Crystal Shape-Dependent Magnetic Susceptibility and Curie Law Crossover in the Spin Ices Dy2Ti2O7 and Ho2Ti2O7

TL;DR

This study measures the magnetic susceptibility of Dy2Ti2O7 and Ho2Ti2O7 spin ices across a wide temperature range, comparing experimental data with theoretical models to understand magnetic behavior and monopole dynamics.

Contribution

It provides accurate susceptibility measurements using spherical crystals, resolving previous discrepancies and highlighting the impact of crystal shape on magnetic studies.

Findings

Excellent agreement with theory above 10 K

Systematic deviations below 10 K

Crystal shape significantly affects susceptibility measurements

Abstract

We present an experimental determination of the isothermal magnetic susceptibility of the spin ice materials Dy2Ti2O7 and Ho2Ti2O7 in the temperature range 1.8-300 K. The use of spherical crystals has allowed the accurate correction for demagnetizing fields and allowed the true bulk isothermal susceptibility X_T(T) to be estimated. This has been compared to a theoretical expression based on a Husimi tree approximation to the spin ice model. Agreement between experiment and theory is excellent at T > 10 K, but systematic deviations occur below that temperature. Our results largely resolve an apparent disagreement between neutron scattering and bulk measurements that has been previously noted. They also show that the use of non-spherical crystals in magnetization studies of spin ice may introduce very significant systematic errors, although we note some interesting - and possibly new - systematics concerning the demagnetizing factor in cuboidal samples. Finally, our results show how experimental susceptibility measurements on spin ices may be used to extract the characteristic energy scale of the system and the corresponding chemical potential for emergent magnetic monopoles.