Giant magnetoelastic response in MnAs

MnAs has been prepared from elemental powders using a modified ball-milling technique, followed by hot pressing and subsequent annealing. Giant magnetic field-induced uniaxial strains of up to 0.7% were observed in a temperature range above the Curie temperature ${\mathrm{(T}}_C\text{=305\hspace{0.05em}}\mathrm{K}\mathrm{).}$ Dilatometric and magnetization measurements showed that the strains were associated with the volume change accompanying a field-induced magnetostructural paramagnetic to ferromagnetic transition above $T_C.$ The occurrence of a tricritical point (TCP) in temperature-field space, where the two low temperature phases and the high temperature NiAs-type phase are identical was shown to be responsible for the decrease of the magnitude of both the maximum strain and magnetization change with increasing temperature while approaching the TCP along the line of the first-order transitions. The coordinates of the TCP are approximately $T_{\mathrm{tr}}\mathrm{=405 K}$ and $H_{\mathrm{tr}}\mathrm{=165 kOe}.$