Effect of Mo capping layers thickness on the perpendicular magnetic anisotropy in MgO/CoFeB based top magnetic tunnel junction structure

A detailed study of the magnetic characterizations of the top structure MgO/CoFeB/Mo is presented.The samples show strong perpendicular magnetic anisotropy(PMA) when the thickness of CoFeB is 0.9 nm and 1.1 nm.The saturation magnetic moment and interface anisotropy constant are 1566 emu/cm~3 and 3.75 erg/cm~2,respectively.The magnetic dead layer(MDL) is about 0.23 nm in this system.Furthermore,strong capping layer thickness dependence is also observed.The strong PMA of 1.1 nm CoFeB only exists in a Mo cap layer thickness window of 1.2-2 nm.To maintain PMA,the metal layer could not be too thin or thick in these multilayers.The oxidation and diffusion of the metal capping layer should be respectively responsibility for the degradation of PMA in these thin or thick metal capping layer samples.     

过量B的Ta/CoFeB/MgO薄膜垂直各向异性和温度稳定性的增强

以CoFeB/MgO为核心单元的垂直各向异性薄膜体系和相关的垂直磁隧道结已获得广泛研究,其中CoFeB的B含量基本都保持为原子比20%.本文采用磁控溅射制备了Ta/(Co0.5Fe0.5)1-xBx/MgO三明治结构及生长顺序相反的系列薄膜,并在573—623K进行真空退火,研究了样品垂直各向异性随B成分的变化.结果显示,当B含量减小到10%时,Ta/CoFeB/MgO体系的垂直各向异性明显降低;相反,当B含量增加至30%时,该体系的垂直各向异性明显增强;发现在高B含量的情形下,样品的垂直各向异性大小与温度稳定性均与三明治结构的生长顺序密切相关;获得了具有优异温度稳定性的垂直磁化MgO/CoFeB/Ta样品.结果表明适当增加B含量是增强CoFeB/MgO体系垂直各向异性和温度稳定性的有效途径之一.     

Mo覆盖层对MgO/CoFeB/Mo结构磁各向异性的影响

研究了Mo覆盖层厚度对MgO/CoFeB结构磁各向异性的影响. 研究发现, 加平行磁场生长出来的MgO/CoFeB/Mo样品表现为面内各向异性, 并且随着CoFeB的厚度减小, 面内各向异性逐渐减弱; 在CoFeB厚度减小到1.1 nm时, 仍可以保持面内各向异性, 垂直方向的外加饱和场逐渐减少; 厚度在0.9 nm及以下的情况下, 面内各向异性消失. 改变Mo覆盖层厚度, 当t_Mo= 1.6 nm时, 垂直方向的饱和场最小. 当生长过程的磁场变为垂直磁场时, 不同厚度的Mo覆盖层对MgO/CoFeB 的磁各向异性影响不同. Mo厚度在1 nm及以下时MgO/CoFeB/Mo样品表现为面内各向异性, Mo覆盖层厚度在1.2和5 nm之间时样品出现了垂直磁各向异性; 并且垂直方向的矫顽力也发生了变化, Mo覆盖层厚度为1.4 nm时样品的磁滞损耗会大一些.     

CoFeB/Pt多层膜的垂直磁各向异性研究

具有垂直磁各向异性的磁性纳米结构是自旋转移力矩器件的重要研究内容, 本文采用反常霍尔效应系统地研究了磁控溅射法制备的[CoFeB/Pt]_n多层膜的垂直磁各向异性. 当CoFeB的厚度小于0.6 nm时, 可以在[CoFeB/Pt]_n多层膜中观察到清晰的垂直磁各向异性, 其垂直磁各向异性强烈依赖于CoFeB和Pt层厚度及多层膜周期数. 当多层膜周期数n ≥ 5时, 出现零剩磁现象. 另外, [CoFeB/Pt]_n多层膜的矫顽力均小于2 kA·m^-1, 有望作为垂直自由层的重要侯选材料应用于垂直磁纳米结构中.     

Anomalous Hall effect in perpendicular CoFeB thin films

Our recent research achievements in the perpendicular magnetic anisotropy （PMA） properties of the CoFeB sand- wiched by MgO and tantalum layers are summarized. We found that the PMA behaviors of Ta/CoFeB/MgO and MgO/CoFeB/Ta thin films are different. The larger PMA in the latter film is related to the lower magnetization of CoFeB deposited on MgO. Furthermore, we have demonstrated a large anomalous Hall effect in perpendicular CoFeB thin fihn. Our results show large anomalous Hall resistivity, large longitudinal resistivity, and low switching field can be achieved, all at the same time, in the perpendicular CoFeB thin film. Anomalous Hall effect with high and linear sensitivity is also found in an MgO/CoFeBFFa thin film with a thick MgO layer, which opens a door tbr future device applications of perpendicular ferromagnetic thin films.     

