XMCD实验技术及其在铁磁薄膜中的应用
摘要
X射线磁性圆二色(X-ray Magnetic Circular Dichroism，XMCD)技术是上世纪九十年代发展起来的一种可以进行元素分辨的同步辐射磁测量实验技术，属于X射线吸收谱的一个分支；它具有元素分辨、化学键选择等优点，是研究复杂磁性样品体系的一种有效方法。另一方面，随着磁性随机存储器和超高密度存储技术的迅速发展，近二十多年来，磁性薄膜和超薄膜的结构、基本磁性、层间耦合、界面磁性及其相关效应一直是人们关注的焦点之一，尤其是光刻成微米和亚微米尺寸的磁性单元阵列的图形薄膜，由于具有与连续膜不同的磁性质，更是引起人们广泛的关注。利用XMCD技术对这些过渡族铁磁金属单质和合金纳米薄膜的研究，对于揭示磁性样品体系中新奇的磁特性所蕴含的内在机理具有重要意义。
软X射线磁性圆二色(soft X-ray magnetic circular dichroism，SXMCD)实验站是国家同步辐射实验室二期工程建设的一个新站，本论文的内容一方面是对实验站和光束线的安装调试、以及实验中遇到的问题和所采取的相应措施；另一方面，以新建成的XMCD实验设备为主，结合其它的结构和磁性表征手段，对铁磁单质薄膜、合金薄膜以及光刻制备的微米亚微米周期的图形化铁磁薄膜的磁矩、磁各向异性以及磁矩的厚度效应等进行了系统研究，主要的研究工作及结果如下：
一、XMCD的实验技术研究
1．合肥国家同步辐射实验室二期工程建立了基于软X射线吸收的光束线和实验站，光束线采用了平面变线距光栅单色器，可以提供100-1000eV的单色光。用气体电离室进行了同步光的波长标定，并用AXUV-100对光子的通量进行了测试，结果表明：样品处的光通量不低于10~8光子／秒，能量分辨本领在1000eV处不小于1000，满足设计要求。
2．在Labview5.1平台上开发了SXMCD实验站的数据采集和控制程序，实现了对硬件的调用以及数据的采集过程，主要功能包括SXMCD的数据采集、磁滞回线的测量等，预留了SXMLD和SMOKE的数据采集接口。
3．以均匀氧化的Al箔为样品，在光斑、磁场和样品的不同几何配置下，研究了外磁场对X射线吸收强度的影响规律，建立模型对实验结果进行了合理解释。吸收信号随磁场与样品的夹角θ增大而变强，其变化趋势与根据模型提出的经验公式1-(1-α)·cosθ符合得很好；吸收谱强度随着外磁场的增加而剧烈的衰减，衰减趋势可用磁场强度B的反比例函数：(P1)／(B+P2)+P3来拟合，拟合参数的变化与所表达的物理意义基本符合；在磁场作用下，观察到光斑在样品上的相对位置变化也会对吸收谱强度造成影响，指出这是XMCD实验中吸收边的边前和边后出现不重合的主要原因。
4．在用样品电流法测量XMCD谱中，外磁场的存在严重降低了样品电流信号；针对实验站内置式可旋转磁铁设计并安装一个紧靠磁极的高压金网，结果发现：不同磁场下随偏压升高样品电流信号迅速增强并趋于饱和，磁场越强，达到饱和需要的偏压越高；比较相同磁场和偏压下均匀氧化的Al箔和CoFe薄膜正反磁场下的吸收谱，发现该装置引入的系统误差仅相当于Co的圆二色值的2.17％；表明用这种方法进行XMCD谱的测量是可行的。
5．XMCD实验数据预处理的方法可以影响到自旋和轨道磁矩的精度，提出通过XPSPEAK4.1对实验数据拟合，写出吸收谱的解析函数；利用解析函数的积分值，提高了磁矩的计算精度。
二、XMCD对铁磁单质薄膜的研究
1．利用XMCD吸收谱研究了在MgO衬底外延的铁单晶薄膜的原子磁矩，根据加和定则得到铁原子的轨道和自旋磁矩分别是0.07μ_B和2.33μ_B，与已发表的实验和理论数据基本吻合；通过XMCD实验得出Fe膜在面内不同晶向的自旋与轨道磁矩，对薄膜面内轨道磁矩的各向异性研究发现：对于15nm厚的Fe膜，双轴磁各向异性与单轴磁各向异性可能叠加在一起。
2．利用XMCD技术研究了Au／Co／Au多晶薄膜的轨道和自旋磁矩随膜厚的变化：随薄膜厚度增加，轨道磁矩由大到小，自旋磁矩则与之相反，轨道磁矩对原子总磁矩的贡献逐渐下降，并趋近于体相Co原子的比值，这与RKKY理论一致。
三、XMCD对铁磁合金薄膜的研究
1．利用XMCD谱研究了50nm Co_(0.9)Fe_(0.1)薄膜，结果发现：样品中Fe、Co元素对磁化强度的贡献比为10.5∶89.5；由XMCD获得的合金平均原子磁矩1.90μ_B与用SQUID磁强计得到的合金平均原子磁矩1.82μ_B基本相符；对Co_(0.9)Fe_(0.1)薄膜面内元素分辨的磁各向异性的研究发现，除了在生长的磁诱导方向存在易磁化轴外，与该轴垂直的方向还存在一个类似易轴的软磁化轴(soft axis)；面内的两个难磁化轴与易磁化轴取向大约成66°夹角，构成面内双轴磁各向异性，并根据XMCD结果对面内双轴形成的原因进行了分析。
