EXTRAORDINARY ANISOTROPIC MAGNETIC-BEHAVIOR IN CERIUM AND ACTINIDE INTERMETALLICS

Both the cerium and light actinide monopnictides with NaCl-structure show anomalous strongly anisotropic 〈100〉 magnetic ordering with similar and unusual highly anisotropic linear magnetic structures. Both theoretically and experimentally it has been shown that the presence and strength of this 〈100〉 magnetic anisotropy is in contrast with the 〈111〉 easy direction for Ce3+ in an octahedral crystal field. There are two possible origins to be considered for such anisotropy effects. One is distortional effects on the single-ion crystal field in conjunction with magnetic ordering arising from isotropic exchange, i.e. the only anisotropic interaction present is single-ion. The other possibility is the affect of anisotropic exchange, i.e. macroscopic anisotropy via a two-ion anisotropic interaction. We discuss the ways of separating out and understanding these two contributions. The weight of evidence is that both effects are significant in the cerium monopnictides; where the first effect's being important follows from the extreme sensitivity of the crystal field to lattice displacement. On the other hand, for the actinide monopnictides, such as UAs and USb, the absence of any distortion within experimental accuracy indicates the probable dominance of anisotropic exchange effects.