Low-Temperature Phase Transitions in Spin Ice Thin Films

Pyrochlore titanates such as Dy2Ti2O7 (DTO) have become paradigmatic examples of spin ice compounds in which geometrical frustration inhibits magnetic ordering and leads to exotic physics. In bulk DTO, magnetic order fails to develop even at temperatures four times below the Curie-Weiss temperature of ~ 2K and upon further cooling the system falls out of equilibrium. We have developed high quality thin films of DTO on non-magnetic Y2Ti2O7 substrates. Sub-kelvin ac-susceptibility and specific heat measurements on a 30nm film of DTO indicate that in contrast to bulk DTO, thin films undergo an unusual phase transition ~0.8K and remain in equilibrium down to 0.35K. We propose to employ muons as sensitive magnetometers with a wide dynamical range to complement susceptibility and specific heat data and clarify the nature of the observed phase transition and low-temperature magnetic relaxation.