Large planar Hall effect in perpendicularly magnetized W/CoFeB/MgO structures

The planar Hall effect (PHE) in W/CoFeB/MgO structure with perpendicular magnetic anisotropy was investigated as a function of CoFeB thickness (t_CoFeB). The PHE is measured by sweeping the in-plane magnetic field at various azimuthal angles as well as by rotating strong magnetic field which is enough to saturate the magnetization. We observed a huge PHE in the W/CoFeB/MgO sample, which is even larger than anomalous Hall effect (AHE). This is distinct from the results in Ta/CoFeB/MgO samples showing a much smaller PHE than AHE. Since the PHE is insensitive to the t_CoFeB while the AHE is proportional to the t_CoFeB, the unprecedented PHE can be attributed to the W layer with a large spin-orbit coupling.     

MgO/Pt界面对增强Co/Ni多层膜垂直磁各向异性及热稳定性的研究

采用直流磁控溅射法在玻璃基片上制备了Pt底层和MgO/Pt双底层的Co/Ni多层膜样品, 通过反常霍尔效应研究了不同MgO厚度和退火温度对样品垂直磁各向异性(perpendicular magnetic anisotropy, PMA)的影响. 随着底层中MgO厚度的逐渐增加, 样品的矫顽力也随之增强, 霍尔电阻变化不大; 对样品进行退火处理后发现, 单纯Pt底层的Co/Ni多层膜随着退火温度的升高, 霍尔电阻逐渐降低, 矫顽力则迅速降低, 热稳定性较差; 而当MgO/Pt双底层的样品在200 ℃退火后矫顽力大幅增加, 霍尔电阻略微有所减小, 更高的退火温度使得Co和Ni合金化, 导致多层膜的PMA特征减弱.     

Magnetic properties of a Pt/Co₂ FeAl/MgO structure with perpendicular magnetic anisotropy

Microstructures and magnetic properties of Ta/Pt/Co 2 FeAl(CFA)/MgO multilayers are studied to understand perpendicular magnetic anisotropy(PMA) of half-metallic full-Heusler alloy films.PMA is realized in a 2.5-nm CFA film with B2-ordered structure observed by a high resolution transmission electron microscope.It is demonstrated that a high quality interface between the ferromagnetic layer and oxide layer is not essential for PMA.The conversions between in-plane anisotropy and PMA are investigated to study the dependence of magnetic moment on temperature.At the intersection points,the decreasing slope of the saturation magnetization(M s) changes because of the conversions.The dependence of M s on the annealing temperature and MgO thickness is also studied.     

Magnetic properties of a Pt/Co2FeAl/MgO structure with perpendicular magnetic anisotropy

Microstructures and magnetic properties of Ta/Pt/Co 2 FeAl（CFA）/MgO multilayers are studied to understand perpendicular magnetic anisotropy（PMA） of half-metallic full-Heusler alloy films.PMA is realized in a 2.5-nm CFA film with B2-ordered structure observed by a high resolution transmission electron microscope.It is demonstrated that a high quality interface between the ferromagnetic layer and oxide layer is not essential for PMA.The conversions between in-plane anisotropy and PMA are investigated to study the dependence of magnetic moment on temperature.At the intersection points,the decreasing slope of the saturation magnetization（M s） changes because of the conversions.The dependence of M s on the annealing temperature and MgO thickness is also studied.     

CoFeB/AlOx/Ta及AlOx/CoFeB/Ta结构中垂直易磁化效应的研究

本文详细研究了在不同氧化层和铁磁层厚度情况下, 底层CoFeB/AlO_x/Ta结构和 顶层AlO_x/CoFeB/Ta结构中的垂直磁各向异性. 在底层CoFeB/AlO_x/Ta结构中观察到了垂直磁化的磁滞回线, 证明了其垂直易磁化效应的存在; 而在顶层AlO_x/CoFeB/Ta结构中却没有观察到类似的磁滞回线. 对这种对称结构中的非对称现象进行了分析. 研究还发现不同的氧化层和铁磁层厚度均会影响层间界面相互作用的强度, 从而导致结构的垂直磁化曲线矫顽力大小发生改变. 这项研究将对基于AlO_x氧化层垂直磁隧道结的研制具有重要的意义. 关键词： 垂直磁各向异性 磁隧道结 随机存储器     

X-ray scattering study on the electric field-induced interfacial magnetic anisotropy modulation at CoFeB / MgO interfaces

