稀土正铁氧体单晶及其异质结构的可控制备与超快磁光效应

Rare-earth orthoferrites RFeO3 (R=rare earth elements) exhibit unique magneto-optical effect owing to their complex magnetic structure and strong spin-orbit-lattice coupling. Although the structure and magnetic structure have been well investigated since 1950’s, recently scientists pay a lot of attention on the ultrafast optical switching and magnetic excitation, multiferroic properties of rare-earth orthoferrites RFeO3. By manipulating and transferring the light, electric and magnetic signals of magneto-optical materials, we can design novel magneto-optic devices. However, up to data, ultrafast magneto-optical properties of RFeO3 were only investigated by the laser with visible light. Meanwhile, it is difficult to trigger spin dynamics of pure RFeO3 orthoferrites single crystals by ultrafast laser heating around room temperature, which limits their application as magneto-optical device.. In the project, we will grow RFeO3 single crystals by means of floating-zone and Fe/RFeO3 heterostructures by molecular beam epitaxy techniques. We also investigate the effect of crystal structure, electronic structure and spin structure on the ultrafast magneto-optical effect by using time-resolved magneto-optical Kerr effect (TRMOKE), and deep Ultraviolet laser based magnetic circular dichroism spectrometers. We investigate room temperature ultrafast-laser-induced spin dynamics in exchange-biased Fe/RFeO3(100) (R=Er and Dy) heterostructures. In addition to the spin precession of Fe with frequency of several GHz, the quasi-ferromagnetic resonance mode, impurity mode, and unexpected coherent phonon mode of AFM in the heterostructures were triggered by ultrafast laser at room temperature. We will explain the improved efficiency of multimode spin dynamics excited by ultrafast laser is attributed to the optical modification of interfacial exchange coupling between Fe/RFeO3(100). Our work will reveal that the optical modification of interfacial exchange coupling between Fe/RFeO3(100) can open a new avenue to engineer the ultrafast spin dynamics of rare-earth orthoferrites and expand the application of ultrafast magneto-optical devices.

稀土正铁氧体晶体RFeO3(R=稀土离子) 由于具有复杂的倾斜反铁磁结构和强的自旋-轨道-晶格耦合，表现出独特的磁光效应而成为凝聚态物理和材料领域的研究热点。尽管对稀土正铁氧体的研究始于上世纪五十年代，但近年来，超快光磁激发工作中取得的进展又激发了人们重新研究其超快磁光效应。利用磁光材料的光电磁的耦合和转换，可制成具有各种功能的新型磁光器件。到目前为止，超快RFeO3磁光效应研究仅在可见光范围，对深紫外区域的磁光效应仍是空白。同时，脉冲激光诱导的自旋进动发生在低温范围，这严重制约了其作为磁光器件的应用。本项研究拟生长正稀土铁氧体单晶RFeO3及其异质结构，阐明晶体结构、电子结构和自旋结构对超快磁光效应的影响；通过时间分辨磁光效应和磁圆二色性，发展对电子结构与自旋结构的超快、超灵敏探测的新手段，为超快磁光材料与器件的应用提供理论指导和技术途径。

稀土RFeO3 正交铁氧体由于其特殊的磁结构、丰富的相转变，它们的单相磁电耦合效应、超快磁动力学、内禀的交换偏置和巨大的各向异性磁热效应等成为人们研究的热点。此外由于铁磁/反铁磁异质结构在磁电子器件和基础磁性研究中的重要地位，铁磁与反铁磁材料中复杂的界面磁耦合效应一直是凝聚态物理磁学领域的热点和争论点。但是现今关于铁磁/RFeO3 异质结构中的交换耦合效应的研究近乎空白。在本项研究中，我们主要利用电子束蒸发在RFeO3 正交重稀土铁氧体单晶衬底上制备了多晶铁薄膜，利用表面磁光克尔效应、各向异性磁电阻、时间分辨磁光克尔效应等表征方法研究了该异质结构中静态与动态磁耦合效应。主要内容包括：.（1）利用原位和非原位表面磁光克尔效应的方式，观察到了无需场冷下，Fe 薄膜中的磁场诱导的可切换符号的交换偏置行为。该单向磁各向异性起源于Fe 薄膜与ErFeO3 中弱的净磁矩之间的反铁磁交换耦合。.（2）利用各向异性磁电阻方法，通过转矩拟合的方式，除了常见的自旋反转耦合效应诱导的单轴磁各向异性，我们在交换耦合的Fe/ErFeO3 异质结构中还观察到了四重和六重高阶磁各向异性。.（3）利用时间分辨的磁光克尔效应， 我们研究了Fe/ErFeO3(100) 与Fe/DyFeO3(100)异质结构中的超快磁动力学。在之前的报道和我们的实验中，利用全光泵浦-探测技术，并不能在室温下有效的探测到RFeO3（R=Er 和Dy）的磁动力学。我们发现在RFeO3 单晶衬底上覆盖一层薄的铁薄膜，能够大大的增强光泵浦反铁磁铁氧体的效率。利用全光泵浦-探测技术，我们不仅仅观察到了以往全光方法只能在自旋重取向温区附近观察到的准铁磁(Q-FM)模式，以往只能用THz 发射谱探测到的杂质（Impurity)模式，还观察到了尚未被广泛报道的一致声子(Phonon)模式。光泵浦RFeO3 反铁磁超快自旋动力学的增强可归因于光对异质结界面交换耦合的改变。发现Fe 薄膜的超快自旋动力学能够被激光功率调控。当外磁场沿着面外时，Fe 薄膜的进动频率随着激光功率的增大而减小。