Anomalous out-of-equilibrium dynamics in the spin-ice material Dy 2 Ti 2 O 7 under moderate magnetic fields

GURUCIAGA, P C; PILI, L; BOYERAS, S; SLOBINSKY, D; GRIGERA, S A; BORZI, R A

JOURNAL OF PHYSICS CONDENSED MATTER

IOP PUBLISHING LTD

Año: 2020 vol. 32 p. 1 - 10

0953-8984

We study experimentally and numerically the dynamics of the spin ice material Dy2 Ti2O7 inthe low temperature (T ) and moderate magnetic field (B) regime (T ∈ [0.1, 1.7] K,B ∈ [0, 0.3] T). Our objective is to understand the main physics shaping the out-of-equilibriummagnetisation vs temperature curves in two different regimes. Very far from equilibrium,turning on the magnetic field after having cooled the system in zero field (ZFC) can increasethe concentration of magnetic monopoles (localised thermal excitations present in thesesystems); this accelerates the dynamics. Similarly to electrolytes, this occurs throughdissociation of bound monopole pairs. However, for spin ices the polarisation of the vacuumout of which the monopole pairs are created is a key factor shaping the magnetisation curves,with no analog. We observe a threshold field near 0.2 T for this fast dynamics to take place,linked to the maximum magnetic force between the attracting pairs. Surprisingly, within aregime of low temperatures and moderate fields, an extended Ohm?s law can be used todescribe the ZFC magnetisation curve obtained with the dipolar spin-ice model. However, inreal samples the acceleration of the dynamics appears even sharper than in simulations,possibly due to the presence of avalanches. On the other hand, the effect of the field nearerequilibrium can be just the opposite to that at very low temperatures. Single crystals, as notedbefore for powders, abandon equilibrium at a blocking temperature TB which increases withfield. Curiously, this behaviour is present in numerical simulations even within thenearest-neighbours interactions model. Simulations and experiments show that the increasingtrend in TB is stronger for B[100]. This suggests that the field plays a part in the dynamicalarrest through monopole suppression, which is quite manifest for this field orientation.