The electric field-induced modifications of magnetic anisotropy in CoFeB/MgO systems are studied using X-ray resonant magnetic scattering and magneto-optical Kerr effect. Voltage dependent changes of the magnetic anisotropy of −12.7 fJ/Vm and −8.32 fJ/Vm are observed for Ta/CoFeB/MgO and Hf/CoFeB/MgO systems, respectively. This implies that the interfacial perpendicular magnetic anisotropy is reduced (enhanced) when electron density is increased (decreased). X-ray resonant magnetic scattering measurements reveal that the small in-plane magnetic component of the remanent state of CoFeB/MgO systems with weak magnetic anisotropy changes depending on the applied voltage leading to modification of the magnetic anisotropy at the CoFeB/MgO interface.     

Perpendicular magnetization and exchange bias in epitaxial NiO/[Ni/Pt]2 multilayers

The realization of perpendicular magnetization and perpendicular exchange bias(PEB)in magnetic multilayers is important for the spintronic applications.NiO(t)/[Ni(4 nm)/Pt(1 nm)]₂multilayers with varying the NiO layer thickness t have been epitaxially deposited on SrTiO;(001)substrates.Perpendicular magnetization can be achieved when t<25 nm.Perpendicular magnetization originates from strong perpendicular magnetic anisotropy(PMA),mainly resulting from interfacial strain induced by the lattice mismatch between the Ni and Pt layers.The PMA energy constant decreases monotonically with increasing t,due to the weakening of Ni(001)orientation and a little degradation of the Ni–Pt interface.Furthermore,significant PEB can be observed though NiO layer has spin compensated(001)crystalline plane.The PEB field increases monotonically with increasing t,which is considered to result from the thickness dependent anisotropy of the NiO layer.     

垂直磁各向异性L10-Mn1.67Ga超薄膜分子束外延生长与磁性研究

具有超强垂直磁各向异性的L1₀-Mn_xGa薄膜由于其与半导体材料结构及工艺的高度兼容性而受到广泛关注, 其超高垂直磁各向异性能和极低的磁阻尼因子预示着L1₀-Mn_xGa薄膜在高热稳定性自旋电子学器件中将发挥重要作用. 而L1₀-Mn_xGa超薄膜对于降低L1₀-Mn_xGa基垂直磁各向异性隧道结中的磁矩翻转临界电流密度有着重要的意义. 本文采用分子束外延的方法, 在半导体GaAs衬底上成功制备出了一系列不同厚度的L1₀-Mn_1.67Ga薄膜, 厚度范围为1-5 nm. 生长过程中反射式高能电子衍射原位检测以及X射线衍射结果均表明了其良好的单晶相. 磁性测量结果表明, 厚度在1 nm以上的L1₀-Mn_1.67Ga薄膜均可以保持垂直磁各向异性特征, 厚度为5 nm的L1₀-Mn_1.67Ga薄膜的垂直磁各向异性能可达到14.7 Merg/cm³. 这些结果为基于L1₀-Mn_1.67Ga的垂直磁各向异性隧道结在自旋转移扭矩驱动的磁随机存储器等低功耗器件的集成及应用提供了重要的实验支持.     

Effects of Fe-Oxide and Mg Layer Insertion on Tunneling Magnetoresistance Properties of CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

To study the influence of CoFeB/MgO interface on tunneling magnetoresistance(TMR),different structures of magnetic tunnel junctions(MTJs) are successfully prepared by the magnetron sputtering technique and characterized by atomic force microscopy,a physical property measurement system,x-ray photoelectron spectroscopy,and transmission electron microscopy.The experimental results show that TMR of the CoFeB/Mg/MgO/CoFeB structure is evidently improved in comparison with the CoFeB/MgO/CoFeB structure because the inserted Mg layer prevents Fe-oxide formation at the CoFeB/MgO interface,which occurs in CoFeB/MgO/CoFeB MTJs.The inherent properties of the CoFeB/MgO/CoFeB,CoFeB/Fe-oxide/MgO/CoFeB and CoFeB/Mg/MgO/CoFeB MTJs are simulated by using the theories of density functions and non-equilibrium Green functions.The simulated results demonstrate that TMR of CoFeB/Fe-oxide/MgO/CoFeB MTJs is severely decreased and is only half the value of the CoFeB/Mg/MgO/CoFeB MTJs.Based on the experimental results and theoretical analysis,it is believed that in CoFeB/MgO/CoFeB MTJs,the interface oxidation of the CoFeB layer is the main reason to cause a remarkable reduction of TMR,and the inserted Mg layer may play an important role in protecting Fe atoms from oxidation,and then increasing TMR.     

