Susceptibility and specific heat of insulating Magnéli phases VnO2n−1

Abstract

The magnetic susceptibilities and the specific heats of single crystals of the vanadium Magnéli phases VnO2n−1(3?n?8) have been measured at low temperatures. Magnetic and thermodynamic properties associated with the antiferromagnetic ordering in the insulating state are strongly affected by spin pairing of vanadium ions. The Néel temperature TN is lowered rather smoothly with increasing n, which corresponds to larger spin pairing effects. A large short range order effect was observed in V3O5, which has no spin pairing. The susceptibility and the specific heat show the behavior of a low dimensional spin system. For V4O7 and V6O11 the magnetic entropy changes up to TN are less than 20% of the value R1n(2S+1) which would be expected if spin pairing were absent; this indicates considerable spin singlet pairing. On the other hand, V7O13 does not display a metal-insulator transition and remains metallic down to 1.2 K. Some remarkable features were observed for V8O15 in the susceptibility and the specific heat. These results suggest that the transition at low temperature (6.1 K) is not an ordinary antiferromagnetic one but another type of transition.