Magnetic Domain Evolution Research Articles

Kerr microscopy study of thermal and athermal training effects in a Co/CoO exchange bias system

The present work reports the magnetic domain evolution during the magnetization reversal and the training effect in a polycrystalline Co/CoO exchange bias system. Co/CoO bilayers with different cobalt (Co) layer thicknesses are being studied. The measurements are carried out using the Kerr microscopy at different temperatures (≥ 80 K) after the field-cooling across the Néel temperature of the antiferromagnetic (AFM) CoO layer. It is observed that with the increasing ferromagnetic (FM) Co layer thickness, the exchange bias reduces and the training effect becomes weaker. Analysis of the temperature variation of the exchange bias field indicates an increasing disorder at the FM-AFM interface with the decreasing FM thickness. Two different training effects, viz., athermal and thermal mechanisms, are observed depending on the thickness of the FM layer. For the lower FM thickness an athermal training effect is observed, whereas for the thicker FM layer thermal training effect is dominated. The domain structure of FM layers drastically changes during the athermal training effect with the cycling and domain size significantly decreasing during the magnetization reversal process below blocking temperature (TB), whereas in the thermal training mechanism no significant changes are found in the magnetic domain evolution throughout the temperature range with cycling. These results are expected to provide key inputs to various theoretical models that are being used to study the exchange bias phenomena in the recent literature.

Programmable magnetic domain evolution in magnetic auxetic systems

In this work, we investigate the evolution of magnetic domains with time in magnetic auxetic systems using the Ising model. We show that the mechanical deformation of the structure affects the way how domains evolve and that their rate of growth does not follow a well‐known power law associated with the Ising model for systems corresponding to a specific distance between Ising spins. We also show that the variation in the rate of deformation of the system leads to completely different results in terms of the change in size of magnetic domains and domain energy. This means that upon changing the speed used to deform the system, it is possible to control the growth of magnetic domains and hence control the magnetic properties of the whole system. This may prove to be very important in the case of potential applications such as magnetic freezers where the magnetocaloric effect resulting with a change in temperature could be affected by the rate of deformation of the discussed auxetic structure.

Mechanical Properties Improvement of Laser Tailor Welded Blanks of DP600 Steel by Magnetic Treatment

The influence of magnetic treatment on the yield strength, the ultimate tensile strength and the elongation of laser tailor welded blanks (TWBs) of DP600 steel was investigated by uniaxial tensile tests. The experimental results showed that the yield strength and the ultimate tensile strength of the TWBs had only a slight change, but the elongation increased by 13.90%–36.23% after the magnetic treatment. The dislocation distributions in the fusion zone (FZ) and the heat-affected zone (HAZ) were observed respectively by transmission electron microscopy (TEM). It was found that after magnetic treatment, the dislocations in both the FZ and the HAZ of the TWBs increased and showed a relatively uniform distribution. The mechanism of the mechanical property improvement of the TWBs by the magnetic treatment was then revealed on the basis of the relationship model between the dislocation and shear strain, considering the evolution of magnetic domains and Frank-Read dislocation multiplication in a magnetic field.

Magnetic domain evolution in Nd–Fe–B:Cu sintered magnet visualized by scanning hard X-ray microprobe

The formation and development of the magnetic domain structures of an Nd2Fe14B sintered magnet doped with 0.1% Cu are studied during the demagnetization process. A scanning hard X-ray microprobe with X-ray magnetic circular dichroism at the Nd L2 edge is applied to investigate the evolution of magnetic domains with the applied magnetic field on a fractured surface. Reversed magnetic domains are initially formed in the Nd–Fe–B main phase grains beside the Cu-abundant regions that superposes on an Nd-rich phase. This characteristic formation of initial domains is reproducibly observed throughout several demagnetization cycles after the magnetization becomes saturated, suggesting the presence of specific nucleation sites that trigger the magnetization reversal.

