Interplay of Spin-Orbit Interaction and Electron Correlation on the Van Vleck Susceptibility in Transition Metal Compounds

We have studied the effects of electron correlation on Van Vleck susceptibility (chi vv) in transition metal compounds. A typical crossover behavior is found for the correlation effect on chi vv as sweeping spin orbit interaction, lambda. For a small lambda, orbital fluctuation plays a dominant role in the correlation enhancement of chi vv; however, the enhancement rate is rather small. In contrast, for an intermediate lambda, chi vv shows a substantial increase, accompanied by the development of spin fluctuation. We will discuss the behavior of chi vv in association with the results of Knight-shift experiments on Sr2RuO4 and an anomalously large magnetic susceptibility observed for 5d Ir compounds.