Magnetic Domain Evolution Research Articles

Magnetization reversal in textured NdFeB–Fe composites observed by domain imaging

Textured composite samples consisting of Nd 13.6Fe 73.6Ga 0.6Co 6.6B 5.6 (MQU-F™) and micron-sized Fe particles with weight ratios from 100:0 to 70:30 have been prepared by hot deformation. Microstructure studies revealed a layered structure of both phases with the layer normal parallel to the pressing direction. Magnetic measurements showed single-phase hysteresis curves for all samples when measured along the pressing direction, which is also the easy axis of magnetization. Coercivity decreased drastically from 1.32 T for pure NdFeB samples to 0.154 T for a sample with 30 wt% Fe. Magneto-optical Kerr microscopy with a digitally enhanced imaging technique has been used to examine the evolution of magnetic domains in the hard and soft phase during demagnetizing a sample consisting of 70 wt% NdFeB and 30 wt% Fe. It is shown that demagnetization takes place via domain rearrangements within the soft phase, which lead to and support the nucleation of reversed interaction domains at phase boundaries. Also nucleation of interaction domains within the hard magnetic phase could be revealed.

Transverse Field-Induced Nucleation Pad Switching Modes During Domain Wall Injection

We have used magnetic transmission soft X-ray microscopy (M-TXM) to image in-field magnetization configurations of patterned Ni80Fe20 domain wall ?nucleation pads? with attached planar nanowires. Comparison with micromagnetic simulations suggests that the evolution of magnetic domains in rectangular injection pads depends on the relative orientation of closure domains in the remanent state. The magnetization reversal pathway is altered by the inclusion of transverse magnetic fields. These different modes explain previous results of domain wall injection into nanowires.

Soft x-ray holographic microscopy

We present a new x-ray microscopy technique based on Fourier transform holography (FTH), where the sample is separate from the optics part of the setup. The sample can be shifted with respect to the holography optics, thus large-scale or randomly distributed objects become accessible. As this extends FTH into a true microscopy technique, we call it x-ray holographic microscopy (XHM). FTH allows nanoscale imaging without the need for nanometer-size beams. Simple Fourier transform yields an unambiguous image reconstruction. We demonstrate XHM by studying the magnetic domain evolution of a Co/Pt multilayer film as function of locally varied iron overlayer thickness.

Temperature dependence of magnetic coupling in ultrathinNiO/Fe3O4(001)films

We have studied the evolution of nanometer-sized magnetic domains in exchange-coupled $\text{NiO}/{\text{Fe}}_{3}{\text{O}}_{4}(001)$ bilayers as a function of temperature. To image magnetic domains in thin antiferromagnetic (AF) NiO and in ferrimagnetic (FM) ${\text{Fe}}_{3}{\text{O}}_{4}(001)$ layers we have used element specific x-ray photoemission electron microscopy. The epitaxial growth of $\text{NiO}/{\text{Fe}}_{3}{\text{O}}_{4}(001)$ leads to a parallel orientation between the two oxide lattices. The magnetic order-disorder transition temperature of a 2-nm-$\text{NiO}/{\text{Fe}}_{3}{\text{O}}_{4}(001)$ film is found to be ${T}_{\text{N}}=375\text{ }\text{K}$, much lower than for bulk $\text{NiO}$ $({T}_{\text{N}}=520\text{ }\text{K})$. Setting the temperature below or above the N\'eel temperature $({T}_{\text{N}})$ of ultrathin NiO films, the influence of the interlayer exchange coupling on the microscopic domain patterns could be studied. Below ${T}_{\text{N}}$, the two layers reveal parallel coupling and an induced twofold degenerated spin configuration. Small nanometer domains are observed inside the larger micrometer magnetic domain of ${\text{Fe}}_{3}{\text{O}}_{4}(001)$. Alternating direction of the spins, pointing along $⟨100⟩$ and $⟨120⟩$ are present in both the AF and FM layers. Above ${T}_{\text{N}}$, the AF contrast in the magnetic image of NiO vanishes and simultaneously large micrometer magnetic domains appear in ${\text{Fe}}_{3}{\text{O}}_{4}(001)$, showing magnetic contrast images of uncoupled spins oriented parallel to $⟨100⟩$. The consequences of the microscopic magnetic domain patterns in FM/AF coupled systems are discussed in terms of macroscopic magnetic properties.

