以雷射脈衝沉積法成長CaMn7 O12薄膜之磁性與介電特性研究

Abstract

近年來，同時擁有鐵電極化與長程有序的磁矩排列之多鐵材料，開始被廣泛的研究。斜方六面體(Rhombohedral)結構的CaMn7O12被發表為一種新穎的磁電耦合多鐵材料。此材料在塊材與單晶上皆存在著兩個相轉變的尼爾溫度(TN1=90K、TN2=48K)。其在較高的尼爾溫度上(第一相轉變溫度)，擁有沿C軸方向上，具有與磁結構耦合巨大的非正規鐵電極化率。因CaMn7O12的鐵電極化為沿C軸方向，因此我們利用雷射脈衝沉積法成長CaMn7O12薄膜於Nb摻雜的STO基板上，希望藉由成長出C軸晶相的薄膜，以研究其本質之磁有序誘發鐵電極化率。我們利用X光繞射儀與超導量子干涉儀分別量測樣品薄膜的結構與磁特性，並針對CaMn7O12樣品薄膜的介電特性深入探討。

Recently, the multiferroic materials with the coexisting electric polarization and long-range magnetic order have been extensively studied. Among the large array of multiferroic materials, CaMn7O12 stands out as one of the most promising novel multiferroic materials owing to its pronounced magnetism-induced ferroelectric polarization recently reported. There are two phase transitions(TN1=90K、TN2=48K) observed both in ceramic and single crystal of rhombohedral CaMn7O12. CaMn7O12 reveals a giant improper ferroelectric polarization strongly coupled to the magnetic structure at rather high Neel temperature (first phase transition), 90K, which is oriented parallel to the c axis. As the ferroelectric polarization is oriented parallel to the c axis, we grew the thin film of CaMn7O12 on Nb-doped SrTiO3 by pulsed laser deposition. It is expected that in thin film form not only the intrinsic magnetism-induced ferroelectric polarization can be unveiled but also the potential applications of this intriguing material can be demonstrated. X-ray diffraction was used to reveal the phase structure of the films deposited on NSTO substrates prepared under various conditions. Magnetic susceptibility measurements were performed using Superconducting Quantum Interference Device (SQUID) to delineate the magnetic properties of the films and their correlations with electric properties. Characterizations on electric transport and dielectric properties as a function of temperature will be presented and discussed.