Magneto-optical Indicator Film Technique Research Articles

Discriminating the physical impacts of various laser pulses on the magnetic structure of oriented electrical steels

Pulsed laser technologies with Long (LPL Irradiation), Short (SPL Scribing) and Ultra-Short (USPL Ablation) pulse duration can be used on SiFe Grain Oriented Electrical Steels (GOES) to refine the domains and reduce the power loss. The corresponding physics related to the magnetic structure is still a subject of research. This paper identifies separate characteristics responsible for the domains structure and the magnetization properties in a GOES sheet processed with the three laser pulses. Magnetic domains based properties are identified thanks to an average dynamic μ-νc-Λ model, the Tensor Magnetic Phase Theory (TMPT), magnetic measurements/observations with the Single Sheet Tester (SST) and the Magneto-Optical Indicator Film (MOIF) technique.

Changes in magnetic domain structure during twin boundary motion in single crystal Ni-Mn-Ga exhibiting magnetic shape memory effect

The complexity of Ni-Mn-Ga single crystal originates from the interplay between ferromagnetic domain structure and ferroelastic twinned microstructure. Magnetic domain structure in the vicinity of single twin boundary was studied using magneto-optical indicator film and magnetic force microscopy technique. The single twin boundary of Type I was formed mechanically and an initial magnetization state in both variants were restored by local application of magnetic field (≈40 kA/m). The differently oriented variants exhibited either stripe or labyrinth magnetic domain pattern in agreement with the uniaxial magnetocrystalline anisotropy of the martensite. The twin boundary was then moved by compressive or tensile stress. The passage of the boundary resulted in the formation of granular or rake domains, respectively. Additionally, the specific magnetic domains pattern projected by twin boundary gradually vanished during twin boundary motion.

Shape critical properties of patterned Permalloy thin films

The effects of shape and edges in magnetic elements with reduced dimensions on the magnetization reversal of cross- and framed cross- shaped Ni79Fe21 (30nm) films were studied. Remagnetization details in the strips of the patterned structures, which had 3 to 30µm widths and ~100µm lengths, were visualized by the magneto-optical indicator film technique. The magneto-optic images revealed three different types of the domain structure formation and evolution in the samples during their magnetization reversal: (i) spin rotation with growth and annihilation of a cross-tie structure in the strips perpendicular to the applied field, (ii) nucleation and fast motion of special boundaries, which consist of a number of coupled vortices located along both edges of the strips parallel to the applied field, and (iii) switching by ripple structure formation with macrodomain nucleation and domain wall motion in the large unpatterned part of the films. It was experimentally revealed that there exists a dependence of the critical field for nucleation and motion of domain walls in the parallel-to-field strips on their width and frame width. In particular, an inverse proportionality between this nucleation field and strip width was found in these strips having micrometer sized widths. Both experimental and simulation results show that, in cases (i) and (ii), the magnetostatic fields, which are formed on the edges of the strips and at their intersections, play a crucial role in the formation of spin inhomogeneities and switching of the samples.

Kinetics of magnetization reversal in a heterophase nanomagnet with spatially modulated anisotropy

Magnetization reversal modes in a thin-film NiFeCuMo ferromagnet (FM) with periodically varying in-plane anisotropy are studied by the magneto-optical indicator film (MOIF) technique. The uni-directional anisotropy in FM regions exchange-coupled to a FeMn antiferromagnet (AFM) film in the form of square mesh stripes is alternated by the uniaxial anisotropy in the FM regions inside this mesh. It is shown that the boundaries formed along the edges of these stripes, which separate FM regions with different anisotropy, crucially influence the kinetics of domain-structure transformation in both types of FM regions. It is established that the lateral exchange anisotropy in the ferromagnet, which is determined by the stabilization of the spin distribution in the FM layer along the FM-(FM/AFM) interface, leads to the asymmetry of the magnetization reversal in FM regions bordered with an FM/AFM structure. Anisotropy of the mobility of 180-degree “charged” and “uncharged” domain walls situated, respectively, perpendicular and parallel to the unidirectional anisotropy axis is revealed. The difference observed between the mobilities of charged and uncharged domain walls is attributed to the difference in the spin distribution in these walls with respect to the unidirectional anisotropy axis and is a key factor for the difference between the magnetization reversal kinetics in horizontal and vertical exchange-biased FM stripes. Drastic differences are revealed in the asymmetry of magnetization reversal processes in mutually perpendicular narrow stripes of FM/AFM structures. Possible mechanisms of magnetization reversal in low-dimensional FM-(FM/AFM) heterostructures are discussed with regard to the effect of domain walls localized on the edges of AFM layers.

