Structural and Magnetic Characterization of RCrO4 oxides (R = Nd, Er, Tm)

The crystal and magnetic structure of RCrO4 oxides (R=Nd, Er and Tm) has been studied by powder neutron diffraction. These compounds crystallize with the zircon-type structure, showing tetragonal symmetry, space group I41/amd. In the case of NdCrO4, magnetic susceptibility measurements reveal the existence of an antiferromagnetic ordering in which both Cr5+ and Nd3+ sublattices are involved. This ordering has been explained on the basis of a propagation vector k = 0 and a collinear structure, described by the symmetry mode Ax, the ordered magnetic moments being 0.62 and 0.66 mB at 2K for Nd3+ and Cr5+, respectively. Magnetic susceptibility and magnetization measurements reveal that both ErCrO4 and TmCrO4 behave as ferromagnetic compounds with a Curie temperature of 15 and 18 K, respectively. Rietveld refinement of the neutron diffraction data for ErCrO4 yields a collinear magnetic structure described with an Fx mode. In the case of the TmCrO4 oxide, the ferromagnetic sublattices of
Tm3+ and Cr5+ are aligned antiparallel in the a-b plane, while along the c-axis the magnetic moments point to the same direction. In both compounds, the rather small values obtained for the Er3+ and Tm3+ ordered moments compared with the theoretical ones have been attributed to crystal field effects. The differences in the ferromagnetic structure of these compounds have been explained as the result of the higher rare-earth anisotropy of Tm3+ when compared with Er3+, for which no magnetic component is present along the c-direction.