Neutron Larmor diffraction investigation of the rare earth pyrochlores $R_2$Ti$_2$O$_7$ ($R$ = Tb, Dy, Ho)

TL;DR

This study uses neutron Larmor diffraction to precisely measure lattice parameters of rare earth pyrochlores, revealing insights into magnetic monopoles, magnetoelastic interactions, and phonon effects across different temperatures.

Contribution

It provides high-precision lattice measurements of $R_2$Ti$_2$O$_7$ compounds, clarifies the behavior of magnetic monopoles, and explores magnetoelastic and phonon interactions in these materials.

Findings

No renormalization of monopole charge due to lattice contraction.

No evidence of negative thermal expansion in Tb$_2$Ti$_2$O$_7$.

Thermal expansion anomalies linked to phonon and crystal field interactions.

Abstract

In this work we present a neutron Larmor diffraction study of the rare earth pyrochlores $R_2$Ti$_2$O$_7$, with $R$ = Tb, Dy, Ho. We measured the temperature dependence of the lattice parameter with precision $10^{-5}$, between 0.5 K and 300 K in each of the three compounds. The lattice parameter of the spin ices Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$ enters into the derivation of the charge of the emergent magnetic monopole excitations suggested to exist in these materials. We found that throughout the range of applicability of the theory of emergent monopoles in the spin ices there will be no renormalization of the monopole charge due to lattice contraction. In Tb$_2$Ti$_2$O$_7$ strong magnetoelastic interactions have been reported. We found no sign of the previously reported negative thermal expansion, but did observe anomalies in the thermal expansion that can be correlated with previously observed interactions between phonon and crystal field excitations. Other features in the thermal expansion of all three compounds can be related to previously observed anomalies the elastic constants, and explained by the phonon band structure of the rare earth titanates. The temperature dependence of the lattice strain in all three compounds can be correlated with the thermal population of excited crystal field levels.