Emergent High-Entropy Phases in Geometrically Frustrated Pyrochlore Magnets

Frustrated magnets provide a platform for exploring exotic phases beyond conventional ordering, with potential relevance to functional materials and information technologies. In this work, we use Monte Carlo simulations to map the thermodynamic phase diagram of pyrochlore iridates R2Ir2O7 (R = Dy, Ho) with three stable magnetic ground-state stable phases: frustrated spin-ice 2 in 2 out (2I2O) phase, frustrated fragmented 3 in 1 out/1 in 3 out (3I1O/1I3O) phase, and antiferromagnetic all in all out (AIAO) phase without frustration. We discovered two additional emergent metastable phases at finite temperatures, located between the boundaries separating those stable phases. These metastable phases exhibit high magnetic susceptibility and high entropy without long-range order. Their stabilization arises from entropic minimization of the free energy, where the entropy dominates energetic competition near phase boundaries at finite temperatures. Our results demonstrate a platform to engineer highly susceptible and degenerated states through frustration and thermal activation, offering a foundation for entropy-based design of metastable phases in correlated systems.