Magnetic Domain Configuration Research Articles

FerrimagneticDyCo5Nanostructures for Bits in Heat-Assisted Magnetic Recording

Increasing the magnetic data recording density requires reducing the size of the individual memory elements of a recording layer as well as employing magnetic materials with temperature-dependent functionalities. Therefore, it is predicted that the near future of magnetic data storage technology involves a combination of energy-assisted recording on nanometer-scale magnetic media. We present the potential of heat-assisted magnetic recording on a patterned sample; a ferrimagnetic alloy composed of a rare earth and a transition metal, DyCo$_5$, which is grown on a hexagonal-ordered nanohole array membrane. The magnetization of the antidot array sample is out-of-plane oriented at room temperature and rotates towards in-plane upon heating above its spin-reorientation temperature (T$_R$) of ~350 K, just above room temperature. Upon cooling back to room temperature (below T$_R$), we observe a well-defined and unexpected in-plane magnetic domain configuration modulating with ~45 nm. We discuss the underlying mechanisms giving rise to this behavior by comparing the magnetic properties of the patterned sample with the ones of its extended thin film counterpart. Our results pave the way for novel applications of ferrimagnetic antidot arrays of superior functionality in magnetic nano-devices near room temperature.

Detection of shape and sizes of magnetic impurity based on the shape of magnetic domain configuration

The phenomenon of domain magnetic shielding by the magnetic sample of external and intrinsic magnetostatic field has been revealed. In this work, we investigate the possibility for detection of the shape and size of magnetic impurity based on the shape of magnetic domain configuration in magnetic film. We show that due to the influence of magnetostatic field of stripe domain structure in magnetic film on the magnetostatic field of magnetic impurity, one can determine the shape and size of this impurity.

The role of the crystal orientation (c-axis) on switching field distribution and the magnetic domain configuration in electrodeposited hcp Co–Pt nanowires

In this report, Co–Pt nanowires (NWs) were produced via potentiostatic electrodeposition into commonly used commercial ordered-alumina and disordered-polycarbonate membranes with similar pore diameters (≈200 nm). The pore diameter of the membranes and the deposition conditions were chosen such that the Co–Pt NWs fabricated into both membranes had a hexagonal close packed (hcp) crystal structure with a crystallographic texturing of the c-axis in the direction perpendicular to the NWs’ long axis; this effect was more pronounced in the alumina membranes. Due to the local fluctuation in electrodeposition conditions (pore diameter, pore shape), we have found a small variation in the c-axis orientations in the plane perpendicular to the NWs’ long axis. Magnetic characterizations suggested that there is uniaxial anisotropy perpendicular to the Co–Pt NWs’ long axis and the small variation in the orientation of the hcp c-axis plays an important role in the switching-field distribution and the magnetic domain structure of the Co–Pt NWs. First order reversal curves (FORCs) revealed week magnetostatic interactions between Co–Pt NWs, thus suggesting that the different pore alignments are not influencing much the magnetic properties in both membranes. The micromagnetic simulation revealed that the transverse-stripe (TS) and longitudinal stripe (LS) domains are energetically most favorable structures in such NWs. This study accentuates the influence of the crystal orientation (c-axis) of the high-anisotropy materials on their functional magnetic properties and thus is of great importance for the fabrication of nanodevices based on such NWs.

Critical thickness for stripe domain formation in FePt thin films: Dependence on residual stress

Magnetically soft FePt thin films of varying thickness (20 nm ≤ d ≤ 100 nm) were sputter-deposited at different Ar pressures in order to systematically modify the residual stress and hence the magnetic anisotropy. The magnetic domain structure of FePt thin films showed a transition from planar to nearly parallel stripes above a critical thickness, dcr, which was found to depend on an anisotropy contribution perpendicular to the film plane, originated essentially in magnetoelastic effects. A careful structural characterization was made in order to obtain the strain and the stress induced magnetic anisotropy in the samples. Vibrating sample magnetometry and magnetic force microscopy were used to investigate the changes occurring in the magnetic domain structure and the critical thickness of each set of films. Joining together structural and magnetic results, we have been able to construct a phase diagram that divided regions of different domain structures, either by changing the film thickness or the perpendicular magnetic anisotropy. The experimental results could be satisfactorily explained by using a model developed by Murayama. The observed dependence of the magnetic properties of soft FePt thin films on the fabrication conditions opens the possibility to tune the magnetic domain configuration from planar to stripe-like domains by changing the argon sputtering pressure used during film deposition.