2．研究了不同厚度的Co_(0.9)Fe_(0.1)薄膜，主要结果如下：随薄膜厚度从5nm增至50nm，Fe的自旋磁矩从1.87μ_B振荡下降至1.70μ_B，而Co表现出相反的变化趋势；根据5nm和50nm的测量数据，近似估算出“磁死层”的厚度为0.83nm，并以此为基础对计算的自旋磁矩进行修正，修正后的Co自旋磁矩几乎不随薄膜厚度变化，而Fe的自旋磁矩变化则随膜厚呈现出单调减少的趋势；结合XRD的测量结果认为，晶化程度的提高可能是修正后Fe的自旋磁矩随膜厚增加而单调减小的主要原因。而由XMCD直接得到的自旋磁矩变化趋势则可能是界面原子互扩散和结晶度随薄膜厚度提高共同作用的结果。
四、XMCD对图形化铁磁薄膜的初步研究
以磁控溅射生长的多晶三明治结构Ta／CoFe／Ta为基础，由激光干涉法制备出微米和亚微米周期的一维铁磁光栅结构，研究的主要成果如下：
1．制备出光栅条纹方向与磁诱导方向垂直的图形薄膜系列，SEM图像显示条纹的实际宽度分别为0.6μm、1.9μm、3.2μm、6.0μm、9.0μm，条纹宽度的均一性良好，小尺寸的磁条宽度与设计尺寸略有出入，大尺寸下符合得很好。
2．通过LMOKE研究了样品沿磁条方向的矫顽力，发现随磁条宽度减小，矫顽力单调增加，与文献中报导的两者近似呈倒数变化的函数描述基本一致。
3．利用XMCD和加和定则分别对Co、Fe的自旋和轨道磁矩进行研究，主要结果如下：随磁条宽度减小，MCD信号降低，主要原因归结为新鲜刻蚀面的氧化；Co、Fe的自旋和轨道磁矩随磁条宽度的变化表现出复杂的趋势：一方面是随着磁条宽度变小，探测范围内表面原子比例增加，表面局域化的3d电子导致自旋和轨道磁矩的增加，另一方面表面氧化导致平均原子磁矩的减小。
X-ray magnetic circular dichroism (XMCD), a powerful technique developed in the 1990s, is an element-specific magnetic measurement technique based on synchrotron radiation. As a variation of X-ray absorption spectrum it has such advantages as element-specific and chemistry sensitive. Especially, XMCD can provide the magnetic contributions of specific components in complex magnetic materials. On the other hand, extensive investigations have been made in past twenty years on the structure, basic magnetic properties, interlayer coupling, interfacial structures and their relative effects in magnetic thin and ultrathin films due to their potential application in magnetic random access memory (MRAM) and ultrahigh-density data storage. In addition, patterned thin films with micron and submicron array have recently attracted increasing interest because of their different behaviors from the continuous thin films. Therefore, it is of great significance to study these ferromagnitic transition-metal films and alloy films by using XMCD technique, and these will be beneficial to disclose the origins of these novel magnetic behaviors.
