  We have investigated the unidirectional spin wave heat conveyer effect in
sub-micron thick yttrium iron garnet (YIG) films using lock-in thermography
(LIT). Although the effect is small in thin layers this technique allows us to
observe asymmetric heat transport by magnons which leads to asymmetric
temperature profiles differing by several mK on both sides of the exciting
antenna, respectively. Comparison of Damon-Eshbach and backward volume modes
shows that the unidirectional heat flow is indeed due to non-reciprocal
spin-waves. Because of the finite linewidth, small asymmetries can still be
observed when only the uniform mode of ferromagnetic resonance is excited. The
latter is of extreme importance for example when measuring the inverse
spin-Hall effect because the temperature differences can result in
thermovoltages at the contacts. Because of the non-reciprocity these
thermovoltages reverse their sign with a reversal of the magnetic field which
is typically deemed the signature of the inverse spin-Hall voltage.
