Elastic anomalies at the second-order phase transition in MnAs

The elastic modulus combinations s11, 2s13 + s44, and s44 — 2s12 of MnAs are measured for temperatures between 290 and 420 K. At Tt = 400 K divergent behaviour of s11 and s13 is found, indicating that a soft phonon mode exists. Structure in the sij(T) near 350 K, in particular for s44(T), is assigned to the high-spin-low-spin transition at Thl. A relation between the first order ferromagnetic-paramagnetic phase transition at Tu = 313 K and the displacive phase transition at Tt is suggested as the energy and entropy of the soft lattice mode competes with those of the spin system.