Neutron-diffraction study of magnetic ordering in the pyrochlore series R2Mo2O7 (R=Nd,Tb,Y).

Abstract

${\mathrm{Nd}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$, ${\mathrm{Y}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$, and ${\mathrm{Tb}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$ are all ordered crystalline oxides with the cubic pyrochlore structure. At 53 K, ${\mathrm{Nd}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$ is long-range ordered with a ferrimagnetic structure. A transition temperature of 95 K is determined from small-angle-neutron-scattering (SANS) data. Antiferromagnetic structures are proposed below 20 K that are consistent with previous magnetization results. No magnetic scattering from ${\mathrm{Y}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$ was observed at low temperature, which is believed to be a result of the small moment on ${\mathrm{Mo}}^{4+}$. For ${\mathrm{Tb}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$ strong diffuse scattering from the large ${\mathrm{Tb}}^{3+}$ moments is observed below 25 K. Fourier analysis of this data yields an approximation to the radial correlation function, which gives information concerning spin-spin correlations for the first four coordination shells. The first-, third-, and fourth-neighbor correlations are ferromagnetic, but the second-neighbor correlation is strongly antiferromagnetic. SANS data show magnetic scattering only at large Q, reenforcing the view that antiferromagnetic interactions dominate. The Q-integrated magnetic SANS shows a temperature dependence similar to that for the correlation functions. The long-range order in ${\mathrm{Nd}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$ and lack of long-range order in ${\mathrm{Y}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Tb}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$ can be understood, to a certain extent, in terms of Landau-theory results for this highly frustrated lattice.