具CaCu5結構之三元稀土銅鋁合金的電磁性質研究

Abstract

在這份論文之中，我們製作了兩個系列的稀土銅鋁合金(RCu4Al與 RCu3Al2，R = Y、La、Ce、Pr、Nd、Sm、Gd、Tb、Dy、Ho、Er、Tm、Lu)並且成功地量測其結構與電磁性質。經由X-ray繞射結構分析得知除了LuCu4Al 樣品為多相之外，其餘樣品皆為六角最密堆積的CaCu5單相結構。在磁性方面，RCu4Al 的樣品中，在低溫具有磁相變的皆為反鐵磁性，而RCu3Al2 的樣品則僅有少部分具有磁性，但是鐵磁性與反鐵磁性互見。電性方面，在大部分樣品裡磁相變的溫度皆可觀察到相對應的電阻變化，而其中兩個含有鈰的樣品(CeCu4Al、CeCu3Al2) 更是具有隨溫度下降而電阻上升的特性，經由模型分析之後應是Kondo系統。若將磁相變溫度對稀土元素的原子序作圖，則發現其曲線大致上符合de Gennes 係數，表示在這兩個系列的合金中，RKKY交互作用為主要的磁性交互作用。

We have successfully synthesized two series of rare earth ternary intermetallics RCu4Al and RCu3Al2 (R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu), which crystallized in a CaCu5-derived hexagonal structure with space group P6/mmm as confirmed from the X-ray diffraction measurements. All the samples are single phase except LuCu4Al which possesses both the CaCu5 and the AuBe5 phases. The lattice parameters are deduced from the x-ray patterns, indicating that the unit cell volumes of the RCu4Al and RCu3Al2 compounds have expanded with respect to that of the corresponding RCu5 compounds due to the smaller size of Cu atoms. Besides, a tendency that the Al atom prefers the 3g site of crystal structure in larger-size rare earth compounds and favors the 2c site in smaller-size rare earth compounds is revealed. Magnetic behavior and electrical resistivity are investigated on these compounds. Curie-Weiss law is used to calculate the effective moments of the rare earth ions in the paramagnetic state, and the Debye temperatures are derived from the resistivity plot. The reisitivity curves of CeCu4Al and CeCu3Al2 are nearly temperature independent in the high temperature range and increase logarithmically with decreasing temperature. The resistivity behaviors of the CeCu4Al and CeCu3Al2 compounds at low temperatures demonstrate both of these two are Kondo systems. The magnetization and resistivity measurements indicate that almost all the RCu4Al compounds (R = Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm) ordered antiferromagnetically at low temperatures, and for R = Sm, Gd, Tb, and Dy, there exists magnetic reorientation at even lower temperatures. On the other hand, magnetism among the RCu3Al2 compounds is not as abundant as compared to the RCu4Al compounds. A magnetic phase transition at 12 K appears in the SmCu3Al2 compound, and GdCu3Al2 is ferromagnetic below 20 K as revealed both in the magnetization and resistivity measurements. Meanwhile, the TbCu3Al2 and DyCu3Al2 compounds become antiferromagnetic below 6 K and 5.5 K respectively. The Neel temperatures for both the RCu4Al and RCu3Al2 compounds scale with the de Gennes factor which suggests that the primary mechanism of interactions leading to the ordering of the magnetic moments may be the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction.