高质量纳米岛状磁电异质结的构建及其纯电场调控垂直磁翻转的研究

The multiferroic magnetoelectric heterojunction, with promising application potential in electric-field-driven magnetization reversal, has been the research focus in the field of both multiferroics and magnetoelectronic devices. However, pure electric-field-driven perpendicular magnetization reversal has not yet been experimentally observed. Ferroelectric nano islands can produce much larger in-plane piezostrain than ferroelectric films. Such piezostrain exhibits as a dynamic strain pulse whose duration is in the magnitude of the nano-second. It can facilitate a complete perpendicular magnetization rotation through precession which has not been achieved by applying a static strain. The present work focuses on the electric field modulated magnetism of high-quality magnetoelectric composite nano islands on the single crystal substrate, which are fabricated by the film deposition and micromachining technology. By varying the lateral size, thickness, microstructure, orientation and the interfacial layer of the heterosturctures, pure electric field control of magnetization reversal is measured and characterized. The underlying coupling mechanism of such heterojunctions is also explored. In the frame of interfacial coupling between the ferroelectric and magnetic phases, the present work aims to fabricate high-quality magnetoelectric composite nano islands, obtain the optimal fabrication parameters, observe the pure electric-field-driven perpendicular magnetization reversal and explore the coupling mechanism. This would also provide experimental evidences for eliminating the overheating in the device miniaturization.

磁电异质结是最有希望实现纯电场调控磁翻转的材料载体之一，近年来成为多铁性磁电材料和磁电子器件两个领域共同的研究热点。迄今为止，尚未有纯电场调控垂直磁翻转的实验报道。铁电纳米岛可产生数倍于铁电薄膜的面内应变，并可获得持续时间低至纳秒的应变脉冲，以操纵时间范畴同在纳秒的磁矩进动翻转，进而实现施加静态应变难以实现的垂直磁矩翻转。本课题以在单晶基片上通过薄膜生长及微加工技术构建的高质量纳米岛状磁电异质结为研究对象，着重研究该结构的构建及纳米岛三维尺寸、微观结构、取向及电极层的界面效应对纯电场作用下垂直磁翻转性能的调控，深入研究其作用规律及磁电耦合机制。总体目标是在铁电、磁性两相界面耦合的框架内，在成功构建纳米岛状磁电异质结的基础上，实现纯电场作用下的垂直磁翻转的实验观察，筛选出纳米岛状磁电异质结的最优参数，初步探明其耦合机制，为根本解决磁电子器件微型化面临的发热问题提供实验依据。

磁电异质结是最有希望实现纯电场调控磁翻转的材料载体之一，近年来成为多铁性磁电材料和磁电子器件两个领域共同的研究热点。本课题以多种基片上通过薄膜生长及微加工技术构建的高质量磁电异质结为研究对象，从Si/Pt基片上高质量多晶锆钛酸铅(PZT)薄膜、[100]PZT薄膜的制备、性能研究及优化；Ni基片上PZT的制备、取向控制及性能研究，多晶PZT-Ni及取向PZT-PbO-Ni的介电、铁电及磁电性能研究；不同生长磁场控制下的CoFeB膜/(011)PMN-PT单晶复合结构的零磁场辅助的电控磁性能研究及机理探索等方面入手，着重研究磁电异质结的构建、尺寸、微观结构、取向及电极层的界面效应对纯电（磁）场作用下磁（电）性能调控及其耦合机制。实验证明：Si/Pt基片上 [100]PZT薄膜的铁电性能虽不如多晶及[111]PZT薄膜，但由于[100]PZT的压电性能更优异，因而更利于以应力为中介的“电-力-磁”的耦合； Ni基片上引入PbO晶种层后诱导的[100] PZT膜具有更高压电性能导致了PZT-PbO-Ni异质结的最大磁电耦合系数为无PbO晶种层的PZT-Ni异质结的3.3倍；实验观察到了CoFeB膜/（011）PMN-PT单晶复合结构非易失的电场调控磁性能，且其磁电耦合系数达到了~8.0*10-6 s m-1，是目前实验观测的最大值。此调制行为来源于磁易轴在面内的90°旋转。与之前工作不同的是：本工作中非易失的磁易轴旋转是在纯电场条件下实现的，不需要外加磁场的辅助，也不需要给PMN-PT基片预先设定面内极化。当在一定范围内改变磁性膜生长过程中施加的磁场方向时，非易失的电场调控磁性能现象依然存在且磁电耦合系数均超过5.5*10-6 s m-1。本课题实现了纯电场作用下非易失性磁性能调控的实验观察，初步探明了磁电异质结的耦合机制，为根本解决磁电子器件微型化面临的发热问题提供了实验依据。