Anomalous out-of-equilibrium dynamics in the spin-ice material Dy$_2$Ti$_2$O$_7$ 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). One of our objectives is to understand the main physics shaping the out-of-equilibrium magnetisation vs. temperature curves. We find that single crystals, as noted before for powdered samples, tend to abandon equilibrium at a blocking temperature T B which increases with applied field. Curiously, this behaviour is present even within the nearest-neighbours interactions model. Simulations and ac-susceptibility experiments show that the increasing trend in T B is stronger for fields applied along [100]. This suggests that the field plays a part in the dynamical arrest through the suppression of magnetic monopoles (localised thermal excitations present in these spin systems), which is quite manifest for this field orientation. On the other hand, very far from equilibrium the effect of B can be the inverse one: turning on the magnetic field after having cooled the system in zero field can increase the monopole concentration through dissociation of bound monopole pairs, thus accelerating the dynamics. The process is similar to that observed in electrolytes. However, for spin ices the polarisation state of the vacuum out of which the monopole pairs are created is a key factor shaping the magnetisation curves that has no analog in the electric case. We observe a threshold field near 0.2 T for this fast dynamics to take place; it is linked to the maximum magnetic force possible between the attracting pair. Surprisingly, within a regime of low temperatures and moderate fields, an extended Ohm's law can be used to describe the zero-field-cooled magnetisation curve obtained