Bloch Wall Motion Research Articles

Computer Simulation of Bloch Line and Bloch Wall Dynamics by Lumped-Constant Model

A lumped constant model, treating forces acting on vertical Bloch lines as a lumped force, was used in a two-dimensional simulation of the motion of Bloch lines and Bloch walls. Relations for forces acting tangentially and normally to the wall were derived, and the mobilities of VBL and Bloch walls for single VBL and for VBL pairs were obtained as functions of the time increment ?t used, with the distance LL between grid points as a parameter. The accuracy of the simulation results as compared with theory was within 1 to 2 percent, depending on both LL and ?t. This method thus enables fairly accurate simulation by microcomputer of VBL and Bloch wall dynamics in a short time.

Simulation of Bloch Wall Motion

Experimenta1 and computer simulation results for Bloch wall motion are compared. While good agreement is obtained for wall displacement under low drive fields, and for the critical fields for thin samples and/or long drive pulses, discrepancies arise for short pulses and thicker samples. To investigate the latter, simulations were performed for a two-wall configuration, using Slonczewski's equations of wall motion. The results, which reflected the effect of the demagnetizing field, were better than for single-wall simulations, but discrepancies still remain for short pulses and high drive fields; this may be partly due to the assumption of a constant wall width.

Computer simulation of domain wall and vertical Bloch line motion in a bubble garnet film

Vertical Bloch lines(VBLs) and wall motion are studied by using computer simulation. The effect of potential wells, necessary for stable VBLs propagation, on the hard wall dynamics is investigated by the simulation. It is proved that the wall velocity decreases or the wall motion becomes oscillatory due to the potential wells. An isolated VBL propagation under a pulsed in-plane field is investigated.

Simulation of Bloch wall motion in bubble films

For straight Bloch walls in uniaxial bubble films, the dependence of the peak velocity and of the saturation velocity on the film thickness is given as v \propto l/h according to Slonczewski's HBL (horizontal Bloch line) model. This dependence could not be verified experimentally. For thick films the saturation velocity seems to increase again with increasing film thickness. Here the results of a direct numerical solution of the equations of motion are presented and compared with the HBL model and experimental results. The saturation velocity resulting from this simulation shows a dependence on the drive field even in case of low loss materials. This dependence increases with increasing film thickness and might explain the experimental results. Comparing the lowest value of the saturation velocity for drive fields slightly larger than necessary for punch-through with the saturation velocity predicted by the HBL model, very good agreement is found. The peak velocity is seen to decrease with increasing film thickness, but seems to tend to a finite value considerably larger than 0.

Bloch wall motion in Si-Fe investigated through surface e.m.f. measurements

Local measurements of the surface e.m.f.s, performed during the magnetization of a Si-Fe single crystal, put in evidence the dynamic behavior of the domain walls at a microscopic level. From the time and spectral analysis of the noise signal detected by means of an array of point-like contacts it is found that 180° longitudinal domain walls move quite independently and in a fairly irregular fashion, while strong spatial correlation effects are associated with the processes involving the flux-closing spike domain structures.

Anelastic Effects in the β-Phase of Niobium Hydride

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Neutron diffraction study of magnetisation processes in magnetite, Fe3O4, both multi-domain and magnetically hardened by milling

Experimentally observed intensity variations in Bragg peaks, which arise from magnetic neutron scattering, indicate that 180 degrees magnetisation reversals dominate the magnetisation processes in low applied magnetic fields. Whereas such reversals can be associated with 180 degrees Bloch wall motion in multi-domain samples of the ferrimagnet magnetite, Fe3O4, a different mechanism seems to be required for milled magnetite, for which 'hysteresis' in its magnetisation is usually attributed to the presence of mono-domain particles. For the milled material, low-field 180 degrees reversals appear to arise from an incoherent process, such as 'curling' or 'buckling' of the magnetisation, rather than from coherent rotation of the moments in unison within uniaxial mono-domain particles. It appears that about half the moment could be incoherently misaligned throughout this reversal process, in zero, as well as in small applied magnetic fields.

Garnet films for micron and submicron magnetic bubbles with low damping constants

Two types of films for small bubbles have been grown without making use of the rare earth ions which are the cause of increased damping of the Bloch wall motion in the usual small bubble film compositions. Firstly, Bi-containing films were grown on (111) orientated GGG for 1 μm bubbles and secondly, Mn3+-containing films were grown under compression on (100) orientated GGG substrates for 0.4 μm bubbles. The damping constants in these films have been obtained by measuring the FMR line width. The correlation between the damping of the FMR and of the Bloch wall motion is determined in films with bubble diameters of, 2 to 4 μm. The damping constants are a factor 10 to 100 smaller than in the existing device films for small bubbles.

