The magnetization and electron spin resonance (ESR) in nanocrystals of Haldane-chain antiferromagnet Gd2BaNiO5 have been investigated. It is revealed that a reduction in crystal size results in an enhancement of magnetization due to a large number of paramagnetic Gd3+ and Ni2+ ions forming on the surfaces of nanocrystals. The smallest nanoparticles with an average size of 45 nm behave like a paramagnet, as evidenced by our ESR data. Upon application of an external magnetic field, the weakly coupled spins can be well aligned along the direction of the magnetic field, giving rise to a giant entropy change of −ΔSm = 36 J⋅kg−1⋅K−1 at 2 K in a field range of 0–7 T. This value is larger than those of most rare-earth-based compounds reported. The large value of −ΔSm, together with the absence of thermal and field hysteresis, makes Gd2BaNiO5 nanocrystals very promising candidates for low-temperature magnetic refrigeration.

中文翻译：

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霍尔丹链化合物 Gd2BaNiO5 纳米晶体的磁化、ESR 和巨磁热效应

研究了霍尔丹链反铁磁体 Gd2BaNiO5 纳米晶体的磁化强度和电子自旋共振 (ESR)。结果表明，由于在纳米晶体表面形成大量顺磁性 Gd3+ 和 Ni2+ 离子，晶体尺寸的减小导致磁化强度的增强。正如我们的 ESR 数据所证明的那样，平均尺寸为 45 nm 的最小纳米粒子表现得像顺磁体。在施加外部磁场时，弱耦合自旋可以沿着磁场方向很好地对齐，从而在 2 K 下产生 -ΔSm = 36 J⋅kg-1⋅K-1 的巨大熵变场范围为 0-7 T。该值大于报道的大多数稀土基化合物的值。-ΔSm 的大值，加上没有热滞后和场滞后，