Possible New Approach for Exchange-Spring Magnet and Continuous Spin State Transition in Half-Doped High-Entropy Cobaltite Perovskite

Here, we report the exotic magnetic behavior of Nd0.5Sr0.5CoO3 and high entropy (Pr1/6Nd1/6Eu1/6Ca1/6Sr1/6Ba1/6)CoO3, (La1/6Sm1/6Gd1/6Ca1/6Sr1/6Ba1/6)CoO3, and (La1/6Nd1/6Dy1/6Ca1/6Sr1/6Ba1/6)CoO3 cobaltites prepared under similar conditions. The cationic composition and chemical homogeneity were confirmed by energy dispersive X-ray analysis and elemental mapping. All the compositions have the same ⟨rA⟩ = 1.236 Å and Co3+/Co4+ ratio. Nevertheless, they exhibit a large variation in TC. A large drop in TC from 220 K for Nd0.5Sr0.5CoO3 to 159 K in (Pr1/6Nd1/6Eu1/6Ca1/6Sr1/6Ba1/6)CoO3 is related to the high entropy effect, whereas the variation in TC for high entropy phases is related to the size disorder parameter σ2. The competing ferromagnetic (FM) Co3+–O–Co4+ and antiferromagnetic Co3+–O–Co3+ and Co4+–O–Co4+ interactions result in phase separation. The FM domains are likely fragmented for higher σ2 compositions due to large local lattice distortion hampering the FM exchange interaction. The kinked-like hysteresis loop may possibly be related to the magnetic exchange-spring effect. The exchange-spring-like magnetic behavior in the present cobaltite system can be attributed to the local lattice deformation, coexistence of the FM and antiferromagnetic interface, as well as the possibility of differential magnetic anisotropy at the cobalt center associated with different spin states. This study unravels the possible new approach for exchange-spring magnets. A gradual spin state transition above TC is observed in high entropy phases, which are due to randomly fluctuating crystal field potential in the panorama of multicationic surroundings.