Magnetic-susceptibility and specific-heat studies of spin-glass-like ordering in the pyrochlore compounds R2Mo2O7 (R=Y, Sm, or Gd)

Abstract

Magnetic-susceptibility and specific-heat measurements have been performed on the cubic pyrochlore compounds ${\mathit{R}}_2$ ${\mathrm{Mo}}_2$ ${\mathrm{O}}_7$ (R=Y, Sm, or Gd). These compounds exhibit magnetic transitions at 18, 68, and 55 K, respectively, indicated by the magnetic-susceptibility data. Broad magnetic specific-heat anomalies, at the corresponding transition temperatures, suggest the absence of long-range magnetic ordering. The behavior of magnetic specific heat and magnetic entropy indicates that magnetic transitions in these compounds are due to spin-glass-like ordering of ${\mathrm{Mo}}^{4+}$ ions. The rare-earth ions in ${\mathrm{Gd}}_2$ ${\mathrm{Mo}}_2$ ${\mathrm{O}}_7$ and ${\mathrm{Sm}}_2$ ${\mathrm{Mo}}_2$ ${\mathrm{O}}_7$ appear to remain in a paramagnetic state down to 1.5 K and give rise to Schottky-like specific-heat anomalies at low temperatures. A model is presented to describe these anomalies quantitatively.