Ce1.66Mg1.34Co3和α″-Fe16N2多层梯度膜磁反转性质的微磁学模拟

为了改善永磁薄膜的磁性能,基于微磁学理论,使用编程软件OOMMF针对Ce_1.66Mg_1.34Co₃和α″-Fe₁₆N₂交换耦合多层梯度膜的磁化过程进行模拟,系统研究磁晶各向异性梯度对多层膜性能的影响,分析体系的剩余磁化强度、矫顽力、磁滞回线和磁化反转过程中的能量变化。研究表明：减小磁晶各向异性梯度或增加界面处磁晶各向异性的差值,可以有效提高薄膜的矫顽力和剩余磁化强度,从而改善磁性能。通过计算磁矩分布发现一种磁涡旋态,这种磁涡旋态的产生过程伴随系统能量的增加。     

c轴垂直取向FePt薄膜的磁和磁光性能研究

采用直流磁控溅射的方法在氧化镁(100)单晶基板上生长了一系列c轴垂直取向的FePt薄膜,通过改变沉积时的基板温度,薄膜从Fe,Pt原子无序排列的面心立方结构逐渐变化到有序排列的L10相面心四方结构.在此基础上,系统研究了FePt薄膜的化学有序度对磁和磁光性能的影响.随着有序度的增加,FePt薄膜的磁晶各向异性能,以及沿垂直方向的矫顽力、剩磁比均增加,在基板温度高于530℃时制备的薄膜中的磁晶各向异性能超过1J/cm3.同时,还观察到有序FePt合金薄膜的磁光克尔光谱(克尔转角的大小和极值所对应的跃迁光子能量)随化学有序度的显著变化.     

Fe_3O_4单晶薄膜磁性电场调控的微磁学仿真研究

磁性薄膜磁学特性电场调控的相关研究对开发新型低功耗磁信息器件具有突出意义.本文基于电场调控磁性的基本理论,以OOMMF(Object Oriented Micro-Magnetic Frame)微磁学仿真软件为工具,研究了电场对生长于PZN-PT单晶衬底上Fe_3O_4单晶薄膜磁学特性的调控.研究结果显示:无外加电场时,薄膜表现出典型的软磁特性;沿衬底[001]方向施加的外加电场可以使得薄膜矫顽力、矩形比等磁学特性发生显著改变:当外加磁场沿[100]([010])时,施加正值(负值)电场强度可以显著增大薄膜的矫顽力与矩形比,当电场强度不小于0.6 MV/m时薄膜矩形比达到1.这是因为外加电场导致薄膜产生单轴应力各向异性,使得薄膜的等效磁各向异性发生了从无外电场下的面内四重磁晶各向异性向高电场下的近似单轴磁各向异性的过渡.外加1 MV/m与-1 MV/m的电场时等效易磁化轴分别沿[100]与[010]方向.另外,外加1 MV/m(-1 MV/m)的电场强度可以使得铁磁共振的频率增大(减小)接近1 GHz.     

铁磁和反铁磁双层膜中铁磁共振的研究

采用微磁学理论研究了铁磁/反铁磁双层膜中的铁磁共振现象.本模型将铁磁薄层抽象为一个单晶,具有立方磁晶各向异性和单轴磁晶各向异性,而反铁磁层视为厚度趋近于半无穷,且只有单轴磁晶各向异性.推导出了该系统的铁磁共振频率和频率谱宽度的解析式.数值计算表明,铁磁共振模式分两支,取决于立方磁晶各向异性.而界面的交换耦合,是磁易轴具有单向性的起因.     

