Two-dimensional micromagnetic simulation of domain structures in films with combined anisotropy

The transformation of the domain structure of micrometer-thick films with variations in the induced uniaxial anisotropy constant with the easy magnetization axis perpendicular to the film surface has been investigated using numerical micromagnetic simulation in the framework of a two-dimensional model of the magnetization distribution. The case where the tetra-axial crystallographic anisotropy exists in the film with uniaxial magnetic anisotropy has been considered. The transformation of the open domain structure into the structure with a magnetic flux closed inside the sample has been investigated in detail, and new types of 109-degree and 90-degree vortex-like domain walls and periodic domain structures have been obtained.     

Remanence due to wall magnetization and counterintuitive magnetometry data in 200-nm films of Ni

200-nm-thick Ni films in an epitaxial Cu/Ni/Cu/Si(001) structure are expected to have an in-plane effective magnetic anisotropy. However, the in-plane remanence is only 42%, and magnetic force microscopy domain images suggest perpendicular magnetization. Quantitative magnetic force microscopy analysis can resolve the inconsistencies and show that (i) the films have perpendicular domains capped by closure domains with magnetization canted at 51 degrees from the film normal, (ii) the magnetization in the Bloch domain walls between the perpendicular domains accounts for the low in-plane remanence, and (iii) the perpendicular magnetization process requires a short-range domain wall motion prior to wall-magnetization rotation and is nonhysteretic, whereas the in-plane magnetization requires long-range motion before domain-magnetization rotation and is hysteretic.     

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)     

Static properties of asymmetric vortex-like domain walls in magnetically uniaxial thick films

The structure and energy of asymmetric vortex-like Bloch and Néel walls in a magnetically uniaxial film with an easy magnetization axis lying in the film plane are investigated by numerically minimizing the total energy within the rigorous micromagnetic approach and the two-dimensional model of the magnetization distribution. The calculations are performed over wide ranges of film thicknesses b (up to b = 1 μm) and magnetic parameters of the films. It is established that the asymmetric vortex-like domain walls are the most universal wall structures in the films under consideration. In magnetically uniaxial films, unlike in magnetically multiaxial films, the asymmetric Bloch walls are always stable.     

Stability of the magnetic domain structure of nanoparticle thin films against external fields

We investigate the effect of external magnetic fields on the magnetic structure of thin films from magnetic nanoparticles (MNP) with dipolar interaction. Such fields are present, for example, if samples are scanned with magnetic probes. Numerical simulations and experimental magnetic force microscopy (MFM) studies are presented. Numerically, we have calculated the magnetization pattern of single-layer and multilayer MNP thin films. The calculations show that unperturbed single-layer MNP films have an in-plane orientation of the magnetization with a flux-closure-domain pattern. An external field generated by a point dipole above the film induces locally an out-of-plane configuration of the magnetization. In the corresponding MFM images, the domain pattern in the film is erased and a stripe-like contrast enhancement at the edges appears. Multilayer films are found to be more robust against external fields than monolayers.     

Magnetization reversal in amorphous gadolinium-cobalt films with a radial gradient of magnetic properties

The integral characteristics of magnetization switching in amorphous gadolinium-cobalt films with perpendicular anisotropy are studied by visualizing the domain structure and measuring magnetooptic hysteresis loops. The films have a radial gradient of magnetic properties that is due to a spatially nonuniform thermal field. Magnetization switching in those film areas where the domain wall motion depends only on the coercive force is simulated in simple terms. In a first approximation, local events of magnetization switching are shown to take place independently of each other and the net hysteresis loop can be represented as a sum of the local loops.     

Hysteresis of the characteristics of magnetostatic waves in ferrite films with stripe domains whose magnetization vectors are oriented close to the plane of the film

The propagation of zero-exchange spin waves (magnetostatic waves) is investigated in yttrium iron garnet films having a regular stripe domain structure with almost in-plane orientation of the domain magnetization vectors. The characteristics of the waves are studied for magnetizations of the film parallel and perpendicular to projections of the [111] crystallographic axes onto the plane of the film. It is established, in contrast with films having the domain magnetization vectors oriented close to the normal to the plane of the film, that both the propagation of magnetostatic waves and the variation of the parameters of the domain structure exhibit a distinctly pronounced hysteretic character as the magnetizing field is varied. The hysteresis of the amplitude-frequency response, equiphase, and dispersion curves of the magnetostatic waves is investigated. The authors examine how the hysteresis of these parameters is related to the hysteresis of the domain structure. The spectrum of magnetostatic waves is found to have an interval of wavelengths (wave numbers) that are not excited in the unsaturated film when the applied field is close to the saturation value, and this phenomenon as well exhibits hysteresis. Zh. éksp. Teor. Fiz. 114, 1430–1450 (October 1998)     

Spin polarized low energy electron microscopy investigations of magnetic transitions in Fe/Cu(1 0 0)

Magnetic transitions in ultrathin Fe films on the Cu(1 0 0) surface have been studied with spin polarized low energy electron microscopy. By monitoring averaged image intensity oscillations and the evolution of magnetization and magnetic domain structure simultaneously and continuously during growth, magnetism and film thickness are correlated with unprecedented precision. The thickness range over which ferromagnetism exists in films grown at room temperature generally increases as the deposition rate is decreased. This trend is attributed to the influence of residual hydrogen. The two-dimensional Ising model with finite size scaling of the Curie temperature accurately describes the evolution of magnetization with increasing film thickness.     

