Cross-tie Walls Research Articles

Investigation on the dynamics of cross-tie walls in elliptical permalloy elements

The magnetization reversal of micrometer-sized permalloy ellipses having cross-tie walls was investigated by means of magnetic-force microscopy and magnetoresistance measurements. Elliptical elements were fabricated using electron-beam lithography, in which an aspect ratio from 2 to 3 and thickness of 72 nm were accordingly designed. The magnetization reversal process illustrated main wall propagation along the short axis with the external field applied in the long-axis direction, while the reversal took place through annihilation of adjacent Ne/spl uml/el-type cores when the external field was applied in the short-axis direction. In addition, the longitudinal magnetoresistance curve showed many steps that are believed to be due to the propagation of the cross-tie wall associated with the annihilation of Ne/spl uml/el-type cores.

X-ray photoelectron spectroscopy and magnetic force microscopy studies of ion-beam deposited Ni/sub 80/Fe/sub 20//Co-oxide bilayers

The correlation between the structural and magnetic properties in Ni/sub 80/Fe/sub 20//Co-oxide bilayers that were prepared using a dual-ion beam deposition technique has been investigated. The pure Permalloy (Ni/sub 80/Fe/sub 20/) film (a = 3.53 /spl Aring/) consisted of face-centered cubic nanocrystallites. Films prepared with 8% O/sub 2/ and 34% O/sub 2/ in the assist beam consisted of rock-salt CoO (a = 4.27 /spl Aring/) and spinel Co/sub 3/O/sub 4/ (a = 8.21 /spl Aring/). The composition and oxidation state of the Co-oxide layers were analyzed with X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). Magnetic force microscopy (MFM) revealed that the Ni/sub 80/Fe/sub 20//Co bilayer exhibited ripple domains, whereas cross-tie domain walls were found in Ni/sub 80/Fe/sub 20//CoO bilayer. At 289 K, the pure Permalloy film exhibited soft magnetic properties while a magnetic hardening with enhanced coercivity was observed in the Ni/sub 80/Fe/sub 20//CoO bilayer. Additionally, the Ni/sub 80/Fe/sub 20//CoO bilayer exhibited an exchange shift H/sub ex//spl sim/-75 Oe at 150 K.

Structural and magnetic properties of evaporated Co/Si(100) and Co/glass thin films

A series of Co thin films have been evaporated onto Si(100) and glass substrates. The Co thickness, tCo, ranges from 50 to 195 nm. The structural and magnetic properties have been investigated by x-ray diffraction, hysteresis curves, Brillouin light scattering and magnetic force microscopy (MFM) techniques. The Co thin films are found to be polycrystalline with (0001) texture. There is an increase of the grain size with increasing film thickness. The coercive fields range from values as low as 2 Oe in thinner films to the highest values, 2500 Oe, in 195 nm thick Co/Si films. The magnetocrystalline anisotropy field Ha decreases as the thickness increases; surface and stress induced anisotropies seem to contribute to the value of Ha. MFM images reveal a well-defined stripe pattern for thicker Co/Si samples. Such domains are not observed in the case of the thinner films. These so-called weak-stripe domains appear in magnetic films with a low or intermediate perpendicular anisotropy. Similar behaviour was observed in Co/glass samples, in addition, cross-tie walls were seen in thinner ones.

Magnetization changes visualized using photoemission electron microscopy

Photoemission electron microscopy was used to visualize the motion of magnetic domains on a sub-nanosecond timescale. The technique exploits the imaging of magnetic domains using soft X-ray circular dichroism, with the special feature that the instrument utilizes a fast image acquisition system with intrinsic 125 ps time resolution. The overall time resolution used is about 500 ps. Different domains and domain movements have been observed in lithographically-produced Permalloy structures on a copper microstrip-line. A current pulse of I=0.5 A with rise times of about 300 ps switched the Permalloy islands from a Landau-Lifshitz type domain configuration into metastable s-state domain configurations. A pulse with opposite direction could reverse these s-type patterns. Photoemission electron microscopy was employed to observe the domain movements while repeating current pulses passed the microstrip-line. Using small unipolar amplitudes, only the cross-tie walls of the s-type patterns disappear and the pattern becomes “fuzzy”, e.g. the domain rims are not sharp and some domains change their size. Before the structure switches at a current threshold, it is more fluctuating as shown by significant differences in sharpness of the islands rim and the inter-domain boundaries show. The device behavior complicates when a bi-polar pulse is applied. Switching and oscillating of domains is observed in various manners, e.g. very tiny domains appear and unify again.

