小尺寸正方形铁磁薄膜边界效应对磁化翻转过程的影响

用自旋动力学方法系统地研究磁偶极相互作用表现的边界效应对小尺寸正方形铁磁薄膜的磁化翻转过程的影响.在确定的磁偶极相互作用强度下,针对不同的单轴各向异性强度和不同的磁化角(外磁场与易轴间的夹角),具体给出矫顽场与磁化角及单轴各向异性强度之间的依赖关系和-些有代表性的磁滞回线,并给出磁化翻转过程中-些有代表性的微观磁结构.模拟结果表明:磁偶极相互作用表现的边界钉扎作用与单轴各向异性场之间的竞争决定磁滞回线的形状和矫顽场的大小,从而在不同磁化角情况下会导致不同的矫顽场机理.本文提出可有效地描述正方形铁磁性薄膜复杂微观磁畴结构的形成与演变的五磁畴模型.这种五磁畴模型既能直接揭示单轴各向异性正方形铁磁薄膜的几何特性和物理特性,也方便于磁化翻转过程的分析.     

On the effective magnetization and uniaxial anisotropy of permalloy films

The values of the effective magnetization and uniaxial induced anisotropy were measured on permalloy films, vacuum deposited on unheated glass slides, by the method of local ferromagnetic resonance. The double resonance curves of these films indicate the existence of two magnetically different components; to both components there correspond different values of the effective magnetization and uniaxial anisotropy. These quantities have different local and time variations for each component. The results of measurement agree with the recently proposed model of the stratification of thin ferromagnetic films.The authors thank J. Kaczér and F. Kroupa for valuable discussions.     

Influence of induced uniaxial anisotropy on the domain structure and phase transitions of yttrium-iron garnet films

The effect of induced uniaxial anisotropy on the properties and parameters of the domain structure and phase transitions in yttrium-iron garnet (YIG) films is investigated. Based on the measurements and the derived formulas we determine the difference between the magnetization and the uniaxial anisotropy field for each of the films. We have also measured the parameters of the domain structures and phase transitions of the films for the magnetization parallel and perpendicular to the projections of the [111] crystallographic axes onto the plane of the film. We find that films of pure YIG films grown in (111) are characterized by the existence of some critical value of the uniaxial anisotropy field. It is found that for films in which the uniaxial anisotropy field is larger than this critical value and films in which it is less than this critical value, such parameters of the domain structures as the ratio of the width of the domains to the film thickness, the orientation of the magnetization of the domains, the orientation of the domain boundaries, and the magnitudes of the phase transition fields differ substantially. Fiz. Tverd. Tela (St. Petersburg) 41, 2034–2041 (November 1999)     

Recovery of soft magnetic properties of FeNiSm films by Ta interlayer

The magnetic properties of FeNiSm thin films with different thicknesses, different Ta interlayer thicknesses and different numbers of Ta interlayers were investigated. The single layer FeNiSm shows in-plane uniaxial anisotropy at a thickness below critical value, but shows weak perpendicular anisotropy with a stripe domain structure at thickness above the critical value. Experiments indicate that one or more Ta interlayers inserted into thick FeNiSm films with weak perpendicular anisotropy were effective not only in canceling the perpendicular anisotropy, but also in recovering the in-plane uniaxial anisotropy. Blocking of the columnar growth of FeNi grains by the Ta interlayer is considered to be responsible for this spin reorientation phenomenon. Moreover, the magnetization reversal mechanism in FeNiSm films with uniaxial anisotropy can be ascribed to coherent rotation when the applied field is close to the hard axis and to domain-wall unpinning when the applied field is close to the easy axis. The dynamic magnetic properties of FeNiSm films with uniaxial anisotropy were investigated in the frequency range 0.1-5 GHz. The degradation of the soft magnetic properties of magnetic thin films due to the growth of columnar grains can be avoided by insertion of a Ta interlayer.     

