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

研究了铁磁性Ni微粒表面包覆顺磁性Pd的加氢特性和界面磁性能。Ni球的平均直径为0.7 μ m，用化学还原法制备了5种不同Pd含量的样品，测量结果总结如下：1）Pd壳层的晶格常数随Pd壳层厚度的变化而变化，壳层越薄，Pd的晶格常数和氢浓度越小。Ni/Pd颗粒和Pd块体的p-x-T曲线表明，Ni/Pd的平台宽度变窄，平台压力变高。2）Pd的磁极化是由Ni的交换增强引起的。Ni/Pd颗粒的磁化强度在80 K附近出现一个宽的最大值，在50 K以下显著增加，氢化抑制了Pd的磁极化; 3）样品具有较大的矫顽力，约为130 Oe，氢化抑制了Ni核与Pd壳之间的界面各向异性，且与Pd壳的厚度无关。矫顽力也随氢化反应的进行而降低，这表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Pd壳层中的应变大于体相Pd中的应变，阻碍了氢化反应的进行。在室温下，Pd的磁极化是在界面处产生的，并不延伸到Pd壳层。极化Pd的磁矩估计为约0.3 μ g。用Stoner理论定性地解释了极化Pd的温度变化：当强交换增强作用于Pd时，Pd的磁化率增强，但保留了顺磁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.