Magnetic properties and magnetic ordering in the Rare Earth Molybdenum(IV) Pyrochlores: R2Mo2O7

Abstract

The 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 μ_B. 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, Θ_c, 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.