SXMCD endstation was just constructed at NSRL. This dissertation includes: the installation and collimation of both endstation and relevant beamline, the problems encountered in experiments and respondent solutions; on the other hand, based on the structure and magnetism measurements performed by XMCD and other techniques, systematic studies have been made in magnetic moment, magnetic anisotropy, and thickness effect of such ferromagnetic films as metal films, alloy films, and patterned films with micron and submicron array. The main results are as follows:
I. Investigations of XMCD experiment techniques
1. SXMCD endstation and beamline, supported by Phase II Project, have been constructed. The beamline with monochromator of varied-line-space gratings can provide monochromatic X-rays with energy range from 100 to 1000eV. Wavelengths are calibrated by using gas ionization chamber, and the flux of photons is measured by AXUV-100 detector that worked in soft X-ray wave band. The result shows that photon flux is over 10~8 photons/s with a resolving power of 1000 at 1000eV. It is satisfied with experimental needs.
2. A program for data acquisition and system control has been developed for SXMCD endstation based on LabVIEW5.1. Its main functions include the data acquisition of SXMCD and the measurement of hysteresis loop. Two ports are reserved for experiments of SXMLD and SMOKE.
3. We took a piece of even-oxidated aluminum foil as specimen to investigate the effect of magnetic field on the absorption intensity. Besides, we also set up a model to explain observed phenomenon. This experiment shows that the model constructed in this paper is applicable for solving this problem.
Regarding the enhancement of absorption intensity with the angle of external
field away from sample plane, the expressions, 1 - (1 - a) · cos θ, is used to fit it
well. Absorption intensity decays quickly with the increase of field intensity. The
decaying trend can be fitted well by the inverse proportion function of field
intensity ((P1/B+P2)+ P3). The variations of these fitting parameters are according
with their physics meaning. Under external field, the relative position of beam spot in the sample surface will affect absorption intensity as well. With the beam spot up or down along sample surface, the signal intensity will change monotonously. According to the presented model this phenomenon has been interpreted successfully.
4. When XMCD spectra are measured by sample current mode, signal currents are strongly affected by external fields. Based on the rotatable electromagnet inside vacuum chamber, we have designed and installed a piece of gold mesh attached to magnetic poles to enhance the signals of sample current and obtained some significant conclusions.
Sample currents increase rapidly and tend to saturation with increasing bias voltage under different magnetic fieldintensities. The stronger external field is applied, the higher bias voltage is required to reach signal saturation. Comparing absorption spectra of oxidated aluminum foil with that of CoFe film in parallel and antiparallel magnetization under the same magnetic field intensity and bias voltage, we find that the systematic error caused by adding magnetic field and bias is 2.17% of MCD intensity for cobalt, which indicates that the developed technique is feasible in XMCD measurements.
5. The experimental data pretreatment can affect the measurement accuracy of orbital and spin moments. By fitting experimental data using XPSPEAK4.1, we get the analytic functions of absorption spectra, which are used as a criterion to determine a suitable numerical integration method under certain experimental conditions, and improve the calculation accuracy.
II The study of ferromagnetic metal films by XMCD
1. XMCD in absorption of the single-crystal iron layer deposited epitaxially on MgO substrate is studied. The main results are as following:
In terms of the data of XMCD and sum rules, the values of spin and orbital moments are calculated to be 0.07μB and 2.33μB respectively, which are consistant with the results published before. The structure and magnetic behavior of a 15nm Fe/MgO film have been studied in detail, and the spin and orbital moments of iron along different crystallographic axis are calculated. The results suggest that biaxial anisotropy and uniaxial anisotropy coexist in this film.
2. The orbital and spin moments are studied of Co films with different thickness. With increasing cobalt film thickness from 2nm to 30nm, the orbital moments decreases and the spin moments increases, while the contribution of orbital moment of cobalt atoms to total moment drops gradually and tends to the value in bulk. This result agrees well with RKKY theory.
