Magnetostriction in Tb.27Dy.73Fe2: Evidence for a low-temperature transition

We report magnetostriction measurements on a single crystal of Tb.27Dy.73Fe2. Length changes parallel and perpendicular to a 111 direction were measured from 298 to 3.1K in magnetic fields from zero to 25kOe. The λ vs H curves show saturation at low fields above 280K; below 280K λ becomes progressively more difficult to saturate until approximately 65K at which temperature the magnetostriction begins to increase again. This upturn is taken as evidence for the existence, at low temperatures and low magnetic fields, of a non-colinear ferrimagnetic phase with the net moment in the (uuw) plane. The temperature and field dependence of λ is discussed by means of a Ginsburg-Landau model of the low temperature transition from a colinear, [100]-easy phase to the [uuw] phase.