Double shifted magnetization curves in magnetic bilayers

Ultrathin ferromagnetic biaxial films exhibit uniaxial anisotropy induced by underlayer and double shifted magnetization curves. Two possible origins of the double shifted curves are identified from experimental data, namely, the step-induced anisotropy and the exchange-coupled induced interfacial anisotropy. Micromagnetic study of modified astroids with starlike shapes indicates that a large uniaxial anisotropy induced by the interface plays a key role in the formation of double shifted magnetization waves. An additional bias field along the easy axis may enhance the shifted field. The present research status, both experimental and theoretical, is briefly discussed.     

Determination of the in-plane anisotropy field in hexagonal systems via rotational magnetization: Theoretical model and Monte Carlo simulations

The magnetic anisotropy field in thin films with in-plane uniaxial anisotropy can be deduced from the VSM magnetization curves measured in magnetic fields of constant magnitudes. This offers a new possibility of applying rotational magnetization curves to determine the first- and second-order anisotropy constant in these films. In this paper we report a theoretical derivation of rotational magnetization curve in hexagonal crystal system with easy-plane anisotropy based on the principle of the minimum total energy. This model is applied to calculate and analyze the rotational magnetization process for magnetic spherical particles with hexagonal easy-plane anisotropy when rotating the external magnetic field in the basal plane. The theoretical calculations are consistent with Monte Carlo simulation results. It is found that to well reproduce experimental curves, the effect of coercive force on the magnetization reversal process should be fully considered when the intensity of the external field is much weaker than that of the anisotropy field. Our research proves that the rotational magnetization curve from VSM measurement provides an effective access to analyze the in-plane anisotropy constant K ₃ in hexagonal compounds, and the suitable experimental condition to measure K ₃ is met when the ratio of the magnitude of the external field to that of the anisotropy field is around 0.2. Supported by the National Natural Science Foundation of China (Grant Nos. 90505007 and 10774061) Recommended by LI FaShen     

Stripe domain statics in garnet films

Magnetic garnet films grown epitaxially on nonmagnetic garnet substrates exhibit a growth or stress-induced uniaxial anisotropy in addition to the cubic magnetocrystalline anisotropy associated with their crystal symmetry. When the uniaxial anisotropy is dominant over the cubic, such films exhibit stripe or bubble domain structures; even a small cubic anisotropy component can have a decisive effect on the behavior of the domains in applied fields. We report an experimental study of the quadistatic behavior of domains in fields applied to a (111) film in the film plane along (11$\text{2}$) and ($\text{1}$10). The experimental results are interpreted by a new theory that gives good agreement with the observed behavior, and yields an accurate measurement of the cubic and uniaxial anisotropy constants.The main qualitative features of the results are: In a ($\text{1}$10) field, the walls are Neél walls perpendicular to the field. In a (11$\text{2}$) field the walls are Bloch walls parallel to the field, the domain magnetization in adjacent stripes is not symmetrical about the film plane, and adjacent stripes are not of equal width; the domain period first shrinks and then expands with increasing field; and even though the applied field has no component perpendicular to the film plane, the film develops a net perpendicular magnetic moment.     

Wellenstruktur der Magnetisierung in dünnen Permalloyschichten

The magnetic ripple of thin uniaxial permalloy films is estimated by the calculus of variations. Two special cases are examined, namely average magnetization and external field in the easy and in the hard direction of the uniaxial anisotropy. When the fluctuations of the magnetization, which are connected with the fluctuations of the anisotropy, are small, the variation problem leads to differential equations which can be integrated. The solutions describe the magnetic ripple in the film. The ripple is a function of the dispersion of the anisotropy directions, the magnetic constants of the film, its thickness, and of the applied field. The agreement between theory and experiment is good.     

Equilibrium states and quasi-static magnetization switching of a multilayer structure

The equilibrium orientations of magnetic moments that correspond to various values and directions of the biasing field are found in a set of magnetic films with cubic crystalline anisotropy and uniaxial induced anisotropy. The films are coupled by exchange interaction of the antiferromagnetic type. Field intervals are established where noncollinear and bistability states causing orientational phase transitions and hysteresis exist. Ninety degree magnetization switching (per switching cycle) of the magnetic moments of the films, as well as an orientational phase transition of bifurcation character, is discovered. Hysteresis loops for 180° in-plane magnetization switching are constructed.     