Periodic magnetic domains in single-crystalline cobalt filament arrays

Magnetic structures with controlled domain wall pattern may be applied as potential building blocks for three-dimensional magnetic memory and logic devices. Using a unique electrochemical self-assembly method, we achieve regular single-crystalline cobalt filament arrays with specific geometric profile and crystallographic orientation, and the magnetic domain configuration can be conveniently tailored. We report the transition of periodic antiparallel magnetic domains to compressed vortex magnetic domains depending on the ratio of height to width of the wires. A ``phase diagram'' is obtained to describe the dependence of the type of magnetic domain and the geometrical profiles of the wires. Magnetoresistance of the filaments demonstrates that the contribution of a series of ${180}^{\ensuremath{\circ}}$ domain walls is over $0.15%$ of the zero-field resistance $\ensuremath{\rho}(H=0)$. These self-assembled magnetic nanofilaments, with controlled periodic domain patterns, offer an interesting platform to explore domain-wall-based memory and logic devices.

Tunable stress induced magnetic domain configuration in FePt thin films

We present a study of the magnetic properties of chemically disordered ferromagnetic FePt films of 100 nm thickness that have been grown by sputtering with different Ar pressures ( mTorr). We found that the residual stress can be controlled by the sputtering pressure, which in turn allows to tune the perpendicular magnetic anisotropy due to magnetoelastic effects. Films deposited at lower Ar pressures display an in-plane compressive stress that favors an out of plane component of the magnetization and the formation of a magnetic domain structure in the form of stripes. For higher pressures the stress is relaxed and the magnetic configuration changes to planar domains. These results show the possibility to accurately tune the initial magnetic state in films with potential applications in magnetoelectrically coupled devices.

Ni80Fe20 nanodisks by nanosphere lithography for biomedical applications

A novel nanofabrication technique based on self-assembling of polystyrene nanospheres and aimed to obtain magnetic nanodisks suspended in ethanol is here presented. Free-standing Ni80Fe20 disks having lateral dimension around 650 nm and thickness 30 nm were obtained by using nanosphere lithography on a sputtered continuous thin film. The multi-step nanofabrication process will be explained, in detail. The process end-product can be used as suitable magnetic carriers having nearly monodispersed size and simultaneously displaying high saturation magnetization and low-coercivity. Magnetisation reversal has been studied by room-temperature hysteresis loop measurements in either dot arrays attached on a substrate or in liquid-dispersed free-standing nanodisks. In both samples, the reversal is marked by magnetic vortex nucleation/annihilation. Such a behavior is confirmed for Ni80Fe20 dot arrays by measuring magnetic domain configuration, while numerical simulation is used for confirming magnetization reversal process in nanodisks.

Microstructure and magnetostrictive behavior of Fe-15 at% Ga alloy with different cooling rates

The main attention of this paper was devoted to the study of the effect of different cooling rates on the magnetic domain configuration and magnetostrictive behavior of heat-treated Fe-15 at% Ga alloy. After annealing at 1,000 °C for 3 h, the samples were subjected to water quenching, air cooling, and furnace cooling treatments. Phase constitution and magnetic domain structures of the samples were studied using X-ray diffraction (XRD) and magnetic force microscopy (MFM). XRD results indicate a single phase of α-Fe with disordered bcc (A2) structure for all samples. MFM results show that both water-quenched (WQ) and air-cooled (AC) samples are mainly made from ordered stripe domain structures, whereas a mixture of irregular stripe, zigzag, and plate domain patterns are observed in furnace-cooled (FC) sample. Magnetostrictive strain was measured in the presence of an externally applied magnetic field. It is found that WQ sample has the highest magnetostriction, while AC and FC samples exhibit moderate and the lowest magnetostriction, respectively, against the applied field. The dependence of initial domain configurations on thermal history is found to be conducive to the change in saturation magnetostrictions of the samples.

Superparamagnetic versus blocked states in aggregates of Fe(3-x)O₄ nanoparticles studied by MFM.

