Cross-tie Walls Research Articles

Micro/magnetic structural development in Co-Ni-Cr and Co-Ni-Cr/Cr films for longitudinal recording media

A series of Co-Ni-Cr and Co-Ni-Cr/Cr films have been examined using transmission electron microscopy in conventional bright/dark-field imaging, selected-area diffraction, convergent beam electron diffraction, as well as in Lorentz electron microscopy imaging mode. The crystallographic orientation and magnetic domain structure have been characterized as a function of film thickness. The results indicate that amorphous Co-Ni-Cr films form prior to the formation of small randomly oriented equiaxed grains during deposition on amorphous substrates. As the film thickness increases, some of the small grains grow preferentially with their c axis parallel to film plane, but others with their c axis at an angle of 62° from the film plane. Longitudinal domain structures of the films were found to consist of Néel walls associated with cross-tie walls and Bloch lines. Each domain contains a large amount of the magnetic ripple-type structure.

Magnetic-structural development in Co-Cr films for perpendicular recording media

Transmission electron microscopy (TEM) was employed to examine the relation between microstructure and the corresponding magnetic domain structure in Co-Cr films. Emphasis has been placed on the influence of crystallographic orientation on the magnetic domain morphology. Examination of thin samples (7.5–35 nm) reveals that randomly oriented small grains form in-plane domain structures with cross-tie walls, magnetic ripple structure, and Bloch lines. As the film thickness increases (40–50 nm), the grains become well oriented along their c axes and the corresponding domain structures are comprised of the stripe-type characteristic of perpendicular domains. The magnetic domain structures will be discussed in terms of crystallographic orientation and microstructural defects.

Magnetic domain walls in thin molybdenum permalloy films observed under external magnetic fields by electron microscopy

Dynamic observations of domain walls have been made using the Lorentz Microscopy technique. The aim of the investigations was to study the influence of magnetic fields on the domain wall stability and also to study the influence of structural features such as grain boundaries, twins' inclusions and second-phase particles on the initial permeability. Most of the thin foils used had thicknesses less than 100 nm hence the specimens could be referred to as thin films. Cross-tie walls which were noted to occur in materials with high permeability, i.e. materials of low crystal anisotropy,Ki, were observed at intermediate thicknesses of the thin foils. Both the cross-tie length and period are dependent upon film thickness, each becoming shorter with increasing thickness. The influence of the applied field on the cross-tie wall stability depends on both the magnitude and direction of the applied field. Secondly, it was observed experimentally that besides the transition from the Neel wall to the Bloch wall, transitions also occur as a function of the applied WA Structura! features such as grain boundaries, twins, inclusions, which may take many forms as holes or cracks, etc., had a very strong pinning effect on the domain wall motion, leading to a reduction in permeability.

The Bloch Lines Dynamics in Permalloy Films

A theoretical calculation of the overall structure of the cross-tie wall is made. The wall static and dynamic parameters (including the cross and circle Bloch lines radii) are found making use of the energy minimization of the wall unit cell. The velocity of the circle Bloch line motion and the annihilation field are calculated. The results of calculation are compared with experimental data. [Russian Text Ignored].

The effect of magnetic fields on crosstie walls in thin iron films observed i n s i t u in an electrostatic TEM

The effect of magnetic fields on the magnetic domain structure in thin iron films (∼200 Å) has been studied in situ by Lorentz microscopy in an electrostatic transmission electron microscope (TEM). The electrostatic instrument provides a specimen region free of magnetic fields. A special solenoid was placed in the specimen region, coaxial with the optic axis. By tilting the specimen holder a known in-plane component of the field could be applied to the specimen. Studies were made with the in-plane field applied parallel to and perpendicular to the domain walls.