具有条纹磁畴结构的NiFe薄膜的制备与磁各向异性研究

具有条纹磁畴结构的磁性薄膜表现出面内转动磁各向异性,对于解决高频电子器件的方向性问题起着至关重要的作用.本文采用射频磁控溅射的方法,研究了NiFe薄膜的厚度、溅射功率密度、溅射气压等制备工艺参数对条纹磁畴结构、面内静态磁各向异性、面内转动磁各向异性、垂直磁各向异性的影响规律.研究发现,在功率密度15.6 W/cm~2与溅射气压2 mTorr(1 Torr=1.33322×102Pa)下生长的NiFe薄膜,表现出条纹磁畴的临界厚度在250 nm到300 nm之间.厚度为300 nm的薄膜比250 nm薄膜的垂直磁各向异性场增大近一倍,从而磁矩偏离膜面形成条纹磁畴结构,并表现出面内转动磁各向异性.高溅射功率密度可以降低薄膜出现条纹磁畴的临界厚度.在相同功率密度15.6 W/cm~2下生长300 nm的NiFe薄膜,随着溅射气压由2 mTorr增大到9 mTorr,NiFe薄膜的垂直磁各向异性场逐渐由1247.8 Oe(1 Oe=79.5775 A/m)增大到3248.0 Oe,面内转动磁各向异性场由72.5 Oe增大到141.9 Oe,条纹磁畴周期从0.53μm单调减小到0.24μm.NiFe薄膜的断面结构表明柱状晶的形成是表现出条纹磁畴结构的本质原因,高功率密度下低溅射气压有利于柱状晶结构的形成,表现出规整的条纹磁畴结构,高溅射气压会导致柱状晶纤细化,面内转动磁各向异性与面外垂直磁各向异性增强,条纹磁畴结构变得混乱.     

c轴垂直取向FePt薄膜的磁和磁光性能研究

采用直流磁控溅射的方法在氧化镁(100)单晶基板上生长了一系列c轴垂直取向的FePt薄膜，通过改变沉积时的基板温度，薄膜从Fe，Pt原子无序排列的面心立方结构逐渐变化到有序排列的L1₀相面心四方结构.在此基础上，系统研究了FePt薄膜的化学有序度对磁和磁光性能的影响.随着有序度的增加，FePt薄膜的磁晶各向异性能，以及沿垂直方向的矫顽力、剩磁比均增加，在基板温度高于530 ℃时制备的薄膜中的磁晶各向异性能超过1 J/cm³.同时，还观察到有序FePt合金薄膜的磁光克尔光谱(克尔转角的大小和极值所对应的跃迁光子能量)随化学有序度的显著变化. 关键词： FePt薄膜 化学有序度参数 磁光克尔光谱     

稀土永磁薄膜材料

简要地介绍在纳米复合稀土永磁薄膜材料、各向异性稀土永磁薄膜材料方面的进展.在纳米复合稀土永磁薄膜材料中实现磁性交换耦合和剩磁增强效应,系统地研究了其结构与磁性的关系.制备成功高磁能积的各向异性稀土永磁薄膜材料,比较了Ti 或Mo 缓冲层对Nd-Fe-B薄膜的表面形貌、磁畴结构和磁性能的影响.发现薄膜的表面形貌强烈地依赖于缓冲层的厚度.由于它极大地影响薄膜的成分,溅射速率被证明是控制薄膜的显微结构、表面形貌和磁性能的一个重要因素.在微磁学模型的基础上,通过分析从5到300 K的矫顽力温度依赖关系.研究了各向异性Pr-Fe-B薄膜的矫顽力机制.在晶粒表面,由于磁各向异性的降低和局域退磁场的提高导致的反转畴的形核被确定为控制各向异性Pr-Fe-B薄膜的磁化反转过程的首要机制.     

氮化铁薄膜晶相合成热分析及其磁性

研究Fe-N体系晶相转变(相变)规律对于高效合成高自旋极化率的g'-Fe4N薄膜材料非常重要.利用同步热分析(TG-DSC)研究了氮化铁薄膜的相变规律. TG-DSC的结果显示,在10℃/min的升温速率下,g'-FeN薄膜在常温到800℃之间共有5次相变,分别为I,g'-FeN→x-Fe2N;II,x-Fe2N→e-Fe3N;III,e-Fe3N→g'-Fe4N; IV, g'-Fe4N→g-Fe;以及V, g-Fe→a-Fe.利用真空退火技术有效调控了氮化铁薄膜的晶相.X-射线衍射测试结果显示,直接在纯氮气中溅射得到的氮化铁薄膜是单相的g'-FeN,经350, 380和430℃退火可分别获得单相的x-Fe2N, e-Fe3N和g'-Fe4N.研究了氮化铁薄膜的磁学性能.振动样品磁强计测试结果显示, g'-Fe4N薄膜在面内/面外表现出明显的磁各向异性,属于典型的磁形状各向异性.     

