MnAs exhibits a first-order phase transition from a ferromagnetic, high-spin metal hexagonal phase to a paramagnetic, lower-spin insulator orthorhombic phase at $T_C=313\mathrm{K}$. Here, we report the results of neutron diffraction experiments showing that an external magnetic field, $B$, stabilizes the hexagonal phase above $T_C$. The phase transformation is reversible and constitutes the first demonstration of a bond-breaking transition induced by a magnetic field. The field-induced phase transition is accompanied by an enhanced magnetoresistance of about $17\%$ at $310\mathrm{K}$. The phenomenon appears to be similar to that of the colossal magnetoresistance response observed in the Mg perovskite family.

• Received 25 January 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.097203