Magnetic domain configurations in two samples containing small aggregates of Fe(3-x)O4 nanoparticles of about 11 and 49 nm in size, respectively, were characterized by magnetic force microscopy (MFM). Two distinct magnetic behaviors were observed depending on the particle size. The aggregates constituted of nanoparticles of about 11 nm in size showed a uniform dark contrast on MFM images, reflecting the predominant superparamagnetic character of these particles and arising from the coherent rotation of the spins within the aggregate as the latter align along the tip stray-field. By applying a variable in-plane field, it is possible to induce magnetic polarization yielding an increasing dark/bright contrast as the strength of the applied field overcomes the stray-field of the tip, although this polarization completely disappears as the remanent state is recovered when the magnetic field is switched off. On the contrary, for aggregates of NPs of about 49 nm in size, dark/bright contrast associated with the existence of magnetic domains and magnetic polarization prevails in MFM images all along the magnetic cycle due to the blocking state of the magnetization of these larger particles, even in the absence of an applied field. All in all, we unambiguously demonstrate the capabilities of magnetic force microscopy to distinguish between blocked and superparamagnetic states in the aggregates of magnetic nanoparticles. Micromagnetic simulations strongly support the conclusions stated from the MFM experiments.

Direct imaging of the magnetic polarity and reversal mechanism in individual Fe(3-x)O4 nanoparticles.

This work reports on the experimental characterization of the magnetic domain configurations in cubic, isolated Fe3-xO4 nanoparticles with a lateral size of 25-30 nm. The magnetic polarity at remanence of single domain ferrimagnetic Fe3-xO4 nanoparticles deposited onto a carbon-silicon wafer is observed by magnetic force microscopy. The orientations of these domains provide a direct observation of the magneto-crystalline easy axes in each individual nanoparticle. Furthermore, the change in the domain orientation with an external magnetic field gives evidence of particle magnetization reversal mediated by a coherent rotation process that is also theoretically predicted by micromagnetic calculations.

Effect of magnetocrystalline anisotropy on the magnetic properties of electrodeposited Co–Pt nanowires

We report on the influence of the magnetocrystalline anisotropy on the easy magnetization axis, magnetization reversal and magnetic domain configurations of electrodeposited Co–Pt nanowires with lengths in the range of 4–6 µm and a diameter of 250 nm. The transmission electron microscopy and the X-ray diffractions revealed that the nanowires are composed of an intermixture of hcp- and fcc-textured crystal structures. The crystallographic orientations of both phases were such that the $$\left[ {00\bar{1}} \right]$$ of the hcp phase and the [111] of the fcc phase are pointing almost perpendicular to the nanowire axis. This observation allows us to understand the perpendicular easy magnetization axis of the nanowire arrays measured with vibrating sample magnetometry. Analytical calculations of the angular dependence of the coercivity revealed that the magnetization reversal changes from vortex to transverse mode at the applied field angle θ = 30°. Fitting of the experiment to these calculations results in a perpendicular effective anisotropy constant (K eff = 2.6 × 104 J/m3) in nanowires which can be ascribed to the strong magnetocrystalline anisotropy. Furthermore, the magnetic domain configurations of individual nanowires of length range 4 < L < 6 µm are studied using magnetic force microscopy. This reveals a spatial magnetization modulation along the length of the nanowires, which was found to be length dependent. Such an intrinsic modulation is attributed to the competition between the magnetocrystalline anisotropy and the shape anisotropy in the nanowires. We believe that this interplay between anisotropies gives rise to a magnetic configuration involving vortex-like structure with alternating chirality along the length of the nanowires.

Magnetic anisotropies in epitaxial Fe3O4/GaAs(100) patterned structures

Previous studies on epitaxial Fe3O4 rings in the context of spin-transfer torque effect have revealed complicated and undesirable domain structures, attributed to the intrinsic fourfold magnetocrystalline anisotropy in the ferrite. In this Letter, we report a viable solution to this problem, utilizing a 6-nm-thick epitaxial Fe3O4 thin film on GaAs(100), where the fourfold magnetocrystalline anisotropy is negligible. We demonstrate that in the Fe3O4 planar wires patterned from our thin film, such a unique magnetic anisotropy system has been preserved, and relatively simple magnetic domain configurations compared to those previous reports can be obtained.

Anisotropic magneto-resistance in Ni80Fe20 antidot arrays with different lattice configurations

Ni80Fe20 antidot arrays having different lattice geometrical properties and irregularities were prepared via electron beam lithography and self-assembling of polystyrene nanospheres. All the samples were experimentally characterised by magnetic force microscopy and room-temperature magneto-resistance measurements in different configurations. The analysis, supported by micromagnetic simulations, has been focused on the effect of lattice geometry on the magneto-resistance behaviour of these systems. The detailed investigation through micromagnetic simulations of the magnetic domain configuration as a function of the applied field allows a complete understanding of the qualitative and quantitative difference of anisotropic magneto-resistance properties that have been measured in samples with different lattice geometries and in different measurement configurations.