利用FePt/Au多层膜结构制备垂直磁记录L10-FePt薄膜

利用磁控溅射方法在100℃的MgO单晶基片上制备了[FePt/Au]10多层膜,并研究了采用FePt/Au多层膜结构对FePt薄膜的有序化温度、矫顽力(HC)、垂直磁各向异性、晶粒尺寸以及颗粒间磁交换耦合作用的影响.磁性测试结果表明:FePt/Au多层膜在退火后具有较高的HC、良好的垂直磁各向异性、较小的晶粒尺寸且无磁交换耦合作用.截面高分辨电镜分析表明:Au可以缓解MgO和FePt之间较大的晶格错配,从而促进薄膜的垂直磁各向异性;同时,采用FePt/Au多层膜结构增加了FePt/Au界面能、应力能以及Au原子在薄膜中的扩散作用,促进了薄膜的有序化,从而有效降低了有序化温度,并且大幅度提高其HC.此外,Au原子部分扩散到FePt相的边界处,起到抑制FePt晶粒生长、隔离FePt颗粒的作用,从而显著降低了FePt晶粒的尺寸和颗粒间磁交换耦合作用.     

衬底层对Fe65Co35合金薄膜结构与磁性的影响

采用磁控溅射法制备了具有不同衬底层(Cu, Co和Ni₈₀Fe₂₀)的FeFe₆₅Co₃₅双层合金薄膜.研究了不同衬底材料以及NiFe衬底层厚度对FeCo合金薄膜结构与磁性的影响.研究结果表明:衬底层的引入可以增加薄膜的面内单轴各向异性,且薄膜的软磁性能显著提升,获得良好软磁性的原因归结为晶粒的细化、层间的偶极相互作用以及表面粗糙度的降低,并且对于相同厚度的不同衬底层, NiFe衬底层对FeCo薄膜软磁性能的提升最为明显;通过改变NiFe衬底层厚度,实现了对薄膜各向异性的调控, NiFe/FeCo表现出良好的高频响应和低的阻尼系数,同时较小的薄膜厚度能够减小涡流损耗,因此,促进了其在高频微波磁性器件方面的应用.     

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.     

A comparison of FePt thin films with HfO2 or MnO additive

The influence of oxide additives on the magnetic and structural properties of FePt L1₀ thin films has been studied. FePt films with HfO₂ additive grown on a 5 nm MgO buffer showed a primarily random texture for both as-deposited and annealed samples. The average grain size was limited to 10 nm and the perpendicular coercivity was 1.3 kOe for a 10 nm thick FePt +20% HfO₂ film annealed at 650°C for 10 min. In direct contrast, MnO additive neither limited grain size nor L1₀ ordering in annealed FePt films. A 10 nm thick FePt+20% MnO film grown on a 5 nm MgO buffer showed a unique discontinuous microstructure composed of clusters of (0 0 1) textured L1₀ grains after being annealed at 650°C for 10 min. The average size of the grains making up these clusters was 50 nm and the perpendicular coercivity of the film exceeded 7 kOe.     

Magnetization reversal in a preferred oriented (111) L1(0) FePt grown on a soft magnetic metallic glass for tilted magnetic recording

