Field-induced magnetic ordering in the Haldane system PbNi2V2O8 -: art. no. 104402

The Haldane system PbNi2V2O8 was investigated by temperature-dependent magnetization M(T) measurements at fields higher than H-c; H-c is the critical field required to close the Haldane gap. It has been revealed that for H > H-c, M(T) exhibits a cusplike minimum at T-min, below which it increases with a decrease in T following a convex curve. These results were observed for both H parallel to c and H perpendicular to c, with the c axis parallel to the chain. These data indicate the occurrence of field-induced magnetic ordering around T-min. The phase boundaries for H parallel to c and H perpendicular to c do not cross each other. This result is consistent with the theoretical calculation for a negative single-ion anisotropy D.