Magnetic properties and magnetic ordering in the Rare Earth Molybdenum(IV) Pyrochlores: R2Mo2O7Journal Articles
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AbstractThe series of cubic pyrochlore structure compounds, R2Mo2O7 (R = Nd—Yb, Y; R ≠ Eu), were prepared as single phase materials by solid state reaction between R2O3 and MoO2 at 1400°C in a CO/CO2 = 1 buffer gas atmosphere. Lattice constants obtained from X‐ray powder data compare well with results from previous studies. Magnetic susceptibility and magnetization data were obtained for all samples between 300 K and 4.2 K (700 K for R = Gd) and a range of applied fields. For R = Nd, Sm, and Gd magnetic ordering is observed at 97 K, 93 K and 83 K respectively which is assigned to ferromagnetism on the Mo(IV) sublattice. The Mo(IV) moment in the ordered state is about 1 μB. At low temperatures, the Gd(III) and Mo(IV) moments are apparently coupled ferromagnetically in Gd2Mo2O7 yet the high temperature susceptibility data seem to indicate a ferrimagnetic (antiparallel) Gd(III)—Mo(IV) coupling. The low‐temperature magnetic properties of Nd2Mo2O7 and Sm2Mo2O7 are complex. For R = Tb—Yb, Y there is no firm evidence for magnetic order although deviations from the Curie‐Weiss Law occur below 20 K for R = Tb, Dy, Y, and Ho. For R = Er and Tm the Curie‐Weiss Law is obeyed to 4.2 K. Yb2Mo2O7 deviates strongly from the Curie‐Weiss Law but this can be assigned to crystal‐field effects on the Yb(III) sublattice and not to magnetic order. There is a remarkable variation in the sign of the Weiss constant, Θc, as R varies through the series from + 115 K (R = Nd) to −25 K (R = Yb) and −60 K (R = Y). This indicates a change in sign of the net exchange interaction in these compounds from ferromagnetic to antiferromagnetic across the rare‐earth series for the R2Mo2O7 phases. This change seems to correlate with the electrical properties (to be published elsewhere) which indicate that the ferromagnetic members (R = Nd, Sm, and Gd) are metals while all others (R = Tb → Yb, Y) are semiconductors.