Dynamics of Néel Wall Transitions into Cross-Tie Walls

Dynamics of Néel Wall Transitions into Cross-Tie Walls

Cross-tie shift register

The principle, design, fabrication and operating conditions of a shift register memory device using cross-tie wall structure in polycrystalline magnetic films will be described. This device is nonvolatile, silicon compatible and can be fabricated in a three mask level process. Each data track of this device consists of a stripline conductor, a 25 μm wide magnetic film data strip and single level data generator/propagator/detector circuitry. This is the first such device to incorporate a functioning generator, propagate circuit, and detector on a single level, thus greatly simplifying fabrication and increasing reliability and yield. A spatially varying magnetic field produced by the periodic wide/narrow propagate bias stripline is used in conjunction with the magnetic field produced by a uniform stripline to achieve propagation. Propagation is accomplished by generating cross-tie/Bloch line pairs and subsequently annihilating trailing pairs. Cross-tie shift registers have been fabricated on both glass and silicon substrates and have been tested.

Electron microscopy of i n-s i t u ion beam sputtered amorphous Gd-Fe alloy thin films

We have investigated the physical microstructure and Lorentz magnetic structure of in-situ ion beam sputtered and RF sputtered amorphous Gd-Fe alloy thin films. Auger electron spectroscopy was used to determine the composition profile and thickness of the films. Films with 18 to 30 at.% Gd were deposited onto amorphous carbon substrates up to 500 Å inside a modified RCA-EMU4 electron microscope. Two diffused electron diffraction halos were observed indicating amorphous phase separation. Using high-resolution dark-field microscopy with a JEOL (JEM-100C) electron microscope, we observed coherent scattering regions of approximately 8–20 Å diameter. In conclusion, amorphous Gd-Fe alloy films exhibit amorphous phase separation/quasi-amorphous structure, in-plane magnetization ripple and crosstie walls, and stress-induced anisotropy.

Barkhausen effect in magnetic thin films: Experimental noise spectra

In this paper Barkhausen noise spectra are shown that were measured on thin uniaxial 83-17 Ni−Fe films and these spectra were compared with the known magnetic behaviour of these films. On these types of sample the Barkhausen effect had been investigated by Lambeck [1], but their Barkhausen noise spectra have not been investigated so far. The film thicknessd m in our experiments ranged between 400 and 2400 A. In this range a change occurs in the dynamics of the magnetization behaviour caused by transitions of the type of domain wall. The results presented here show the very strong dependence of the Barkhausen noise spectra on the type of domain wall. Films with Bloch walls always show a frequency dependence off −1.7 in the higher frequency range. Samples in the thickness range where the transition occurs from the Neel wall via the cross-tie wall to the Bloch wall, have exponents between −1 and −2. The frequencyf c above which the measured noise intensity begins to decrease varies very much for the different films. The curve off c versus film thicknessd m has the same form as the curve of the domain wall mobilitym versusd m.

The perfection of flux grown crystals

Three types of crystal grown by slow cooling from the flux have been studied by X-ray topography and optical birefringence microscopy and shown to be nearly perfect. Results show that barium titanate and ferric borate crystals may contain large regions free from dislocations. Good magnetic domain contrast has been observed in ferric borate, and the cross-tie wall has been imaged by X-ray topography for the first time. Crystals of the rare earth vanadates, including crystals of solid solutions containing two rare earth ions, were found to have dislocation densities less than 100 lines/cm 2 and give good anomalous transmission of X-rays. No evidence was found in any of the crystals for dislocations associated with the spontaneous nucleation propagating large distances into the crystals.

Bloch line interaction in coarsely crystalline permalloy films

Data storage in thin permalloy films has failed to reach its promise partly because domain walls do not maintain their configuration in the presence of disturb fields. The phenomena associated with the motion of a domain wall, both creep and coercivity, are generally determined by the interactions between the wall and its host magnetic film. Some of these interactions are isolated and studied. When a domain wall moves across a specimen, it interacts with many defects at a given time. In order to minimize the complexity of the system, a physical model is described, in which a Bloch line moving along a cross-tie wall in a coarsely crystalline permalloy film is treated like a segment of a Bloch wall. In this model adjacent Bloch wall segments are completely decoupled from each other. The Bloch line is then studied with Lorentz microscopy to determine factors that strongly affect its motion. The experimental work is compared to a mathematical model in which a partial breakoff of exchange coupling across grain boundaries is used to explain the observed behavior. Some implications of these results are discussed.

