Measurement of magnetic parameters of nanometer-thick conducting magnetic films using anisotropic magnetoresistive effect

The angular dependences of anisotropic magnetoresistance (AMR) are measured in conducting ferromagnetic films of nanometer thickness and layered structures containing such films and having the shape of narrow ribbons. These structures are used for preparing spin-dependent magnetic tunnel junctions possessing a giant magnetoresistance. The possibility of determining the main magnetic parameters, which are important for preparing magnetic junctions, by AMR angular measurements is demonstrated experimentally. The magnetic anisotropy axis, the saturating magnetic field, and the coercivity are determined in a 25-nm-thick permalloy (Py) film, in the structures FeMn film (15 nm)-Py film (10 nm) deposited by RF magnetron sputtering on a oxidized silicon substrate, as well as in the structure FeMn (15 nm)-Py (10 nm)-SiC (1.5 nm)-Py (10 nm) deposited on a sitall substrate. It is shown that, under the same conditions of Py films deposition, the magnetic anisotropy axis in the FeMn-Py structure is turned through 90° relative to the anisotropy axis of Py in structures without FeMn layers. The value of the exchange bias fields of the magnetization reversal measured in the structure FeMn (15 nm)-Py (10 nm)-SiC (1.5 nm)-Py (10 nm) by the AMR method is in good agreement with the result of measurement by the inductive method.     

Soft magnetization of a semi-hard/soft magnetic bilayer produced by oblique-incidence evaporation

The possibility of achieving soft magnetization in semi-hard magnetic films such as Fe, Fe_93.5Si_6.5, Fe₅₀Co₅₀ and Fe₇₀Co₃₀ is investigated by depositing films on an Fe₂₀Ni₈₀ underlayer by oblique-incidence evaporation. The magnetic anisotropy of the underlayer is strengthened to a depth of several lattice parameters by vapor deposition of the film at an oblique angle to the substrate surface. This method also allows magnetic anisotropy to be induced in strongly isotropic semi-hard magnetic overlayers to a thickness of a few thousands Angstroms. The coercive force of bilayer films measured along the hard-axis is reduced remarkably by this process, and the strength of the anisotropy field is demonstrated to be readily controllable. When magnetic anisotropy exists in both magnetic layers, a significant change is observed in the magnetization processes of the semi-hard magnetic layer and the coercive forces in the hard magnetization direction is dramatically reduced. Soft magnetization of the semi-hard magnetic layer cannot be achieved when magnetic anisotropy exists in only one of the magnetic layers.     

Layer-resolved microscopy of magnetic domains in multi-layered systems

Photoelectron emission microscopy in connection with magnetic circular dichroism in soft X-ray absorption can be used for the microscopic imaging of magnetic domains in layered thin film structures consisting of several magnetic layers. Due to the element-selectivity of the method, the different magnetic layers in such a structure can be imaged separately, provided that they contain different elements. This has been applied for the investigation of Co/Cu/Ni trilayers, epitaxially grown on Cu (001). The magnetic coupling between the Co and Ni layers can be directly visualized from comparing layer-resolved magnetic domain images of both layers. As a consequence of the competition between the anisotropy energies of the two magnetic layers and the magnetic coupling energy, spin-reorientation transitions between collinear and non-collinear magnetic configurations are observed. Apart from this globally observable magnetic interlayer coupling a micromagnetic coupling mechanism is also evident from the layer-resolved domain images. It is caused by magnetostatic interaction of local stray fields from domain walls. Received: 22 August 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-345/5511-223, E-mail: kuch@mpi-halle.de     

Microstructure and magnetic properties of multilayered [Fe/Pt]n structures prepared by successive deposition

The structure and magnetic properties of multilayered [Fe/Pt]n structures prepared by successive magnetron sputtering of Fe and Pt plates and deposition of Fe and Pt layers on a preliminarily heated glass substrate have been studied as functions of the number n and thickness of the layers. Mössbauer studies and measurements of magnetic hysteresis loops (MH) have established that [Fe/Pt]n films for n = 16 exhibit primarily magnetic anisotropy normal to the film plane. Data obtained by X-ray photoelectron spectroscopy (XPS) strongly suggest that the films have an interface between the substrate and the multilayered structure. Our micromagnetic modeling leads to the conclusion that the magnetic anisotropy oriented normal to the [Fe/Pt]n film plane (for n = 16) is induced by formation of an anisotropic interface.     

各向异性场对三明治膜巨磁阻抗效应的影响

根据具有巨磁阻抗(GMI)效应的实际三明治样品的状况，提出三明治结构作为理论计算的模型，考虑了磁性层的各向异性场，利用一定边界条件下的Maxwell方程和Landau-Lifshitz方程对模型进行了理论计算，得到了阻抗与频率、各向异性场等因素间的函数关系.在此基础上，着重讨论了各向异性场H_k的作用.H_k的大小、方向均会对GMI效应造成影响，最佳的H_k应在400A/m左右，位于面内并与电流垂直.还证实了三明治结构中磁性层的磁致伸缩只会减弱G 关键词： 巨磁阻抗(GMI)效应 三明治膜 Maxwell方程 Landau-Lifshitz方程     

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.     

Effect of thermal annealing on the magnetic anisotropy of GaMnAs ferromagnetic semiconductor

We have systematically investigated the influence of annealing on the magnetic anisotropy properties of GaMnAs film using an epilayer with a Mn concentration of 6.2%. The GaMnAs epilayer was grown by molecular beam epitaxy and the planar Hall effect measurement was used to monitor the magnetic anisotropy of the film. We found significant annealing-induced changes in the magnetic anisotropy properties of the GaMnAs film that depended on the annealing conditions. For example, the cubic anisotropy that gave a four-fold symmetry of magnetic easy axes decreased while the uniaxial anisotropy that gave a two-fold symmetry of magnetic easy axes increases in the samples annealed temperature below 300 °C. In particular, the uniaxial anisotropy along the [010] direction in as-grown GaMnAs film changed to the [100] direction by rotating by 90° after the sample was annealed at 300 °C for 3 h. This investigation thus indicates that the magnitude and the direction of the magnetic anisotropy in the GaMnAs film can be effectively controlled by choosing an appropriate annealing time and temperature.     

Magnetoresistive Fe19Ni81/Tb-Co medium with an internal magnetic bias

The magnetization reversal and magnetoresistance of two-layer exchange-coupled Fe19Ni81/Tb-Co films are studied. Amorphous Tb _x Co_{100 − x} layers with 30 < x < 35 are found to have a uniaxial magnetic anisotropy and a rather high coercive force, which ensures magnetic biasing of the adjacent permalloy layers. In addition, the permalloy layers subjected to selective annealing exhibit a significant anisotropic magnetoresistance and a small magnetic hysteresis. These properties make it possible to consider the developed film structure as an effective magnetoresistive medium. This structure is used to form magnetic sensor samples that have an odd transfer function in the absence of external magnetic biasing.     

Shape instability in out of equilibrium magnetic domains observed in ultrathin magnetic films with perpendicular anisotropy

The magnetic configurations induced by the growth process in a thin film with perpendicular magnetisation have been observed by magnetic force microscopy (MFM). The FePd thin film has been grown by molecular beam epitaxy. A high uniaxial chemical ordering of the alloy into the tetragonal L1₀ structure induces the development of a large perpendicular anisotropy. As the growth process is performed below the Curie temperature of the FePd alloy, domain nucleation occurs during the growth process. The magnetic configuration has been imaged in the as grown state. As the equilibrium size of the magnetic domains decreases when the thickness of the layer increases, the domains obtained from spontaneous nucleation at the beginning of the growth of the thin film are submitted to very large strains as the layer thickness increases. At low thicknesses (low strains), the domain wall instability gives rise to an undulation of the domain walls. Thereafter, it leads to the formation of well-defined magnetic fingers, thus giving birth to the coexistence of two length scale in the domain structure. A quantitative estimation of the strain leading to the fingering instability is obtained. Last, the implications of these observations on the kinetic of domain wall distortion in ultrathin layers are discussed.     