Micromagnetic Simulation of Strain-Assisted Current-Induced Magnetization Switching

We investigated the effect of substrate misfit strain on the current-induced magnetization switching in magnetic tunnel junctions by combining micromagnetic simulation with phase-field microelasticity theory. Our results indicate that the positive substrate misfit strain can decrease the critical current density of magnetization switching by pushing the magnetization from out-of-plane to in-plane directions, while the negative strain pushes the magnetization back to the out-of-plane directions. The magnetic domain evolution is obtained to demonstrate the strain-assisted current-induced magnetization switching.

Dynamic in situ visualization of voltage-driven magnetic domain evolution in multiferroic heterostructures

Voltage control of magnetism in multiferroic heterostructures provides a promising solution to the excessive heating in spintronic devices. Direct observation of voltage-modulated magnetic domain evolution dynamics is desirable for studying the mechanism of the voltage control of magnetism at mesoscale, but has remained challenging. Here we explored a characterization method for the dynamic in situ evolution of pure voltage modulated magnetic domains in the heterostructures by employing the scanning Kerr microscopy function in the magneto optic Kerr effect system. The local magnetization reorientation of a Ni/PMN-PT heterostructure were characterized under sweeping applied voltage on the PMN-PT single crystal, and the results show that the magnetization rotation angle in the local regions is much greater than that obtained from macroscopic magnetization hysteresis loops.

Magnetic domain tuning and the emergence of bubble domains in the bilayer manganiteLa2−2xSr1+2xMn2O7(x=0.32)

We report a magnetic force microscopy study of the magnetic domain evolution in the layered manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ (with $x=0.32$). This strongly correlated electron compound is known to exhibit a wide range of magnetic phases, including a recently uncovered biskyrmion phase. We observe a continuous transition from dendritic to stripe-like domains, followed by the formation of magnetic bubbles due to a field- and temperature dependent competition between in-plane and out-of-plane spin alignments. The magnetic bubble phase appears at comparable field- and temperature ranges as the biskyrmion phase, suggesting a close relation between both phases. Based on our real-space images we construct a temperature-field phase diagram for this composition.

Study of magnetic domain evolution in an auxetic plane of Galfenol using Kerr microscopy

Galfenol (FexGa100−x), a magnetostrictive alloy (3/2λ 110–400 ppm) of Iron and Gallium exhibits an in-plane auxetic response in the ⟨110⟩ crystallographic direction. Negative Poisson's ratios have been observed in response to application of stress fields, where values of as low as −0.7 have been reported for compositions of greater than roughly 20% Ga [Zhang et al., J. Appl. Phys. 108(2), 023513 (2010)] and in response to application of magnetic fields, where values of as low as −2.5 have been reported to be expected for compositions of less than roughly 20% Ga [G. Raghunath and A. B. Flatau, IEEE Trans. Magn. (in press)]. Several models have been proposed to understand these two distinct phenomena. Galfenol samples with less than 20% Ga also exhibit an unusual response to an increasing magnetic field applied along the ⟨110⟩ direction. The longitudinal strain which increases initially with applied field experiences a dip (until ∼10 mT) before increasing again to reach saturation. The transverse strain increases and reaches a maximum value (at the same field of ∼10 mT) and then drops from the maximum by 5%–10% as magnetic saturation is approached [G. Raghunath and A. B. Flatau, IEEE Trans. Magn. (in press)].This work deals with discussing the evolution of magnetic domains in a 16 at. % Ga single crystal Galfenol sample when subjected to magnetic fields in the ⟨110⟩ direction in the (100) plane. The magnetic domains on the surface of mechanically polished Galfenol samples were imaged using Magneto-Optic Kerr Effect microscopy. Simultaneously, the strains along the longitudinal and transverse ⟨110⟩ directions were recorded using a bi-directional strain gauge rosette mounted on the unpolished bottom surface of the planar samples. The energy from the applied magnetic field is expected to grow the ⟨110⟩ oriented domains at the expense of domains oriented along all other directions. But since the plane has an easy ⟨100⟩ axis, we expect the domains to orient along the easy direction before saturating along the applied magnetic field direction. A correlation between the images recorded and the strains observed will be used to understand this shift of domains and bump in strain at low fields.