A study of digitally recorded transitions in thin magnetic films

Digital recordings were placed on thin magnetic films which covered a range of coercivities and thicknesses. The details of the resulting transitions between bits were observed by transmission electron microscopy and found to be defined by cross-tie walls in a saw-tooth like structure (STS). This is unlike the magnetization distributions assumed by most recording theories. The theories of several other researchers, based on their observations of STS could not explain the results observed in this work. This is due to theoretical energy expressions based on magnetization distributions somewhat different from those observed here. The STS tip angle was found to be proportional to H c -0.5. The STS amplitude was proportional to film thickness times H c -1.5. Attempts at formulating a theory to explain these results were not successful.

Coercivity spectrum and domain wall creep in 200 to 1000 Å permalloy films

Investigations of cross-tie wall deformation under the influence of magnetic field carried out by magnetic suspension have revealed the nonuniform interaction of wall elements with the magnetic film potential relief. The Néel segment coercive field HcN is the less (HcN ≈︁ 0.1 Oe) and that may be explained by the theory of ripple induced coercivity. The cross-tie coercive field Hct changes with film thickness within the limits of 0.3 to 0.7 Oe (under stray field influence) and reaches its minimum at d ≈︁ 600 Å. Following a decrease in field Hct with thickness the creep kinematics varies due to changes in the effective distance between the wall points fixed at defects. The maximum creep efficiency is shown to be connected not with the greatest cross-tie density but with the growth of the distance le. The macroscopic coercive field of domain wall motion Hst≈︁1.0 Oe is defined by the nature of separate large imperfections. [Russian Text Ignored]

The structure and the energy of the cross-tie domain walls

The paper gives the model of the cross-tie walls in thin ferromagnetic films. The distance between side walls, in our model, increases for increasing thickness D of the film, which is in accordance to the experimental data.

A new model for cross-tie walls using parabolic coordinates

A model for the cross-tie wall is proposed which is based on parabolic coordinates. The model is also based on the experimental and theoretical work of several authors. The several parts of the wall are brought together and calculations are made. It is expected that the results obtained will be helpful in obtaining a complete solution on an analytical basis.

Lorentz electron microscopic studies on the motion threshold field of cross-tie walls and on the behaviour of cross-tie walls at inhomogeneities in thin magnetic films

Investigations are made on the motion of cross-tie walls in thin permalloy films with uniaxial anisotropy. In particular, the influence of larger film inhomogeneities which are visible in the electron microscope at Lorentz microscopic representation is studied with respect to the behaviour of cross-tie walls. It is found that the threshold field of a wall jump, which is defined as the motion threshold field, Hw, generally is smaller than the coercive force, Hc, of the film. If the wall jumping is hindered by a film inhomogeneity, Hw can rise. The effect of wall pinning causes an increase of the wall motion threshold field as well as a change of wall curvature in the vicinity of the film inhomogeneity. Inhomogeneities in the film influence the magnetization distribution in their vicinity. A result of the local variation of the magnetization distribution can be the formation of inhomogeneity spikes. The direction of cross-tie walls follows the local variation of the magnetization direction. This is also the case at cracks and film edges. Es wird über Untersuchungsergebnisse zur Stachelwandbewegung in dünnen Permalloy -Schichten mit einachsiger Anisotropie berichtet. Insbesondere wird der Einfluß größerer Schichtinhomogenitäten. die bei lorentzmikroskopischer Abbildung im Elektronenmikroskop sichtbar sind, auf das Verhalten von Stachelwänden untersucht. Es wird gefunden, daß die als Wandbewegungsfeldstärke Hw definierte Sprungfeldstärke einer Wand im allgemeinen kleiner als die Schichtkoerzitivfeldstärke Hc ist. Sie kann stark erhöht werden, wenn die Wand durch eine Schichtinhomogenität am Springen gehindert wird. Der Effekt des “pinning” besteht neben der Erhöhung der Wandbewegungsfeldstärke in einer Beeinflussung der Wandkrümmung an der Inhomogenität. Schichtinhomogenitäten beeinflussen den Magnetisierungsverlauf in ihrer Umgebung. Durch die örtliche Änderung der Magnetisierungsrichtung kann es zur Ausbildung von Inhomogenitätenstacheln kommen. Stachelwände folgen der örtlichen Änderung der Magnetisierungsrichtung. Dies gilt auch für Risse und Schichtränder.