Neutron diffraction study of antiferromagnetic garnet Ca 3Mn 2Ge 3O 12 in a magnetic field

Neutron diffraction observations in magnetic fields may be explained if, in addition to spin-flop occurring at a critical field, Bloch-wall motion with change of tetragonal axis occurs for applied field at polar angle θ < 35°.

Thermally activated Bloch wall motion

On the basis of the theory of thermal activation, a model is presented, which describes the motion of a 180°-Bloch wall through an array of obstacles. This model agrees well with experiments.

Magnetization jumps in simple domain structures

We recorded magnetization jumps during the very slow motion of Bloch walls. Complex and negative jumps are observed and can be reproduced by analog simulation of Preisach grains. Magnetostatic couplings arise from wall bending and allow changes of configuration.

Blochwandresonanz in einer 0.3 μm dicken, 81.5% Ni-Fe schicht

We report the observation of Bloch wall motion in a thin permalloy film. The uniaxial anisotropy in the film has been produced by oblique incidence evaporation. The magnetic susceptibility spectrum has been measured within the frequency range of 1–150 MHz using an experimental set-up which exhibits a very high sensitivity. The alternating magnetic field amplitude of about 1 A/m has been applied along the easy axis. Since the characteristic frequency for skin effect is 30 times larger than the measuring frequency, eddy current effects could be neglected. A specific 180° Bloch wall mass of 23 × 10 -9 kg m -2 has been calculated from the measuring frequency dependence of complex susceptability. For comparison the specific wall mass for bulk material according to Döring and Enz is calculated to δm/ δF = 1 × 10 -9 kg m -2.

Theory of Bloch-line and Bloch-wall motion

A theory of Bloch-line (BL) dynamics in magnetic bubble materials is proposed and applied to the problem of domain collapse with a small number of BLs. We derive from the Landau-Lifshitz-Gilbert equation the dynamic reaction force, composed of gyrotropic and viscous terms, on a single moving BL. From this follows easily the wall mobility for equally spaced BLs. However, we show that BLs tend to cluster statically at distances comparable to the BL width. If the BLs are clustered on one side of a circular-cylinder domain, then a change of the bias field causes the centers of bubble and cluster to move on spiral trajectories. The predicted displacement of the bubble center is large enough for visual confirmation, but the bound on coervicity required to confirm the spiral shape may be too stringent. The collapse mobility for the clustered case is typically much greater than that for the case of uniform spacing. The theory is supported by collapse-time data of Malozemoff for a garnet film of medium viscosity (α=0.12). In garnets of low viscous damping of (α≲0.01) we predict that coercivity Hc gives rise to nonlinearity of the velocity-drive relation, with a characteristic ``knee'' veolcity, (2/π)γHcr, for collapse of a domain of radius r.

On the bloch wall motion in multilayer films

A Bloch wall is considered which is pinned along its intersection line with the film surface and bent due to the action of an external field. It can be shown that beyond a critical field the wall starts to move irreversibly. In film systems consisting of hard and soft films the pinning may be caused by the connection between the walls in the different films. In this case the calculation of the critical field gives a value which is the flux-weighted average of the coercive fields of the hard and soft films in agreement with previous experimental results of Crowther.

Influence of coercive force on low-frequency creep in bloch-wall permalloy films

A very high resolution Bitter pattern technique is used to observe the motion of Bloch walls in Permalloy films excited by slowly rising and slowly falling hard-axis fields. For low coercive force films, the basic motion consists of approximately equal and opposite jumps induced by simultaneous wall structure changes at two transition fields. Net motion begins with the application of small easy-axis bias fields. For high coercive force films, although the wall structure changes continue to occur, the wall jumps and a net displacement do net occur until the easy-axis bias field exceeds a certain critical value which is a function of the wall coercive force.

Bloch Wall Motion in Ferromagnetic Films Excited by Fast-Rising Hard-Axis Pulses

Bloch wall motion along the hard direction of Permalloy films due to fast-rising hard-axis pulses is studied. Experimentally, it is found that for a given value of ḣ⊥, the time rate of change of the applied field, wall motion occurs only for fields in excess of a certain minimum or threshold value hth. Two distinct regions exist in this characteristic threshold curve. When ḣ⊥ is large hth is constant and when ḣ⊥ is small hth is a linear function of ḣ⊥. Above hth the wall displacement or movement per pulse increases linearly with driving field until this field approaches the value where the effect of Néel lines becomes important. A theoretical model taking account of both Bloch-Néel wall transitions and the gyromagnetic effect of the spins in the walls and in the surrounding domains is proposed to account for the experimental results. The calculation assumes a reasonable wall structure and that Bloch walls are allowed to move freely except for a coercive force, i.e., a possibly more complicated interaction between the Bloch walls and Néel lines is neglected. With appropriate values of the constants and film parameters, the computation gives very good agreement with experimental results.

