  Quantized spin waves, or magnons, in a magnetic insulator are assumed to
interact weakly with the surroundings, and to flow with little dissipation or
drag, producing exceptionally long diffusion lengths and relaxation times. In
analogy to Coulomb drag in bilayer two dimensional electron gases, in which the
contribution of the Coulomb interaction to the electric resistivity is studied
by measuring the interlayer resistivity (transresistivity), we predict a
nonlocal drag of magnons in a ferromagnetic bilayer structure based on
semiclassical Boltzmann equations. Nonlocal magnon drag depends on magnetic
dipolar interactions between the layers and manifests in the magnon current
transresistivity and the magnon thermal transresistivity, whereby a magnon
current in one layer induces a chemical potential gradient and/or a temperature
gradient in the other layer. The largest drag effect occurs when the magnon
current flows parallel to the magnetization, however for oblique magnon
currents a large transverse current of magnons emerges. We examine the effect
for practical parameters, and find that the predicted induced temperature
gradient is readily observable.
