Magnon-drag Peltier effect in a Ni-Cu alloy

Peltier-effect measurements to an accuracy of 1% have been made on Ni₆₆{Cu}₃₄$ and ${Ni}₆₉Fe₃₁ at 1.3-4.2 K and magnetic fields to 5.5 T. The Peltier coefficient π is proportional to T² as expected for electron diffusion. However, the absolute value of π decreases by 7% in Ni-Cu and 4% in Ni-Fe when the field is increased from just above ferromagnetic saturation to 5.5 T, at 4.2 K. This decrease is consistent with a magnon-drag contribution to the Peltier coefficient, which is quenched by the field, at least in the case of Ni-Cu. The magnitude of the contribution, when combined with Hall data, indicates that the magnon gas drifts at a speed close to that of the $4s$ electron gas. Using a simple kinetic theory this implies that magnons relax faster on $4s$ electrons than on impurities. Interpretation of Ni-Fe data is not so clear.