Magneto-optical visualization of three spatial components of inhomogeneous stray fields

The article deals with the physical principles of magneto-optical visualization (MO) of three spatial components of inhomogeneous stray fields with the help of FeCo metal indicator films in the longitudinal Kerr effect geometry. The inhomogeneous field is created by permanent magnets. Both p- and s-polarization light is used for obtaining MO images with their subsequent summing, subtracting and digitizing. As a result, the MO images and corresponding intensity coordinate dependences reflecting the distributions of the horizontal and vertical magnetization components in pure form have been obtained. Modeling of both the magnetization distribution in the indicator film and the corresponding MO images shows that corresponding to polar sensitivity the intensity is proportional to the normal field component, which permits normal field component mapping. Corresponding to longitudinal sensitivity, the intensity of the MO images reflects the angular distribution of the planar field component. MO images have singular points in which the planar component is zero and their movement under an externally homogeneous planar field permits obtaining of additional information on the two planar components of the field under study. The intensity distribution character in the vicinity of sources and sinks (singular points) remains the same under different orientations of the light incidence plane. The change of incident plane orientation by π/2 alters the distribution pattern in the vicinity of the saddle points.     

Magneto-optic study of the behavior of magnetic domains walls in ferrimagnetic garnet films placed over samples with in-plane magnetization

Magneto-optic (MO) imaging is based on Faraday rotation of a linearly polarized incident light beam illuminating a sensitive MO layer placed in close contact to the sample. For in-plane magnetized layers of Lu_3−xBi_x Fe_5−yGa_yO₁₂ ferrimagnetic garnet films, zig-zag domain formation occurs whenever the sample stray parallel field component changes sign. In this work we study the behavior of zig-zag domain walls that appear when the garnet is placed over samples with in-plane magnetization like audio tapes recorded with different signals. We describe the zig-zag walls considering the anisotropy, exchange and magnetostatic energies in the Neel tails and the contribution of an applied magnetic field. Using different recorded signals we have been able to control the gradient of stray parallel field component on the garnet, changing the distance between domains and the size of zig-zag walls. We could even avoid the appearance of these zig-zag domain walls and obtain closed domains structures. We also study the behavior of the domain walls when an external magnetic field is applied parallel to the sample.     

Investigation of small-scale stray magnetic fields by magnetooptical methods

Domain structures with in-plane magnetization are investigated in magnetically hard films. A magnetooptical setup designed for studying stray magnetic fields combines the advantages of the vibrating-sample magnetometer and the magnetooptical method of signal detection. The sensitivity of measurements of the stray field normal component is ～0.1 Oe. The criteria for choosing the optimal parameters of magnetooptical media for information readout are established.     

兰州重离子治癌装置同步加速器切割磁铁研制

切割磁铁作为同步加速器注入引出的关键部件之一,对磁场及切割板结构都有严格的要求。介绍了兰州重离子治癌装置(HIMM)同步加速器切割磁铁的设计情况,基于磁场优化软件OPERA,对切割磁铁磁场均匀度及杂散场的分布进行了详细的分析。根据磁场要求优化结构设计,完成了磁铁的加工。磁场测量结果分析显示,有效场区范围内磁场均匀度优于设计结果。同时通过对磁铁端部屏蔽处理,在环内侧管道处杂散磁场降到2mT以内,满足物理设计要求。     

Vortex states of a superconducting film from a magnetic dot array

Using Ginzburg-Landau theory, we find novel configurations of vortices in superconducting thin films subject to the magnetic field of a magnetic dot array, with dipole moments oriented perpendicular to the film. Sufficiently strong magnets cause the formation of vortex-antivortex pairs. In most cases, the vortices are confined to dot regions, while the antivortices can form a rich variety of lattice states. We propose an experiment in which the perpendicular component of the dot dipole moments can be tuned using an in-plane magnetic field. We show that in such an experiment the vortex-antivortex pair density shows broad plateaus as a function of the dipole strength. Many of the plateaus correspond to vortex configurations that break dot lattice symmetries. In some of these states, the vortex cores are strongly distorted. Possible experimental consequences are mentioned.     

Photogalvanic current in a parabolic well

We study the in-plane stationary photocurrent in a parabolic potential well. The well has vertical asymmetry due to inhomogeneous distribution of scatterers. The electric field of light has both vertical and in-plane components. The photogalvanic effect originates from the periodic oscillation of electrons in a vertical direction caused by the normal component of the alternating electric field with simultaneous in-plane acceleration/deceleration by the in-plane component of electric field. The problem is considered in classical approximation assuming inhomogeneously-distributed friction. Photocurrent has a resonance character. Resonance occurs at light frequencies close to a characteristic well frequency. The effect of in-plane magnetic field is also studied.     