Magnetic domain formation within patterned NiFe/Cu/Co ellipses

Neutron reflectometry was used to study the formation and evolution of magnetic domains within a patterned array of NiFe/Cu/Co ellipses. The measurements directly show that domains form upon relaxation away from hard axis magnetic saturation, and their size and shape distributions are invariant throughout the process. Modeling of the data demonstrates that uniform magnetic domains are commensurate with the ellipse structure, but are approximately 75 nm smaller in radius. Together these findings suggest that there is one magnetic domain per nanoparticle whose constituent moments rotate collectively as the field is varied.

Stripe-to-bubble transition of magnetic domains at the spin reorientation of (Fe/Ni)/Cu/Ni/Cu(001)

Magnetic domain evolution at the spin reorientation transition (SRT) of (Fe/Ni)/Cu/Ni/Cu(001) is investigated using photoemission electron microscopy. While the (Fe/Ni) layer exhibits the SRT, the interlayer coupling of the perpendicularly magnetized Ni layer to the (Fe/Ni) layer serves as a virtual perpendicular magnetic field exerted on the (Fe/Ni) layer. We find that the perpendicular virtual magnetic field breaks the up-down symmetry of the (Fe/Ni) stripe domains to induce a net magnetization in the normal direction of the film. Moreover, as the virtual magnetic field increases to exceed a critical field, the stripe domain phase evolves into a bubble domain phase. Although the critical field depends on the Fe film thickness, we show that the area fraction of the minority domain exhibits a universal value that determines the stripe-to-bubble phase transition.

Thickness dependence of magnetic domain formation in La 0.6Sr 0.4MnO 3 epitaxial thin films studied by XMCD–PEEM

We have used photoelectron emission microscopy (PEEM) and X-ray magnetic circular dichroism (XMCD) to study the effect of thin film thickness on the magnetic domain formation in La 0.6Sr 0.4MnO 3 samples that were epitaxially grown on stepped SrTiO 3 (0 0 1) substrates. The magnetic image exhibited a stripe structure elongated along the step direction, irrespective of film thickness, suggesting that uniaxial magnetic anisotropy induced by step-and-terrace structures plays an important role in the magnetic domain formation. Additional domains evolved gradually with increasing film thickness. In these domains, the direction of magnetization differed from the step direction due to biaxial magneto-crystalline anisotropy. The evolution of additional magnetic domains with increasing film thickness implies that a competition exists between the two anisotropies in LSMO films.

Effect of Magnetic Field on the Magnetic Domain Structure of MnAs Film on GaAs(001)

We have observed the magnetic domain evolution under an external magnetic field in an epitaxial MnAs film on GaAs(001) by magnetic force microscopy. Owing to the strain involved, the ferromagnetic alpha-MnAs and the paramagnetic beta-MnAs phases coexist as self-aligned stripes at room temperature. It was found that a complex magnetic domain structure appeared in the ferromagnetic alpha-phase region at the demagnetized state (H=0 Oe). As the magnetic field increased, the magnetic domain structure was gradually changed, and reached a completely saturated state at H=600 Oe. Especially, at H=300 Oe, the observed magnetic domain wall distribution is well matched with the topographical shape

Magneto-optical imaging of the magnetization process in colossal magnetoresisitive lanthanum manganite