III Investigations of ferromagnetic alloy films by XMCD
1. By using XMCD and SQUID magnetometry, the spin and orbital moments of Fe and Co in the Co_(0.9)Fe_(0.1) (50nm) are studied. Compared with single-element film, the spin moment of Co remains constant while that of Fe reduced from 1.98 to 1.63μB. The contributions of different elements in the film is, m_(Fe) : m_(Co) - 10.5 : 89.5. The average magnetic moment (1.90μB) determined by XMCD is in agreement with that (1.82μB) obtained from SQUID measurements. Utilizing XMCD spectra in-plane element-dependent magnetic anisotropy in Co_(0.9)Fe_(0.1) films has been investigated. Apart from known field-induced easy axis during film growth, a soft axis perpendicular to it was observed, and hard axes are 66° away from the easy axis. Therefore in-plane bi-axis anisotropy is constructed. According to XMCD results, the origin of this bi-axis anisotropy has been analyzed.
2. Co_(0.9)Fe_(0.1) films with thickness of 5nm, 10nm, 20nm, 50nm were investigated by XMCD spectra. As film thickness increases from 5nm to 50nm, the variations of spin moments for Fe is an oscillatory decrease from 1.87μB to 1.70μB, while for Co it increases from 1 .41μB to 1.69μB monotonously. In terms of the data at 5nm and 50nm, the approximative thickness of "magnetic dead layer" is estimated to be 0.83nm. After correction, the spin moments for Co keep a constant approximately while those for Fe decrease gradually. According to the XRD results, the crystallinity improvement of films is the main reason that results in a monotonous decrease of spin moment for Fe with the increase of film thickness. However, the initial spin moment variation gained by XMCD should attribute to the integrated results from both substrate atom diffusion at interface and the improvement of the crystallinity of films.
IV Investigations of Ta/CoFe/Ta sandwich unpatterned and patterned films by XMCD
The initial specimens are polycrystalline Co_(0.9)Fe_(0.1) films fabricated by RF magnetron sputtering, and that patterned are one-dimension ferromagnetic lattice fabricated by laser interference lithography. The stripe orientation is perpendicular to the direction induced by magnetic field during growth. The main results are as follows.
1. SEM results indicate that the actual width of stripes are 0.6, 1.9, 3.2, 6.0, 9.0μm, respectively. These stripes seem quite uniform in bigger periods, but a litter bad in small periods.
2. By LMOKE, it is found that the coercive forces of these patterned films are
changed in inverse proportionality with its stripe width. It is in agreement with the results published before.
3. By using XMCD and sum rules, the spin and orbital moment of Co and Fe are studied. With the decrease of stripe widths, MCD intensity decreases gradually. The main cause is probably due to the oxidation of fresh etching surface. The variations of spin and orbital moment for Co and Fe display a complex trend. With the magnetic stripe width decreases, the ratio of fresh surface to total surface increases relatively, and the 3d electrons of surface atom become more local, which leads to the increase of spin and orbital moments, while surface oxidation has an adverse effect on the variation of magnetic moment.
软X射线磁性圆二色(soft X-ray magnetic circular dichroism，SXMCD)实验站是国家同步辐射实验室二期工程建设的一个新站，本论文的内容一方面是对实验站和光束线的安装调试、以及实验中遇到的问题和所采取的相应措施；另一方面，以新建成的XMCD实验设备为主，结合其它的结构和磁性表征手段，对铁磁单质薄膜、合金薄膜以及光刻制备的微米亚微米周期的图形化铁磁薄膜的磁矩、磁各向异性以及磁矩的厚度效应等进行了系统研究，主要的研究工作及结果如下：
