In-plane thermal conductivity of [formula presented]: Evidence for magnon heat transport

We report the temperature and magnetic field dependence of the in-plane thermal conductivity ([Formula presented]) of high-quality monocrystalline [Formula presented]. Isothermal measurements of the field dependence of [Formula presented] at low temperatures ([Formula presented]) show no change in [Formula presented] below a critical magnetic field [Formula presented] ([Formula presented] for [Formula presented]) and [Formula presented] for [Formula presented]). Above [Formula presented], [Formula presented] more than doubles as [Formula presented] is increased to 9 T. At [Formula presented], there is a transition from a noncollinear to a collinear arrangement of the Nd and Cu spins and a collapse of the gap, [Formula presented], in an acoustic magnon branch at [Formula presented]. Closure of this gap appears to allow the conduction of substantial amounts of heat by acoustic magnons.