We are pleased to announce the winner of the Best Poster Award, sponsored by Entropy for 45th Conference of the Middle European Cooperation in Statistical Physics (MECO45), held on 14–16 September 2020.

“Phase transitions in three-dimensional random anisotropy Heisenberg model: two case studies” by Maxym Dudka, Yurij Holovatch and Juan J. Ruiz-Lorenzo

The poster created by Maxym Dudka, Yurij Holovatch (Institute for Condensed Matter Physics, Lviv, Ukraine) and Juan J. Ruiz-Lorenzo (Extremadura University, Badajoz, Spain) concerns the study of ordering in random anisotropy magnets. Such magnets constitute a wide class of magnetic systems, with structural disorder described by a random anisotropy model that was introduced in the early 1970s by Harris, Plischke and Zuckermann. Despite extensive studies, the problem of the nature of a low-temperature phase of random anisotropy systems remains a very intriguing issue. While, for large values of local anisotropy strength, the majority of studies predict spin-glass, there is much discussion about ordering for small and moderate values of such strength. It appears that the answer to this question depends also on the local axis distribution. Analytical and numerical approaches predict an absence of the ferromagnetic order for uniform continuous distribution while preserving long-range order for discrete distribution, except in the numerical study, where second-order phase transition to long-range order was claimed for both kinds of distributions with the same correlation length critical exponent. We study phase transitions in the three-dimensional random anisotropy model with three-component order parameter by means of extensive Monte Carlo simulations, using the parallel tempering method for two different random anisotropy axis distributions and two different values of local anisotropy strength for each disorder distribution case. We observe clear signatures of the second order phase transition (paramagnetic–ferromagnetic) for both distributions, although extracted critical exponents support different collective behaviors for different random anisotropy axis distributions. Moreover, while we find strong evidence of universality for the case of the discrete disorder distribution, results for the uniform case do not present universality, showing the dependence of the critical exponents on the disorder strength, as well as on the lattice size, therefore questioning the nature of low-temperature state in the thermodynamic limit. The work was initiated within the FP7-PEOPLE, IRSES project SPIDER. Maxym Dudka acknowledges the financial support of the Polish National Agency for Academic Exchange (NAWA) on the final stage of the project.