Theory of multiple spin density wave and lattice distortion in FCC antiferromagnet

The phase diagram including the cubic, two kinds of tetragonal and the orthorhombic phases of the antiferromagnets Mn1-xAx (A = Ni, Ga, Rh and Au) which are known as the first-kind antiferromagnets, is explained theoretically. Our Hamiltonian is composed of the polynomial of the variables describing multiple spin density wave (MSDW) states, the coupling between MSDW states and lattice distortion and the elastic energy. The polynomial is derived from symmetry consideration. By calculating the partition function and the free energy, we show that the phase diagram is reproduced and elucidate the structure of MSDW at each phase and the condition of the appearance of the orthorhombic phase.