Evidence for a Second Magnetic Phase Transition in Godolinium

The heat capacity and temperature derivative of the electrical resistivity of Gd have been measured simultaneously in the vicinity of 226 K, where the easy axis of magnetization tilts away from the crystallographic $c$ axis. In addition to the known anomaly in the $a$-axis resistivity, a small step change in the specific heat was observed. Application of magnetic fields above 1000 Oe along the $c$ axis or 120 Oe along the $a$ axis suppressed both the resistive and specific-heat anomalies. This behavior is discussed in terms of a molecularfield model which treats the anisotropy energy as a term in the magnetic free energy. The tilting of the easy axis is driven by the temperature dependence of the magnetization, which causes the lowest-order anisotropy constant $K_1$ to change sign. The magnitudes of the step in the specific heat and the critical field in the $c$-axis direction calculated in this model are in good agreement with experiment.