L1(0) FePt is an important material for the fabrication of high density perpendicular recording media, but the ultrahigh coercivity of L1(0) FePt restricts its use. Tilting of the magnetic easy axis and the introduction of a soft magnetic underlayer can solve this problem. However, high temperature processing and the requirement of epitaxial growth conditions for obtaining an L1(0) FePt phase are the main hurdles to be overcome. Here, we introduce a bilayered magnetic structure ((111) L1(0) FePt/glassy Fe(71)Nb(4)Hf(3)Y(2)B(20)/SiO(2)/Si) in which the magnetic easy axis of L1(0) FePt is tilted by ~36° from the film plane and epitaxial growth conditions are not required. The soft magnetic underlayer not only promotes the growth of L1(0) FePt with the preferred orientation but also provides an easy cost-effective micro/nanopatterning of recording bits. A detailed magnetic characterization of the bilayered structure in which the thickness of (111) L1(0) FePt with the soft magnetic Fe(71)Nb(4)Hf(3)Y(2)B(20) glassy underlayer varied from 5 to 60 nm is carried out in an effort to understand the magnetization switching mechanism. The magnetization switching behavior is almost the same for bilayered structures in which FePt layer thickness is >10 nm (greater than the domain wall thickness of FePt). For FePt film ~10 nm thick, magnetization reversal takes place in a very narrow field range. Magnetization reversal first takes place in the soft magnetic underlayer. On further increase in the reverse magnetic field, the domain wall in the soft magnetic layer compresses at the interface of the hard and soft layers. Once the domain wall energy becomes sufficiently large to overcome the nucleation energy of the domain wall in L1(0) FePt, the magnetization of the whole bilayer is reversed. This process takes place quickly because the domain walls in the hard layer do not need to move, and the formation of a narrower domain wall may not be favorable energetically. Our results showed that the present bilayered structure is very promising for the fabrication of tilted bit-patterned magnetic recording media.     

Ag和Ti底层对[Fe/Pt]n多层膜有序化的影响

利用磁控溅射的方法，在热玻璃基片上制备了以Ag，Ti，Cu，Cr，Pt和Ta为底层的［Fe/Pt］_n多层膜，后经不同温度真空热处理，得到L1₀有序结构的FePt 薄膜（L1₀-FePt）.实验结果表明，以Ag和Ti为底层，通过采用基片加温，同时 利用［Fe/Pt］_n多层膜结构，可以促进FePt薄膜的有序化过程，使FePt-L1_０有序化温度从500℃降低到350℃.在较高的温度下退火，以Ag为底层对薄膜的磁性 能影响较小，而以Ti为底层在高于500℃退火后，矫顽力明显下降.在400℃退火20min后，以 Ag和Ti为底层的样品平行膜面的矫顽力分别达到597kA/m和645kA/m，剩磁比分别达到0.81和 0.94，为将来FePt-L1_０有序相合金薄膜用于未来超高密度磁记录介质打下基础. 关键词： ０-FePt薄膜')" href="#">L1_０-FePt薄膜 有序化温度 底层 多层膜结构     

The role of short exchange length in the magnetization processes of L10-ordered FePt perpendicular media

A three-dimensional micromagnetic model with non-uniform grain size distribution has been built up to study the magnetization process in FePt L1₀ perpendicular media. A 3D model of a single FePt magnetic grain is also set up for comparison. The high magneto-crystalline anisotropy K_u results in a short exchange length l_ex in FePt nanograins. Therefore a magnetic grain is divided into smaller grids on the order of l_ex. The simulated perpendicular and longitudinal loops are consistent with experiments, and it is explained why the measured perpendicular H_c is relatively smaller compared with the saturation field of the longitudinal loop in the FePt perpendicular medium.     

Influence of thickness on current-induced magnetization switching in L1_0-FePt single layer

The thickness dependent spin–orbit torque(SOT) in an L1_0-Fe Pt single layer is investigated in this work. As the thickness increases from 8 nm to 16 nm, the magnetization switching ratio in the L1_0-Fe Pt film with higher chemical ordering becomes smaller. It is noted that compared with 3-nm-thick L1_0-Fe Pt film, 8-nm-thick L1_0-Fe Pt film can switch much magnetization with the increase of chemical ordering. When the Fe Pt film is thick enough, the SOT in Fe Pt is closely related to the L1_0-ordered structure, which indicates a bulk nature. Therefore, the disordering plays an important role in the magnetization switching only for the ultra-thin Fe Pt films, while the structural gradient may play an important role for thicker films. However, both of the two mechanisms cannot fully explain the process of magnetization switching and the spin current generation. Although many factors influence SOT, here in this work we emphasize only the bulk nature of strong SOC in L1_0-Fe Pt through density functional theory calculations, which should generate large spin current due to spin Hall effect.     

