Exploring new functionalities of magnetic materials utilizing nanoplasmonics and multiferroics

本提案的目的是利用纳米等离子体和多铁性来探索磁性材料的新功能。特别是，我们对开发光或电控制磁性的新方法感兴趣。在研究的第一部分，我们将研究光与磁之间的相互作用，这在基础科学和技术应用方面是至关重要的。一个具体的目标是超快光控制磁性。然而，光与磁性介质相互作用的强度非常弱，这种弱相互作用一直是光学探测或控制磁性的主要挑战。为了克服这个缺点，我们将利用磁等离子体，它结合了磁性和等离子体的功能。在我们之前的研究中，我们观察到在铁磁薄膜中加入贵金属纳米颗粒可以显著增强磁光（MO）效应。系统研究等离子体共振对MO效应的影响，进一步了解MO活性增强的潜在机制。我们还将研究光激发高能电子引发的超快磁性。要回答的问题是：我们能否通过局部表面等离子体共振增强热电子的光学产生？如果是这样，我们如何有效地操纵热电子诱导的超快磁过程？另一个项目是利用超快光激发产生大波数自旋波。已知自旋波数是由抽运光的空间强度分布决定的。通过使用近场光学，我们将能够将泵浦光斑尺寸减小到纳米级。更高的自旋波数也可以通过局部表面等离子体共振产生，其中空间强度分布由纳米颗粒的几何形状决定。***研究的第二部分是探索利用磁电多铁性来实现磁场控制的机会，从而实现磁性和铁电级之间的耦合。然而，室温多铁性材料非常罕见，BiFeO3 （BFO）是一个例外。它的缺点是磁矩很弱。最近，人们发现一种双钙钛矿材料Bi2FeCrO6 （BFCO）具有与BFO相似的铁电性，并增加了强磁矩的好处。我们的主要目标是回答BFCO是否可以为磁场的电场开关提供新的途径。我们还将研究门控BFCO薄膜的电场触发磁化动力学。考虑到由多铁体实现的磁场控制有可能彻底改变当今的电子技术，所提出的研究结果将是重要的。

The purpose of this proposal is to explore new functionalities of magnetic materials utilizing nanoplasmonics and multiferroics. In particular, we are interested in developing new ways to control magnetism optically or electrically.*** In the first part of the research, we will investigate the interaction between light and magnetism, which is of utmost importance in terms of fundamental science and technological applications. A specific goal is the ultrafast light control of magnetism. The strength of the interaction of light with a magnetic medium, however, is very weak, and this weak interaction has been the main challenge in optically probing or controlling magnetism. In order to overcome this drawback, we will exploit magneto-plasmonics, which combines magnetic and plasmonic functionalities. In our previous studies, we observed that the incorporation of noble-metal nanoparticles in ferromagnetic films significantly enhances magneto-optical (MO) effect. We will conduct systematic studies on the influence of plasmon resonance on MO effect, and develop a better understanding of the underlying mechanism of the enhanced MO activities. We will also study the ultrafast magnetism triggered by optically excited high energy electrons. The questions to be answered are: can we enhance the optical generation of hot electrons via localized surface plasmon resonance? If so, how effectively can we manipulate the hot-electron induced ultrafast magnetic process? Another project is to generate large wavenumber spin waves using ultrafast optical excitation. It is known that the spin wavenumber is determined by the spatial intensity distribution of the pumping light. We will be able to reduce the pump spot size to the nanoscale by using near-field optics. Higher spin wavenumbers can also be generated by using the localized surface plasmon resonance, in which the spatial intensity distribution is determined by the geometries of the nanoparticles.*** The second part of the research is to explore the opportunity of electric-field control of magnetism utilizing magnetoelectric multiferroics, which enables the coupling between magnetic and ferroelectric orders. However, room temperature multiferroics are very rare, with BiFeO3 (BFO) being an exception. Its drawback is that it has a very weak magnetic moment. Recently, it was found that a double perovskite material Bi2FeCrO6 (BFCO) has ferroelectricity similar to BFO with the added benefit of a strong magnetic moment. Our main goal is to answer the question whether BFCO can provide a new avenue for electric-field switching of magnetism. We will also investigate the electric-field triggered magnetization dynamics in gated BFCO thin films. Considering that the electric-field control of magnetism enabled by multiferroics has the potential to revolutionize today's electronics technology, the outcome of the proposed research will be significant.