Synthesis of antiferromagnetic gadolinium dihydride nanoparticles and its large cryogenic magneto-caloric effect

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Abstract
Summary form only given. Due to significant application in medical treatment, magnetic record and cryogenic magneto-refrigeration, a great amount of researches on ferromagnetic nanoparticles were executed [1]. With decrease of the scale of ferromagnetic material, the magnetic order and magnetization gradually decrease and such decrease is attributed to the increasing surface disorder or surface spin canting. However, for the anti-ferromagnetic nanoparticles, their magnetization will increase when compared with its corresponding bulk material because its large surface will induce more un-compensated spins to increase the magnetization of the nanoparticles. For example, when the size of anti-ferromagnetic CoRh 2 O 4 decreases to 16nm, the increased surface spins induced the increased magnetization [2]. For exploiting more anti-ferromagnetic nanoparticles with high magnetization, the Gd-based anti-ferromagnetic GdH 2 nanoparticles were prepared by arc-discharge technique. The GdH 2 nanoparticles show irregularly polygonal shape and have size distribution of 30-80nm with the average particle size of 50nm. In further, the GdH 2 nanoparticle has no core-shell structure and can be stably exist in the atmosphere (Figure1). At 5K, the GdH 2 nanoparticles show magnetization of 109 emu/g under applied field of 50 kOe . With the high magnetization and low anisotropy constant, the GdH 2 nanoparticles show large cryogenic entropy change (-ΔSM). The -ΔSM slowly increases with decreasing temperature from 80K to 30 K, and then rapidly increases from 30K to 5K. At 5K, the value of -ΔSM shows largest value of 12 .3J/( kg K) under the change of 50 kOe field (Figure 2). The large entropy change is attributed to large amount of un-compensated spins on the surface and weak anisotropy of the GdH 2 nanoparticles.

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