Abstract
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.
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