磁性颗粒膜法拉第转角的研究

在外磁场作用下,复合介质的法拉第磁光效应依赖于颗粒膜电介质张量。而复合介质的电介质张量的计算相当复杂。运用了有效介质近似理论,利用非均匀复合介质的有效电场等于单个颗粒中局域场的平均值的自恰条件,由电介质张量εe方程及自洽条件导出了计算磁性颗粒膜系统磁光法拉第转角的解析公式。并应用导出的关系,以Cu金属颗粒为例,讨论了颗粒膜中金属颗粒含量及对应的基质、离子浓度、颗粒形状对法拉第转角的影响,结果表明,利用有效介质近似理论计算的结果与实验结果一致。     

利用法拉第效应进行永磁体磁场二维光学成像

借助于法拉第磁致旋光效应,以ZF6玻璃为磁旋光介质,设计了适当的光学系统,对永磁体磁极附近磁场的分布进行了光学二维成像.利用自编的图像采集软件采集了一系列原始图片,并利用自编的图像处理软件对这些原始图片进行了处理,完成了检偏过程.分别得到了透射成像方式和反射成像方式下永磁体磁场的分布图像,这些图像正确反映了永磁体磁场的实际分布.本研究工作为宏观尺度磁场的观测与测量提供了一种有效的手段.     

Magnetic anisotropy determined by differential magnetization measurements in twisted amorphous ribbons

A new method of measurement, which enables a determination of the in-plane anisotropy in magnetic metallic glass ribbons, has been developed. It can be used for materials with the easy axis of magnetization parallel as well as perpendicular to the ribbon axis. The method is based on measurements of the stress dependence of the inhomogeneous magnetization obtained by twisting the ribbon around the ribbon axis. The required experimental equipment is very simple, and the value of the anisotropy constant comes out directly as an equivalent internal stress field.     

Free magnetization oscillations in garnet ferrite films with quasi-planar anisotropy

Free magnetization oscillations in garnet ferrite films with quasi-planar anisotropy was studied. The oscillations were excited by a pulse of an in-plane magnetic field. An analytic expression relating the oscillation frequency to the film parameters and the external magnetic field was derived; the expression is in good agreement with the experimental data. The planar anisotropy is shown to increase the free-oscillation damping.     

Local ferroelectricity in SrTiO3 thin films

The temperature-dependent polarization of SrTiO3 thin films is investigated using confocal scanning optical microscopy. A homogeneous out-of-plane and an inhomogeneous in-plane ferroelectric phase are identified from images of the linear electro-optic response. Both hysteretic and nonhysteretic behavior are observed under a dc bias field. Unlike classical transitions in bulk ferroelectrics, local ferroelectricity is observed at temperatures far above the dielectric permittivity maximum. The results demonstrate the utility of local probe experiments in understanding inhomogeneous ferroelectrics.     

Experimental determination of ultra-sharp stray field distribution from a magnetic vortex core structure

The fine magnetic stray field from a vortex structure of micron-sized permalloy (Ni₈₀Fe₂₀) elements has been studied by high-resolution magnetic force microscopy. By systematically studying the width of the stray field gradient distribution at different tip-to-sample distances, we show that the half-width at half-maximum (HWHM) of the signal from vortex core can be as narrow as ∼21 nm at a closest tip-to-sample distance of 23 nm, even including the convolution effect of the finite size of the magnetic tip. a weak circular reverse component is found around the center of the magnetic vortex in the measured magnetic force microscope (MFM) signals, which can be attributed to the reverse magnetization around the vortex core. Successive micromagnetic and MFM imaging simulations show good agreements with our experimental results on the width of the stray field distribution.     

EMIR: a photothermal tool for electromagnetic phenomena characterization

The heat-photon conversion phenomenon can be used to obtain a thermal image of an electromagnetic field. The electromagnetic field is partially absorbed by a sensitive paint or by a coating deposited on structures or on thin films. A map of the temperature increase of this absorbing medium is an image of the electric or magnetic intensity field distribution, depending on the electric and magnetic properties of the medium. A brief history of the various techniques used to obtain thermal images of electromagnetic fields is first presented. Emphasis is then put on infrared thermography which has been preferentially used in the past 20 years. An analysis of the thermal problems involved is presented. It appears that the solution to these problems is the key for the enhancement of the technique and for really quantitative work. Original solutions have been developed at ONERA, based on the combined use of optimised thin films with controlled electric conductivity, very sensitive infrared cameras, lock-in infrared thermography, and microwave interferometry. In these conditions, quantitative images of both amplitude and phase are obtained. Such an electromagnetic field imaging technique is a powerful tool which has no equivalent and which can be used for several types of applications such as: i) antenna radiation pattern characterization; ii) mode propagation characterization in waveguides; iii) study of absorption phenomena in complex materials; iv) nondestructive evaluation of dielectric structures (electromagnetic windows) or radar absorbing materials; v) knowledge of surface currents distribution on metallic structures.     

