正交晶系多鐵性稀土錳氧化物薄膜之磁與介電特性研究

Abstract

多鐵材料之所以吸引了許多人的注意，是由於具有磁電耦合特性，並且有發展與應用於儲存技術的潛力。值得一提的是在磁場下誘發鐵電之實驗結果中，磁電耦合機制的重大發現更是令人振奮。隨後在E-type正交晶系多鐵錳氧化物中，過往的理論文獻，更估計此種材料之自發性鐵電之電耦強度將比TbMnO3高出百倍以上；實驗的結果證實了鐵電強度確實有增強的現象，但卻也發現一些沒有預期之結果；例如磁性相變或電耦強度減弱之現象。因此，在此論文中我們將探討一系列E-type正交晶系稀土錳氧化物薄膜 (o-ReMnO3, Re= Ho, Er, Tm, Yb) 之磁、電特性以瞭解這些偏離理論預測的結果。 首先，為了製備錳氧化物薄膜，我們利用脈衝雷射(PLD)成長不同軸向之錳氧化物薄膜於鈦酸鍶(SrTiO3)或鋁酸鑭(LaAlO3)基板上。我們利用XRD分析薄膜之軸向與結構後，再使用超導量子干涉儀(SQUID)與變溫阻抗量測儀(LCR meter)研究薄膜之磁電特性。 我們實驗室早先的磁性研究中表示磊晶應力可能造成E-type磁性之磁有序排列發生轉變，但本研究第一部分結果則顯示除了應力影響外，稀土元素f-軌域電子之磁矩可能在影響E-type磁有序上扮演決定性的角色。由本研究實驗結果得出在E-type正交晶系稀土錳氧化物磁有序相變中，稀土元素之磁矩對於E-type磁有序方向改變是不可忽略的考量。此外，我們也發現隨著稀土元素的4f電子總角動量量子數增加，磁有序方向將從原本的b軸 (Yb3+)到a軸 (Tm3+)而最後到c軸上 (Er3+ 與 Ho3+)。而後，在其他文獻在介電常數與鐵電的觀察結果，發現了尼爾溫度下E-type與cycloidal spins state磁相共存的證據，顯示了E-type稀土錳氧化物在尼爾溫度下的磁相變比早期的研究更複雜。而我們的第三部分研究結果認為cycloidal spins state可能存在於尼爾溫度下E-type正交晶系稀土錳氧化物薄膜中，並且支持其他的參考文獻中磁相共存之發現。

The potential of applications to modern storage technology is attracting a lot of attention on multiferroics. Most intriguingly, the observation of magnetic field induced polarization is an important link to understand the internal or external mechanism of magnetoelectric coupling. In E-type multiferroic orthorhombic rare-earth manganites (o-ReMnO3, Re= Y, Ho, Er, Tm, Yb, Lu), the prediction of gigantic spontaneous polarization, two orders larger then TbMnO3, was calculated by several teams of theorists. Later, the experiments show that the magnitude of polarization was encouraging; however, unexpected observations of magnetic phase transition and weakening of polarization was also disclosed. To uncover the abovementioned unpredicted observations, in this thesis, we study a series of E-type o-ReMnO3 (Re= Ho, Er, Tm, Yb) thin films by determining their magnetic and dielectric properties. First, to produce manganites thin films, we use pulse laser deposition for growing different crystal orientation thin films on SrTiO3 and LaAlO3 substrates. Then, we determine the structure orientation and lattice constants of o-ReMnO3 thin films by XRD. The magnetic and dielectric properties of o-ReMnO3 thin films are measured by using a Quantum Design superconducting quantum interference device system (SQUID) and Agilent 4980A (LCR meter). The earlier studies in our lab unveil that the reordering transition of E-type o-ReMnO3 thin films appeared along unexpected axis might due to the epitaxially induced strain effect. However, instead of following our earlier argument on the strain effect, new evidence shows the f-electrons moments of rare-earth elements may play a crucial role on the unexpected magnetic reordering transition. Further studies of our results suggest the rare-earth elements moments are non-negligible considerations for magnetic phase transition in E-type o-ReMnO3 thin films. Moreover, with the increase of total angular momentum J of the 4f-electrons of rare-earth element the orientation of magnetic reordering was changed from the b-axis for Yb3+ to the a-axis for Tm3+ and, eventually to the c-axis for Er3+ and Ho3+. Additionally, the observation of dielectric constant uncovers the plausible evidence of the coexistence between E-type and cycloidal spins state below Neel temperature. It is obvious that the spins reordering transition in E-type o-ReMnO3 thin films are more complex than we knew before. The third part of our results suggests the cycloidal spins state is found below Neel temperature in E-type o-TmMnO3 thin films, which also support the other findings on cycloidal spins state or coexistence state. These interesting observations may refresh the information of magnetic phase transition in orthorhombic rare-earth manganites thin films.