兰州大学机构库 >物理科学与技术学院
外延铁磁/铁电异质结中电场调控磁性的研究
Alternative TitleElectric field control of magnetism in epitaxial ferromagnetic/ferroelectric heterostructures
郭晓斌
Thesis Advisor王涛 ; 席力
2018-03-15
Degree Grantor兰州大学
Place of Conferral兰州
Degree Name博士
Keyword外延铁磁薄膜 磁各向异性 电场 非易失性 磁化强度翻转
Abstract

非易失性、高读写速度、高存储密度、低功耗和微型化是未来存储器发展的主要趋势,其中磁性随机存储器是目前最有希望实现这一目标的存储器。在磁信息读取方面,基于隧穿磁电阻效应的磁性隧道结已成功应用于存储设备且表现出非常明显的优势。在磁信息写入方面,通电导线产生的磁场、自旋转矩效应和自旋-轨道转矩效应都属于较强的电流强度通过相关效应来实现磁矩的可控翻转,只有施加的电流密度大于临界电流密度时才能改变磁化强度方向,这导致器件的能耗比较高。相比较而言,利用电场调控磁性的方法来实现磁化翻转显著降低了焦耳热的影响,可实现低功耗条件下的高密度信息存储。电场调控磁性的研究主要集中在同时拥有铁电性、铁磁性和铁弹性的复合多铁异质结构及其磁电耦合效应。把具有磁致伸缩效应的铁磁材料和具有压电效应的铁电材料耦合在一起形成铁磁/铁电异质结,通过逆磁电耦合效应可实现异质结中铁磁层磁化强度的调控。调控模式包括界面电荷调制、交换偏置调制和应力调制,其中常温下的应力调制操作简单且调控效果好,近年来受到了广泛的关注。

目前在铁磁/铁电异质结利用磁电耦合效应调控磁性的研究中,通常选择具有各向同性的CoFeB或者单轴各向异性的FeNi,FeCo,Co等铁磁性材料来制备多铁异质结。在电场作用下,通过铁电层中极化方向的改变来实现相邻铁磁层磁化强度的易失或非易失性翻转。然而对于具有磁晶各向异性的外延薄膜的磁性及电场调控磁性方面的研究还很少,主要问题是应力诱导的单轴磁各向异性太小不能克服薄膜原始的磁晶各向异性在难轴方向的势垒。在本论文中,我们首先研究了外延铁磁薄膜的结构、静态磁性、动态磁性的关系,以及外磁场作用下的磁矩翻转机制。在此基础上,我们利用具有压电记忆效应的[Pb(Mg1/3Nb2/3)O3]x-[PbTiO3]1-x (PMN-PT)作为异质结的铁电层,利用具有合适磁晶各向异性能的外延FeSi磁性薄膜作为异质结的铁磁层研究电场对磁化强度翻转和磁各向异性的可逆、非易失性调控。主要的研究内容如下:

(1) 利用射频磁控溅射方法在(001)MgO单晶衬底上制备了Co61Fe26Si13 (CoFeSi)、Fe86Si14和Fe80Si20 (FeSi)薄膜。研究了不同退火温度对CoFeSi薄膜的结晶性、磁各向异性、动态高频磁性的影响。利用X射线衍射和透射电子显微镜表征了FeSi薄膜在(001)MgO衬底上的外延关系:(001)[100]MgO//(001)[110]FeSi。静态磁性的测试结果表明FeSi薄膜的磁性具有面内四重对称性,这也表明具有立方磁晶各向异性FeSi薄膜的获得。利用磁光克尔效应研究了外延FeSi薄膜在面内不同方向施加磁场时的磁化过程。高频磁性的测试结果表明在Fe86Si14/MgO(001)薄膜中,面内任意方向的自然共振频率约为8.0 GHz,弥补了传统单轴各向异性薄膜在特定方向自然共振现象消失的缺陷。

(2) 利用射频磁控溅射方法在(001)PMN-0.32PT单晶衬底上制备了外延Fe86Si14薄膜,在Fe86Si14/(001)PMN-0.32PT异质结中,研究发现了负向电场作用后的四重磁各向异性到单轴磁各向异性主导的非易失性、不可逆转变。根据理论模拟的结果,我们改变了铁磁层的成分和铁电衬底的选择,在外延Fe80Si20/(001)PMN-0.3PT异质结中,通过角度依赖的面内剩磁曲线测试,研究了在±6 kV/cm的电场作用下单轴磁各向异性主导下的可逆、非易失性的90o磁化强度翻转。利用纵向磁光克尔效应测量其在不同电场下的磁滞回线,进一步证明了磁化翻转的非易失性以及磁晶各向异性和单轴磁各向异性的竞争关系。

(3) 通过计算模拟了外延FeSi/(011)PMN-PT异质结中四重磁各向异性和单轴磁各向异性之间的可逆、非易失性翻转的可行性。在300 oC退火之后的外延Fe80Si20/(011)PMN-0.3PT 异质结中,静态磁性测量表明可通过施加饱和电场和逆向矫顽电场的方法实现四重磁各向异性和单轴磁各向异性的可逆、非易失性转变,定性的与理论模拟结果相一致。同时在辅助磁场的作用下实现了电场调控磁化强度的可逆、非易失性、稳定的90o和180o翻转。最后在实验上证明了铁电衬底的选择、铁磁薄膜的结晶性对Fe80Si20/(011)PMN-PT异质结中电场调控磁性的重要性。

Other Abstract

Non-volatile, high read/write speed, high storage density, low power consumption and device miniaturization are the main developing trends for magnetic memory in the future. Magnetic random access memory is the most promising candidate to achieve this goal. In terms of magnetic information reading, the magnetic tunnel junctions based on the tunneling magnetoresistance effect have been successfully applied to the storage devices and exhibit obvious advantages. In the field of magnetic information writing, the magnetic field generated by conduction wires, the spin transfer torque and the spin-orbit torque can be used to realize the controllable magnetization reversal through current induced relevant effect. However, the applied current density must be larger than the critical current density to achieve the goal. These approaches will cause a significant increase in energy consumption. In contrast, through electric field control of magnetic properties to realize the magnetization switching can not only significantly decrease the effect of the joule heat, but also realize the high density data storage with low power consumption. The research on the electric field control of magnetism mainly focuses on the composite multiferroic heterostructures with combined ferroelectricity, ferromagnetism and ferroelasticity and magnetoelectric coupling effects. In an artificial ferromagnetic/ferroelectric (FM/FE) heterostructures with the coupling of ferromagnetic layer having high magnetostrictive to ferroelectric layer having large piezoelectric effect, the magnetism of FM/FE heterostructures can be adjusted by inverse magnetoelectric coupling effect. The coupling mechanisms in such FM/FE heterostructures include charge mediated effect through charge accumulation/dissipation at the interface, exchange bias-mediated effect resulting from ferromagnetic-antiferromagnetic coupling, and strain mediated effect by means of transferring strain from the piezoelectric FE layer to the FM layer. Among them, the strain-mediated mechanism has received a lot of attention recent years, since the strain-mediated mechanism is particularly convenient and effective at room temperature.

In the current study, the isotropic CoFeB or the FeNi, FeCo, and Co with uniaxial magnetic anisotropy was usually selected as FM layer to fabricate FE/FM heterostructures. Among them it has realized the volatile and non-volatile magnetization switching of FM layer by modifying the ferroelectric polarization direction of FE layer under appropriate electric fields. However, few works have been performed regarding the magnetic properties and electric field control magnetism of FM with cubic magnetocrystalline anisotropy. The main problem is that the strain induced uniaxial magnetic anisotropy is too weak to overcome the original hard axis barrier of cubic magnetocrystalline anisotropy energy. In this thesis, we first study the structure, static magnetic and dynamic magnetic properties of the epitaxial ferromagnetic thin films, and the magnetization switching mechanism under the external magnetic field. Then the single crystal [Pb(Mg1/3Nb2/3)O3]x-[PbTiO3]1-x (PMN-PT) with piezoelectric memory effect are used as the ferroelectric layer of the heterostructures, and epitaxial FeSi film with appropriate magnetocrystalline anisotropy are used as ferromagnetic layer. We study the reversible and non-volatile magnetization switching, as well as the magnetic anisotropy variation under the applied electric field. The research contents are as follows:

(1) The Co61Fe26Si13 (CoFeSi), Fe86Si14 and Fe80Si20 (FeSi) thin films were deposited on (001)MgO substrates by radio frequency magnetron sputtering. The influences of post annealing temperature on crystallization, magnetic anisotropy and dynamic high frequency magnetic properties of CoFeSi films were investigated. The epitaxial relationship of (001)[100]MgO//(001)[110]FeSi is characterized by x-ray diffraction and transmission electron microscopy. The static magnetic properties measurements show the four-fold symmetric cubic magnetic anisotropy of FeSi thin films, demonstrating the (001) epitaxial relation of FeSi films through the magnetic viewpoint. The magnetization reversal process is also investigated by measuring mico-focused magneto optical Kerr hysteresis loops. Moreover, in the Fe86Si14/MgO(001) thin films, the large cubic anisotropy field significantly increases the natural resonance frequency up to 8.0 GHz, and the natural resonance phenomena can be observed in any in-plane directions. It exhibits the advantage comparing with the soft magnetic thin films with in-plane uniaxial anisotropy, in which the natural resonance phenomenon disappears in some specific directions.

(2) The epitaxial Fe86Si14 films were fabricated on (001) PMN-0.32PT single crystal substrate by radio frequency magnetron sputtering. The magnetic anisotropy transition from four-fold cubic anisotropy to two-fold uniaxial magnetic anisotropy occurs after applying negative electric fields in Fe86Si14/(001)PMN-0.32PT heterostructures. Unfortunately, the magnetic anisotropy transition is non-volatile and irreversible. Based on theoretical analysis and simulation results, we modified the composition of the ferromagnetic layer and the ferroelectric substrate. In the fabricated epitaxial Fe80Si20/(001)PMN-0.3PT heterostructures, we study the non-volatile and reversible 90o magnetization switching with the uniaxial magnetic anisotropy dominated under ±6 kV/cm by measuring the angular dependent remanent magnetization. By measuring mico-focused magneto optical Kerr hysteresis loops under electric fields, the non-volatile magnetization switching and internal competitive relation between cubic magnetocrystalline anisotropy energy and in-plane uniaxial anisotropy energy are further confirmed.

(3) The feasibility of reversible and non-volatile magnetic transition between four-fold and two-fold magnetic anisotropy by electric field is demonstrated through free energy analysis and simulation in epitaxial FeSi/(011)PMN-PT heterostructures. In the epitaxial Fe80Si20/(011)PMN-0.3PT heterostructures with 300 oC post annealing process, the reversible and nonvolatile transition between the four-fold cubic magnetocrystalline anisotropy and uniaxial magnetic anisotropy under saturated electric field and converse coercive electric field is demonstrated by means of static magnetic properties measurements. Meanwhile, we also achieved electric-field mediated reversible and non-volatile 90o and 180° magnetization switching with the assistance of the magnetic field pulse. Finally, the importance of the selection of the ferroelectric substrate and the crystallization of the ferromagnetic film on the electric field modulation of the magnetic properties in Fe80Si20/(011)PMN-PT heterostructures were also verified experimentally.

URL查看原文
Language中文
Document Type学位论文
Identifierhttps://ir.lzu.edu.cn/handle/262010/229237
Collection物理科学与技术学院
Recommended Citation
GB/T 7714
郭晓斌. 外延铁磁/铁电异质结中电场调控磁性的研究[D]. 兰州. 兰州大学,2018.
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