angular momentum without an
accompanying charge current, represent a new, promising avenue in modern
spintronics from both a fundamental and an application point of view. Such pure
spin currents can not only flow in electrical conductors via mobile charge
carriers, but also in magnetically ordered electrical insulators as a flow of
spin excitation quanta. Over the course of the last years remarkable results
have been obtained in heterostructures consisting of magnetically ordered
insulators interfaced with a normal metal, where a pure spin current flows
across the interface. This topical review article deals with the fundamental
principles, experimental findings and recent developments in the field of pure
spin currents in magnetically ordered insulators. We here put our focus onto
four different manifestations of pure spin currents in such heterostructures:
The spin pumping effect, the longitudinal spin Seebeck effect, the spin Hall
magnetoresistance and the all-electrical detection of magnon transport in
non-local device concepts. In this article, we utilize a common theoretical
framework to explain all four effects and explain important material systems
(especially rare-earth iron garnets) used in the experiments. For each effect
we introduce basic measurement techniques and detection schemes and discuss
their application in the experiment. We account for the remarkable progress
achieved in each field by reporting the recent progress in each field and by
discussing research highlights obtained in our group. Finally, we conclude the
review article with an outlook on future challenges and obstacles in the field
of pure spin currents in magnetically ordered insulator / normal metal
heterostructures.
