NiFe/Pt薄膜中角度相关的逆自旋霍尔效应

NiFe/Pt双层薄膜样品在铁磁共振时, NiFe磁矩进动所产生的自旋流注入到Pt层中, 由于逆自旋霍尔效应产生直流电压V_ISHE, 此电压会叠加到NiFe薄膜由于自旋整流效应而产生的电压V_SRE 上, 实验测量所得电压为V_ISHE和V_SRE的叠加. 为了区分这两种不同机理对电压的贡献, 本文采取旋转外加静磁场的方法, 通过分析所测电压随磁场角度的变化从而分离出V_ISHE 的大小. 研究结果表明, 相比于单层NiFe(20 nm)薄膜样品, NiFe(20 nm)/Pt(10 nm)双层膜样品中由于NiFe自旋注入到Pt 中导致铁磁共振线宽增加. 与逆自旋霍尔效应产生的电压相比, 自旋整流效应的贡献较小, 但不可忽略. 本文工作有助于认清铁磁/非磁性金属材料中的自旋相关效应, 并提供了一种准确的分析逆自旋霍尔效应的方法.     

Magnetic nanostructures as amplifiers of transverse fields in magnetic resonance

We introduce the concept of amplifying the transverse magnetic fields produced and/or detected with inductive coils in magnetic resonance settings by using the reversible transverse susceptibility properties of magnetic nanostructures. First, we describe the theoretical formalism of magnetic flux amplification through the coil in the presence of a large perpendicular DC magnetic field (typical of magnetic resonance systems) achieved through the singularity in the reversible transverse susceptibility in anisotropic single domain magnetic nanoparticles. We experimentally demonstrate the concept of transverse magnetic flux amplification in an inductive coil system using oriented nanoparticles with uni-axial magnetic anisotropy. We also propose a composite ferromagnetic/anti-ferromagnetic core/shell nanostructure system with uni-directional magnetic anisotropy that, in principle, provides maximal transverse magnetic flux amplification.     

Intra-well relaxation process in magnetic fluids subjected to strong polarising fields

We report on the frequency and field dependent complex magnetic susceptibility measurements of a kerosene-based magnetic fluid with iron oxide nanoparticles, stabilized with oleic acid, in the frequency range 0.1-6 GHz and over the polarising field range of 0-168.4 kA/m.By increasing polarising field, H, a subsidiary loss-peak clearly occurs in the vicinity of the ferromagnetic resonance peak, from which it remains distinct even in strong polarising fields of 168.4 kA/m. This is in contrast to other reported cases in which the intra-well relaxation process is manifested only as a shoulder of the resonance peak, which vanishes in polarising fields larger than that of 100 kA/m.The results of the XRD analysis connected to the anisotropy field results confirm that the investigated sample contains particles of magnetite and of the tetragonal phase of maghemite.Taking into account the characteristics of our sample, the theoretical analysis revealed that the intra-well relaxation process of the small particles of the tetragonal phase of maghemite may be responsible for the subsidiary loss peak of the investigated magnetic fluid.     

Increase in temperature of a ferromagnetic substance in rf magnetic field

If one applies an rf magnetic field parallel to a strong static field, temperature of a ferromagnetic substance rises due to ferromagnetic resonance. This phenomenon finds immediate application in the field of hyperthermic oncology. In this work, we have found expression for the increase in temperature when a ferromagnetic material is placed under a static and a varying magnetic field of high frequency through spin wave approach. The numerical value of this increment of local temperature has been estimated for yttrium iron garnet (YIG). We also have examined the possibility of enhancing the temperature of a ferromagnetic material only by applying a strong static field.     

双层磁性薄膜系统中铁磁共振性质

利用能量极小原理研究了双层磁性薄膜系统中的铁磁共振特性,给出共振频率与线宽随外加磁场的变化关系.以及考虑应力各向异性、交换各向异性和单轴各向异性后,系统共振频率与线宽的变化情况.数值计算结果表明:外应力场和交换各向异性场对铁磁共振频率、频谱宽度以及磁化行为均有影响.     

Dependence of the ferromagnetic resonance modes on the coupling strength in exchange-coupled trilayer structures

The dependence of the dispersion relation and the magnetization on the exchange coupling strength was calculated for a system consisting of two ferromagnetic layers exchange-coupled through a nonmagnetic spacer layer. The magnetic layers are characterized by both uniaxial and cubic magnetocrystalline anisotropies. A minimization procedure was developed which allows the resonance modes to be obtained for any magnetic field orientation and strength, as well as for any exchange coupling strength. If the antiparallel coupled film is unsaturated at resonance, the dispersion relation for both acoustic and optic modes could be rather complex, especially when the field is applied in the plane of the film.     