Suppression of Spike Noise from FeCSi Film for Backlayer of Double-Layered Perpendicular Magnetic Recording Medium

The relationships among magnetic properties, noise characteristics, and structures of FeCSi laminated films developed as soft magnetic backlayers of double-layered perpendicular magnetic recording media were examined to find a way to suppress spike noise. FeCSi laminated films were fabricated by alternately stacking soft magnetic layers with non-magnetic intermediate layers. The periods of the cross-tie walls in FeCSi monolayers lengthened as the film thickness decreased. The periods of the cross-tie walls in the laminated films were longer than those in the monolayers. This indicated that the lamination technique was effective in preventing free poles from forming on the surface of the laminated films and that amplitudes of spike noise from the laminated films should have been suppressed. By using FeCSi soft magnetic layers with a thickness of 20 nm and carbon intermediate layers with a thickness of 5 nm, amplitudes of spike noise from the laminated films were suppressed sufficiently for practical use. The laminated films had a high saturation magnetization of 1.6 T and low media noise that was on the same level as in conventional longitudinal magnetic recording media.

Variation of domain formation in a 15 nm NiFe layer exchange coupled with NiO layers of different thicknesses

The correlation between ferromagnetic domain formation and exchange bias in a series of NiFe/NiO samples with varying NiO thicknesses has been investigated using the magneto-optic Kerr effect and magnetic force microscopy. Below a critical thickness (15 nm) of NiO, the exchange bias HE is zero and ripple domains exist in the NiFe layer. Above this critical thickness, cross-tie type domain walls appear concurrently with the appearance of exchange bias. Both the number of cross-tie domain walls and the exchange bias increase with an increase in NiO thickness, reaching a maximum at 35 nm NiO, after which both show a gradual decrease. This variation of domain wall formation in the NiFe layer with the NiO thickness possibly reflects the variation of the domain structure in the NiO layer through interfacial exchange coupling.

Magnetic domain wall trapping by in-plane surface roughness modulation

Using the influence of surface roughness on the coercivity of thin magnetic films, we have produced an artificially modulated magnetic domain configuration in a chemically homogeneous polycrystalline thin magnetic film. This is achieved by evaporating a thin Fe film on top of a smooth substrate that was previously covered with an array of considerably rougher Ag lines. Magneto-optical Kerr effect measurements revealed the presence of two distinct switching fields in the hysteresis loops. In the intermediate state, a stable magnetic domain configuration of antiparallel aligned domains is achieved. Magnetic force microscopy images indicate that interacting cross-tie domain walls are separating the magnetic domains.

Repulsive Interaction of Néel Walls, and the Internal Length Scale of the Cross-Tie Wall

Neel walls and cross-tie walls are two structures commonly seen in ferromagnetic thin films. They are interesting because their internal length scales are not determined by dimensional analysis alone. This paper studies (a) the repulsive interaction of one-dimensional Neel walls and (b) the internal length scale of the cross-tie wall. Our analysis of (a) is mathematically rigorous; it provides, roughly speaking, the first two terms of an asymptotic expansion for the energy of a pair of interacting walls. Our analysis of (b) is heuristic, since it rests on an analogy between the cross-tie wall and an ensemble of Neel walls. This analogy, combined with our results on Neel walls and a judicious choice of parameter regime, yields a specific prediction for the internal length scale of a cross-tie wall. This prediction is consistent with the experimentally observed trends.

Cross-tie walls in thin permalloy films

Cross-tie walls in permalloy films have been simulated as a function of thickness in the range 10-70 nm using direct integration of the Landau-Lifshitz-Gilbert equation in a Cartesian lattice with periodic boundary conditions along the length of the wall. A cross-tie wall is a Bloch-like transition along the wall [or vertical Bloch line (VBL)] between Neel Walls of opposite chirality in a film with in-plane magnetization. This transition is a low-energy state, which allows the long tails of Neel walls to partially close their magnetic flux in the vicinity of the walls. The magnetization configuration of these transitions is either elliptical (vortex) or hyperbolic (antivortex). The pi-VBL transitions are 50 nm in diameter and the distance from each other increases with film thickness. The cross-tie wall energy per unit area was lower than that of a pure Neel or Bloch wall between 15 and 50 nm film thickness.

Magnetic force microscopy observation of antivortex core with perpendicular magnetization in patterned thin film of permalloy

The cross-tie wall is a kind of magnetic domain wall composed of a main straight wall and crossing subwalls and observed in magnetic thin films. This wall contains two kinds of magnetic vortex structures: “circular vortex” and “antivortex.” At the cores of both vortices, the existence of a spot with perpendicular magnetization has been theoretically predicted. We have detected the perpendicular magnetization spots at each vortex core and identified the direction of it by applying magnetic force microscopy imaging to cross-tie walls in patterned rectangular thin permalloy (Ni80Fe20) films. We also fabricated magnetic structures that contain only antivortex by engineering the shape of thin films.