A constraint-free phase field model for ferromagnetic domain evolution.

A continuum constraint-free phase field model is proposed to simulate the magnetic domain evolution in ferromagnetic materials. The model takes the polar and azimuthal angles (ϑ1,ϑ2), instead of the magnetization unit vector m(m1,m2,m3), as the order parameters. In this way, the constraint on the magnetization magnitude can be exactly satisfied automatically, and no special numerical treatment on the phase field evolution is needed. The phase field model is developed from a thermodynamic framework which involves a configurational force system for ϑ1 and ϑ2. A combination of the configurational force balance and the second law of thermodynamics leads to thermodynamically consistent constitutive relations and a generalized evolution equation for the order parameters (ϑ1,ϑ2). Beneficial from the constraint-free model, the three-dimensional finite-element implementation is straightforward, and the degrees of freedom are reduced by one. The model is shown to be capable of reproducing the damping-dependent switching dynamics, and the formation and evolution of domains and vortices in ferromagnetic materials under the external magnetic or mechanical loading. Particularly, the calculated out-of-plane component of magnetization in a vortex is verified by the corresponding experimental results, as well as the motion of the vortex under a magnetic field.

Hard-X-ray magnetic microscopy and local magnetization analysis using synchrotron radiation.

X-ray measurement offers several useful features that are unavailable from other microscopic means including electron-based techniques. By using X-rays, one can observe the internal parts of a thick sample. This technique basically requires no high vacuum environment such that measurements are feasible for wet specimens as well as under strong electric and magnetic fields and even at a high pressure. X-ray spectroscopy using core excitation provides element-selectivity with significant sensitivities to the chemical states and atomic magnetic moments in the matter. Synchrotron radiation sources produce a small and low-divergent X-ray beam, which can be converged to a spot with the size of a micrometer or less using X-ray focusing optics. The recent development in the focusing optics has been driving X-ray microscopy, which has already gone into the era of X-ray nanoscopy. With the use of the most sophisticated focusing devices, an X-ray beam of 7-nm size has successfully been achieved [1]. X-ray microscopy maintains above-mentioned unique features of X-ray technique, being a perfect complement to electron microscopy.In this paper, we present recent studies on magnetic microscopy and local magnetic analysis using hard X-rays. The relevant instrumentation developments are also described. The X-ray nanospectroscopy station of BL39XU at SPring-8 is equipped with a focusing optics consisting of two elliptic mirrors, and a focused X-ray beam with the size of 100 × 100 nm(2) is available [2]. Researchers can perform X-ray absorption spectroscopy: nano-XAFS (X-ray absorption fine structure) using the X-ray beam as small as 100 nm. The available X-ray energy is from 5 to 16 keV, which allows nano-XAFS study at the K edges of 3d transition metals, L edges of rare-earth elements and 5d noble metals. Another useful capability of the nanoprobe is X-ray polarization tunability, enabling magnetic circular dichroism (XMCD) spectroscopy with a sub-micrometer resolution. Scanning XMCD imaging, XMCD measurement in local areas, and element-specific magnetometry for magnetic particles/magnetic devices as small as 100 nm can be performed. Nano-XAFS application includes visualization of the chemical state in a particle catalyst [3] and phase-change memory devices [4]. For magnetic microscopic study, magnetization reversal processes of an individual magnetic CoPt dot in bit-patterned media have directly been observed [2]. Imaging of the chemical distribution and magnetic domain evolution in a Nd-Fe-B sintered magnet in demagnetization processes is presented.

On the elastically coupled magnetic and ferroelectric domains: A phase-field model

A phase-field model is developed to study local elastic coupling between magnetic and ferroelectric domains that show one-to-one pattern match. A multiferroic layered heterostructure of Co0.4Fe0.6/BaTiO3 is considered as an example. Dynamics of the local elastic coupling is investigated by simulating a time-dependent electric-field driven changes in local magnetization/polarization/strain distributions and by comparing the associated velocities of the magnetic and ferroelectric domain walls. It is found that the electric-field-driven dynamic magnetic domain evolution manifests itself as an alternating occurrence of local magnetization rotation and coupled motion of magnetic and ferroelectric domain walls with almost identical velocities.