Statistical and Multifractal Properties of Barkhausen Jumps in Exchange-Coupled Antiferromagnetic/Ferromagnetic Bilayers

Statistical and multifractal properties of Barkhausen jumps in exchange-coupled ferromagnet/antiferromagnet bilayers are studied on Co/IrMn and NiFe/NiO heterostructures using a magneto-optical indicator film technique. The statistical analysis proves a nonstochastic character of magnetization jumps. In particular, power-law behavior is observed for Co/IrMn samples. Furthermore, the statistics depends on the ferromagnet layer thickness and antiferromagnet layer material structure. The average jump size displays an asymmetry between the forward and backward branches of the hysteresis loop, particularly pronounced for the structure with a thin Co layer. In spite of the observation of such an asymmetry in the activity of the domain nucleation and pinning centers, the statistical distributions of jumps do not show any significant differences for two branches of the hysteresis loop. The conclusion on a nonrandom character of the magnetization process is supported by the multifractal analysis which reveals the presence of correlations in the time arrangement of the Barkhausen jumps.

Direct experimental study of the effect of dislocations on the magnetization reversal in a quasi-two-dimensional ferromagnet with unidirectional anisotropy

The effect of dislocations on the elementary acts of the magnetization reversal in the epitaxial heterostructure NiFe/NiO/MgO(001) has been studied using the magneto-optical indicator film technique. It has been found that the edge dislocations grouped along the 〈110〉 slip planes lead to the formation of quasi-one-dimensional domains in the permalloy film with the induced uniaxial anisotropy oriented along these planes the direction of which differs by 90° from that of the uniaxial anisotropy in the dislocation-free part of the heterostructure. The micromechanism of the observed effect has been discussed taking into account the effect of dislocations on the orientation of spins in the antiferromagnet and their exchange interaction with the spins of the ferromagnet on the interphase plane.

Domain structure and magnetization of the permalloy/niobium bilayers

Domain structure and kinetics of the magnetization reversal in the Py/Nb bilayers grown with lift-off lithography have been studied using visualization by magneto-optical indicator film technique. Various types of domain walls and switching behavior are observed depending both on film and substrate thickness. The observed transformations of the domain structure are in agreement with the model, which takes into account effective surface anisotropy governed by surface roughness, while influence of the misfit-induced elastic strain is found to be negligible.

Edge and Shape Dependence of the Magnetization Reversal of Patterned Exchange-Biased Bilayers

The magnetization reversal processes in an exchange-coupled nanocomposite system, consisting of a continuous film of the ferromagnet Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">77</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> Mo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> with a square mesh of the antiferromagnet FeMn on top, as a function of an external magnetic field were investigated by the magneto-optical indicator film technique. It was found that the boundary conditions near the antiferromagnetic (AFM) stripes crucially influence the remagnetization kinetics in the ferromagnetic film. Unusual behavior was revealed for different topologically stable structures at 180 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> domain walls in the ferromagnetic film. Also, the magnetization reversal of the ferromagnetic regions underneath the antiferromagnetic stripes is different for applied fields parallel and perpendicular to the sample's unidirectional anisotropy. In both cases, an asymmetry in the magnetic structure transformation during remagnetization of the AFM-covered parts of the ferromagnetic film was observed for the forward and backward branches of the hysteresis loop.

Growth and shape directionality of domains in exchange-coupled ferromagnet/antiferromagnet bilayers

Real time domain wall (DW) propagation processes were observed using the magneto-optic indicator film technique (MOIF) in both single crystal and polycrystalline epitaxial exchange-coupled ferromagnet/antiferromagnet (FM/AFM) bilayers. In both cases a new phenomenon of preferable directions of a DW's expansion under a canted magnetic field was found. Upon application of a magnetic field oriented slightly (4°÷10°) off the unidirectional anisotropy axis clockwise or counterclockwise, unlike along this axis, during remagnetization from the ground state the domain growth occurred in that same direction at an angle of (14°÷45°). During remagnetization to the ground state upon field reduction, but no inversion, the preferred domain growth direction was at the angle with reversed sign, however in case the field reduction to zero or inversion, the preferred DW growth direction disappeared or was at the original angle, respectively. It was revealed the angle of the domain wall orientation in canted field does not depend on the value of field angle.

