纳米结构薄膜的磁共振和磁耦合

发布时间：2018-05-14 05:30

本文选题：坡莫合金-稀土 + 自旋泵浦　；参考：《东南大学》2016年博士论文

【摘要】：磁性材料的研究长久以来一直是磁学研究的重要组成部分,其中软磁材料因为其高饱和磁化强度和低矫顽力等特点也广泛的应用于各个领域。近年来,随着自旋电子学的快速发展,软磁薄膜材料再次迎来了研究热潮,软磁材料与非磁金属的多层膜结构被大量的应用于如自旋泵浦效应,自旋霍尔效应,自旋塞贝克效应等研究。坡莫合金作为经典的软磁材料,拥有更低的矫顽力和磁各向异性,是目前研究最多的自旋电子学器件材料。但是,由于坡莫合金的轨道淬灭,自旋轨道耦合作用相对较弱,造成阻尼因子偏小,很大程度上限制了它的应用。稀土元素作为重要的战略资源,在硬磁和软磁领域都有很重要的应用。稀土元素通常在室温下表现出弱磁性,并且本身具有较强的自旋轨道耦合,其与坡莫合金构成的多层膜结构,在保持较好的软磁特性的基础上,可以改变样品整体的阻尼因子,这一点对构成自旋转移力矩振荡器等自旋器件是十分关键的。另一方面,强自旋轨道耦合材料一直是自旋电子材料的一个研究热点,包括自旋泵浦效应以及自旋霍尔效应均有涉及。自旋轨道耦合作用又可以分为内禀和外禀两类,内禀是由材料本身的自旋-轨道相互作用引起的,强的内禀自旋轨道耦合作用会表现出强的自旋泵浦效应和自旋霍尔效应等；外禀则是由于掺杂等外因引起的自旋轨道耦合加强,如Skew Scattering和Side Jump等等,外禀自旋轨道耦合也会表现出强的自旋霍尔效应,但是不会产生大的自旋泵浦效应。随着军事及通讯产业的发展,软磁材料在高频率段的应用也日益得到重视。传统的软磁材料如铁磁金属及其合金,或者铁氧体等等,都存在一定的缺陷,导致材料在高频下会有较大的磁滞损耗和涡流损耗,不利于应用。随着铁基纳米非晶带材/薄膜的提出,这些问题得到了解决,通过带材或者薄膜结构来减小涡流损耗,改变退火条件控制晶粒生长来得到合适的矫顽力,材料的电阻率也可以通过退火氧化变大,从而可以得到高磁导率,高磁化强度和大电阻率的高频软磁材料。本论文主要研究了坡莫合金和稀土(Tb和Nd)的多层膜结构的铁磁共振信息,讨论了多Py/RE界面对自旋泵浦效应的影响；通过研究坡莫合金与Cu基底Ta元素掺杂的双层膜结构的自旋霍尔效应,对掺杂引起的自旋轨道耦合增强做出研究分析；采用单辊甩带方法制备Fe-Hf-Cr-B合金带材,通过改变Cr的成分含量和材料的退火条件,研究材料的磁性、电阻率和磁导率。研究结果如下：一、坡莫合金/稀土异质结构的界面自旋泵浦效应研究制备了Py/RE,Py/RE/Py两个结构的薄膜样品,稀土元素分别选用重稀土Tb和轻稀土Nd,主要针对Py/RE的界面泵浦效应,开展了对双层膜和三层膜的磁化动力阻尼的研究,同时比较Py/RE以及Py/RE/Py的实验结果,探索了多界面对自旋泵浦效应和磁化动力阻尼的影响。得到的研究结果如下：1、在Py/Tb(Py/Tb/Py)系列样品中,对双层膜,样品的矫顽力随着Tb的插入增厚总体呈增大趋势,从2 Oe增大到9 Oe,最后趋于稳定,饱和磁化强度随Tb增厚而减小,从787emu/cm3降低到700emu/cm3左右；而对于三层膜,总体变化趋势跟双层膜一致,Hc从20e增大到80e左右,Ms从820降低至680emu/cm3,但是会在dTb≥12nm时出现双Hc现象,说明三层膜样品中的两个坡莫合金层的矫顽力出现差异,可能是由于Py/Tb界面和Tb/Py界面对两磁层的影响不同所致。2、在Py/Nd(Py/Nd/Py)系列样品中,双层膜和三层膜的矫顽力和饱和磁化强度都基本维持不变,双层膜的Hc一直保持在3 Oe左右,三层膜则有轻微变化,从3 Oe降到1 Oe后又增大稳定在2 Oe,Ms则分别在770和790 emu/cm3左右震荡。这是由于Nd居里温度较低,室温下没有磁性,不会被相邻的Py诱导出磁矩,因此对薄膜的磁性不会造成影响,反观Tb的居里温度与室温接近,由于紧邻效应诱导的反方向磁矩会降低薄膜的Ms。3、用变频VNA-FMR铁磁共振对样品的共振场和共振线宽进行研究分析。通过对共振场和共振线宽的拟合发现：对Py-Tb系列,随着Tb层厚度增加,同一频率的共振场逐渐降低,面内的单轴各向异性常数逐渐增大,共振线宽则随着dTb增大变宽最终趋于饱和；Py-Nd薄膜同一频率的共振峰位置则兀乎不随dNd而变化,这和前面观测到的Ms几乎不变相吻合,另外也说明Py-Nd系列样品的面内单轴各向异性没有显著变化,共振线宽会随dNd增加而增大最终趋于饱和。4、根据线宽和阻尼的关系式得出两个系列样品的阻尼因子随非磁层厚度的变化趋势,分别利用双层膜和三层膜模型进行拟合。对于Py-Tb系列,双层膜和三层膜的自旋泵浦阻尼因子均随dTb增加而从0.006左右增大并最终饱和稳定在0.045左右,但是双层膜的增长速度更快。根据拟合结果,双层膜和三层膜的自旋扩散长度均约为λ=6nm,界面混合电导则分别为gpy/Tb=50.5×1015cm-2和58.2×1015cm-2。对Py-Nd系列,其阻尼变化情况和Tb时基本一致,从0.006增大到0.020左右,比Tb的增幅要小,拟合得到Nd的自旋扩散长度约为λ=11nm,界面混合电导约为gpy/Th=25×1015cm-2左右。5、从拟合结果看,Tb和Nd的自旋泵浦效应都很明显：Nd的自旋轨道耦合要比常见的强L-S耦合元素如Pt,Pd更强,因为其有更大的界面混合电导,而Tb的自旋轨道耦合则更强,不仅自旋扩散长度要更小,界面混合电导也是Nd的两倍左右。此外,我们提出了有效自旋扩散长度的概念,三层膜引入的第二个RE/Py界面会增大自旋回流的几率,使得能量耗散降低,阻尼的增加更加缓慢,因此表观上三层膜有更大的自旋扩散长度。二、Cu基材料掺杂Ta引起的强自旋轨道耦合研究采用磁控溅射仪制备了由坡莫合金Py和Cu基中掺杂不同成分的Ta元素构成的双层膜结构,通过铁磁共振和平面霍尔效应的测量方法对薄膜的自旋轨道耦合作用进行了研究,得到结果如下：1、利用磁控溅射仪制备了Py/Cu-Ta的双层膜结构,其中采用Ti作为缓冲层,SiO2作为保护层。固定Py层的厚度为2nm,改变Cu-Ta的厚度来研究厚度依赖性,并通过共溅射的方法实现不同掺杂成分的Cu-Ta薄膜层,质量组分依次为7%,10%和12%。2、自旋轨道耦合作用可以分为内禀和外禀两类,内禀来自于非金属本身由于f电子引起的强自旋轨道耦合作用,而外禀则是由于掺杂等引起的Side Jump和Skew Scattering等作用造成的自旋轨道耦合加强。通过铁磁共振研究了不同成分Py/Cu-Ta样品的自旋泵浦效应,结果表明所有成分的Py/Cu-Ta样品的共振线宽和阻尼因子并没有明显的增大,阻尼因子的大小均约为0.007左右,与单层的Py的本征阻尼近似,说明样品中没有明显的自旋泵浦效应,Cu-Ta薄膜的内禀自旋轨道耦合作用很弱。3、采用平面霍尔效应的方法来研究不同成分的Py/Cu-Ta双层膜的自旋霍尔效应。利用光刻掩膜技术刻蚀成Hall Bar结构进行输运测量,推导计算并拟合横向电压值,最终得到自旋霍尔系数和自旋霍尔角。研究发现Cu-Ta能产生和Pt大小近似的较强的自旋霍尔效应,在Cu-Ta薄膜厚度为2nm左右时,掺杂成分最低7%的Cu-2Ta薄膜的自旋霍尔角可以达到0.066,这说明Cu基中掺杂Ta元素确实增强了外禀自旋轨道耦合作用。三、掺Cr-FeHfBO带材的高频磁特性通过单辊甩带的方法制备了非晶态Fe-Hf-Cr-B合金带材,利用Cr元素的掺入来进一步抑制Fe的氧化,并通过改变Cr的成分配比以及样品的退火条件来获取可应用于高频下的软磁材料,得到研究结果如下：1、通过单辊快淬甩带的方法制备了不同成分的Fe-Hf-Cr-B非晶带材样品,并对样品进行不同条件的退火。退火时采用低温起泡器来降低退火环境的氧分压,而Cr元素的引入也可以进一步抑制Fe在退火过程中的氧化,以其获得更好的高频软磁带材样品。2、对Fe80Hf9Cr8B3和Fe85Hf10Cr2B3两个系列样品的形貌结构表征和静态磁性的测量,发现Fe85Hf10Cr2B3的样品矫顽力随退火温度呈先减小后增大的趋势,并在600℃退火1.5小时后表现出最好的静磁特性,其矫顽力Hc为0.560e,饱和磁感应强度Bs可以达到1.33T,并且该样品的晶粒尺寸也符合Fe元素的交换耦合长度,说明Fe85Hf10Cr2B3随着晶粒生长,在晶粒尺寸达到29nm左右时产生了交换耦合作用,使得样品表现出最小的矫顽力。3、为了进一步验证Fe85Hf10Cr2B3样品作为高频软磁材料的有力竞争者,电阻率和高频磁导率也进行了测量分析。电阻率最大发生在500℃退火1小时的样品中,约为200μΩ·cm,静磁特性最好的600℃退火1.5小时的样品电阻率为165μΩ-cm,这一结果虽然没有预想的电阻率大,但是比一般的金属软磁材料的电阻率要高两个数量级,并且跟课题组之前的Fe89Hf8B3工作相比,电阻率也增加了一倍,可以很大程度上降低高频下工作时的涡流损耗。4、通过阻抗分析仪对上述电阻率最高的两个样品进行了磁导率的测量分析,发现当频率达到10MHz时,样品的实部磁导率仍能保持250以上,这一结果从稳定性和高磁导率两方面验证了样品是很好高频软磁材料。