Anisotropy and dynamic properties of Co2MnGe Heusler thin films

Co₂MnGe films of 30 and 50 nm in thickness were grown by RF-sputtering. Their magnetic anisotropies, dynamic properties and the different excited spin wave modes have been studied using conventional ferromagnetic resonance (FMR) and Microstrip line FMR (MS-FMR). From the in-plane and the out-of-plane resonance field values, the effective magnetization (4πM_eff) and the g-factor are deduced. These values are then used to fit the in-plane angular-dependence of the uniform precession mode and the field-dependence of the resonance frequency of the uniform mode and the first perpendicular standing spin wave to determine the in-plane uniaxial, the four-fold anisotropy fields, the exchange stiffness constant and the magnetization at saturation. The samples exhibit a clear predominant four-fold magnetic anisotropy besides a smaller uniaxial anisotropy. This uniaxial anisotropy is most probably induced by the growth conditions.     

Ferromagnetic resonance study of the effect of elastic stresses on the anisotropy of magnetic films

The distribution of the magnitude and direction of the uniaxial magnetic anisotropy field over the area of permalloy films obtained by thermal evaporation in vacuum was studied using a scanning ferromagnetic resonance spectrometer. The films were deposited on glass substrates subjected to bending stress in the presence of an in-plane static magnetic field. Depending on the character of the elastic-stress distribution over a local spot of a film, the uniaxial anisotropy induced by a magnetic field is either enhanced or completely compensated, which is accompanied by a significant change in the direction of the easy axis of magnetization. The observed effects are in good agreement with phenomenological calculations.     

Optical measurement of magnetic anisotropy in thin garnet films

This paper describes an improved optical method for measuring locally the cubic and uniaxial magnetic anisotropy fields in thin garnet films. The derivative of the in-plane component of the magnetization is measured, using a double modulation technique which combines polarization modulation with field modulation. A simple graphical method is devised to calculateH _k andH _u from the extrema in this derivative curve. The results of measurements on magnetic garnet films obtained by different methods are compared. Local measurements of the anisotropy induced by substrate facet strain are described.     

Exchange bias for ferromagnetic/antiferromagnetic bilayers with the uniaxial anisotropy being misaligned with the exchange anisotropy

Using the principle of minimal energy and S-W model, the exchange bias for ferromagnetic/antiferromagnetic bilayers has been investigated when the uniaxial anisotropy is misaligned with the exchange anisotropy. According to the relation between the energy of the bilayer and the orientation of ferromagnetic magnetization, it is found that the bilayer will be in the monostable state or bistable state when the external field is absent in the initial magnetization state. The monostable state or bistable state of the bilayer, which determines the angular dependence of exchange bias directly, is controlled by the competition between the exchange anisotropy and uniaxial anisotropy. When the applied field is parallel to the intrinsic easy axes and intrinsic hard axes, one of the switching fields of the hysteresis loop shows an abrupt change, while the other keep continuous by analyzing the magnetization reversal processes. Consequently, the exchange bias field and the coercivity will show a jump phenomenon. The numerical calculations indicate that both the magnitude and direction of the exchange anisotropy will significantly affect the angular dependence of exchange bias. The jump phenomenon of exchange bias is an intrinsic property of the bilayer, which is dependent on the interfacial exchange-coupling constant, the orientation of the exchange anisotropy, the thickness and uniaxial anisotropy constant of the ferromagnetic layer.     

Singularity in high-frequency susceptibility of thin magnetic films with uniaxial anisotropy

A sharp peak of magnetic susceptibility has been observed in the ferromagnetic resonance spectra of uniaxial magnetic films placed in a planar field directed orthogonal to the easy magnetization axis, along which a pumping high-frequency magnetic field has been oriented. The peak width is considerably narrower than the line width of the uniform ferromagnetic resonance, and its position in a field equal to the film anisotropy field does not depend on the pumping frequency. The nature of the peak is associated with a drastic increase in the static transverse susceptibility of the film in the vicinity of the anisotropy field. It is shown phenomenologically that the peak can be observed only for quality samples with small angular and amplitude dispersion of the uniaxial anisotropy.     