Multiple nuclear spin echo in thin polycrystalline ferromagnetic films

The formation of multiple nuclear spin echo signals has been studied in thin ferromagnetic polycrystalline films of 3d-metals and their alloys with induced anisotropy at temperatures between 2.2 and 300 K using two-pulse and three-pulse excitation. A method is proposed for the experimental determination of the contributions made by different mechanisms to the formation of spin-echo signals in magnets with strongly inhomogeneous Zeeman and quadrupole interactions. It is shown that in ferromagnets with a high rf field gain at the nucleus, the frequency modulation mechanism has a substantial influence in observations of nuclear spin-echo signals at nuclei with a high magnetic moment, even at liquid-helium temperatures. Fiz. Tverd. Tela (St. Petersburg) 40, 1056–1061 (June 1998)     

Ferroelectricity in spiral magnets

It was recently observed that the ferroelectrics showing the strongest sensitivity to an applied magnetic field are spiral magnets. We present a phenomenological theory of inhomogeneous ferroelectric magnets, which describes their thermodynamics and magnetic field behavior, e.g., dielectric susceptibility anomalies at magnetic transitions and sudden flops of electric polarization in an applied magnetic field. We show that electric polarization can also be induced at domain walls and that magnetic vortices carry electric charge.     

Measurement of micro weld joint position based on magneto-optical imaging

In a laser butt joint welding process,it is required that the laser beam focus should be controlled to follow the weld joint path accurately.Small focus wandering off the weld joint may result in insufficient penetration or unacceptable welds.Recognition of joint position offset,which describes the deviation between the laser beam focus and the weld joint,is important for adjusting the laser beam focus and obtaining high quality welds.A new method based on the magneto-optical(MO)imaging is applied to measure the micro weld joint whose gap is less than 0.2 mm.The weldments are excited by an external magnetic field,and an MO sensor based on principle of Faraday magneto effect is used to capture the weld joint images.A sequence of MO images which are tested under different magnetic field intensities and different weld joint widths are acquired.By analyzing the MO image characteristics and extracting the weld joint features,the influence of magnetic field intensity and weld joint width on the MO images and detection of weld joint position is observed and summarized.     

Optimization of MO Bragg diffraction of guided optical waves by MSW in YIG films by using inclined magnetic bias field

We describe a new scheme of noncollinear interaction geometry for magneto-optical (MO) Bragg cells based on inelastic scattering of guided optical wave beams by magnetostatic waves in yttrium–iron–garnet (YIG) films. A great increase of the diffracted light intensity was obtained when using an inclined magnetization of the film, in the case when static in-plane magnetization component is directed along the light propagation direction. It is shown that the diffraction efficiency can be increased more than two times, at a specific value of the angle (≈35°) between the saturation magnetization vector and the normal to the film surface. The effect can be explained through a four-wave model of the diffraction process, which can take place in optical waveguides with MO gyrotropy. The results obtained by a simple analytical solution of the diffraction problem are found to be in good qualitative agreement with the experimental observations.     

Noncollinear spin reorientation transition in S?=?1 ferromagnetic thin films

An in-plane spin reorientation transition in thin ferromagnetic films is discussed in terms of the thermodynamics of inhomogeneous low-dimensional systems based on a Néel sublattices concept while using a spin 1 Heisenberg Hamiltonian. The model allows us to investigate in a straightforward manner the layer-dependent phenomena. In this context, we propose a model of noncollinear magnetization structure based on the appropriate distribution of the anisotropy parameters inside the Fe films on W(110). The spin reorientation transition originates at the Fe/W(110) interface and proceeds via noncollinear magnetization structure toward the surface with increasing film thickness in accordance with the experimental findings. The temperature-driven spin reorientation transition in freestanding Fe films and in Fe/W(110) systems is also discussed in detail.     

An open magnet utilizing ferro-refraction current magnification

Ferro-refraction is the field magnification that is obtained when a current segment is near a high magnetic permeable boundary. It is shown that ferro-refraction may be used in the design of magnets for NMR or MRI to increase the efficiency of these magnets. The field may be modeled analytically with the Biot--Savart law and the inclusion of mirror image currents. Ferro-refraction is particularly useful in the design of monohedral magnets, magnets producing a remote homogeneous region which have the magnetic sources arranged to one side. These magnets have also been called planar magnets. Two designs for a monohedral magnet which produce good agreement between experimental and analytic results are presented.     

Mǒsbauer study of the field induced uniaxial anisotropy in electro-deposited FeCo alloy films

Thin ferromagnetic films with in-plane magnetic anisotropy are promising materials for obtaining high microwave permeability.The paper reports a Mo¨ssbauer study of the field induced in-plane uniaxial anisotropy in electro-deposited FeCo alloy films.The FeCo alloy films were prepared by the electro-deposition method with and without an external magnetic field applied parallel to the film plane during deposition.Vibrating sample magnetometry and Mo¨ssbauer spectroscopy measurements at room temperature indicate that the film deposited in external field shows an in-plane uniaxial anisotropy with an easy direction coinciding with the external field direction and a hard direction perpendicular to the field direction,whereas the film deposited without external field does not show any in-plane anisotropy.Mo¨ssbauer spectra taken in three geometric arrangements show that the magnetic moments are almost constrained in the film plane for the film deposited with applied magnetic field.Also,the magnetic moments tend to align in the direction of the applied external magnetic field during deposition,indicating that the observed anisotropy should be attributed to directional ordering of atomic pairs.