Mixed-valence induced Griffiths phase and large magnetocaloric effect in Eu2B2O5 + δ system

• Griffiths phase (GP) induced by the formation of a mixed Eu 3+ /Eu 2+ valence state in a Eu 2 B 2 O 5 + δ system. • Giant magnetic entropy changes of up to 50.8 J kg −1 K −1 near the Curie temperature and 36.09 J kg −1 K −1 near the GP temperature (Δ H = 5 T). • Critical behavior analysis reveals the coexistence of long- and short-range orders in the Eu 2 B 2 O 5 + δ system. • The reasonable GP construction can produce an excellent MCE in cryogenic magnetic refrigeration materials. This study presents the Griffiths phase (GP) induced by the formation of a mixed Eu 3+ /Eu 2+ valence state in a low-temperature Eu 2 B 2 O 5 + δ system. The existence of a short-range ferromagnetically ordered GP is confirmed through modified Curie-Weiss and critical behavior analysis based on temperature dependence of heat capacity. Large magnetic entropy changes of up to 50.8J kg −1 K −1 near the Curie temperature T C and 36.09 J kg −1 K −1 near the GP temperature are achieved under a 5T magnetic field. The obtained nearest neighbor exchange energy J 1 reveals the weak coupling between spins. A Brillouin function analysis revealed free-spin behaviors of Eu 2+ near T C and constrained spin dynamics of Eu 2+ due to mixed-valence interactions within the GP regime. Spin polarization in the PM matrix stimulated by the applied magnetic field generates a large magnetocaloric effect (MCE), implying that reasonable GP construction can produce an excellent MCE in cryogenic magnetic refrigeration materials.