[Abstract]:The study of magnetic materials has long been an important part of magnetic research. Soft magnetic materials have been widely used in various fields because of their high saturation magnetization and low coercivity. In recent years, with the rapid development of spintronics, soft magnetic thin film materials have ushered in the upsurge of research, soft magnetic materials and non magnetic gold. The multilayer structure of the genus is widely used in the study of spin pump effect, spin Holzer effect, spin cibeck effect and so on. As a classic soft magnetic material, permalloy has lower coercivity and magnetic anisotropy. It is the most studied spintronics device material at present. However, because of the orbit quenching of permalloy, the spin orbit of permalloy The coupling of the channel is relatively weak, which makes the damping factor smaller and restricts its application to a large extent. As an important strategic resource, rare earth elements are very important in both hard and soft magnetic fields. Rare earth elements usually exhibit weak magnetic properties at room temperature and have strong spin orbit coupling, which are composed of permalloy. The multi layer membrane structure can change the damping factor of the whole sample on the basis of good soft magnetic properties, which is crucial to spin devices such as spin transfer torque oscillator. On the other hand, the strong spin orbit coupling material has always been a hot spot of spintronic material, including the spin pump effect. The spin Holzer effect is involved. The spin orbit coupling can be divided into two types of intrinsic and external, which are caused by the spin orbit interaction of the material itself. The strong intrinsic spin orbit coupling will show a strong spin pump effect and a spin Holzer effect, and the external factor is caused by doping and other external causes. The spin orbit coupling is strengthened, such as Skew Scattering and Side Jump, etc., the intrinsic spin orbit coupling will also show a strong spin Holzer effect, but it will not produce a large spin pump effect. With the development of the military and communication industry, the application of the soft magnetic materials at the high frequency rate is also paid more attention. There are some defects in their genera and their alloys, or ferrite, and so on, which results in large hysteresis loss and eddy current loss at high frequency, which is not conducive to application. With the introduction of iron based nanoamorphous ribbons / films, these problems have been solved. The loss of eddy current is reduced by the strip or thin film structure, and the annealing conditions are controlled. The suitable coercive force is obtained by grain growth, and the resistivity of the material can also be increased by annealing, thus the high magnetic permeability, high magnetization and large resistivity can be obtained. This paper mainly studies the ferromagnetic resonance information of the multilayer film structure of permalloy and rare earth (Tb