Ferromagnetism in Thermally Decomposed Polyimide Films

Magnetic resonance studies of polyimide films thermally decomposed in flowing N₂ at 520°C reveal the presence of two very different magnetic resonance spectra at room temperature. One spectra is a sharp temperature independent paramagnetic resonance line having a g value of 1.990, typical of a free radical. The other much broader line centered at lower field displays a marked broadening and shift to lower magnetic field as the temperature is lowered, characteristic of a ferromagnetic resonance (FMR) signal. Measurements of the AC susceptibility as a function of magnetic field strength confirm the existence of ferromagnetism at room temperature. Magnetic force microscope (MFM) imaging at room temperature show evidence of long thin ferromagnetic regions in the decomposed polymer.     

Magnetic field effects on surgical ligation clips

Magnetic forces exerted on surgical clips and the magnetic resonance imaging distortion they create in phantoms and rabbits at magnetic field strengths of 1.5 Tesla were investigated. Results are reported for both ligation and aneurysm clips manufactured from three types of stainless steel as well as titanium, tantalum and niobium metals. Paramagnetism and eddy currents were measured in a customized moving Gouy balance. Direct measurements of other magnetic forces were carried out in a 1.5T MRI system. The titanium and tantalum clips showed the least interaction with the magnetic field, both in terms of forces exerted and the observed image distortion with the larger clips generating the larger interactions. The strongest field distortions and attractive forces occurred with 17-7PH stainless steel clips. These interactions were ferromagnetic in origin and of sufficient strength to present significant risk to patients having this type of clip present during an MRI scan.     

"MR conditional" respiratory ventilator system incident in a 3-T MRI environment

The misunderstanding of the labeling for an “MR conditional” respiratory ventilator system resulted in the placement of this device, which had substantial ferromagnetic components, too close to a 3-T magnetic resonance (MR) system, causing a projectile incident. Magnetic resonance imaging health care professionals should be aware of the potentially dangerous consequences when such medical devices that have Food and Drug Administration-approved MR conditional labeling are brought into the MR system room. Recommendations to prevent future incidents are provided herein.     

Active ferromagnetic shimming of the permanent magnet for magnetic resonance imaging scanner

This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic resonance imaging (MRI) magnet.It is designed to reduce inhomogeneity of magnetostatic field of C-type permanent magnet to meet the stringent requirement for MRI applications.An optimal configuration (locations and thicknesses) of active ferromagnetic pieces is generated through calculation according to the initial field map and the demanded final homogeneity specifications.This approach uses a minimisation technique which makes the sum of squared magnetic moment minimum to restrict the amount of the active ferromagnetic material used and the maximal thickness of pieces stacked at each hole location in the shimming boards.Simulation and experimental results verify that the method is valid and efficient.     

Phase domain structures in cylindrical magnets under conditions of a first-order magnetic phase transition

The magnetic and resonance properties of cylindrical magnets at first-order phase transition from paramagnetic to ferromagnetic state were theoretically studied. It has been shown that in the external magnetic field directed perpendicularly to the rotation axis, formation of a specific domain structure of paramagnetic and ferromagnetic layers can be energetically favorable. The parameters of cylindrical phase domains as well as their dependences on temperature, magnetic field and material characteristics have been calculated. Peculiarities of the magnetic resonance spectra appearing as a result of the phase domain formation have been considered. Dependence of the resonance field of the system of ferromagnetic domains on magnetization and temperature has been obtained.     

铁磁共振磁交换力显微镜

原子间的自旋相互作用力对原子级别磁性纳米构造体的表面磁性质的理解是极为重要的. 磁交换力显微镜是测量表面自旋作用力的重要手段, 但它的缺点一是需要加外部强磁场, 二是不能分离表面形貌和自旋信息, 这就导致材料表面受外部磁场的影响, 而且表面形貌和自旋信息之间相互影响, 使自旋间的相互作用力的检测和成像研究受到限制.为了解决上述问题, 利用原子力显微镜, 并采用微波照射的方法, 根据铁磁共振原理, 分别独立提取磁性材料表面形貌和自旋信息, 称之为铁磁共振磁交换力显微镜, 理论和实验结果表明此方法可以有效地分离两种信息. 铁磁共振磁交换力显微镜可以促进对原子级磁性材料机能的理解以及磁性相关科学领域的进步, 特别是对自旋电子元件的发展有很大的促进作用, 是新世纪高度信息化社会不可缺少的测量技术. 关键词： 原子力显微镜 磁交换力显微镜 自旋 铁磁共振     

一种实用的使用铁磁材料匀场的开放式超导MRI磁体的优化方法(英文)

磁共振成像(MRI)系统是一种重要的医学诊断设备。作者提出了一种针对使用铁磁材料匀场的开放式超导MRI主磁场匀场的优化方法。在这种方法中,为了使这种包含非线性铁磁轭结构的MRI磁体产生高均匀度的磁场,集成了改进的多输入多输出负反馈控制理论和有限元方法,来计算匀场区非线性铁磁材料的形状。特别要指出的是,这种方法对初始值的要求不高,一定可以收敛,如果配置得当,可以快速收敛。另外,由于收敛速度快,这种方法也可以应用于大型的非轴对称三维模型优化分析。在文中,作者在一个二维轴对称磁体模型上测试了该方法,表现良好。     