To gain insight into the origin of the colossal reduction of resistance in response to magnetic field in colossal magnetoresistance manganite, the magnetic field induced transition in ferromagnetic La0.7Ca0.3MnO3 was studied using a high-resolution magneto-optical imaging (MOI) technique. The MO images were captured in various magnetic fields over a wide temperature range for both highly dense samples with strong-link grain boundaries and porous samples with weak-link boundaries. Formation and evolution of magnetic domains as a function of field or temperature were clearly observed around and far below the Curie temperature TC=240K. Ferromagnetic areas tend to grow to large sizes and finally join together at the expense of paramagnetic areas as the field increases or temperature decreases for strong-link samples. A sharp magnetoresistive transition is observed when the sample changes from a paramagnetic insulator to a metallic ferromagnetic phase in the vicinity of TC. In contrast, the porous samples showed magnetoresistance over a wide temperature range and exhibited a remarkable grain boundary related magnetization process in addition to magnetization within grains. A close correlation is found between the magnetization process observed by MOI and magnetoresistance measurements. Our MOI results indicate that the strong-link or weak-link grain boundaries are responsible, respectively, for magnetoresistance occurring either only in the vicinity of the ferromagnetic transition or over a very wide temperature range.

Magnetic domain evolution with applied field in a Ni–Mn–Ga magnetic shape memory alloy

Abstract The evolution of a magnetic domain pattern with an applied field in two perpendicular directions was studied in a Ni–Mn–Ga magnetic shape memory alloy by means of optical and scanning electron microscopy (SEM). The optical contrast of magnetic domains arises from surface undulation. This surface relief is coherent at the (1 0 1) twin boundary. However, the surface relief causes the trace of the (0 1 1) twin boundary to follow a zigzag pattern. These optical observations were confirmed by SEM Type II magnetic contrast.

Observation of Magnetic Ripple and Nanowidth Domains in a Layered Ferromagnet

We investigated ferromagnetic domain structures on nanometer to micrometer scale for single crystals of a layered ferromagnet, La(2-2x)Sr(1+2x)Mn2O7 (0.32 < or = x < or = 0.40), as functions of x and temperature by means of Lorentz electron microscopy. We have succeeded in observing the evolution of magnetic ripple structure, dynamically, related to a spin reorientation transition where the magnetization direction switches between parallel and perpendicular to the layers. Our high-resolution magnetic domain imaging revealed that the ripple state is characterized by the evolution of magnetic nanowidth domains.

Quasistatic x-ray speckle metrology of microscopic magnetic return-point memory.

We have used coherent, resonant, x-ray magnetic speckle patterns to measure the statistical evolution of the microscopic magnetic domains in perpendicular magnetic films as a function of the applied magnetic field. Our work constitutes the first direct, ensemble-averaged study of microscopic magnetic return-point memory, and demonstrates the profound impact of interfacial roughness on this phenomenon. At low fields, the microscopic magnetic domains forget their past history with an exponential field dependence.

Multilayered magnetic nanostrips studied by transmission X-ray microscopy

Transmission x-ray microscopy with the x-ray magnetic circular dichroism as a contrast mechanism was used to image the field dependent evolution of magnetic domains in artificially nanostructured strips of a multilayered Fe/Gd system. A diversity of domain configurations have been imaged for different strengths of the applied magnetic field and different widths of the strips, varying between 100 nm and 1 μm. Undulating domain patterns, analogous to those observed in other condensed matter systems could be found.

Imaging magnetic microstructures of with soft X-ray microscopies

X-ray magnetic circular dichroism (X-MCD) was used as magnetic contrast mechanism to image at high lateral resolution the domain structure in thin magnetostrictive Terfenol-D layered systems both with full field magnetic X-ray transmission microscopy (M-TXM) at the Advanced Light Source (ALS) in Berkeley CA (USA) and a new photoemission electron microscope (PEEM) at the Swiss Light Source SLS in Villigen (Switzerland). The switching behavior of the magnetic domain structure could be studied with the transmission X-ray microscope at the ALS by recording the evolution of magnetic domains in external magnetic fields. A spin reorientation from out-of-plane to in-plane was observed both after tempering the as-grown amorphous system at elevated temperatures and with decreasing thickness. The coupling mechanism to an adjacent Co layer could be observed due the inherent element-specificity of X-MCD by images taken at the Fe and Co sites, resp. The complementary of x-ray-in/x-ray-out and x-ray-in/electron-out microscopies to image magnetic microstructures could be demonstrated

