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The magnetocaloric effect (MCE) of half-metallic CrO2 particles has been studied with respect to particle size on the nanometre scale. Results from superconducting quantum interference device (SQUID) measurements indicate that acicular CrO2 particles with a length of 400 nm yield a large magnetic entropy change ΔSM of 5.1 J kg−1 K−1 at an applied field of 15 kOe and an adiabatic temperature change ΔTad of 2.0 K near the Curie temperature (∼390 K). These results are among the highest for magnetic oxides, and are comparable to that for pure Gd. However, smaller CrO2 particles with a length of 260 nm only exhibit ΔSM = 2.25 J kg−1 K−1 and ΔTad = 0.95 K. The difference in MCE between these two sizes of CrO2 particles results primarily from disordered spins on the surface of the particles. In addition, measurements and calculations of the specific heat capacity for the CrO2 particles are presented. These results indicate that the total specific heat capacity is dominated by the magnetic specific heat contribution. Therefore, we believe that these CrO2 nanoparticles may hold future promise in the development of new magnetic refrigerants.

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A simple method for the synthesis of carbon-coated Ni/SiO 2 core/shell nanocomposites is reported. The Ni nanoparticles were coated with silica layers via a combined procedure of sol-gel fabrication and hydrogen reduction prior to carbon coating via acetylene decomposition at an appropriate temperature. It was found that the anti-acid ability of the Ni/SiO 2 composites was greatly enhanced after carbon coating. The results of magnetization measurement show that the real part (μ′) of complex permeability of the as-obtained sample is almost independent of frequency, and the imaginary part (μ″) stays small up to a frequency of 1 GHz. The encapsulation of Ni particles with SiO 2 results in the rise of Ni nanoparticles resistivity. The outcome is the reduction in effect of eddy current at high frequency, making the real part μ′ almost constant and the imaginary part μ″ very small. Thus, this simple method may be effective for preparing composites of soft magnetic properties, especially in the high-frequency range.carbon, nickel, core/shell structure, nanocomposites, magnetic properties, stability, soft magnetic properties

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