铁磁共振法测磁各向异性

采用铁磁共振方法,研究了铁磁/反铁磁双层膜系统中,因交换耦合以及磁晶各向异性而产生的有效各向异性场.结果表明:被测系统有无交换偏置场以及其正负号性质等均能在共振谱中得到辨析.结果还显示:沿着不同结晶方向施加外磁场,共振场的行为与磁晶各向异性以及铁磁/反铁磁交换耦合作用而诱发的单向各向异性等密切相关.将共振频率的变化看成外磁场(包括其方向和大小)的函数,研究得到了单向各向异性,立方各向异性等对共振频率的影响,并同实验结果做了很好的比较. 关键词： 铁磁/反铁磁双层膜 交换耦合 铁磁共振 单向各向异性     

Comparison of ferromagnetic resonance between amorphous wires and microwires

Measurements of magnetoimpedance in amorphous wires and microwires at the GHz region are presented here. The maxima observed in the magnetoimpedance of different samples in the high frequency range are attributed to the ferromagnetic resonance (FMR) that occurs when a sample is submitted to a longitudinal static magnetic field and an oscillating transversal field. While the appearance of a peak on the resistance and the drop of the inductance is explained by means of FMR, the dependence of the width of that resonance with different parameters (magnetic field, stress distribution, sample size, etc.) is not clearly understood, and therefore additional works to explain the value of the resonance width are needed. It is interesting to consider firstly, the influence of the diameter of the sample and thus the stress distribution on the FMR. The size and position of the FMR is found to be completely different for diameters ranging from 24 to 171 μm. The dependence of the frequency value at which the peak of the resistance is found can be explained by the different values of the anisotropy field in the sample. The width of the FMR changes drastically with the diameter of the sample and with the applied magnetic field and they will be discussed in this paper.     

应力各向异性下耦合三层薄膜铁磁共振

研究铁磁/铁磁/反铁磁三层薄膜中应力各向异性场对共振场的影响.得到系统存在声学和光学两种模,并且随着应力场的增强,在某些区域声学模和光学模共振场均向高值方向移动.此外发现声学模共振场对两铁磁层厚度的依赖较大,对光学模而言,随着两铁磁层厚度的改变,应力各向异性场对其共振场的影响甚微.     

Ferromagnetic resonance and magnetization in permalloy films with nanostructured antidot arrays of variable size

We present the results of structural and magnetic characterization of Permalloy (Fe₂₀Ni₈₀) films deposited by sputtering on self-organized nanoporous alumina previously treated with phosphoric acid to vary the pore diameters. SEM and AFM images of the top film surfaces show a triangular array of pores with diameters similar to the untreated porous alumina. However, the underlying pore enlargement is evidenced by the magnetic study. Indeed magnetization measurements reveal a decrease in the easy-plane anisotropy energy with elapsing time of chemical etching. Consistent with these results, ferromagnetic resonance measurements perpendicular to the film indicate a systematic reduction of the resonance field which can be directly related to an increase in the pore diameter. The effect of lateral confinement (due to the pores) in the spin-wave resonance is evidenced in multiple absorption lines when the applied field is in the film plane and perpendicular to it. This contrasts with the results for the continuous test film.     

Magnetic properties of Fe/ZnSe/Fe trilayers

The magnetic properties of Fe/Zn/Fe trilayers have been studied by ferromagnetic resonance and magnetization measurements. These measurements have been used to investigate the magnetic anisotropy of the iron layers and the magnetic coupling across the semiconductor spacer. The angular dependence of the resonance spectra has been measured in-plane and out-of-plane in order to deduce magnetic anisotropy constants of the samples. Experimental data were fitted by using an energy-density expression that includes bulk cubic anisotropy, growth-induced uniaxial in-plane anisotropy and perpendicular-surface anisotropy terms. A small ferromagnetic coupling is observed in the trilayers with spacer thickness up to .     

Investigation of ferromagnetic resonance and damping properties of CoFeB

In this study, the influences of thin film thickness and post-annealing process on the magnetic properties of CoFeB thin films were investigated. The angular dependency and linewidth of the ferromagnetic resonance signal were used to explore the magnetic behavior of sputtered single-layer and trilayer thin film stacks of CoFeB. A micromagnetic simulation model was employed based on the metropolis algorithm comprising the demagnetizing field and in-plane induced uniaxial anisotropy terms with all relevant contributions. Our results reveal that the direction of magnetization changes from in-plane to out-of-plane as a result of the annealing process and induces a perpendicular magnetic anisotropy in the 1-nm thick CoFeB thin film. The ferromagnetic resonance (FMR) linewidth can be defined well by the intrinsic Gilbert damping effect and the magnetic inhomogeneity contribution in both as-grown and annealed samples. The difference between the linewidths of the single and trilayer film is mainly caused by the spin pumping effect on damping which is associated with the interface layers.