and Nd), and discusses the multiple Py/RE interface. The effect of spin pump effect is studied by studying the spin Holzer effect of the double membrane structure of the Cu base Ta element doped with permalloy and substrate. The spin orbit coupling enhancement caused by doping is studied and analyzed. The Fe-Hf-Cr-B alloy strip is prepared by single roll strip rejection method, and the material content and the annealing conditions of the material are changed to study the material. The study results are as follows: first, the interfacial spin pump effect of permalloy / rare earth heterostructure is studied to prepare the Py/RE, Py/RE/Py two structure film samples. The rare earth elements are selected as heavy rare earth Tb and light rare earth Nd, respectively, for the boundary surface pumping effect of Py/RE, and the double layer and three layer films are carried out. The study of the magnetization dynamic damping, at the same time compared the experimental results of Py/RE and Py/RE/Py, explored the effects of multi boundary on the spin pump effect and the magnetization damping. The results obtained are as follows: 1, in the Py/Tb (Py/Tb/Py) series, the coercive force of the sample increases with the thickness of the Tb, and increases from the 2 Oe to the double layer. As large as 9 Oe, it tends to be stable at the end. The saturation magnetization decreases with the thickening of Tb and decreases from 787emu/cm3 to about 700emu/cm3. For the three layer film, the overall change trend is consistent with the double layer membrane. Hc increases from 20E to 80e, Ms decreases from 820 to 680emu/cm3, but the dual Hc phenomenon occurs at dTb > 12NM, indicating two of the three layer membrane samples. The coercivity difference of the permalloy layer may be caused by the difference between the Py/Tb interface and the Tb/Py interface on the two magnetosphere. In the Py/Nd (Py/Nd/Py) series, the coercive force and the saturation magnetization of the double layer and three layers are basically maintained, the Hc of the double layer remains at about 3 Oe, and the three layer film has a slight change from 3. Oe decreases to 1 Oe and increases steadily at 2 Oe, and Ms is oscillating around 770 and 790 emu/cm3, respectively. This is due to the low temperature of Nd Curie, no magnetic properties at room temperature, no magnetic moments induced by adjacent Py, so the magnetic properties of the film will not be affected, but the Curie temperature of Tb is close to the room temperature, due to the reverse direction magnetic moment induced by the immediate effect. The resonance field and resonance line width of the sample are analyzed by the frequency conversion VNA-FMR ferromagnetic resonance (Ms.3). By fitting the resonance field and the resonance line width, the resonance field of the Py-Tb series decreases gradually with the increase of the thickness of the Tb layer, the uniaxial anisotropy of the surface increases gradually, and the resonance line width is followed by the resonance line width. The resonance peak position of dTb becomes more saturated, and the resonance peak position of the same frequency of Py-Nd film does not change with the dNd, which is almost not consistent with the observed Ms. It also shows that the uniaxial anisotropy of the Py-Nd series is not significantly changed, and the resonance line width will increase with the dNd increase and eventually become saturated.4. The relationship between the line