Ultrahigh vacuum scanning tunneling microscopy/magnetic force microscopy study of ultrathin iron films grown on polycrystalline nickel oxide films

The thickness dependence of the topographic and magnetic structure of ultrathin Fe films grown on polycrystalline NiO films under ultrahigh vacuum (UHV) conditions was studied to investigate the growth mechanism of the ferromagnetic film and the corresponding magnetic interaction with the antiferromagnetic substrate. Externally prepared NiO films of 60 nm thickness were cleaned by heating in UHV. Ultrathin layers of Fe in the range of 1–27 nm were deposited on top of the NiO film and were analyzed at specific coverages. Iron grows as a polycrystalline film with the grains increasing in size with the thickness. The contours of the underlying NiO crystallites were evident at low coverages but gradually disappeared as the Fe grains coalesced at thicker coverages. Magnetic force microscopy images of the 1 nm thick film show randomly oriented magnetic grains with an average domain size of 30 nm. With an increase in film thickness the size of the domains grows to about 200 nm at 15 nm of iron. At a film thickness of 19 nm cross-tie domain walls become visible, indicating the crossover of some parts of the film from random magnetic grains into continuous domains with in-plane magnetization. A further increase in the film thickness leads to larger in-plane domains, while there are some areas with localized grains on the surface.

Néel and Cross-Tie Wall Energies for Planar Micromagnetic Configurations

We study a two-dimensional model for micromagnetics, which consists in an energy functional over S 2 -valued vector fields. Bounded-energy configurations tend to be planar, except in small regions which can be described as vortices (Bloch lines in physics). As the characteristic “exchange-length” tends to 0, they converge to planar divergence-free unit norm vector fields which jump along line singularities. We derive lower bounds for the energy, which are explicit functions of the jumps of the limit. These lower bounds are proved to be optimal and are achieved by one-dimensional profiles, corresponding to Neel walls, if the jump is small enough (less than π/2 in angle), and by two-dimensional profiles, corresponding to cross-tie walls, if the jump is bigger. Thus, it provides an example of a vector-valued phase-transition type problem with an explicit non-one-dimensional energy-minimizing transition layer. We also establish other lower bounds and compactness properties on different quantities which provide a good notion of convergence and cost of vortices.

Simple analytical description for the cross-tie domain wall structure

A closed form analytical expression for the magnetization vector distribution within the cross-tie domain wall in an isotropic ferromagnetic thin film is given. The expression minimizes the exchange energy functional exactly, and the magnetostatic energy by means of an adjustable parameter (wall width). The equilibrium value of the wall width and the film thickness corresponding to the transition between the Néel and the cross-tie walls are calculated. The results are compared with the recent experiments and are in good qualitative agreement.

A magnetic force microscopy and Kerr effect study of magnetic domains and cross-tie walls in magnetoresistive NiFe shapes

Magnetic domains were observed in 350 Å NiFe films. Cross-tie domain walls and ripple structures were observed inside the domains by MFM. The perturbative effect of the standard CoCr MFM tip on the magnetic microstructure was brought to evidence. A demagnetized, low-coercivity Fe tip gave the best images.

In-situ Lorentz TEM cooling study of magnetic domain configurations in Ni/sub 2/MnGa

Magnetic domain configurations in the ferromagnetic shape memory alloy Ni/sub 2/MnGa are analyzed by means of Lorentz microscopy and noninterferometric phase reconstruction methods. Domain structures in the cubic phase consist of cross-tie walls in the thinnest portions of the foil, and more complex configurations in thicker regions. At low temperature, the magnetization configurations change as the structure transforms martensitically to a tetragonal phase. A simple model for the magnetization changes is proposed.

Observation of asymmetric Bloch walls in epitaxial Co films with strong in-plane uniaxial anisotropy

Combined studies involving magnetic force microscopy and micromagnetic simulations are used to investigate the domain wall structure in epitaxial Co(101̄0) thin films with strong in-plane uniaxial magneto-crystalline anisotropy. This letter shows experimental evidence that, for such a system, the domain wall structure transforms from an asymmetric Bloch wall into an asymmetric Néel wall upon decreasing the film thickness from 100 to 20 nm. This transition occurs without cross-tie wall formation. Furthermore, it is found that from the four possible energetically equivalent asymmetric Bloch wall configurations, only two are stabilized along a single domain wall. For a given wall, the transition from one configuration to the other involves the simultaneous reversal of the polarity of the Bloch core and the Néel cap.