Absence of magnetic domain wall motion during magnetic field induced twin boundary motion in bulk magnetic shape memory alloys

A detailed study of twin boundary motion in NiMnGa single crystals together with in situ magnetic domain observation is presented. Optical polarization microscopy in connection with a magneto-optical indicator film technique was used to investigate the reorganization of the magnetic domains during twin boundary motion over a wide magnetic field range. Images at different field strengths demonstrate that no magnetic domain wall motion within the twins takes place, even during the structural reorientation by twin boundary movement. This absence of interaction of magnetic and structural domains is different from currently proposed models, which assume domain wall movement under an external field.

Magnetooptical study of thickness dependences of magnetization reversal of thin NiFe and Co films

Effect of thickness on the magnetic structure formation and the elementary events of magnetization reversal processes in ferromagnetic (FM) NiFe and Co films were investigated using the magnetooptical indicator film technique. It was found that there are two stages of the reversal along the easy direction in the thicker films. It occurs by first quasi-uniform magnetization rotation and then by nucleation and growth of new domains with unusual non180° domain walls. It was shown that reversal of the thinnest films is controlled by new domain nucleation inside them, whereas the thicker film reversal is done by same process at the edges. It was proposed that thinnest film magnetization reversal is determined by inhomogeneities at the FM/substrate interface.

Annealing temperature dependences of magnetization reversal in exchange-biased bilayers

The thermal stability of the hysteretic properties and domain structures of exchange-coupled Co–Fe/Ir–Mn and Ni–Fe/Ir–Mn bilayers was studied. Using the magneto-optical indicator film technique, X-ray diffractometry and VSM, we have revealed that the layers’ crystal structures are more regular, and also play the main role in the bilayer remagnetization process after annealing procedures. It was found that post-annealing crystal structure leads to changes in the magnetization reversal process. This effect is greater in the Ni–Fe bilayer than in the Co–Fe bilayer.

Chirality of a forming spin spring and remagnetization features of a bilayer ferromagnetic system

Distribution of a magnetic moment in an exchange-coupled bilayer Fe/SmCo epitaxial structure grown on a (110) MgO substrate is visualized by the magnetooptic indicator film technique. The direction and the magnitude of the effective magnetization in this structure are determined both under external magnetic fields of variable magnitude and direction and after the removal of these fields. It is shown that such a heterostructure is remagnetized by a nonuniform rotation of a magnetic moment both along the thickness of a sample and in its plane. A field antiparallel to the axis of unidirectional anisotropy gives rise to spin springs with opposite chiralities in different regions of the magnetically soft ferromagnetic layer. The contributions of these springs to the net magnetization cancel out, thus decreasing the averaged magnetic moment and the remanent magnetization without their rotation. When the external field deviates from the easy axis, the balance is violated and the sample exhibits a quasi-uniform rotation of the magnetic moment. Asymmetry in the rotation of the magnetic moment is observed under the reversal of the field as well as under repeated remagnetization cycles. It is established that a monochiral spin spring is also formed in a rotating in-plane magnetic field when the magnitude of the field exceeds the critical value. Possible mechanisms of remagnetization in this system are discussed with regard to the original disordered orientation of magnetization of the magnetically soft layer with respect to the easy axis, which is defined by the variance of unidirectional anisotropy axes of this layer on the interface.

Magneto-optic indicator film observations of domain motion in magnetostrictive materials under stress

The magneto-optic indicator film technique was used to image the magnetic domain structure in a Tb40Fe60/Fe50Co50 multilayer as either a stress or magnetic field was applied 90° to the material’s easy axis. These observations showed that the magnetization vectors rotated under application of the stress, and recovered upon removal of that stress, in general without domain wall motion. However, magnetization rotation under field application was observed to include wall motion, and the magnetic state was found to have been changed upon removal of the field contrary to expectations. In both cases, both inhomogeneous and homogeneous rotations were observed.

Thermal Stability and Domain Structure in Spin Valve Films with IrMn Exchange Biased Layers

We have investigated the magnetic domain structure and the thermal stability of magnetotransport properties of IrMn biased spin-valves containing Co, CoFe and NiFe. The magnetic domain structures were imaged using a magneto-optical indicator film(MOIF) technique. To investigate the thermal stability, magnetoresistance(MR) was measured at annealing temperature(TANN) and room temperature(<TEX>$T_{RT}$</TEX>) followed by the annealing. Domain imaging reveal that the increase of annealing temperature led to changes in the exchange coupling between the two ferromagnet(FM) layers through nonmagnetic layer rather than between FM and antiferromagnet. unlike the NiFe biased IrMn spin valve with large domains, MOIF pictures of Co and CoFe biased IrMn spin valve structures show the formation of many small microdomains. The magnetic structure, as revealed by the domain images, appeared unchanged while the MR dropped dramatically. From the combined giant magnetoresistance(GMR) and MOIF results, it was apparent that the decrease of MR ratio was not related to the spin valve magnetic structure up to about <TEX>$350^{\circ}C$</TEX>(<TEX>$T_{RT}$</TEX> ).

