FeCo基磁各向异性三层膜的光学模铁磁共振研究

The ferromagnetic resonance frequency of optical mode (OM) is usually far higher than that of acoustic mode (AM) in traditional exchange coupled FM/NM/FM trilayers. Nevertheless, the OM resonance is still difficult to be utilized in practice for a long time due to its tiny permeability. We prepared a magnetic anisotropy FeCoB/Ru/FeCoB trilayer with antiferromagnetic coupling between FeCoB layers. It is interesting to note that a pure OM ferromagnetic resonance with both high resonance frequency and high permeability was achieved. The magnetic anisotropy trilayer with high ferromagnetic resonance frequency, high permeability and low loss is promising for practical utilization. However, up to now, the enhancement mechanism of OM ferromagnetic resonance and the manipulation of the OM microwave performances are still unclear in the special trilayer. Therefore, in this project, firstly we will fabricate an antiferromagnetically coupling magnetic configuration with magnetic anisotropy ferromagnetic layers. The effects of interface and interlayer coupling on enhancement of OM resonance are studied using various nonmagnetic spacer materials with different thicknesses for optimizing the microwave performances of OM resonance. Secondly, the electron structures and magnetic interaction of the magnetic anisotropy trilayers will be calculated using the first-principle, and the experimental data of ferromagnetic resonance in the studied trilayers will be simulated using micro-magnetics method simultaneously. Based on the above analysis, the enhancement mechanism of OM resonance can be clarified and corresponding physical picture of OM resonance can be figured out. Thirdly, the various values of magnetic anisotropy field and interlayer exchange coupling field for the trilayer will be achieved by choosing suitable ferromagnetic and nonmagnetic materials, optimizing trilayer structure and fabrication process. Thus, the transformation rule between optical and acoustic modes will be obtained by analyzing the relationship between resonance mode and effective fields. As a result, the mode transformation will be realized in a controllable way. At last, we will emphasize on a special artificial structure with a controllable nonlinear magnetic configuration for understanding the transformation between OM and AM in common case. The magnetic configuration variation in the special artificial structure is realized by pinning the magnetic moments in one ferromagnetic layer at antiferromagnetic interface, and electrically rotating the ones in the other ferromagnetic layer at ferromagnetic/ferroelectric interface. As a result, the transformation mechanism of resonance modes in common case can be clarified via controlling the relatively moment orientation between ferromagnetic layers. This project would clarify the enhancement mechanism of optical mode resonance, manipulate the microwave performances, and provide theory and application basis for optical mode resonance.

虽然传统FM/NM/FM三层膜的光学模铁磁共振频率常远高于声学模，但因其磁导率低，无法实际应用。申请者通过控制磁构型，在磁各向异性且反铁磁耦合的FeCoB/Ru/FeCoB三层膜中得到了频率和磁导率双高的纯光学模共振，具备了实用化前景，但光学模共振增强机理及微波性能控制方法尚不清楚。本项目①构建磁各向异性反铁磁耦合磁构型，研究界面效应和层间耦合效应对光学模共振的影响，获得光学模共振微波性能优异的结构；②利用第一性原理研究体系的电子结构，微磁学模拟铁磁共振性能，从微观和宏观两方面阐明光学模共振增强机理及其物理图像；③研究磁各向异性对模式转换场、交换耦合作用对共振频率的影响，实现声/光模式可控转换；④利用反铁磁钉扎固定一个FM层磁矩，磁电耦合效应电控调节另一FM层磁矩取向，构建磁构型连续可控变化的人工结构，研究声/光全模式转换机理和综合利用途径。本项目将为光学模的实际应用奠定理论和实验基础。

本项目开展了超高频高磁导率光学模共振的机理、材料制备和性能调控研究，以期能够将其应用于自旋电子学器件和微波器件。主要内容和创新点如下：.1、超高频光学模共振的实现。利用成分梯度溅射方法制备了自偏置（外加磁场为零）单轴磁各向异性铁磁膜，并利用铁磁/非磁/铁磁（FM/NM/FM）三层膜的层间180°或90°耦合，成功将声学模抵消，使光学模共振增强，取得了突破性进展。获得了22.77 GHz且磁导率高达10左右的超高频光学模共振薄膜。突破了传统声学模共振频率低的瓶颈，为寻找超高频软磁材料开辟了新途径。.2、磁各向异性特征对光学模共振的决定作用。研究发现，单轴磁各向异性FM层耦合将得到单轴分布的很高的光学模共振频率；伞状分布的磁各向异性FM层耦合则得到光学模共振频率较高且全向分布的光学模共振；磁各向同性FM层耦合则无法得到共振信号。磁各向异性轴呈伞状分布的样品具有全向光学模共振特点，既具有很高的光学模共振频率，又具有各向几乎均等的特点。这种特点恰好解决了高频软磁薄膜的单轴磁各向异性和微波器件要求软磁材料各向同性的矛盾，必将为微波器件的设计提供更好的材料和更多的灵活性，促进该领域的快速发展。.3、光学模共振超晶格厚膜的研制。微波器件所需的软磁薄膜往往需要微米级厚度，而本项目研究的FM/NM/FM光学模共振单元只有50 nm。为了获得微米级厚膜，本项目提出了一种新方法：利用单元内强耦合获得高的光学模共振性能，利用单元间退耦合，消除单元干扰，在保证单元结构精确一致的前提下构建多层膜。目前实验室已制备20个光学模共振单元构成的微米级厚度的超晶格结构，且其共振频率依然高于14 GHz。超高频厚膜的成功研制解决了微波器件应用的后顾之忧，必将推动磁性微波器件的快速发展。.4、光学模共振的磁控声/光模式转换和电控转向。外磁场改变FM/NM/FM结构的反铁磁或90°耦合态将破坏光学模共振，使超高频（>10 GHz）光学模变成低频（~几 GHz）声学模。但所需外场（~100 Oe）只有将声学模推高到10 GHz以上所需磁场（>1200 Oe）的1/10，因此，通过层间耦合结构的磁构型改变实现声/光模式的转换是一种方便节能的方法。另外，利用磁电耦合效应可以在光学模共振不被破坏的前提下，将光学模共振转角90°。这些磁控模式转换和电控光学模转向将为多功能器件的设计提供很强的支撑作用。