Proposal on cross-tie wall and Bloch line propagation in thin magnetic films

A serial access memory is proposed based on the propagation of cross-tie walls and Bloch lines along domain walls in thin magnetic films. The techniques used to propagate bubbles may be applied to cross-tie walls and Bloch lines. This method shows promise because of the speed and density, the polycrystalline material, the large temperature range, and easy detection with a loop. The expected speed makes possible cheap detection, and as a result the need for magnetic logic is decreased. With propagation velocities within the range of domain tip velocities, a 100 MHz stepping rate can be expected. At this rate a 0.5 millivolt signal can be obtained from a loop as a Bloch line passes beneath it.

Temperature Dependence of Magnetic Domains and Wall Structures

The high voltage Lorentz microscopy was successfully used to observe changes with temperature; of domain structures and metallurgical structures in an iron film set on the hot stage combined with a goniometer. The microscope used was the JEM-1000 EM which was operated with the objective lens current cut off to eliminate the magnetic field in the specimen position. Single crystal films with an (001) plane were prepared by the epitaxial growth of evaporated iron on a cleaved (001) plane of a rocksalt substrate. They had a uniform thickness from 1000 to 7000 Å.The figure shows the temperature dependence of magnetic domain structure with its corresponding deflection pattern and metallurgical structure observed in a 4500 Å iron film. In general, with increase of temperature, the straight domain walls decrease in their width (at 400°C), curve in an iregular shape (600°C) and then vanish (790°C). The ripple structures with cross-tie walls are observed below the Curie temperature.

Cross-tie—Néel domain wall transition in thin magnetic films

A theoretical consideration of the hysteresis change of the domain wall structure in flat uniaxial thin magnetic films under the influence of a magnetic field parallel to the hard axis has been carried out. The theory is based on a cross-tie wall model defined in a more precise manner than that of Middelhoek. The circular Bloch line shift as a function of a magnetic field and estimates for the critical fields of both the direct “cross-tie Néel” wall transition and the inverse transition have been obtained.

Lorentz electron microscopic studies on cross-tie density and on formation and annihilation of bloch line pairs in cross-tie walls

The investigations of the dependence of cross-tie density on thickness and anisotropy field strength of thin permalloy films known from literature give no information about the magnetic history of the samples. Using Lorentz electron microscopy studies of cross-tie walls it is shown that the cross-tie density in the absence of a magnetic field is a function of the preceding magnetic sample treatment and that no any sharply defined equilibrium cross-tie density exists. The change of the number of Bloch lines is observed after the influence of hard-axis fields, demagnetizing acfields, and in connection with local inhomogeneities. Die aus der Literatur bekannten Untersuchungen zur Abhangigkeit der Blochliniendichte von der Dicke und Anisotropiefeldstarke dunner Permalloy-Schichten lassen eine Angabe der magnetischen Vorgeschichte der Proben vermissen. In lorentzmikroskopischen Untersuchungen von Stachelwanden wird nachgewiesen, das die Stacheldichte im feldfreien Zustand von der vorangegangenen magnetischen Behandlung der Proben abhangt und das keine scharf begrenzte Gleichgewichtsstacheldichte existiert. Es wird die Veranderung der Blochlinienanzahl nach dem Einflus von Senkrechtfeldern, entmagnetisierenden Wechselfeldern und im Zusammenhang mit ortlichen Inhomogenitaten beobachtet.