Crystal-Field Effects and Anomalous Susceptibility of Antiferromagnets: Application to Ce—Group-V Compounds

Abstract

Crystal-field effects can drastically change the temperature variation of the susceptibility in antiferromagnets. The powder inverse susceptibility need no longer increase monotonically as temperature decreases below the Néel temperature, and can have a maximum and an additional minimum. We illustrate these effects for antiferromagnetic compounds containing ${\mathrm{Ce}}^{3+}$ in an octahedral crystal field. The anomalous behavior arises because there is a range of exchange field for which the ${\Gamma }_7$ ground-state doublet splitting decreases with increasing field until the doublet levels cross. The maximum in the powder inverse susceptibility can occur because the decrease in energy splitting leads to a rapid increase of the transverse susceptibility with decreasing temperature, and this outweighs the decrease in the longitudinal susceptibility. Our model leads to the expectation of approximately normal susceptibility behavior at low and high values of the ratio of exchange to crystal-field strength, i.e., where the doublet splitting increases with increasing exchange field. It shows anomalous behavior in the intermediate region where the splitting decreases with increasing exchange field. Since the exchange field increases with decreasing temperature, the ratio of the exchange field to crystal-field strength increases with decreasing temperature. For a certain range of exchange and crystal-field parameters, this can lead to a change from anomalous to normal energy level and susceptibility behavior with decreasing temperature. This return to normal behavior gives rise to an upturn (i. e., a second minimum) in the polycrystalline inverse susceptibility at low temperature. The behavior of the Ce—group-V compounds of NaCl structure covers the entire range of the ratio of crystal-field strength to exchange interaction. Corresponding to this variation, the susceptibility behavior is normal for the light compounds (CeP and CeAs), most anomalous for CeSb, and returns toward normal for CeBi.