Tunable Magnonic Thermal Hall Effect in Skyrmion Crystal Phases of Ferrimagnets

We theoretically study the thermal Hall effect by magnons in skyrmion crystal phases of ferrimagnets in the vicinity of the angular momentum compensation point (CP). To this end, we start by deriving the equation of motion for magnons in the background of an arbitrary equilibrium spin texture, which gives rise to the fictitious electromagnetic field for magnons. As the net spin density varies, the resultant equation of motion interpolates between the relativistic Klein-Gordon equation at the CP and the nonrelativistic Schrödinger-like equation away from it. In skyrmion crystal phases, the right- and the left-circularly polarized magnons, with respect to the order parameter, are shown to form the Landau levels separately within the uniform skyrmion-density approximation. For an experimental proposal, we predict that the magnonic thermal Hall conductivity changes its sign when the ferrimagnet is tuned across the CP, providing a way to control heat flux in spin-caloritronic devices on the one hand and a feasible way to detect the CP of ferrimagnets on the other hand.