一、XMCD的实验技术研究
1．合肥国家同步辐射实验室二期工程建立了基于软X射线吸收的光束线和实验站，光束线采用了平面变线距光栅单色器，可以提供100-1000eV的单色光。用气体电离室进行了同步光的波长标定，并用AXUV-100对光子的通量进行了测试，结果表明：样品处的光通量不低于10~8光子／秒，能量分辨本领在1000eV处不小于1000，满足设计要求。
2．在Labview5.1平台上开发了SXMCD实验站的数据采集和控制程序，实现了对硬件的调用以及数据的采集过程，主要功能包括SXMCD的数据采集、磁滞回线的测量等，预留了SXMLD和SMOKE的数据采集接口。
3．以均匀氧化的Al箔为样品，在光斑、磁场和样品的不同几何配置下，研究了外磁场对X射线吸收强度的影响规律，建立模型对实验结果进行了合理解释。吸收信号随磁场与样品的夹角θ增大而变强，其变化趋势与根据模型提出的经验公式1-(1-α)·cosθ符合得很好；吸收谱强度随着外磁场的增加而剧烈的衰减，衰减趋势可用磁场强度B的反比例函数：(P1)／(B+P2)+P3来拟合，拟合参数的变化与所表达的物理意义基本符合；在磁场作用下，观察到光斑在样品上的相对位置变化也会对吸收谱强度造成影响，指出这是XMCD实验中吸收边的边前和边后出现不重合的主要原因。
4．在用样品电流法测量XMCD谱中，外磁场的存在严重降低了样品电流信号；针对实验站内置式可旋转磁铁设计并安装一个紧靠磁极的高压金网，结果发现：不同磁场下随偏压升高样品电流信号迅速增强并趋于饱和，磁场越强，达到饱和需要的偏压越高；比较相同磁场和偏压下均匀氧化的Al箔和CoFe薄膜正反磁场下的吸收谱，发现该装置引入的系统误差仅相当于Co的圆二色值的2.17％；表明用这种方法进行XMCD谱的测量是可行的。
5．XMCD实验数据预处理的方法可以影响到自旋和轨道磁矩的精度，提出通过XPSPEAK4.1对实验数据拟合，写出吸收谱的解析函数；利用解析函数的积分值，提高了磁矩的计算精度。
二、XMCD对铁磁单质薄膜的研究
1．利用XMCD吸收谱研究了在MgO衬底外延的铁单晶薄膜的原子磁矩，根据加和定则得到铁原子的轨道和自旋磁矩分别是0.07μ_B和2.33μ_B，与已发表的实验和理论数据基本吻合；通过XMCD实验得出Fe膜在面内不同晶向的自旋与轨道磁矩，对薄膜面内轨道磁矩的各向异性研究发现：对于15nm厚的Fe膜，双轴磁各向异性与单轴磁各向异性可能叠加在一起。
2．利用XMCD技术研究了Au／Co／Au多晶薄膜的轨道和自旋磁矩随膜厚的变化：随薄膜厚度增加，轨道磁矩由大到小，自旋磁矩则与之相反，轨道磁矩对原子总磁矩的贡献逐渐下降，并趋近于体相Co原子的比值，这与RKKY理论一致。
三、XMCD对铁磁合金薄膜的研究
1．利用XMCD谱研究了50nm Co_(0.9)Fe_(0.1)薄膜，结果发现：样品中Fe、Co元素对磁化强度的贡献比为10.5∶89.5；由XMCD获得的合金平均原子磁矩1.90μ_B与用SQUID磁强计得到的合金平均原子磁矩1.82μ_B基本相符；对Co_(0.9)Fe_(0.1)薄膜面内元素分辨的磁各向异性的研究发现，除了在生长的磁诱导方向存在易磁化轴外，与该轴垂直的方向还存在一个类似易轴的软磁化轴(soft axis)；面内的两个难磁化轴与易磁化轴取向大约成66°夹角，构成面内双轴磁各向异性，并根据XMCD结果对面内双轴形成的原因进行了分析。
2．研究了不同厚度的Co_(0.9)Fe_(0.1)薄膜，主要结果如下：随薄膜厚度从5nm增至50nm，Fe的自旋磁矩从1.87μ_B振荡下降至1.70μ_B，而Co表现出相反的变化趋势；根据5nm和50nm的测量数据，近似估算出“磁死层”的厚度为0.83nm，并以此为基础对计算的自旋磁矩进行修正，修正后的Co自旋磁矩几乎不随薄膜厚度变化，而Fe的自旋磁矩变化则随膜厚呈现出单调减少的趋势；结合XRD的测量结果认为，晶化程度的提高可能是修正后Fe的自旋磁矩随膜厚增加而单调减小的主要原因。而由XMCD直接得到的自旋磁矩变化趋势则可能是界面原子互扩散和结晶度随薄膜厚度提高共同作用的结果。
四、XMCD对图形化铁磁薄膜的初步研究
以磁控溅射生长的多晶三明治结构Ta／CoFe／Ta为基础，由激光干涉法制备出微米和亚微米周期的一维铁磁光栅结构，研究的主要成果如下：
1．制备出光栅条纹方向与磁诱导方向垂直的图形薄膜系列，SEM图像显示条纹的实际宽度分别为0.6μm、1.9μm、3.2μm、6.0μm、9.0μm，条纹宽度的均一性良好，小尺寸的磁条宽度与设计尺寸略有出入，大尺寸下符合得很好。
2．通过LMOKE研究了样品沿磁条方向的矫顽力，发现随磁条宽度减小，矫顽力单调增加，与文献中报导的两者近似呈倒数变化的函数描述基本一致。
3．利用XMCD和加和定则分别对Co、Fe的自旋和轨道磁矩进行研究，主要结果如下：随磁条宽度减小，MCD信号降低，主要原因归结为新鲜刻蚀面的氧化；Co、Fe的自旋和轨道磁矩随磁条宽度的变化表现出复杂的趋势：一方面是随着磁条宽度变小，探测范围内表面原子比例增加，表面局域化的3d电子导致自旋和轨道磁矩的增加，另一方面表面氧化导致平均原子磁矩的减小。
X-ray magnetic circular dichroism (XMCD), a powerful technique developed in the 1990s, is an element-specific magnetic measurement technique based on synchrotron radiation. As a variation of X-ray absorption spectrum it has such advantages as element-specific and chemistry sensitive. Especially, XMCD can provide the magnetic contributions of specific components in complex magnetic materials. On the other hand, extensive investigations have been made in past twenty years on the structure, basic magnetic properties, interlayer coupling, interfacial structures and their relative effects in magnetic thin and ultrathin films due to their potential application in magnetic random access memory (MRAM) and ultrahigh-density data storage. In addition, patterned thin films with micron and submicron array have recently attracted increasing interest because of their different behaviors from the continuous thin films. Therefore, it is of great significance to study these ferromagnitic transition-metal films and alloy films by using XMCD technique, and these will be beneficial to disclose the origins of these novel magnetic behaviors.
