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

研究了顺磁钯包覆铁磁镍微粒的加氢特性和界面磁性能。采用化学还原法制备了5个不同Pd质量的Ni球，平均粒径为0.7 μ m。测量结果总结如下：1) Pd壳层加氢后晶格常数的变化取决于Pd壳层的厚度，且壳层越薄，Pd的晶格常数和氢浓度越小。Ni/Pd粒子和体体Pd的p-x-T曲线表明，Ni/Pd的平台宽度比体体Pd的平台宽度更窄，平台压力更高。2)Ni的交换增强诱导Pd的磁极化。Ni/Pd粒子的磁化强度在80k附近达到最大值，在50K以下显著增加。Pd的这种磁极化被氢化抑制。3)试样矫顽力较大，约为130 Oe。氢化作用抑制了Ni核与Pd壳层之间界面的各向异性，而这种各向异性与Pd壳层厚度无关。氢化作用使矫顽力减小，说明矫顽力是由界面各向异性引起的。平台的变窄和平台压力的升高表明钯壳内的应变大于体内的应变，阻碍了钯的氢化。在室温下，钯的磁极化是在界面处产生的，不会延伸到钯的壳层上。极化Pd的磁矩估计约为0.3马克。用Stoner理论定性地解释了极化Pd的温度变化，即当强交换增强作用于Pd时，Pd的磁化率增强，保持了准磁性Pd的热变化形状。

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.