Non-ergodic Phase Transition in the Global Hysteresis of the Frustrated Magnet DyRu2Si2

Some frustrated magnets exhibit a huge hysteresis called "global hysteresis (GH)", where the magnetic plateaus appearing in the increasing field process are skipped in the decreasing field process from the high magnetic field state. In this paper, we focused on the frustrated magnet DyRu2Si2 and measured magnetization relaxations from two plateau states inside the GH loop, the phases III and IV, and investigated the phase transitions into them. As a result of the relaxation measurements, no relaxation is observed in the phase III, whereas long-time relaxations of more than 105 sec are observed at the phase IV plateau. Moreover, a Mpemba-effect-like relaxation phenomenon where the relaxation from an initial state prepared in the zero-field-cooled condition overtakes that from an initial state prepared in the field-cooled condition is observed. These results indicate that the phase IV is the non-ergodic state with a complex free-energy landscape with multiple local minima, while the phase III has a simple free energy structure. Therefore, the III-IV phase transition is considered to be the ergodic to non-ergodic phase transition. Although this type of phase transition typically occurs in random glassy systems, the phase IV in DyRu2Si2 has a regular long-range ordered magnetic structure and yet exhibits non-ergodic properties, which is highly nontrivial. Our findings open the possibility of observing non-ergodic states in frustrated magnets with regular long-range orders.