Effect of frozen spin on the magnetocaloric property of La0.7Ca0.3CoO3 polycrystalline and single crystal samples
Polycrystalline (PC) and single crystalline (SC) samples of La0.7Ca0.3CoO3 (LCCO) with the perovskite structure were synthesized by conventional solid-state reaction and the floating-zone growth method. We conducted isothermal magnetization measurements of the PC and SC samples at temperatures from 2.8 K to 140 K, and evaluated the magnetic entropy change (−ΔSM) under zero field cooling (ZFC) and field cooling (FC) to 2.8 K. An interesting result has been obtained, where the −ΔSM–T curves in the low temperature range show totally different features between the ZFC and FC cooling procedures. The −ΔSM shows a large inverse irreversibility value for the ZFC process, while the −ΔSM also shows a normal positive value, but one that is slightly larger at 2.8 K for the FC process for both samples. We also present the results of a comprehensive investigation of the magnetic properties of the LCCO system. Systematic measurements have been conducted on DC magnetization, AC susceptibility, and exchange-bias. These findings suggest that complex structural phases, including ferromagnetic and spin-glass/cluster-spin-glass (SG/CSG)) states and their transitions, exist in PC samples, while there is a much simpler magnetic phase regime in SC samples. It was also of interest to discover that the CSG induced a magnetic field memory effect and an exchange-bias-like effect.
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