CoFeB/MgO不连续多层纳米软磁薄膜微波电磁特性

采用直流/射频磁控溅射方法和不连续多层交替沉积工艺,制备了CoFeB/MgO系列纳米不连续多层薄膜,研究了微波频段下的电磁性能.结果表明,通过调整CoFeB合金相和MgO介质相的相对含量可有效调控薄膜的微结构和电磁性能,且在［Co₆₄Fe₂₄B₁₂(0.7nm)/MgO(0.4nm)］₄₀薄膜中获得了优良的微波软磁性能和高电阻率,其饱和磁化强度1.3 T,难轴矫顽力130 A/m,电阻率3.4mΩ·cm,且共振频率高达2.1 GHz,磁导率实部μ′和虚部μ″在1.59 GHz处均高于240,并且在0.9—2 GHz宽频带范围同时大于100.该薄膜可用于微波吸收材料和电磁兼容的设计中. 关键词： 纳米磁性颗粒膜 磁导率频谱 电磁性能 吸波材料     

Ferromagnetic resonance in monocrystalline spin valves CoFeB/Ta/CoFeB and CoFeB films with perpendicular magnetic anisotropy

Orientation dependences of ferromagnetic resonance in a MgO/CoFeB/MgO/Ta film with one ferromagnetic layer (monolayer) and in a MgO/CoFeB/Ta/CoFeB/MgO/Ta spin valve containing two single-crystal ferromagnetic CoFeB layers divided by a nonmagnetic Ta layer (bilayer) were investigated. Analysis of the orientation dependences of the structures with perpendicular magnetic anisotropy allowed calculating constants of magnetic anisotropy and damping factors. Physical reasons underlying the differences between these parameters in one- and two-layered structures are discussed.     

Pt974Mn26/Co多层膜的磁性与磁光性质研究

用磁控溅射法制备了Mn含量一定、不同PtMn层厚度的Pt974Mn26/Co磁性多层膜系列，通过x射线衍射对该多层膜系列进行结构分析；测定了不同PtMn层厚度系列样品的磁滞回线、有效垂直各向异性，分析了饱和磁化强度和有效垂直各向异性变化的原因；通过测定该多层膜体系的克尔谱，分析了一定波长下克尔角随PtMn层厚度变化的规律.认为克尔角的变化是由于界面的合金化以及原子的极化减小所致. 关键词： 多层膜 磁性 磁光     

多层膜［Co85Cr15/Pt］20的磁性、垂直磁记录特性和微结构的关系

采用磁控溅射法制备了性能优良的以Pt为缓冲层的［Co85₈₅Cr15_{15/Pt］2020 多层膜，研究了溅射气压对［Co8585Cr1515/Pt］2020多层膜微结构和磁性的 影响.研究结果表明，Ar溅射气压对［Co8585Cr1515/Pt］2020多层膜的微结构 、垂直磁各向异性和矫顽力有重要的影响 关键词： 溅射气压 多层膜 垂直磁各向异性 有效磁各向异性常数}     

Ultrathin (CoFe/Pt)n multilayers with tuned magnetic properties

Perpendicular magnetic anisotropy (PMA) has been investigated in ultrathin (CoFe [0.2] nm/Pt [0.2] nm)_n multilayers. The Pt layers show an fcc crystal structure with a preferred [111] orientation. The multilayers with n=3, 4 show PMA in the as-grown state, which can be enhanced by thermal annealing. However, no PMA is observed in the as-grown state with higher repetitions (n>&=5), although it is observed after thermal annealing. For 1=&<n=&<8, the anisotropy energy is around 10⁵ J/m³ for all (CoFe [0.2]/Pt [0.2])_n stacks. The perpendicular anisotropy is related to layer thickness and interface roughness.     

Co/Ni多层膜垂直磁各向异性的研究

采用直流磁控溅射法在玻璃基片上制备了Pt底层的Co/Ni多层膜样品, 对影响样品垂直磁各向异性的各因素进行了调制, 通过样品的反常霍尔效应系统的研究了Co/Ni多层膜的垂直磁各向异性. 结果表明, 多层膜中各层的厚度及周期数对样品的反常霍尔效应和磁性有重要的影响. 通过对多层膜各个参数的调制优化, 最终获得了具有良好的垂直磁各向异性的Co/Ni多层膜最佳样品Pt(2.0)/Co(0.2)/Ni(0.4)/Co(0.2)/Pt(2.0), 经计算, 该样品的各向异性常数K_eff 达到了3.6×10⁵ J/m³, 说明样品具备良好的垂直磁各向异性. 最佳样品磁性层厚度仅为0.8 nm, 样品总厚度在5 nm以内, 可更为深入的研究其与元件的集成性.