Phase field modeling and simulation of particulate magnetoelectric composites: Effects of connectivity, conductivity, poling and bias field

A phase field model was developed to study the relationships between particulate microstructure and the magnetoelectric properties of magnetostrictive–piezoelectric composites. The model explicitly treats grain structures and domain processes in two-phase composites and automatically takes into account boundary conditions at inter-phase interfaces and grain boundaries. The model was used to investigate certain fundamental issues important for both rational design and practical application of magnetoelectric composites, such as connectivity of constituent phases, electrical conductivity of the magnetostrictive phase, in situ electric poling of ferroelectric phase within composite and the bias magnetic field during sensing. The evolution of magnetic and electric domains and their associated magnetostriction and electrostriction were simulated to study the strain-mediated domain-level coupling mechanisms that exist between magnetization and polarization. The simulations show that in situ poling of the ferroelectric phase within the composite microstructure is essential for piezoelectricity and thus the magnetoelectric effect, which requires 0–3 connectivity of the magnetostrictive–ferroelectric microstructure. Contrary to conventional wisdom that regards the conductivity of the magnetostrictive phase as detrimental to the magnetoelectric effect, it was found that a finite electrical conductivity of an isolated magnetostrictive phase can significantly enhance the magnetoelectric coefficient by both assisting effective poling and helping to stabilize the poled ferroelectric state. The simulations also show that the polarization response markedly lags behind the magnetization response, and a bias magnetic field effectively enhances the magnetoelectric effect.

Kerr microscopy studies of the effects of bending stress on galfenol

This work deals with using a magneto-optic Kerr effect (MOKE) microscope to optically analyze the evolution of magnetic domains in a rolled and Goss textured galfenol (Fe81Ga19 + 1.0% NbC) sample when subjected to a bending stress. The initial magnetization state of the cantilevered sample was fixed along its length by a 0.3 T permanent magnet. The magnetic state was monitored with the MOKE microscope as a tip load was applied to bend the sample. The magnetic state of galfenol depends on its magneto-elastic properties. A finite element model that incorporates an energy based formulation of magnetostriction [W. D. Armstrong, J. Magn. Magn. Mater. 263(1–2), 208–218 (2003)] was used to investigate the stresses in the sample and the corresponding change in the magnetic induction as bending occurred. A qualitative comparison with the domain pictures is presented, and the experimental micromagnetic behavior results are shown to correlate well to the macro scale bending stress and magnetization results obtained in the FEM simulations.

Strain induced magnetic domain evolution and spin reorientation transition in epitaxial manganite films

The evolution of magnetic domain structure in epitaxial La0.625Ca0.375MnO3 films on (001) NdGaO3 is monitored as a function of temperature and magnetic field using magnetic force microscopy. We see two distinct regions of magnetic orientational order: one in-plane displaying contrast-less image and the other tilted away from the film plane forming a distinct stripe pattern. A strong domain splitting is observed at the boundary of two regions, which is resilient to reorientation with temperature and magnetic field. We propose a model magnetic free energy functional to explain the mechanism of domain splitting seen in manganite films.

Magnetization reversal mechanism in perpendicular exchange-coupled Fe/L10–FePt bilayers

The magnetization reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers has been investigated as a function of the soft layer thickness (tFe = 2, 3.5, 5 nm) combining magnetization loops at variable angle, magnetic domain analysis by magnetic force microscopy and numerical micromagnetic simulations. The analytical model proposed in the literature can properly account for some features of the reversal mechanism, such as positive nucleation fields and the reduction of the perpendicular coercive field and remanence by increasing the soft layer thickness, but cannot satisfactorily describe the magnetization process of real systems. We showed that for a thickness of the soft layer exceeding the FePt exchange length (∼2 nm), numerical micromagnetic calculations are needed to reproduce experimental observations. Indeed, just above the coercive field, the magnetization reversal does not proceed in single step switching, as predicted by the analytical model, but according to a more complex process: evolution of nucleated magnetic domains whose magnetization is approximately along the surface normal in the hard layer and slightly out of the film plane in the soft layer, followed by rotation of Fe moments along the field direction.