Quenching Experiments with High‐Purity Iron

AbstractIron wire of very high purity was quenched in a helium cryostat. Changes in electrical resistivity were used to measure the recovery during isochronal and isothermal annealing in the temperature range 4.2 to 1000 °K. Two sharp recovery stages were observed (first stage: 300 °K &lt; T &lt; 400 °K, second stage: 400 °K &lt; T &lt; 520 °K). The activation energy was determined by the change in slope and the Meechan‐Brinkman test (first stage: 0.48 to 0.51 eV, second stage: 1.05 to 1.1 eV). Both recovery processes follow a first‐order kinetic law. These two stages are observed after cold work as well. The first stage is attributed to the migration of carbon atoms alone and paired with vacancies, the second stage corresponds to the migration of vacancies. The recovery spectrum exhibits in three different temperature ranges a resistivity increase and a final recovery stage in the range 850 to 1000 °K. The last increase and recovery stage may be due either to the redissolution of carbon atoms or to Bloch wall motion.

Wall streaming, creeping, and a new parade motion in Ni-Fe films excited by hard-axis pulses

The basic mechanism of Bloch wall motion associated with very-fast-rise-time hard-axis pulses appears to be the same for wall streaming or worm motion, creep, and a new uniform motion. Creep under these conditions as distinguished from creep excited by relatively low-frequency sinusoidal excitation must involve important gyromagnetic effects and exhibits the same threshold behavior as wall streaming and the new uniform motion. Creep may be associated with either an applied easy-axis bias or with an equivalent stray-field bias. The uniform motion is more easily produced in thicker films (1500 Å) with very small or no bias and consists of a "parade" of tip domains, first nucleated at the film edge and then moving in the hard-axis direction until annihilated at the edge. The threshold field for wall motion in all of these processes depends on the rate of change of the pulse field, increasing with increasing rise time. The threshold field decreases with easy-axis bias and is related to a characteristic coercive force. The movement per pulse increases with increasing magnitude of drive field to a maximum and then decreases. Several mechanisms are suggested for this behavior.

Mobility and Loss Mechanisms for Domain Wall Motion in Thin Ferromagnetic Films

Domain-wall mobility for Ni–Fe alloy films has been measured as a function of film thickness from 300 to 1650 Å. Between 300 and 800 Å, the mobility decreases with increasing film thickness, ranging from 8×103 cm/sec·Oe at 300 Å to 3×103 cm/sec·Oe at 800 Å. Between 900 and 1000 Å, the mobility increases rapidly with increasing film thickness to about 7×103 cm/sec·Oe. Above 1000 Å, the mobility increases slowly with film thickness. These mobility data have been compared with predictions based on intrinsic and eddy-current loss mechanisms utilizing available static domain wall-shape information. Predictions based on a 180° Bloch or Néel wall model in which the magnetization rotates linearly through the wall are in disagreement with the present data over the entire range of film thickness. For the 300–800 Å thickness range, predictions based on Lorentz microscopy wall-shape measurements are in good agreement with the measured mobility values. The crosstie structure associated with Néel walls in this thickness range does not appear to affect the mobility. Bitter patterns indicate that the sharp increase in mobility between 900 and 1000 Å may be associated with a wall structure transition in this region. For thicker films, mobility predictions based on static Bloch wall-shape calculations are an order of magnitude lower than the observed mobilities. Lorentz microscopy wall-shape measurements for 1200-Å films yield wall widths of about 3000 Å significantly wider than calculated Bloch wall widths. Predictions based on these measured widths are in good agreement with the data. Although the calculation includes both intrinsic and eddy-current losses, intrinsic losses dominate for crosstie or Néel wall motion in the 300–800-Å thickness range and for Bloch wall motion in the 1000–1600 Å thickness range.

Time development of magnetization

A simple model of Bloch wall motion is described for materials in which there exists a magnetic diffusion after-effect. The model is applied to the statistical system of independent Bloch walls. A qualitative explanation of the time development of magnetization in the d.c. magnetic field, based on this model, is given.