width and the damping is obtained. The damping factor of the two series of samples is changed with the thickness of the non magnetic layer, and the double layer and three layer membrane model are used respectively. The spin pump damping factor of the Py-Tb series, the double layer film and the three layer film increases from about 0.006 with the increase of the double layer and the three layer film, and the final saturated stability is around 0.045. The growth rate of the double layer film is faster. According to the fitting results, the spin diffusion length of the double layer and three layer films are all about =6nm, and the interface conductivity is gpy/Tb=50.5 x 1015cm-2 and 58.2 x 1015cm-2. to Py-Nd series respectively. The damping change is basically the same as Tb, and the increase from 0.006 to about 0.020 is smaller than that of Tb, and the Nd is fitted to Nd. The spin diffusion length is about lambda =11nm and the interface conductivity is about gpy/Th=25 x 1015cm-2.5. From the fitting results, the spin pump effect of Tb and Nd is obvious: the spin orbit coupling of Nd is stronger than the common strong L-S coupling elements such as Pt and Pd, because it has a larger boundary mixing conductivity and the Tb spin orbit coupling is stronger. Only the spin diffusion length is smaller and the interface conductivity is about two times that of Nd. In addition, we have proposed the concept of effective spin diffusion length. The second RE/Py interfaces introduced by the three layer film will increase the probability of spin reflux, which makes the energy dissipation lower and the damping increase more slowly, so the apparent spin diffusion of the three layers has a larger spin diffusion. The length. Two, the strong spin orbit coupling caused by Cu based materials doped with Ta, a double layer membrane structure composed of Ta elements doped with different components in the Py and Cu base of permalloy is prepared by a magnetron sputtering instrument. The spin orbit coupling effect of the film is studied by the method of ferromagnetic resonance and plane Holzer effect. The results are as follows. 1, the double layer membrane structure of Py/Cu-Ta was prepared by magnetron sputtering, in which Ti was used as a buffer layer and SiO2 as a protective layer. The thickness of the fixed Py layer was 2nm, the thickness of Cu-Ta was changed to study the thickness dependence, and the Cu-Ta film with different doping components was realized by co sputtering. The mass components were in turn to be 7%, 10% and 12%.2, and spin. The orbital coupling can be divided into two kinds of intrinsic and extrinsic factors, which are derived from the strong spin orbit coupling caused by the f electron due to the non metal itself, but the outside is strengthened by the spin orbit coupling caused by the effect of Side Jump and Skew Scattering caused by doping. The Py/Cu-Ta samples of different components are studied by ferromagnetic resonance. The spin pump effect shows that the resonance line width and damping factor of the Py/Cu-Ta samples of all components do not increase obviously. The damping factor is about 0.007, which is approximate to the intrinsic damping of the single layer Py, indicating that there is no obvious spin pump effect in the sample. The intrinsic spin orbit coupling effect of the Cu-Ta thin film is very weak.3. The plane Holzer effect is used to study the spin Holzer effect of the Py/Cu-Ta double layer with