Investigation of the response of a new amorphous ferromagnetic MFM tip coating with an established sample and a prototype device

A new type of amorphous ferromagnetic MFM tip coating has been used to obtain high-resolution images of domain wall structures in a magnetically soft amorphous thin film and a prototype recording head. The new tip coating has a composition close to that of commercial METGLAS®2605SC (FeSiBC) amorphous ribbon and is produced by RF-sputtering on to Nanosensors™ Si probes. A cross-tie wall found in an amorphous alloy thin film of composition Co91Nb6Zr3 (at%) was taken as an established sample. Detailed study of the periodicity of the main wall, and features of the sub-structure under a variety of tip conditions, established the robustness of the images obtained. This is an important step in the use of MFM on soft magnetic materials. For comparison, images of the same section of cross-tie wall have been obtained with a commercial CoCr tip, showing an unacceptable degree of image perturbation. Having established the conditions for robust imaging, a prototype recording head produced from two layers of Co91Nb6Zr3 with an SiO2 isolation interlayer was successfully imaged by MFM. A classic closure domain structure adjacent to the gap was observed, in conjunction with pinned walls along the edges of the device where the etching is imprecise.

Domain formation in Fe–N multilayers

We prepared a series of sputtered FeN single layer films, and bilayer and multilayer films laminated with alumina, in which both the Fe–N thickness and the alumina interlayer thickness varied. The films were characterized by quantitative magneto-optical Kerr-microscopy and inductive measurements. We found a strong influence of interlayer coupling and FeN thickness on the domain formation in these layers. For single layers a transition from cross-tie walls to Bloch-type walls for increasing ferromagnetic layer thickness was observed. The multilayer films displayed a wide range of different types of coupled Neel walls depending on interlayer thickness. These include superimposed, compensated, and 360° walls. The change in domain wall structure strongly influences the coercivity of the thin film systems. The relative coupling strength between the magnetic films is calculated from the domain wall width of the superimposed Neel walls. Magnetic in-plane deviation strongly depends on the sign of magnetostriction. The observed magnetic dispersion leads to an increase in the minimum magnetic switching time. The observed micromagnetic structures result in unwanted irregular domain patterns in structured samples. Both are indicating limitations for applications in thin film recording heads.

Magnetic force microscopy and micromagnetic study of cross-tie wall structures in Co91Nb6Zr3 amorphous thin films

High resolution images of cross-tie domain wall structures have been obtained by magnetic force microscopy (MFM) for a 37.5 nm Co91Nb6Zr3 film using a NiFe thin film coated tip. Between successive cross ties, the main or spinal wall was found to be consistently subdivided unequally into pairs of oppositely oriented Néel wall sections separated by circular Bloch lines. Main and wing walls intersect at cross Bloch lines. A reversed-contrast MFM image of the same uneven cross-tie wall structure was obtained after reversing the tip magnetization. MFM images reflect only the field from the divergence of the underlying magnetization M and contain no direct information on curl M. Accordingly they are best interpreted by comparison with the magnetization pattern of a similar cross-tie structure obtained by micromagnetic computation. This enables the cross and circular Bloch line singularities to be distinguished in the MFM images of the cross-tie structure. By combining repeated observations made with opposite tip magnetizations, disturbance of the main and wing wall structures by the tip was extracted from the MFM signal which was then compared with the signal computed for a two-dimensional model wall. The main wall was found to be an asymmetric Néel wall with a weak S shaped magnetic structure. The wing walls were found to be Néel walls of acute angle, decreasing with distance from the spine.

Lorentz Microscopy Observation of Magnetic Domain Structure Variation in NiFe/Au Multilayer Films Caused by Au Layer Thickness

Lorentz electron microscopy offers a very useful technique by which the magnetic domain structure of a thin film can be observed. In order to develop GMR materials with high sensitivity, it is desirable to understand the magnetic domain structure of the material. In this study, the magnetic domain structure of a series of NiFe/Au multilayer films (MLFs) with Au layer thickness between 1 nm and 4nm has been observed using Lorentz microscopy. The MLFs showed a complicated domain structure, which contained twin walls and cross-tie walls in the MLFs. As the Au layer thickness increased so the ferromagnetic coupling between the NiFe layers decreased resulting in a more complex domain structure. In-situ magnetizing experiments showed that magnetic domain wall motion was the dominant mechanism for reversing the direction of magnetization. It is believed that reversal of the magnetization in the MLFs with thick Au layers was greatly affected by the structural defects in the films.