Magnetic properties and huge magnetic impedance of permalloy/ copper/permalloy film elements

Magnetic properties, special features of magnetic anisotropy, magnetic domain structure, and huge magnetic impedance of permalloy/copper/permalloy film elements fabricated by the method of ion-plasma sputtering are investigated. Elements with different plane geometry of layers are analyzed. In the first case, all layers of the element have identical sizes of 10 × 0.5 mm, and in the second case, the width of the copper layer is narrowed down to 0.2 mm. The magnetic properties and the magnetic impedance of the examined film elements allow them to be used as detectors of low magnetic fields. The elements of the first type are more adapted to low excitation current frequencies (up to 150 MHz), and those of the second type are more adapted to high frequencies (above 150 MHz).     

Structure and magnetic anisotropy of Co/Cu/Co films

The energy of the magnetic anisotropy of Co/Cu/Co polycrystalline ultradisperse films is investigated as a function of the thickness of copper and cobalt layers. The influence of the structure parameters (the size and distribution of defects, the period and amplitude of roughnesses) on the surface and volume components of the magnetic anisotropy is analyzed. The parameters of the structure inhomogeneities and their distribution over the film surface are determined from two-dimensional Fourier spectra and electron microscope images of the films.     

Magnetic reversal mechanism in strong perpendicular magnetic anisotropic CoPt/AlN multilayer film

Magnetic reversal mechanism of the Sub/AlN5 nm/[CoPt2 nm/AlN5 nm]₅ nano multilayer film, which shows strong perpendicular magnetic anisotropy (K_u=6.7×10⁶ erg/cm³), has been studied. The angle-dependent magnetic hysteresis loops of this highly perpendicular anisotropic CoPt/AlN multilayer film were measured in the present work, applying a magnetic field along different angles φ with respect to the film normal. It demonstrates that the magnetic reversal of the CoPt ultrathin layers in the CoPt/AlN multilayer film is occurred by the reversible magnetization rotation and the irreversible displacement of domain walls. The φ-dependent part of coercive field is resulted from the internal stress according to the Kondorsky and Kersten model. The φ-independent part of coercive field implies some random and isotropy pinning centers (e.g., vacancies, dislocations, grain boundaries) in the ultrathin CoPt layers. Our work is useful for coercivity control of metal/ceramics layered structures, in particular the perpendicular magnetic tunneling junctions.     

Ferromagnetic resonance in monocrystalline spin valves CoFeB/Ta/CoFeB and CoFeB films with perpendicular magnetic anisotropy

Orientation dependences of ferromagnetic resonance in a MgO/CoFeB/MgO/Ta film with one ferromagnetic layer (monolayer) and in a MgO/CoFeB/Ta/CoFeB/MgO/Ta spin valve containing two single-crystal ferromagnetic CoFeB layers divided by a nonmagnetic Ta layer (bilayer) were investigated. Analysis of the orientation dependences of the structures with perpendicular magnetic anisotropy allowed calculating constants of magnetic anisotropy and damping factors. Physical reasons underlying the differences between these parameters in one- and two-layered structures are discussed.     

Radial magnetic anisotropy in amorphous electrolytically prepared CoP and FeP cylindrical layers

Conclusion The presence of radial magnetic anisotropy with the local axis of the easy magnetization perpendicular to the surface of the layers may be identified in the amorphous cylindrical CoP layers by means of the study of the domain structure using the Bitter technique. This anisotropy is maintained also after undercritical isothermal annealing. In amorphous cylindrical FeP layers the radial magnetic anisotropy is formed — in contrast to CoP layers — only after isothermal annealing below the crystallization temperature. However, the amorphous-crystalline transformation leads in both layers to an apparent disturbance of radial anisotropy, which may be connected with the character of the crystallization of layers and with the new distribution of internal stresses.Dedicated to Jan Kaczér DrSc on the occasion of his 65th birthday.     

Effects of oblique-incidence evaporation on magnetic properties of Fe20Ni80/Fe70Co30 bilayer film

Evaporative deposition at oblique incidence is shown to enhance the magnetic anisotropy of an Fe₂₀Ni₈₀ magnetic film and induce magnetic anisotropy in an overlying, strongly isotropic Fe₇₀Co₃₀ film. This deposition method for the formation of an underlayer of several lattice parameters in thickness and semi-hard overlayer of a few thousands Angstroms in thickness achieves a significant change in the magnetization process and strong suppression of the coercive forces of Fe₇₀Co₃₀ in the hard magnetization direction. Soft magnetization of the Fe₇₀Co₃₀ overlayer is not achieved when one of the layers is deposited at oblique incidence. It is anticipated that shape magnetic anisotropy is responsible in part for the magnetic anisotropy induced in both in Fe₂₀Ni₈₀ under- and Fe₇₀Co₃₀ overlayer by oblique incidence evaporation.     