different components. Using the photolithography mask technique to etch the Hall Bar structure to carry out the transport measurement, the calculation and fitting of the transverse voltage value are derived and the spin Holzer coefficient and spin Holzer angle are finally obtained. It is found that the Cu-Ta can produce a stronger approximation of the Pt size. The spin Holzer effect, when the thickness of the Cu-Ta film is about 2nm, the spin Holzer angle of the Cu-2Ta film with the lowest doping content of 7% can reach 0.066, which indicates that the Ta element in the Cu matrix does enhance the intrinsic spin orbit coupling. Three, the high frequency magnetic properties of the Cr-FeHfBO doped strip are prepared by the single roll band rejection method. -B alloy strip is used to further inhibit the oxidation of Fe by adding Cr elements, and to obtain soft magnetic materials which can be applied to high frequency by changing the composition of Cr and the annealing conditions of the samples. The results are as follows: 1, the samples of Fe-Hf-Cr-B amorphous ribbons with different components were prepared by the method of single roll fast quenching zone, and the samples were prepared. Annealing in different conditions. The low temperature foaming device is used to reduce the oxygen partial pressure in the annealing environment, and the introduction of Cr can further inhibit the oxidation of Fe during the annealing process, so as to obtain better high-frequency soft magnetic strip sample.2 and to characterize the morphology and static magnetism of two series of samples of Fe80Hf9Cr8B3 and Fe85Hf10Cr2B3. It is found that the coercivity of Fe85Hf10Cr2B3 samples decreases first and then increases with the annealing temperature, and shows the best magnetostatic characteristic after 1.5 hours annealing at 600 C. The coercive force Hc is 0.560e and the saturation magnetic induction strength Bs can reach 1.33T, and the grain size of the sample also conforms to the exchange coupling length of the Fe element, indicating Fe With the growth of grain, 85Hf10Cr2B3 produced the exchange coupling effect when the grain size was about 29nm. The sample showed the smallest coercive force.3. In order to further verify the Fe85Hf10Cr2B3 sample as a powerful competitor of the high frequency soft magnetic material, the resistivity and the high frequency permeability were also measured and analyzed. The maximum resistivity occurred at 500 C. Among the samples annealed for 1 hours, it is about 200 Omega cm, and the best static magnetic properties at 600 C for 1.5 hours is 165 uh -cm. This result is two orders of magnitude higher than the resistivity of the ordinary metal soft magnetic material, although the resistivity is not expected, and the resistivity is increased compared with the Fe89Hf8B3 work before the project group. With one double, the eddy current loss.4 at high frequency can be reduced to a large extent. The permeability of the two samples with the highest resistivity above is measured and analyzed by the impedance analyzer. It is found that the real permeability of the sample can remain above 250 when the frequency is up to 10MHz. This result is verified from the two aspects of stability and high permeability. It is proved that the sample is a very good high frequency soft magnetic material.

【学位授予单位】：东南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O482.54;O484

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