真空磁场热处理温度对不同厚度的Ni88Cu12薄膜畴结构及磁性的影响

利用射频磁控共溅射方法,在Si衬底上制备了Ni88Cu12薄膜,并且研究了膜厚以及真空磁场热处理温度对畴结构和磁性的影响. X射线衍射结果表明热处理后的薄膜晶粒长大,扫描电子显微镜结果发现不同热处理温度下薄膜表现出不同的形貌特征.热处理前后的薄膜面内归一化磁滞回线结果显示,经过热处理的Ni88Cu12薄膜条纹畴形成的临界厚度降低,未热处理的Ni88Cu12薄膜在膜厚为210 nm时出现条纹畴结构,而经过300℃热处理的Ni88Cu12薄膜在膜厚为105 nm就出现了条纹畴结构.高频磁谱的结果表明,随着热处理温度的增加, Ni88Cu12薄膜的共振峰会有小范围的移动.     

Effect of reversible adsorption on the magnetic properties of iron garnet films

The reversible change in the domain structure and the magnetic domain width in bismuth-containing iron garnet films with an easy magnetization axis oriented normal to their surface during adsorption caused by hydrogen bonds is studied by a magnetooptical method. The dependence of the domain width on the vapor pressure of methyl alcohol or water in a cell with a sample is determined, and the time dependence of the domain width induced by the adsorption-desorption processes occurring between methyl alcohol molecules or water molecules on the film surface is studied. A model is proposed to explain the detected effects.     

Activation volumes of wall-motion and nucleation processes in Co-based ferromagnetic multilayer films

Activation volumes of the wall-motion and nucleation processes in Co-based multilayer films were characterized from time-resolved domain evolution patterns. These activation volumes were both sensitive to the multilayer structure as well as the film preparation condition. The two activation volumes were generally unequal with each other and the inequality directly influenced on magnetization reversal behavior.     

Magnetization reversal process and intrinsic coercivity of sputtered Sm2Co17-based films

The Sm₂Co₁₇-based intermetallic films with additives of Fe, Cu, and Zr have been deposited on Si(1 0 0) substrates by dc magnetron sputtering process. Subsequent thermal treatment and the film thickness are found to have significant contribution to the crystal structure and grain structure, which determines the magnetization reversal process and intrinsic coercivity (H_C) of these films. The conventional thermal annealing (CTA) treatment almost failed to crystallize the as-deposited films, leading to a very low H_C. Continuous and homogeneous domain walls cannot form in this deteriorated microstructure, so that the pinning mechanism can be excluded. Contrarily, the films with thickness exceeding 0.8 μm treated by rapid recurrent thermal annealing (RRTA) show an improved H_C, which is attributed to the observed completed crystallization and compact microstructure. It is suggested that this film structure is responsible for providing continuous and homogeneous domain walls, leading to a magnetization reversal process controlled by domain wall pinning model. In special, the H_C of the RRTA-treated film with thickness of 1.8 μm shows a good temperature dependence from 25 to 300 °C, with intrinsic coercivity temperature coefficient β of −0.23%/°C.     

Domain-wall guided nucleation of superconductivity in hybrid ferromagnet-superconductor-ferromagnet layered structures

Domain-wall superconductivity is studied in a superconducting Nb film placed between two ferromagnetic Co/Pd multilayers with perpendicular magnetization. The parameters of top and bottom ferromagnetic films are chosen to provide different coercive fields, so that the magnetic domain structure of the ferromagnets can be selectively controlled. From the dependence of the critical temperature Tc on the applied magnetic field H, we have found evidence for domain-wall superconductivity in this three-layered F/S/F structure for different magnetic domain patterns. The phase boundary, calculated numerically for this structure from the linearized Ginzburg-Landau equation, is in good agreement with the experimental data.     

The description of the domain structure in thin ferromagnetic films with thickness near the critical value

The magnetization distribution is considered in a thin ferromagnetic film of a thickness near the critical value, for which the continuous phase transition from the homogeneous magnetization state to the domain structure appears. The easy axis of the magnetization is perpendicular to the film plane and the anisotropy constant β < 4π. On the basis of the self-consistent theory the vortex-like magnetization distribution in the interdomain wall was obtained. The value of the critical thickness, the period of the domain structure and the amplitudes of the magnetization vector components as dependent on the film parameters were derived.     