SXMCD endstation was just constructed at NSRL. This dissertation includes: the installation and collimation of both endstation and relevant beamline, the problems encountered in experiments and respondent solutions; on the other hand, based on the structure and magnetism measurements performed by XMCD and other techniques, systematic studies have been made in magnetic moment, magnetic anisotropy, and thickness effect of such ferromagnetic films as metal films, alloy films, and patterned films with micron and submicron array. The main results are as follows:
I. Investigations of XMCD experiment techniques
1. SXMCD endstation and beamline, supported by Phase II Project, have been constructed. The beamline with monochromator of varied-line-space gratings can provide monochromatic X-rays with energy range from 100 to 1000eV. Wavelengths are calibrated by using gas ionization chamber, and the flux of photons is measured by AXUV-100 detector that worked in soft X-ray wave band. The result shows that photon flux is over 10~8 photons/s with a resolving power of 1000 at 1000eV. It is satisfied with experimental needs.
2. A program for data acquisition and system control has been developed for SXMCD endstation based on LabVIEW5.1. Its main functions include the data acquisition of SXMCD and the measurement of hysteresis loop. Two ports are reserved for experiments of SXMLD and SMOKE.
3. We took a piece of even-oxidated aluminum foil as specimen to investigate the effect of magnetic field on the absorption intensity. Besides, we also set up a model to explain observed phenomenon. This experiment shows that the model constructed in this paper is applicable for solving this problem.
Regarding the enhancement of absorption intensity with the angle of external
field away from sample plane, the expressions, 1 - (1 - a) · cos θ, is used to fit it
well. Absorption intensity decays quickly with the increase of field intensity. The
decaying trend can be fitted well by the inverse proportion function of field
intensity ((P1/B+P2)+ P3). The variations of these fitting parameters are according
with their physics meaning. Under external field, the relative position of beam spot in the sample surface will affect absorption intensity as well. With the beam spot up or down along sample surface, the signal intensity will change monotonously. According to the presented model this phenomenon has been interpreted successfully.
4. When XMCD spectra are measured by sample current mode, signal currents are strongly affected by external fields. Based on the rotatable electromagnet inside vacuum chamber, we have designed and installed a piece of gold mesh attached to magnetic poles to enhance the signals of sample current and obtained some significant conclusions.
Sample currents increase rapidly and tend to saturation with increasing bias voltage under different magnetic fieldintensities. The stronger external field is applied, the higher bias voltage is required to reach signal saturation. Comparing absorption spectra of oxidated aluminum foil with that of CoFe film in parallel and antiparallel magnetization under the same magnetic field intensity and bias voltage, we find that the systematic error caused by adding magnetic field and bias is 2.17% of MCD intensity for cobalt, which indicates that the developed technique is feasible in XMCD measurements.
