Van Vleck Paramagnets in High Magnetic Fields

As a result of the crystal-field splitting of the lowest multiplet, the electronic ground state of the even 4f-rare-earth ions in many compounds is a nonmagnetic singlet, the excited states being raised by more than 10 K or even 100 K in energy by the crystalline field. Because of the very strong hyperfine coupling between electrons and nuclei in Van Vleck compounds, there is a profound influence of the magnetic field on both the electronic and nuclear low-lying energy level schemes. Besides inducing a large hyperfine enhanced magnetic moment by admixing the excited electronic states into the singlet ground state (SGS) wave function, an external magnetic field also gives rise to an additional electric field gradient which results from the 4f-charge cloud of the rare-earth ion, no matter what the crystal symmetry is. Even in cubic symmetry, a field-induced nuclear quadrupole splitting will be superimposed on the hyperfine pseudo-Zeeman splitting. Experimental evidence for this and some other effects may come from accurate measurements of the hyperfine specific heat of Van Vleck paramagnets in sufficiently high magnetic fields.