CAREER: Hexagonal Ferrite Thin Films for the High-Temperature Magnetoelectric Memory Effect

电场和磁场之间的动态耦合——电磁波的发现和利用给人类社会带来了革命性的变化，特别是在无线通信领域。材料中电场和磁场之间的“静态”耦合（例如，使用电场切换磁铁的南北两极）预计将在紧凑和节能的信息存储和处理、传感器和执行器中有主要应用。由于对信息存储和处理的需求不断增加，而当前技术的能力正在耗尽，因此需要这些应用程序。这个学院早期职业发展（Career）项目探索了新材料（如六方铁氧体）中电场和磁场之间可能的静态耦合，通过阐明它们的电、磁和结构特性之间的联系，并使用先进的材料制备对材料进行微调。结合本研究，本项目的教育目标是：通过前沿研究和探索以学生为中心的新教学方法，促进本科生基础物理教学；通过系统的创新研究训练，指导和激励学生研究人员；并吸引K-12学生激发他们对科学的兴趣。技术总结：利用电场开关磁偶极子（磁电记忆效应）——这种效应可以成为下一代信息处理和存储技术的工作原理——不幸的是，在已知材料中很少见，而且仅限于低温。这个学院早期职业发展（Career）项目通过发现新材料或通过实验调整已知材料的特性，探索在高温下高效和稳定的磁电记忆效应。特别地，本项目研究了六方铁氧体（h-RFeO3; R =Y, Ho, Lu）等具有非正常铁电性和非正常铁磁性的材料中可能存在的磁电记忆效应。本文的具体研究目标是：1)阐明六方铁氧体磁性有序的起源。2)实验确定了六方铁氧体的磁电效应，并确定了其潜在的机制。3)采用外延薄膜生长的方法，通过调整六方铁氧体的结构来调整其磁性能以及磁性能和电性能之间的耦合。采用脉冲激光沉积法制备了调谐结构的单晶外延薄膜材料。利用中子散射、x射线光谱学和x射线衍射分别研究了磁性、电子和晶格结构的细节。通过测量铁磁特性在电场作用下的变化，研究了材料的电、磁特性之间的耦合关系。该项目的成功除了促进了对复杂氧化物中磁电耦合的理解外，还可能在实验上建立一个由非正常铁电性和非正常铁磁性引起的磁电效应的新范式

NON-TECHNICAL SUMMARYThe discovery and utilization of the dynamic coupling between the electric and magnetic fields - electromagnetic waves - has revolutionized human society, particularly in the wireless communications. The "static" couplings between the electric and magnetic fields in a material (e.g. switching the north and south poles of a magnet using an electric field) are expected to have major applications in compact and energy efficient information storage and processing, sensors, and actuators. These applications are desired since the demand for information storage and processing is ever increasing while the capabilities of current technology are being exhausted. This Faculty Early Career Development (CAREER) project explores the possible static couplings between the electric and magnetic fields in new materials, such as hexagonal ferrites, by elucidating the connections between their electric, magnetic, and structural properties, and fine-tuning the materials using advanced material preparations. Integrated with the research, the educational objectives of this project are, to promote teaching undergraduate students fundamental physics by exposing them to cutting-edge research and by exploiting new student-centered pedagogical approaches, to mentor and inspire student researchers with systematic trainings for innovative research, and to engage K-12 students to stimulate their interests in science.TECHNICAL SUMMARY Switching a magnetic dipole using an electric field (magnetoelectric memory effect) - an effect that can be a working principle of the next-generation technology for information processing and storage - is unfortunately rare in known materials and restricted to low temperature. This Faculty Early Career Development (CAREER) project explores the magnetoelectric memory effect that is efficient and stable at high temperature, by the discovery of new materials or by tuning the properties of known materials experimentally. In particular, this project investigates the possible magnetoelectric memory effect in the materials that exhibit both improper ferroelectricity and improper ferromagnetism, such as hexagonal ferrites (h-RFeO3; R =Y, Ho, Lu). The specific research objectives of the proposed work are: 1) Elucidate the origin of the magnetic orderings in hexagonal ferrites. 2) Experimentally determine the magnetoelectric effect and identify the underlying mechanism in hexagonal ferrites. 3) Adjust the magnetic properties and the coupling between the magnetic and electric properties in hexagonal ferrites by tuning their structures using epitaxial thin film growth. Pulsed laser deposition method is employed to prepare single crystalline epitaxial thin film materials of tuned structures. The details of magnetic, electronic, and lattice structures are investigated using neutron scattering, x-ray spectroscopy, and x-ray diffraction respectively. The couplings between the electric and magnetic properties are studied by measuring the change of ferromagnetic properties in an electric field. Besides advancing the understanding in the magnetoelectric couplings in complex oxides in general, the success of the project may experimentally establish a new paradigm of magnetoelectric effect originated from improper ferroelectricity and improper ferromagnetism