Magnetic order-order phase transitions in magnets with collective electrons: MnCoSi

First principles calculations of the electronic structure of MnCoSi and a spiral state model are used to analyze magnetic-field induced magnetic order-order phase transitions in MnCoSi at pressures up to 2kbar. This shows that the magnetic moments of Mn and Co in MnCoSi are created by collective d-electrons and that pressure stimulation of induced order-order transitions in this material can be a consequence of structural changes in the density of d-electron states as the pressure is raised. Induced order-order transitions are described by a single-band model for the spiral state in which the band filling and the nonmagnetic density of electron states are based on first principles calculations of the electronic structure of MnCoSi. This model shows that the low-temperature states induced by a magnetic field are not collinear ferromagnetic structures, but may represent the coexistence of a homogeneous ferromagnetic component and a periodic spiral component of the total magnetic moment of the d-band. A pure...