Anomalous out-of-equilibrium dynamics in the spin-ice material Dy₂Ti₂O₇ under moderate magnetic fields

Abstract

We study experimentally and numerically the dynamics of the spin ice material Dy 2 Ti 2 O 7 in the 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-equilibrium magnetisation 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 increase the concentration of magnetic monopoles (localised thermal excitations present in these systems); this accelerates the dynamics. Similarly to electrolytes, this occurs through dissociation of bound monopole pairs. However, for spin ices the polarisation of the vacuum out 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 a regime of low temperatures and moderate fields, an extended Ohm's law can be used to describe the ZFC magnetisation curve obtained with the dipolar spinice model. However, in real 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 nearer equilibrium can be just the opposite to that at very low temperatures. Single crystals, as noted before for powders, abandon equilibrium at a blocking temperature T B which increases with field. Curiously, this behaviour is present in numerical simulations even within the nearest-neighbours interactions model. Simulations and experiments show that the increasing trend in T B is stronger

Topological metamagnetism: Thermodynamics and dynamics of the transition in spin ice under uniaxial compression

“…The dumbbell approximation has successfully explained several dynamical [14,17,27,31,32] and thermodynamical facts [3,14,16,32,33]. Since the Kasteleyn transition takes place in the limit of no monopolar excitations, in the absence of demagnetization effects it should be identical in both the NN and dumbbell models.…”

“…We begin by analyzing the unstrained samples. The isothermal magnetization M for both compounds was studied at temperatures above the dynamical freezing regimes (T > 0.7 K) [23][24][25][26][27]. Figure 1 theoretical value [28].…”

Dynamics and thermodynamics of a topological transition in spin ice materials under strain

“…Additionally, by polarising the magnetic moments, the static magnetic field changes the background in which the magnetic monopoles move. All this slows down the magnetic response of the system to an external field, reducing the blocking temperature [26]. The peak in χ DC observed in Fig.…”

“…The dumbbell approximation has successfully explained several dynamical [11,19,[25][26][27] and thermodynamical facts [9-11, 27, 28]. Since the Kasteleyn transition takes place in the limit of no monopolar excitations, it should be identical in both the NN and the dumbbell model.…”

“…The change of dynamic regime marked by the maximum in χ (T, B) is rooted in several facts. The depletion of magnetic charges is particularly important for B||[001], and quite abrupt near the transition [26]. This lowers the number of spin flips at low temperatures, and reduces the screening [11].…”