Pressure Effects in Ferromagnetism of Orbital Ordering Ferromagnet

We have studied pressure effects on layered Perovskite-type orbital ordering ferromagnet K 2 CuF 4 , 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 CuF 2 plane in K 2 CuF 4 the magnetic phase diagram is derived by perturbational procedure. Judging from the parameter values estimated by FLAPW band calculations for K 2 CuF 4 under high pressure as well as at ambient pressure, it is concluded that the pressure-induced F-AF transition may be realized in K 2 CuF 4 without structural transition from antiferrodistortive to ferrodistortive phase. Further the observed pressure dependence of T C of K 2 CuF 4 can be explained qualitatively by our theory.