Study of the branching domain structures in epitaxial films of yttrium iron garnet by end face magnetic force microscopy

The distribution of the magnetization vector on the free surface and end (cleaved facet) of thick epitaxial yttrium iron garnet films has been studied by the scanning magnetic-force microscopy method. Volume fractal-like branching of the domain structure has been found at the interface of the film and the free space with the refinement of the formed partial branches of the stripe (labyrinth) domains. Triangular closing domains have been observed at the interface between the film and the substrate. Direct experimental proof of the existence of the static horizontal Bloch lines within the stripe domain walls has been obtained.     

Electrical properties of epitaxial magnesium-manganese-zinc ferrite films

The resistivity of Mg-Mn-Zn ferrite films from 2.4 to 12 μ thick was studied as a function of the temperature during cyclic heating from 20 to 450 °C and subsequent cooling. The resistivity and activation energy were found as functions of the film thickness. A study was made of the effect of annealing in air on the resistivity, activation energy, exchange-interaction parameter, saturation magnetization, and domain structure of the Mg-Mn-Zn ferrite films.     

Structure and properties of deposited yttrium iron garnet films

Ferromagnetic resonance (FMR) and vibrating-sample magnetometer techniques were used to study the nature of the structural characteristics of yttrium iron garnet films deposited through either liquid phase epitaxy or laser evaporation on a (111)-oriented gallium gadolinium garnet substrate. It was proved that, based on the experimentally observed cubic magnetic anisotropy, deposited films should be considered to be single crystals. However, the absence of the FMR domain branch in a nonsaturated film and the shape of the magnetization curve indicate that a deposited film when demagnetized does not have a domain structure, as would be expected for a single-crystal film. According to the model proposed, a deposited film consists of close-packed single-crystal fragments with equal crystallographic orientation, the boundaries between which are in a partially atomically disordered state. As a result, such a film is both locally and macroscopically anisotropic, like a continuous single crystal. This film can split into domains only within a fragment (as is the case in a magnetic granular polycrystal); however, this does not happen, because the linear dimensions of a submicroscopic fragment are smaller than the equilibrium domain width.     

Kinetics of magnetization reversal in a thin La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite film

The kinetics of magnetization reversal of a thin LSMO film has been studied for the first time. It is shown that the magnetic domain structure critically depends on the conditions of structure formation. In the demagnetized state (after zero-field cooling from T _c ), a maze-like domain microstructure with perpendicular magnetization is formed in the film. However, after field cooling and/or saturating magnetization by a field of arbitrary orientation, the [110] direction of spontaneous magnetization in the film plane is stabilized; this pattern corresponds to macrodomains with in-plane magnetization. Further film magnetization reversal (both quasi-static and pulsed) from this state is implemented via nucleation and motion of 180° “head-to-head” domain walls. Upon pulse magnetization reversal, the walls “jump” at a distance proportional to the applied field strength and then undergo thermally activated drift. All dynamic characterisitcs critically depend on the temperature when the latter varies around the room temperature.     

Effect of film morphology on the magnetic properties for Nd-Fe-B thin films

The microstructure and magnetic properties of Nd-Fe-B thin films with a particulate structure were investigated. The nominal thickness of the Nd-Fe-B layer (t_N) was varied from 2 to 50 nm on a (0 0 1) Mo buffer layer. The films were grown with their c-axis perpendicular to the plane, and the morphology of the film with t_N=2 nm was shown to be particulate from an atomic force microscope image. The slope of the initial magnetization curve became steeper by increasing the t_N value in the initial magnetization curve, indicating that the film morphology composed of single domain particles changed to that of multi-domain particles with growth. The film with t_N=8 nm, which had a structure consisting of a mixture of single and multiple domain particles, showed the maximum value of the coercivity measured in the direction perpendicular to the film plane (H_c) as 19.5 kOe.     

Observation and theory of strong stripe domains in amorphous sputtered FeB films

Domain structures in thin sputtered amorphous FeB films are studied by means of the longitudinal Kerr effect. In addition to the irregular domain structure characteristic of soft magnetic materials, we observe in certain regions a fine equilibrium domain structure with periodicity of a few micrometers. The Kerr contrast indicates that the magnetization at the surface of the film lies partially along the stripe direction. These characteristics and the behavior in applied fields suggests that the domains are similar to type II “strong stripe domains” observed earlier in permalloy films. Extending an earlier theory by Hara, we use a stray-field-free model with tilted orthorhombic anisotropy to show that there are at least two qualitatively different strong stripe structures: type IIa with surface magnetization perpendicular to the stripes and type IIb with surface magnetization at least partially parallel to the stripes. Type IIb is favored when K_p/K₀<cos 2θ ₀ where K₀ is the anisotropy component with axis tilted by θ₀ out of the film plane, and K_p is an in-plane anisotropy perpendicular to K₀. Strong stripes in amorphous FeB appear to be type IIb while those in permalloy are usually type IIa.