After the trail of a true ground state in spin ice

MARZIALI BERMUDEZ, MARIANO; GRIGERA, SANTIAGO ANDRÉS

La Plata

Workshop; Topology in Magnetism: Theory Meets Experiments; 2019

PSI; IFLySIB

Thermal equilibrium properties of spin-ice material Dy2Ti2O7, including specific heat, magnetization and spin correlations, could be successfully reproduced by a model featuring magnetic interactions up to third nearest neighbor and long-ranged dipolar forces. With the best-fit parameters, the model predicts an ordered ground state which breaks the cubic symmetry of the lattice [Borzi et al., Nature Communications 7:12592 (2016)]. In the presence of long-range magnetic order, neutron scattering experiments  were expected to yield sharp Bragg peaks. However, even after very long relaxation times, such ordered state could not be observed down to 300 mK. Instead a diffuse pattern was obtained, indicating no long-range order.In this work, we show that such an ordered state could be accessible from configurations that are compatible with the experimentally observed structure factor by walking the path backwards: starting from an ordered state, about 10⁴ monopole pairs need to be created and let annihilate in a 16³-cell lattice to reach spin configurations which reproduce the experimental neutron data. In addition, we found that experiment-compatible states already imply some degree of order, since the experimental structure factor cannot be reproduced simply by random two-in-two-out configurations. The small energy difference (~ 70 mK) between ground and the disordered states compatible with the experiment in comparison with the high energy cost of single monopole-pair excitations (~ 4K) and the large number of sequential excitations required may account for the system?s not reaching the true ground state.