Induced magnetic form factor of Sm in mixed-valence compounds

Abstract

The induced moment form factor of Sm has been measured in a number of compounds using the polarized neutron technique. For integral-valence compounds, experimental and theoretical evidence is presented showing that Sm2+ and Sm3+ usually have large differences in their form factors. For the mixed-valence systems, SmS at high pressure and Sm0.76Y 0.24S, the observed form factor is nearly identical to that of the 2+ ion. If we adopt the viewpoint that the Sm ions are fluctuating between 4f 5 and 4f6 configurations, then our measurements imply that the induced moment from the 4f5 configuration is dominated by the Van Vleck contribution to the susceptibility arising from an admixture of states with different J values or that the 4f5 configuration makes a negligible contribution to the form factor. The suppression of the Curie (1/T) terms is consistent with low-temperature susceptibility measurements on these systems. The implications of these observations are examined within the framework of a simple valence-fluctuation model.