  Spin current, i.e. the flow of spin angular momentum or magnetic moment, has
recently attracted much attention as the promising alternative for charge
current with better energy efficiency. Genuine spin current is generally
carried by the spin wave (propagating spin precession) in insulating
ferromagnets, and should hold the chiral symmetry when it propagates along the
spin direction. Here, we experimentally demonstrate that such a spin wave spin
current (SWSC) shows nonreciprocal propagation characters in a chiral-lattice
ferromagnet. This phenomenon originates from the interference of chirality
between the SWSC and crystal-lattice, which is mediated by the relativistic
spin-orbit interaction. The present finding enables the design of perfect spin
current diode, and highlights the importance of the chiral aspect in SWSC.
