Electronic mechanism of structural phase transitions in manganese arsenide

The spin-fluctuation theory of magnetism is generalized to magnets with complex crystal and magnetic structure. An expression is obtained for the free energy of a system of itinerant p and d electrons, which are described by the Hubbard Hamiltonian with the static displacements of the ion cores, which lead to a change of the lattice symmetry, taken into account. The calculation of the free energy is done using the approximation of uniform local fields. The approach developed is applied to a study of the concrete magnetic and structural order parameters in ferromagnetic MnAs at T=0. It is shown that the appearance of structural distortions of the initial nickel arsenide lattice (nonzero structural order parameter) leads to a lowering of the average energy of the band electrons. The competition of the energy of the electrons and of the ion–ion coupling can stabilize these distortions. The interaction of the structural and magnetic order parameters tends to suppress the structural distortions. It is shown that the appearance of magnetization in the system (spontaneous or magnetic-field induced) can lead to restoration of the initial nickel arsenide structure. Such a magnetostructural transition is observed experimentally.