Inhibition of Nd magnetic order in NdFe1−xCoxO3 (x⩽0.5)

The low-temperature specific heat of the NdFe1−xCoxO3 (x=0, 0.03, 0.1, 0.25, and 0.5) solid solutions has been measured in order to study the magnetic ordering of the Nd sublattice as a function of the Co content. The experimental results show the inhibition of the Nd magnetic ordering for x⩾0.1. Bertaut's theory of spin configurations for ionic structures is used to propose a simple mean-field model which is able to explain the main features of the low-temperature heat capacity of NdFe1−xCoxO3 for x=0, 0.03 and 0.1 in terms of the ratio between the NdNd and FeNd interactions. In order to properly explain the specific heat curves we find satisfactory to include a distribution of internal fields acting on the Nd3+ ions. Indeed, low-spin Co ions in NdFe1−xCoxO3 are equivalent to magnetic vacancies and the Nd atoms are polarized additionally by an internal field caused by the FeNd isotropic interaction between the non compensated Fe moments and the Nd.