  MnAs exhibits a first-order phase transition from a ferromagnetic, high-spin
metal NiAs-type hexagonal phase to a paramagnetic, lower-spin insulator
MnP-type orthorhombic phase at T_C = 313 K. Here, we report the results of
neutron diffraction experiments showing that an external magnetic field, B,
stabilizes the hexagonal metallic phase above T_C. The phase transformation is
reversible and constitutes the first demonstration of a bond-breaking
transition induced by a magnetic field. At 322 K the hexagonal structure is
restored for B > 4 tesla. The field-induced phase transition is accompanied by
an enhanced magnetoresistance of about 17 % at 310 K. We discuss the origin of
this phenomenon, which appears to be similar to that of the colossal
magnetoresistance response observed in some members of the manganese perovskite
family.
