Structural transformation induced by magnetic field and colossal   magnetoresistance response above 313 K in MnAs
  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. 
J. Mira, F. Rivadulla, J. Rivas, A. Fondado, R. Caciuffo, F. Carsughi,   T. Guidi, J. B. Goodenough