Dynamic behaviors of domain wall in FM/AFM bilayers

The magnetic domain evolution behaviors of ferromagnetic (FM) monolayer and ferromagnetic (FM)/antiferromagnetic (AFM) bilayer are compared and analyzed. The results indicate that the equivalent width, mass and velocity of magnetic domain wall are changed, then the coercivity is enhanced and the exchange bias is present due to the exchange coupling between anti- ferromagnetic and ferromagnetic layer. The results also show that the equivalent width, mass and velocity of magnetic domain wall for FM /AFM bilayers system could be varied with the change of the net magnetization of antiferromagnetic layer, the magnetic anisotropy constants of FM and AFM layer, the exchange coupling constant of antiferromagnetic layer, interface exchange coupling constant and the temperature, and the relevant influences on the coercivity and exchange bias are discussed. So, the physical mechanisms of the emergence of exchange bias and enhancement of coercivity are discovered by the formation and evolution of the domain wall.

Altering magnetostrictive strain pathways via morphology of spontaneously aligned domains

In contrast to the use of a mazelike (randomly oriented) magnetic domain morphology or the application of a prestress, it is shown that spontaneously aligned domain morphology is capable of reducing the switching fields and producing a variety of magnetostriction strain pathways that are otherwise not possible by conventional materials approaches (composition and microstructure) alone. Using phase field micromagnetic microelastic modeling, the underlying magnetic domain evolution and the resultant strain behavior of giant magnetostriction materials with uniaxial magnetic anisotropy is explained by analyzing elastostatic interactions across domain walls arising from magnetostriction-induced strain mismatch.

Evolution of magnetic bubble domains in manganite films

We report a thickness-dependent evolution of magnetic domains from long stripe-like to bubble-like entities in La1−xSrxMnO3 (x ∼ 0.3) (LSMO) films grown on LaAlO3 substrates. By using 2-D fast Fourier transformation of magnetic force microscopy images and power spectral density function, we accurately determine the domain width in LSMO films with a wide range of thickness (50–325 nm). We find that the domain size scales with the Kittel’s square root law [C. Kittel, Phys. Rev. 70, 965 (1946).] only when reduced film thicknesses are used, which suggests the critical role of substrate-film interaction in domain formation.

Correlation of magnetic domains and magnetostrictive strains in Terfenol-D via magnetic force microscopy

Local evolution of magnetic domain structures in Terfenol-D under variable magnetic fields is characterized using magnetic force microscopy and the resulting magnetostrictive strains parallel and perpendicular to the applied magnetic field are simultaneously measured. It is found that both magnetic domain structure and magnetostrictive response depend upon the loading history and a substantial change in magnetic domains is observed for Terfenol-D under opposite magnetic field, resulting in notable asymmetry in magnetostrictive strain. The technique will be useful in characterizing the evolution of magnetic domains for a wide range of ferromagnetic materials under an external field.

Local two-way magnetoelectric couplings in multiferroic composites via scanning probe microscopy

Local two-way magnetoelectric (ME) couplings of a multiferroic composite have been characterized at nanoscale using novel scanning probe microscopy techniques we developed. A bilayer multiferroic composite consisting of lead zirconate titanate (PZT) and TbDyFe (TDF) has been fabricated, and the evolution of ferroelectric domains in PZT induced by an external magnetic field is observed by piezoresponse force microscopy, while the evolution of magnetic domains in TDF induced by an external electric field is observed by magnetic force microscopy, confirming the two-way ME couplings in the multiferroic composite. The technique will be useful in characterizing nanoscale ME couplings in a wide range of multiferroic composites.