Exploration of the magnetocaloric effect in SrRuO3 and BiFeO3/SrRuO3 films via vacancy and interface design

Abstract

Harnessing additional degrees of freedom in magnetic oxides offers new strategies for energy-efficient cooling. Here, we report the magnetocaloric effect (MCE) in Ru-deficient SrRuO₃ thin films and BiFeO₃/SrRuO₃ heterostructures. Controlled Ru vacancies introduce dual ferromagnetic transitions (T_C1 and T_C2), giving rise to multiple entropy-change peaks in ΔS(T). Compared to bulk SrRuO₃ (ΔS_max = 77 mJ kg⁻¹ K at T_C = 160 K), our films exhibit tunable Curie temperatures (71–138 K), broadened transitions, and enhanced entropy changes up to 101 mJ kg⁻¹ K⁻¹ at 1 kOe. X-ray photoelectron spectroscopy indicates Ru⁴⁺ valence with vacancies, while density functional theory reveals that Ru vacancies induce gap states and enhance magnetization at ∼12.5%, strengthening the MCE and stabilizing a local AFM environment near 25% vacancy concentration. In BiFeO₃/SrRuO₃ heterostructures, interfacial coupling preserves the intrinsic SRO response while introducing an inverse MCE near the BiFeO₃ Néel temperature (T_N ≈ 50 K) with ΔS_max enhancing another 50% to 154 mJ kg⁻¹ K⁻¹ and coercivity enhanced to ∼1.7 kOe. These results establish vacancy engineering and interfacial design as powerful approaches for tunable, low-field MCE in oxide heterostructures.