A multiscale model for magneto-elastic behaviour including hysteresis effects

Magnetic and mechanical behaviour are strongly coupled: an applied stress modifies the magnetic behaviour, and on the other hand, magnetic materials undergo a magnetisation-induced strain known as the magnetostriction strain. These coupling effects play a significant role on the overall performance of electromagnetic devices such as magnetostrictive transducers or high-performance electric machines. In order to provide engineers with accurate design tools, magneto-elastic effects must be included into constitutive laws for magnetic materials. The origin of the magneto-elastic coupling lies in the competitive contributions of stress and magnetic field to the definition of magnetic domain configurations in magnetic materials. The magnetic domain scale is then suitable to describe magneto-elastic interactions, and this is the reason why multiscale approaches based on a micro-mechanical description of magnetic domain structures have been developed in the last decades. We propose in this paper an extension of a previous anhysteretic multiscale model in order to consider hysteresis effects. This new irreversible model is fully multiaxial and allows the description of typical hysteresis and butterfly loops and the calculation of magnetic losses as a function of external magneto-mechanical loadings. It is notably shown that the use of a configuration demagnetising effect related to the initial domain configuration enables to capture the non-monotony of the effect of stress on the magnetic susceptibility. This configuration demagnetising effect is also relevant to describe the effects of stress on hysteresis losses and coercive field.

Synthesis and property tunability of interparticle exchange-decoupled L10-FePt:Au/Fe perpendicular ECC films

L10-FePt based perpendicular exchange coupled composite (ECC) films with both weak FePt interparticle exchange coupling (IPEC) and strong hard/soft exchange coupling (HSEC) are virtually crucial to improve the signal-to-noise ratio and the writability of recording media concurrently. This article reports synthesis and magnetic property tunability of the L10-FePt:Au/Fe ECC film with low IPEC between neighboring L10-FePt particles and strong HSEC between the hard L10-FePt:Au composite layer and the soft Fe layer. Moreover, the magnetic domain configuration study reveals the domain wall assisted magnetization reversal mechanism. The coupled Fe layer facilitates reverse nucleation and magnetization reversal in the L10-FePt:Au composite layer, leading to remarkable reduce of the switching field while maintaining high thermal stability.

Tailoring the magnetic anisotropy and domain patterns of sputtered TbFeGa alloys

We report the magnetic anisotropy and domain configuration of cosputtered TbFeGa alloys. The layers were deposited from two targets with compositions TbFe2 and Fe3Ga, respectively. The structural and magnetic properties do not only depend on the composition but also on the growth conditions. Alloys with the same composition but deposited using a DC or a pulsed power source in the TbFe2 target exhibit a different magnetic anisotropy. The perpendicular magnetic anisotropy, the size and topology of domain patterns can be tailored by changing the evaporation parameters of TbFe2. The width of the stripe domain increases from 235 to 835 nm when using the DC source in the TbFe2. We correlate this effect with Tb enrichment of the TbxFe1−x phases present in the samples.

On the YFe11Mo intermetallic characterization

Rare-earth intermetallic compounds adopting the tetragonal ThMn12–type structure and containing high Fe concentrations have attracted considerable attention in the field of permanent magnets. Among them, the Y-Fe-Mo series has been extensively investigated, especially by X-ray diffraction (XRD), but the microstructural characterization was very limited.In the present work, Y:11Fe:Mo has been prepared by melting Y, Fe and Mo in an arc furnace followed by splat-quenching and/or annealing treatments. The structure, chemistry and magnetic domain configurations of the resulting polycrystalline aggregates have been investigated by XRD, scanning and transmission electron microscopies (SEM and TEM, respectively), energy dispersive X-ray spectroscopy (EDS) and Lorentz microscopy.A special emphasis was given to Lorentz microscopy, where contrast is based on the Lorentz deflection imposed on electrons by the passage through a magnetic specimen. The magnetic domains have been imaged by the intermediate lens in Fresnel mode, with the objective lens switched off. In these conditions the intermediate lens is defocused so that out-of-focus images of the specimen are formed: the magnetic domain walls are imaged as alternate bright (convergent) and dark (divergent) lines. For the overfocused image, bright lines occur at the position of domain walls for which the magnetisation on either side is deflecting the electrons towards the wall, whereas dark lines are observed at the walls for which the magnetisation on either side is deflecting the electrons away from the wall. The opposite contrast is observed at the underfocused image.The results have shown that the tetragonal YFe11-xMoy phase is predominant, with a cellular dendritic morphology (Figure 1), but a considerable presence of -Fe(Mo) could be inferred, forming a coarse intercellular eutectic mixture. Significant Fe segregation occurred during annealing. However, this composition variation corresponded to an extremely limited evolution of the lattice parameters, the Rietveld analysis pointing to Fe vacancies at the 8i sites on the annealed material. XRD and EDS results indicate that the fraction of point defects in the ThMn12-type structure adapts to the processing route and that the stable configuration depends on the temperature. The current study also showed that grain boundaries are usually associated with domain walls and that YFe11Mo grains present internal domain walls forming stripe/maze patterns characteristic of high anisotropy materials (Figure 2), while residual -Fe(Mo) grains exhibit vortex configurations (see arrows in Figures 2 (a-c)).The work was supported by the Portuguese Science Foundation through the CTM/48617/2002 and PEst-OE/CTM-UI0084/2011 grants.

