多磁體RMnO3 (R=Ho, Er, Tm, Yb, Lu and Y)的物性研究

Abstract

在固態反應合成下，稀土錳氧化物RMnO3 ( R = 稀土元素或釔 )依據R3+ 離子半徑的大小可形成二種晶體結構，分別為：（1）當R為半徑較大的La、Ce、Pr、Nd、Sm、Eu、Gd、Tb、Dy時，則會形成正方結構的RMnO3；（2）當R為半徑較小的Ho、Er、Tm、Yb、Lu、Y，則會形成六方結構的RMnO3。這個研究利用固態反應法備製了一系列六方結構的RMnO3（R = Ho、Er、Tm、Yb、Lu、Y）的樣品，藉由X光繞射儀斷定其結構，繼而測量其磁性、電性和比熱的特性，探討不同R3+離子對於整個化合物的物性影響。 研究發現，在磁性的數據中顯示，只有在YMnO3、YbMnO3和LuMnO3清楚出現了反鐵磁的轉變溫度，分別為73 K、87 K和83 K；而由比熱的量測發現，磁性轉變溫度清楚的反應在比熱的性質中，由比熱的轉變溫度可以歸納出，轉變溫度隨R的離子半徑變小而增高。在介電常數的量測中，發現在所有磁轉變溫度附近都出現不正常的轉折現象，證實磁和電性的強關聯性。综而言之，我們的這個研究提供了重要的實驗數據，有助於未來多磁體的理論發展。

The rare earth manganites RMnO3 (R = rare earth element or Y) exhibit strong magnetic exchange interactions between the magnetic moments of the Mn3+ ions as well as some of the magnetic R3+. Depending on the size of rare earth ion, RMnO3 crystallizes into two different structures：（1） orthorhombic phase for R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, or Dy which has a larger ionic radius compared with that of Ho；and （2）hexagonal phase for R = Ho, Er, Tm, Yb, Lu or Y which possesses a smaller ionic radius. In this work, we have prepared a series of hexagonal RMnO3 with R = Ho, Er, Tm, Yb, Lu and Y and employed the techniques of X-ray powder diffraction, magnetic susceptibility, heat capacity and dielectric permeability in order to systematically study their R-dependent structural, magnetic and electric properties. In the data of magnetic susceptibility of these six samples, only YMnO3, YbMnO3 and LuMnO3 show antiferromagnetic transitions near 73 K, 87 K and 83 K respectively. However, the magnetic transition temperatures of all samples are clearly observed in the data of specific heat. It is found that the transition temperature increases with decreasing the radius of R-ion, which is attributed to the enhancement of exchange interaction between Mn3+-ions. In the data of temperature dependent dielectric permeability vs. temperature, anomalies appear near TN in all samples, implying a strong coupling between ferroelectric and magnetic orders in the hexagonal RMnO3 compounds. The results of this study provide important information for the future development of theoretical model for multiferroism.