5. The experimental data pretreatment can affect the measurement accuracy of orbital and spin moments. By fitting experimental data using XPSPEAK4.1, we get the analytic functions of absorption spectra, which are used as a criterion to determine a suitable numerical integration method under certain experimental conditions, and improve the calculation accuracy.
II The study of ferromagnetic metal films by XMCD
1. XMCD in absorption of the single-crystal iron layer deposited epitaxially on MgO substrate is studied. The main results are as following:
In terms of the data of XMCD and sum rules, the values of spin and orbital moments are calculated to be 0.07μB and 2.33μB respectively, which are consistant with the results published before. The structure and magnetic behavior of a 15nm Fe/MgO film have been studied in detail, and the spin and orbital moments of iron along different crystallographic axis are calculated. The results suggest that biaxial anisotropy and uniaxial anisotropy coexist in this film.
2. The orbital and spin moments are studied of Co films with different thickness. With increasing cobalt film thickness from 2nm to 30nm, the orbital moments decreases and the spin moments increases, while the contribution of orbital moment of cobalt atoms to total moment drops gradually and tends to the value in bulk. This result agrees well with RKKY theory.
III Investigations of ferromagnetic alloy films by XMCD
1. By using XMCD and SQUID magnetometry, the spin and orbital moments of Fe and Co in the Co_(0.9)Fe_(0.1) (50nm) are studied. Compared with single-element film, the spin moment of Co remains constant while that of Fe reduced from 1.98 to 1.63μB. The contributions of different elements in the film is, m_(Fe) : m_(Co) - 10.5 : 89.5. The average magnetic moment (1.90μB) determined by XMCD is in agreement with that (1.82μB) obtained from SQUID measurements. Utilizing XMCD spectra in-plane element-dependent magnetic anisotropy in Co_(0.9)Fe_(0.1) films has been investigated. Apart from known field-induced easy axis during film growth, a soft axis perpendicular to it was observed, and hard axes are 66° away from the easy axis. Therefore in-plane bi-axis anisotropy is constructed. According to XMCD results, the origin of this bi-axis anisotropy has been analyzed.
2. Co_(0.9)Fe_(0.1) films with thickness of 5nm, 10nm, 20nm, 50nm were investigated by XMCD spectra. As film thickness increases from 5nm to 50nm, the variations of spin moments for Fe is an oscillatory decrease from 1.87μB to 1.70μB, while for Co it increases from 1 .41μB to 1.69μB monotonously. In terms of the data at 5nm and 50nm, the approximative thickness of "magnetic dead layer" is estimated to be 0.83nm. After correction, the spin moments for Co keep a constant approximately while those for Fe decrease gradually. According to the XRD results, the crystallinity improvement of films is the main reason that results in a monotonous decrease of spin moment for Fe with the increase of film thickness. However, the initial spin moment variation gained by XMCD should attribute to the integrated results from both substrate atom diffusion at interface and the improvement of the crystallinity of films.
IV Investigations of Ta/CoFe/Ta sandwich unpatterned and patterned films by XMCD
The initial specimens are polycrystalline Co_(0.9)Fe_(0.1) films fabricated by RF magnetron sputtering, and that patterned are one-dimension ferromagnetic lattice fabricated by laser interference lithography. The stripe orientation is perpendicular to the direction induced by magnetic field during growth. The main results are as follows.
1. SEM results indicate that the actual width of stripes are 0.6, 1.9, 3.2, 6.0, 9.0μm, respectively. These stripes seem quite uniform in bigger periods, but a litter bad in small periods.
2. By LMOKE, it is found that the coercive forces of these patterned films are
changed in inverse proportionality with its stripe width. It is in agreement with the results published before.
3. By using XMCD and sum rules, the spin and orbital moment of Co and Fe are studied. With the decrease of stripe widths, MCD intensity decreases gradually. The main cause is probably due to the oxidation of fresh etching surface. The variations of spin and orbital moment for Co and Fe display a complex trend. With the magnetic stripe width decreases, the ratio of fresh surface to total surface increases relatively, and the 3d electrons of surface atom become more local, which leads to the increase of spin and orbital moments, while surface oxidation has an adverse effect on the variation of magnetic moment.
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