Strongly in-plane magnetized FePt thin film on ultrathin Fe underlayer

The effect of ultrathin Fe underlayer on the strong in-plane magnetization of FePt magnetic thin film was investigated. This FePt thin film could be attained using the ultrathin Fe underlayer with 1 nm thickness. The in-plane coercivity of FePt film with 20 nm thickness grown on ultrathin Fe underlayer was high up to 7400 Oe. However, its out-of-plane coercivity was extremely low to 350 Oe compared to those of FePt thin films in other conventional studies. This result indicates that FePt thin film was strongly in-plane magnetized by ultrathin Fe underlayer. The strong ordering phase transformation kinetics and the high texturing to in-plane direction of the FePt thin film by ultrathin Fe underlayer were confirmed by Kinetics Monte Carlo (KMC) simulation and XRD measurement result, respectively. It is also supposed that they are associated with the reduction of an interface free energy between the film and the substrate with an introduction of ultrathin underlayer.     

氧化物隔离对Si基片上生长L1_0相FePt薄膜磁性的影响

用MgO和SiO_2两种氧化物将FePt薄膜与Si(100)基片隔离,分析隔离层在FePt层发生A1→L1_0转变过程中的作用,寻找用Si母材涂敷L1_0-FePt磁性层来提高磁力显微镜针尖矫顽力的合理方案.采用磁控溅射法在400?C沉积Fe Pt薄膜,在不同温度进行2 h的真空热处理,分析晶体结构和磁性的变化.结果表明:没有隔离层,Si基片表层容易发生扩散,50 nm厚FePt薄膜的矫顽力最大只有5kOe(1 Oe=10~3/(4π)A·m~(-1));而插入隔离层,矫顽力可以超过10 kOe;MgO在Si基片上容易碎裂,热处理温度不能高于600?C,用作隔离层,FePt的最大矫顽力为12.4 kOe;SiO_2与Si基片的晶格匹配更好,热膨胀系数差较小,能承受的最高热处理温度可以超过800?C,使得Fe Pt的矫顽力可以在5 kOe到15 kOe范围内调控,更适合用于制作矫顽力高并可控的磁力显微镜针尖.     

Microstructure and magnetic properties of nanocomposite FePt/MgO and FePt/Ag films

An in-plane magnetic anisotropy of FePt film is obtained in the MgO 5 nm/FePt t nm/MgO 5 nm films (where t=5, 10 and 20 nm). Both the in-plane coercivity (H_c∥) and the perpendicular magnetic anisotropy of FePt films are increased when introducing an Ag-capped layer instead of MgO-capped layer. An in-plane coercivity is 3154 Oe for the MgO 5 nm/FePt 10 nm/MgO 5 nm film, and it can be increased to 4846 Oe as a 5 nm Ag-capped layer instead of MgO-capped layer. The transmission electron microscopy (TEM)-energy disperse spectrum (EDS) analysis shows that the Ag mainly distributed at the grain boundary of FePt, that leads the increase of the grain boundary energy, which will enhance coercivity and perpendicular magnetic anisotropy of FePt film.     

Effects of Cr underlayer and Pt buffer layer on the interfacial structure and magnetic characteristics of sputtered FePt films

This work develops a new method for growing L1₀ FePt(0 0 1) thin film on a Pt/Cr bilayer using an amorphous glass substrate. Semi-coherent epitaxial growth was initiated from the Cr(0 0 2) underlayer, continued through the Pt(0 0 1) buffer layer, and extended into the L1₀ FePt(0 0 1) magnetic layer. The squareness of the L1₀ FePt film in the presence of both a Cr underlayer and a Pt buffer layer was close to unity as the magnetic field was applied perpendicular to the film plane. The single L1₀ FePt(1 1 1) orientation was observed in the absence of a Cr underlayer. When a Cr underlayer is inserted, the preferred orientation switched from L1₀ FePt(1 1 1) to L1₀ FePt(0 0 1) and the magnetic film exhibited perpendicular magnetic anisotropy. However, in the absence of an Pt intermediate layer, the Cr atoms diffused directly into the FePt magnetic layer and prevented the formation of the L1₀ FePt(0 0 1) preferred orientation. When a Pt buffer layer was introduced between the FePt and Cr underlayer, the L1₀ FePt(0 0 1) peak appeared. The thickness of the Pt buffer layer also substantially affected the magnetic properties and atomic arrangement at the FePt/Pt and Pt/Cr interfaces.