Hydrogenation and Interface Magnetic Properties of Fine Ni Particles Coated with Pd

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The hydrogenation characteristics and the interface magnetic propirties of ferromagnetic Ni fine particles coated with paramagnetic Pd were investigated. The average size of Ni Spheres is 0.7 mum in diameter, and five samples with different weight of Pd were prepared by a chemical reduction method.The measurement results are summarized as follws ;1) The change in a lattice constant of Pd Shells by hydrogenation depends on the thickness of the Pd shell, and the lattice constant of the Pd and a hydrogen concentration are smaller for the thinner shell. The p-x-T curves for Ni/Pd particles and bulk Pd show that the width of the plateau of Ni/Pd becomes narrpwer and the plateau pressure becomes higher than those of bulk Pd.2) The magnetic polarization of Pd is induced by an exchange enhacement of Ni. The Magnetization of Ni/Pd particles exhibits a broad maximum in the vicinity of 80 K,and considerably increases below 50K.This magentic polarization of Pd is suppressed by hydrogenation.3) The sample exhibit relatively large coercive force of about 130 Oe. The interface anisotropy between the Ni core and the Pd Shell is Suppressed by hydrogenation, which is independent of the thickness of Pd shell. The coercive force also decreases by hydrogenation, which implies that the coercive force is caused by the interfacek anisotropy.The narrowing of the plateau and the high plateau pressure indicate that the strain in the Pd shell is larger than in bulk Pd and impedes the hydrogenation. The magnetic polarization of Pd is induced at the interface and does not extend over the Pd shell in room temperature. The magnetic moment of the polarized Pd is estimated to be about 0.3 mug. The temperature variation of the polarized Pd is qualitatively explained by the Stoner theory that when strong exchange enhancement acts on Pd, the susceptibility of Pd is enhanced with retaining a shape of thermal variation of Pramagnetic Pd.