Pressure induced antiferromagnet-ferromagnet transition in La0.5Ba0.5CoO2.8 cobaltite

The anion deficient cobaltite La 0.5 Ba 0.5 CoO 2.8 with the formal cobalt valence state close to 3+ has been studied as function of pressure up to 6.5 GPa at different temperatures by neutron powder diffraction. At ambient pressure the crystal structure of this compound has cubic symmetry (space group Pm3 ̅ m ) and is found to become antiferromagnetic with T N close to 250 K. Applied pressure induces a gradual transition from the antiferromagnetic into a ferromagnetic state through a mixed magnetic state. The transition is not accompanied by obvious changes in the macroscopic crystal symmetry. It is suggested that the magnetic ground state strongly depends on the unit cell volume and that the transition is associated with a spin state crossover of the cobalt ions whereas the formal Co 3+ /Co 4+ ratio is less important than expected following the double exchange scenario for the appearance of ferromagnetism.