Stationary antiferromagnetic domains during magnetization reversal in an exchange-biased FeMn/Fe76Mn6C18 bilayer

Domain processes were observed at 300 K using the magneto-optic indicator film technique (MOIF) in an exchange-coupled ferromagnet (FM)/antiferromagnet (AF) bilayer Fe76Mn6C18 (150 Å)/FeMn (100 Å) deposited under the presence of a 0.4 mT magnetic field (H) applied in the plane of the sample. The hysteresis loop for this sample was comprised of two half-loops symmetrically shifted in opposite directions from the origin. At H=0, MOIF observations showed the presence of domains in the FM with magnetization (M) vectors along the axis of the preparation field separated by 180° walls. Upon field application along that axis, saturation of the FM was achieved by the nucleation and growth of domains. In this state, at not very high fields, it was possible to observe an unusual MOIF contrast at the location of the original FM domain walls in the as-prepared ground state, associated with the intersection of domain walls in the AF with the FM. Upon field reduction M reversed only in regions which had reversed during the prior field application, so that at H=0 the domain structure was the same as that in the original ground state. If H was off-axis, during the original field application domain growth occurred in that same direction; upon field reduction, the preferred domain growth direction was at an angle with reversed sign. These results prove the AF domain walls do not move during the motion of FM domain walls, and that an exchange spring is created parallel to the AF/FM interface as the ferromagnetic layer reverses.

Nucleation and evolution of hybrid spin spiral in soft/hard ferromagnetic bilayer

The hybrid exchange spring behavior in a bilayer consisting of an epitaxial film of Fe (500 Å)/Sm2Co7 (350 Å) deposited onto a Cr (200 Å) buffered MgO(110) substrate was investigated using the magneto-optical indicator film technique. Two critical magnitudes of the remagnetizing magnetic field were found that dramatically change the bilayer remanent magnetization. One of them is determined by exchange spring penetration in the interface; another is determined by hybrid spin spiral transformation to domain walls in the hard ferromagnet layer. Unexpected dependence of the direction of average remanent magnetization on the external magnetic field direction and value is revealed.

Peculiarities of the magnetization reversal in heterophase nanocomposite permanent magnets

We have studied the process of magnetization reversal in a thin-film Fe/Sm2Co7 exchange coupled bilayer structure under the action of an in-plane external field. An analysis of the local magnetization changes, as measured using the magnetooptical indicator film technique, showed that the magnetization reversal proceeds by inhomogeneous rotation of the magnetic moments in Fe and SmCo layers, both in plane and in the perpendicular direction. It is established that, because of the exchange interaction between layers, the magnetization reversal along the easy axis in the entire structure is determined primarily by the formation of exchange-induced spin helices and domain walls in the magnetically soft layer, whereas the magnetization reversal at an angle of α with respect to the easy axis plays a significant role in the magnetically hard layer and becomes dominating for α=90°.

Antiferromagnetic spin structure and domains in exchange-coupled multilayers

As revealed by the observation of memory effects and domain imaging, the antiferromagnetic (AFM) spin structure in exchange bias is not static. During reversal, the AFM spins form an exchange spring connected with the ferromagnet (FM). We have observed hybrid domain walls consisting of FM and AFM sections and their evolution using the magnetooptical indicator film technique. The external magnetic field moves only the FM section of the hybrid domain walls, leading to the formation of an exchange spring parallel to the interface. The nucleation and unwinding of the exchange spring occur at different locations, and the propagation depends strongly on the chirality of the FM domain walls.

Anomalous switching behavior of antiparallel-coupled Co layers separated by a super thin Ru spacer

The details of the magnetization reversal in coupled ferromagnetic Co/Ru/Co trilayers deposited on obliquely sputtered Ta underlayers were studied using the magneto-optical indicator film technique. The ground states of the sandwich are characterized by noncollinear magnetization orientations in the two Co layers, which can be remagnetized by the motion of 180° and non-180° domain walls. In the latter case, the angle between the magnetization vectors in the adjacent domains was revealed to be about ±100°, and an anomalous magnetization reversal was observed. The canted magnetization states and their mutual transformations are discussed in terms of the competition between ferromagnetic coupling through pinholes and antiferromagnetic coupling across the Ru layer.