Pressure effects in ferromagnetism of orbital ordering ferromagnet

We have studied pressure effects on layered Perovskite-type orbital ordering ferromagnet K2CuF4, particularly the pressure-induced transition from ferromagnetic (F) to antiferromagnetic (AF) state. First the magnetic phase diagram of simple one-dimensional multi-band Hubbard Hamiltonian with antiferrodistortive order is constructed with exact diagonalization procedure. It is found that AF and helical (HI) states can be stable in a region of large transfer energy even with keeping antiferrodistortive order. For more realistic two-dimensional models relevant to CuF2 plane in K2CuF4 the magnetic phase diagram is derived by perturbational procedure. Judging from the parameter values estimated by FLAPW band calculations for K2CuF4 under high pressure as well as at ambient pressure, it is concluded that the pressure-induced FAF transition may be realized in K2CuF4 without structural transition from antiferrodistortive to ferrodistortive phase. Further the observed pressure dependence of T-C of K2CuF4 can be explained qualitatively by our theory.