Instability of a quasi-magnetic structure in a layer compound, Cs 0.5 Rb 0.5 VF 4

Magnetic properties of Cs0.5Rb0.5VF4 are studied by neutron diffraction. The layer compound shows an antiferromagnetic phase transition at about 34 K (TN). Its normal antiferromagnetic unit cell is 2ap×2bp×2cp, where ap×ap×cp is an ideal unit cell of TlAlF4-type. It is found that a quasi-antiferromagnetic phase having a unit cell of 2ap×2ap×cp is easily induced under a suitable applied field after doing a field cooling of 3 T from 50 K to 2.5 K. The quasi-magnetic structure shows a relaxation dependent on the field strength and the temperature. In the relaxation process, it is found that there is a coexistence of the normal and quasi-antiferromagnetic phases and that the two magnetic phases show a good compensation for a population ratio between two phases made in a used crystal. A metastable magnetic phase transition between the normal and quasi-magnetic phases occurs at about 13 K (TM). It is also found that only the quasi-magnetic phase shows a critical behavior in the vicinity of the antiferromagnetic phase transition (TN). It is interpreted that the residual normal antiferromagnetic phases below TM are made by local structural modulations, which mainly result from an inhomogeneous substitution of Cs with Rb ions in the crystal.