  The observation of the spin Seebeck effect in insulators has meant a
breakthrough for spin caloritronics due to the unique ability to generate pure
spin currents by thermal excitations in insulating systems without moving
charge carriers. Since the recent first observation, the underlying mechanism
and the origin of the observed signals have been discussed highly
controversially. Here we present a characteristic dependence of the
longitudinal spin Seebeck effect amplitude on the thickness of the insulating
ferromagnet (YIG). Our measurements show that the observed behavior cannot be
explained by any effects originating from the interface, such as magnetic
proximity effects in the spin detector (Pt). Comparison to theoretical
calculations of thermal magnonic spin currents yields qualitative agreement for
the thickness dependence resulting from the finite effective magnon propagation
length so that the origin of the effect can be traced to genuine bulk magnonic
spin currents ruling out parasitic interface effects.