Structure Properties of the ${\rm YFe}_{11}{\rm Mo}$ Intermetallic Compound

The structure, chemistry and magnetic domain configurations of YFe11Mo polycrystalline aggregates have been investigated by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and Lorentz microscopy. Although extensive reports on the magnetic properties of this compound can be found in the literature, its exact equilibrium structure remains elusive. The compound was prepared by arc-melting followed by splat-quenching and subsequent annealing. The quenched material consisted of a ThMn12-type matrix and α-Fe(Mo) impurity phase. Annealing lead to iron segregation from the matrix, as evidenced by a clear shift to higher angles of the α -Fe(Mo) peak observed in the diffraction pattern. Nevertheless, the composition variation in the intermetallic compound was accompanied by an extremely limited evolution of the lattice parameters, suggesting the presence of Fe vacancies after annealing. Rietveld analysis carried out on the annealed material points to Fe vacancies at the 8i sites. The present results indicate that the fraction of point defects in the ThMn12-type structure adapts to the processing route and that the stable configuration depends on the temperature. YFe11Mo grains present internal domain walls forming stripe/maze patterns characteristic of high anisotropy materials, while residual α -Fe(Mo) grains exhibit vortex configurations. Annealing hardly affected the magnetic domain configurations of the YFe11Mo and α-Fe(Mo) aggregates.

Charged magnetic domain walls as observed in nanostructured thin films: dependence on both film thickness and anisotropy

The magnetic domain configurations of soft magnetic, nanostructured, pulsed laser-deposited Co films were investigated. Their dependence on both the thickness t (20 nm ≤ t ≤ 200 nm) and the anisotropy was studied. Charged zigzag walls, with a characteristic saw-tooth vertex angle θ, were observed. θ changed with t from θ ≈ 17° to ≈25°, presenting an intermediate sharp maximum that has not been described before. The reduced length of the zigzag walls also exhibited a peak at t ≈ 70 nm. The relationship between the total reduced length and the density energy of the magnetic wall allowed us to establish a change from a Néel-type to a Bloch-type core of the zigzag walls at this thickness, t ≈ 70 nm. We also accounted for the magnetic energy arising from the surface roughness of the thinner films after imaging the film surface morphologies. Moreover, this distinctive behaviour of the zigzag walls of these low-anisotropy films was compared to that of high-anisotropy films.

Magnetic entropy change and transport properties in Ni45Co5Mn36.6In13.4 melt-spun ribbons

Heusler alloy Ni45Co5Mn35.6In13.4 ribbons were prepared by melt spinning with different wheel linear speeds. It was found that melt spinning is an effective way to change atomic order of the crystal structure, which leads to a significant change in magnetic and transport properties as well as the martensitic transition behavior. Austenite with cubic B2 structure was confirmed for the melt-spun ribbons at room temperature. Compared to the as-prepared ingots, martensitic transition and reverse transition shift to lower temperature, and metamagnetic behavior becomes less significant due to enhanced ferromagnetic properties in martensitic phase, thus resulting in a reduced magnetic entropy change. However, an interesting transport behavior was observed in the ribbons. Under a magnetic field of 5T, a large difference in resistance appears on heating and cooling process, which is closely related to the large difference of ZFC–FC magnetization. Our analysis indicates that the change of magnetic domain configuration and the FM spin structure de-pinning under external cooling field should be responsible for the separation of ZFC–FC resistance and ZFC–FC magnetization.