Properties of amorphous CoZr(RE) (RE=Gd,Sm, Dy) films with various uniaxial anisotropies,prepared by a new process

If a DC magnetic field is applied parallel to the plane of amorphous CoZr(RE) thin films during sputter depositing, a uniaxial anisotropy is formed the direction of which depends upon the choice of RE substituted and its concentration. When RE=Gd a perpendicular anisotropy K_p forms over a large concentration range, a spin reorientation process being at the origin of the process. A well-defined K_p is developed also in CoFeZrGd and CoZrGdSm films. CoZrGdDy films exhibit simultaneously a perpendicular and an in-plane uniaxial anisotropy. The related magnetization process and domain structures are quite peculiar.     

Magnetization processes of CuMn below the freezing temperature Tf

A simple classical model [5] that is based on a blocking of magnetic clusters in a uniaxial anisotropy field explains characteristic properties of magnetization processes of Cu(5–15 at.%) Mn as transition states between the “zero point magnetization” M₀ = M(T → OK) and the thermal equilibrium magnetization M_∞ = M(t → ∞, T). Further it shows two facts the experimental confirmations of which we report in this paper: (a) a magnetization decrease with increasing temperature in high fields and (b) a superposition rule for the magnetization processes in small fields and at low temperatures.     

Large-radius dynamic topological solitons in uniaxial ferromagnets

In thin ferromagnetic films with perpendicular anisotropy, there exist stable large-radius dynamic topological solitons, which are stabilized owing to conservation of the orbital angular momentum of the magnetization field. The solitons can also exist if the projection of the total spin moment of the magnet onto the easy axis is not conserved, i.e., in the absence of pure uniaxial symmetry of the model. Even in the pure uniaxial case, the magnetization distribution in a soliton with a sufficiently large radius differs significantly from centrosymmetric.     

Bifurcation magnetic resonance in films magnetized along hard magnetization axis

We study low-frequency ferromagnetic resonance in a thin film magnetized along the hard magnetization axis performing an analysis of magnetization precession dynamics equations and numerical simulation. Two types of films are considered: polycrystalline uniaxial films and single-crystal films with cubic magnetic anisotropy. An additional (bifurcation) resonance initiated by the bistability, i.e. appearance of two closely spaced equilibrium magnetization states is registered. The modification of dynamic modes provoked by variation of the frequency, amplitude, and magnetic bias value of the ac field is studied. Both steady and chaotic magnetization precession modes are registered in the bifurcation resonance range.     

Magnetization reversal measurements in mesoscopic amorphous magnets by magneto-optical Kerr effect

A magneto-optical setup based on the transverse Kerr effect has been designed to study the magnetization reversal processes by vector magnetometry in arrays of magnetic nanostructures with a reduced total volume. This system allows the measurement of both the parallel and perpendicular to the field components of the magnetization. It has been used to analyze the behavior of amorphous Co _x Si_1-x lines fabricated by electron beam lithography that present a very well defined shape induced uniaxial anisotropy. When the field is applied near to the hard direction, coherent rotation processes are found to occur with a collapse of this reversal mode at fields very close to the hard axis that allows to estimate the very low anisotropy dispersion of these samples. The analysis of the vector hysteresis loops reveals that the magnetization switches via an incoherent process that starts prior to the Stoner-Wohlfarth instability and that can be described in terms of a localized curling-like reversal mode.Received: 16 June 2004, Published online: 24 September 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.60.Jk Magnetization reversal mechanisms - 75.50.Kj Amorphous and quasicrystalline magnetic materials     

Ferromagnetic resonance features in anisotropic magnetic films with a metastable state of magnetic moment

In experiments on single-domain magnetic films with uniaxial in-plane anisotropy, a new homogeneous ferromagnetic resonance peak was observed in a planar magnetic field oriented at an angle to the easy magnetization axis and directed opposite to the magnetization projection onto the field direction. The peak was observed in fields smaller than the magnetization reversal field of the film, and the origin of the peak was found to be related to the metastable state of the magnetic moment. A good agreement was obtained between phenomenological calculations and experimental data.