Magnetocaloric Study of Spin Relaxation in `Frozen' Dipolar Spin Ice Dy2Ti2O7

TL;DR

This study investigates the magnetocaloric effect in Dy2Ti2O7 spin ice, revealing slow spin relaxation, metastable states at very low temperatures, and the impact of magnetic dilution on relaxation dynamics, providing insights into spin ice physics.

Contribution

It introduces the magnetocaloric effect as a tool to study slow spin relaxation and metastable states in dipolar spin ice Dy2Ti2O7 at millikelvin temperatures.

Findings

Irreversible temperature rise after demagnetization at T < 0.4 K

Relaxation time increases rapidly down to 0.3 K

Magnetic dilution prolongs dynamical response at millikelvin temperatures

Abstract

The magnetocaloric effect of polycrystalline samples of pure and Y-doped dipolar spin ice Dy2Ti2O7 was investigated at temperatures from nominally 0.3 K to 6 K and in magnetic fields of up to 2 T. As well as being of intrinsic interest, it is proposed that the magnetocaloric effect may be used as an appropriate tool for the qualitative study of slow relaxation processes in the spin ice regime. In the high temperature regime the temperature change on adiabatic demagnetization was found to be consistent with previously published entropy versus temperature curves. At low temperatures (T < 0.4 K) cooling by adiabatic demagnetization was followed by an irreversible rise in temperature that persisted after the removal of the applied field. The relaxation time derived from this temperature rise was found to increase rapidly down to 0.3 K. The data near to 0.3 K indicated a transition into a metastable state with much slower relaxation, supporting recent neutron scattering results. In addition, magnetic dilution of 50 % concentration was found to significantly prolong the dynamical response in the milikelvin temperature range, in contrast with results reported for higher temperatures at which the spin correlations are suppressed. These observations are discussed in terms of defects and loop correlations in the spin ice state.