Low temperature behavior of magnetic domains observed using a magnetic force microscope

A commercial atomic force microscope/magnetic force microscope (MFM) was modified to cool magnetic samples down to around 100 K under a high vacuum while maintaining its routine imaging functionality. MFM images of a 120 nm thick La0.7Ca0.3MnO3 film on a LaAlO3 substrate at low temperature show the paramagnetic-to-ferromagnetic phase transition. Evolution of magnetic domains and magnetic ripples with decreasing temperature are also observed near the edge of a 20 nm thick patterned Co film on a Si substrate.

Imaging of magnetic domains by transmission x-ray microscopy

The combination of the high-resolution transmission x-ray microscope (TXM) based on the zone plate technique with the x-ray magnetic circular dichroism (X-MCD) providing a huge magnetic contrast is a new technique to image magnetic domain structures. It is inherently element specific and contains information on the local spin and orbital moments of the absorbing species that can be obtained by applying magneto-optical sum rules. A lateral spatial resolution depending on the quality of the zone plates down to 30 nm can be achieved. We report on first results at the Fe edges of Fe both in amorphous and in multilayered Gd-Fe systems. With a TXM set-up at BESSY I adapted to record magnetic images in varying magnetic fields the evolution of magnetic domains within a complete hysteresis loop and magnetic aftereffects have been studied.

Magnetic domains in epitaxial Fe/GaAs micro-patterned wires

The magnetic domain evolution during the magnetization reversal in in-plane magnetized epitaxial Fe/GaAs(001) wire elements with dimensions of 15 μm width × 500 μm length × 300 Å thickness has been studied using a scanning Kerr microscope. The two jump switching processes, characteristic of the magnetization reversal in continuous epitaxial Fe(001) films with four-fold in-plane anisotropy, are observed. However, the domain nucleation and growth processes which mediate the discontinuous and irreversible magnetization switching at the two critical reverse fields Hc1 and Hc2 are different from those in the continuous film. They are found to be determined by the orientation of the applied field with respect to the short and the long wire axis and hence are “shape”-anisotropic. This shape-anisotropy is a combined local dipolar field and anisotropic finite size effect, allowing the growth of domains only from the long/short wire axis and the propagation of domains only in direction of the short/long wire axis respectively according to the applied field direction.

Magnetic domain percolation in granular Co-Ag

The most dramatic changes in the properties of granular metals occur near the percolation threshold xc, which generally occurs at a volume fraction of approximately 0.50. There has been much speculation concerning the evolution of magnetic domains at xc, however, no direct observations have been produced. Using scanning electron microscopy with polarization analysis, we investigate the formation of domains in a series of granular Co-Ag samples. We find xc to lie in the range [0.50, 0.55], and we report on the size and morphology of the observed domains. Below xc, a domain pattern appears which may be due either to interparticle correlations or to residual Co in the silver matrix; we have previously discussed the ramifications of this observation on an understanding of GMR. In this paper, we place limits on the latter of these two models. Above xc, the domains resemble ‘‘stripe domains’’ which have been observed in amorphous materials. This observation suggests the presence of anisotropy perpendicular to the film plane.

Evolution of spiral dynamic magnetic domains in Anger states of iron garnet films

Dynamic domain structures in the Anger state of iron garnet films are analyzed by a video micrographic technique. Data are obtained on the evolution of individual spiral domains over the lifetime of each domain. A new mechanism for the destruction of this state is discovered.

Magnetic domain evolution and wall energy in Nd 2Fe 14B melt spun allos

The evolution of magnetic domains in Nd 2Fe 14B melt spun alloys is observed by Lorentz electron microscopy in grains the C-axis of which is nearly normal to the ribbon plane. A domain wall energy value of 38 mJ m −2 is estimated from the domain wall thickness revealed by Foucault mode. The values of magnetic bubble diameters and collapse fields are discussed.