Change of magnetism in the two-dimensional Heisenberg ferromagnet K2CuF4 observed at high pressures

The mechanism responsible for determining the exchange interaction to be ferromagnetic or antiferromagnetic in Cu2+ compounds is closely correlated with the antiferrodistortive (AFD) or ferrodistortive (FD) order of Cu2+ hole orbitals. One of the representative two-dimensional Heisenberg ferromagnets, K2CuF4, has an ideal AFD orbital ordering. To investigate the possibility of the magnetic phase transition due to a change in the orbital ordering from AFD to FD at high pressures, susceptibility measurements of this compound have been made under various pressures up to 13 GPa over the temperature range from 1.5 to 18 K. A sudden decrease in the Curie temperature starting from 7 GPa was observed. Furthermore, a large decrease in the susceptibility was observed at pressures above 10 GPa. These results indicate a pressure induced ferromagnetic to antiferromagnetic phase transition occurring at about 8-9 GPa as a result of a change of orbital state from AFD to FD order.