Coexistence of direct and inverse magnetocaloric effects at low fields via exchange–Zeeman competition in La 2−x Gd x FeMnO 6

The coexistence of conventional and inverse magnetocaloric effects (MCEs) at low magnetic fields is highly desirable for broadening the operational temperature window and facilitates self-regulated magnetic refrigeration. Beyond practical implications, the coexistence of MCE provides insights to elucidate the magnetic ground state. However, in most perovskites reported to exhibit such MCE switching, the response is either weak at low fields, suffers from significant thermal hysteresis, or occurs only at high fields, owing to a comparatively strong internal exchange field. Here, we report that with suitable Gd substitution in A-site of La 2 FeMnO 6 effectively tunes the competing 3d–3d, 3d–4f and 4f –4f exchange interactions, realizing a robust low-field concurrence of conventional and inverse MCE. The field-tunable entropy compensation point further suggests the potential for intrinsic thermal regulation. Combined DC magnetization, specific-heat, electron-spin-resonance, and first-principles calculations provide a consistent picture in which the 3d–4f exchange interaction generates an internal exchange field that competes with the external Zeeman energy, resulting in the observed entropy-sign switching. These results identify A-site substitution as an effective strategy for engineering the low-field coexistence of conventional and inverse MCE in rare-earth double perovskites.