Magnetic susceptibility of multiferroics and chemical ordering

Magnetic susceptibility of two types of perovskite compounds A(Fe0.5M0.5)O3 A=Ba,Sr with M=Nb and Sb differing in the degree of chemical ordering was studied between 2 and 380 K. In the low temperature region, these compounds exhibit antiferromagnetic and spin-glass transition at T = 16−30 K. For the ordered compounds (M = Sb) the inverse susceptibility depends on T linearly up to the highest temperatures used. This enables the determination of the Weiss temperature θ and effective moment of the Fe3+ ion expressed by a number of Bohr magnetons, peff. In contrast to it, the compounds with M = Nb are characterized by a higher degree of the glassines and by a concave character of the inverse susceptibility vs T dependence in the high temperature region. In these cases, the values of peff and θ could be estimated only very roughly by extrapolation to higher temperatures. The observed behavior was qualitatively explained from the Curie-Weiss law taking into account a distribution of the values θ, which reflects the presence of different exchange interactions between the Fe3+ ions. The disordered samples (M = Nb) exhibit a roughly two times larger |θ|/TN ratio values as compared to the ordered (M = Sb) ones.