Monte Carlo study on bilayer thin films magnetic properties dependence with discontinouos anisotropy parameters

In the last few years there has been significant interest in the field of thin films, due to numerous specific phenomena related to the low dimension of these systems, and to the large opportunities in development of high technologies based on their specific magnetic and electronic properties. When dealing with systems of reduced dimensionality it is important to take into account the influence of magnetic anisotropies. In this paper we investigate the magnetic properties of bilayer thin film. This behavior is modeled using Monte Carlo simulations, in the Extended Anisotropic Heisenberg Model. The magnetization, out-of-plane and in-plane magnetic susceptibilities, and also the specific heat bearings according to temperature are investigated in order to find the potential magnetic ordering phases and the critical temperatures, for two sets parameter assignments. For quasi-uniform anisotropy parameters of the film we detect the ferromagnetism-paramagnetism transition and then, by changing the model parameters values, we relieve a short range ferromagnetic ordering phase arising from the antiferromagnetic base layer coupling influence and from easy-plane anisotropy discontinuity on the layers interface.      

Brillouin light scattering study of the magnetic hysteresis loop in Fe-Ni/Si(100) film with induced magnetic anisotropy

The magnetic hysteresis of Fe₅₇Ni₄₃/Si(100) with magnetic anisotropy induced by an external field has been studied by Brillouin light scattering (BLS). The results are compared with those of the magneto-optic-Kerr-effect (MOKE) measurement and the vibrating sample magnetometer (VSM). The BLS results show that the sample film has strong in-plane anisotropy. The angle between the magnetization and a 4.6 G applied magnetic field H reaches a maximum value of 45° when H lies along the hard axis. The coercivity and magnetic anisotropy field for the film obtained by the BLS are compared with the values obtained by the VSM and MOKE measurement.     

Field-induced spin-reorientation transitions in magnetic superlattices with uniaxial anisotropy and biquadratic exchange

Phase transitions induced by an external field are investigated in magnetic multilayer systems with uniaxial anisotropy and biquadratic exchange. A magnetic field directed perpendicular to the plane of the layers changes the effective anisotropy and exchange constants, determining the orientation of the magnetization in the plane of the layers, and can give rise to spin-reorientation transitions. All possible types of such transitions are investigated for the case of uniaxial anisotropy, which differs substantially from the case of cubic anisotropy by the different renormalization of the effective anisotropy constants. Fiz. Tverd. Tela (St. Petersburg) 41, 461–463 (March 1999)     

Unidirectional anisotropy in ferromagnetic-ferrimagnetic film structures

A mechanism of unidirectional anisotropy formation in an exchange-coupled ferromagnetic-ferrimagnetic film structure with orthogonal effective magnetizations in the layers is investigated. The reason for unidirectional anisotropy is the magnetic heterogeneity of the ferrimagnetic layer in the compensation range. Magnetization reversal in the magnetically soft layer of an (REE–transition metal)/NiFe film structure is discussed based on a model of uniform rotation of magnetization. It is found that unidirectional anisotropy sharply decreases the magnetic noise level in the magnetically soft layer. The field of application of these materials is outlined.     

Magnetic anisotropy engineering in square magnetic elements

Square magnetic elements with side in the 100–500 nm range have been fabricated using the focused ion beam (FIB) milling technique from a 10 nm thick, single-crystal Fe film, epitaxially grown on MgO(0 0 1). Thanks to the good crystal quality of the film, magnetic elements with well-defined magnetocrystalline anisotropy have been prepared, while the fine control of the size and shape of the magnets allows for the effective engineering of the anisotropic behavior of the magnetostatic energy that determines the so-called configurational anisotropy. Micromagnetic calculations and experiments show that the angular dependence of the transverse susceptibility has a strong dependence on the material parameters as well as on the static applied field. This allows the effective engineering of the total anisotropy of the magnets.