Free Domain Wall Research Articles

The Study of Closure Domain Structure Dynamics in Bistable Microwires Using the Technique of Three-Level Field Pulses

The process of release of a single domain wall from the closure domain structure at the microwire ends and the process of nucleation of the reversed domain in regions far from the microwire ends were studied using the technique that consists in determining the critical parameters of the rectangular magnetic field pulse (magnitude— $H_{\mathrm {pc}}$ and length— $\tau _{c}$ ) needed for free domain wall production. Since these processes can be influenced by the magnitude of the magnetic field before or after the application of the field pulse ( $H_{p}$ , $\tau$ ), we propose a modified experiment in which the so-called three-level pulse is used. The three-level pulse starts from the first level, then continues with the second measuring rectangular pulse ( $H_{p}$ , $\tau$ ), which ends at the third field level. Based on the results obtained in experiments using three-level field pulses, it has been shown that reversed domains are not present in the remanent state in regions far from the microwire ends. Some modification of the theoretical model of a single domain wall trapped in a potential well will be needed for an adequate description of the depinning processes.

Stress-Induced Changes in Closure Domain Structure Dynamics in Bistable Ferromagnetic Microwire

A new experimental method for the study of closure domain structure dynamics in bistable ferromagnetic microwires is proposed. The simple experimental set-up enables detection of the presence of a free domain wall in the wire after application of a well-de ned rectangular magnetic eld pulse. The changes in closure domain structure dynamics, caused by applied tensile stress, are demonstrated by measurements on Fe77.5B15Si7.5 glasscoated microwire. Minimum critical eld for the release of a domain wall from the closure domain structure increases with increasing stress, while on the other hand, the minimum time needed for this process rapidly decreases with increasing stress.

Dynamics of closure domain structure in bistable ferromagnetic microwire

Dynamics of closure domain structure in bistable glass-coated Fe77.5B15Si7.5 amorphous ferromagnetic microwire was studied experimentally and theoretically. A new experiment for this study was proposed. The basic idea of this consisted in checking whether a well-defined rectangular magnetic field pulse applied to the microwire end can produce a stable and free domain wall in the microwire. A theoretical model previously used for describing the wall depinning process from the wire end was adapted for interpretation of our experimental results. Information about parameters of the potential well in which the wall is initially trapped as well as about wall dynamics was obtained. Values of these parameters are in good agreement with those obtained from different types of experiments.

Influence of structural disorder on magnetic domain formation in perpendicular anisotropy thin films

Using a combination of resonant soft x-ray scattering, magnetometry, x-ray reflectivity and microscopy techniques we have investigated the magnetic properties and microstructure of a series of perpendicular anisotropy Co/Pt multilayer films with respect to structural disorder tuned by varying the sputtering deposition pressure. The observed magnetic changes in domain size, shape and correlation length originate from structural and chemical variations in the samples, such as chemical segregation and grain formation as well as roughness at the surface and interfaces, which are all impacted by the deposition pressure. For low pressure samples we find evidence of a random "gas-like" distribution of magnetic domains, while in the higher pressure samples the domain structure exhibits only short range "liquid-like" positional ordering. The structural and chemical disorder induced by the higher deposition pressure first leads to an increase in the number of magnetic point defects that limit free domain wall propagation. Then, as the sputtering pressure is further increased, the domain wall energy density is lowered due to the formation of local regions with reduced magnetic moment, and finally magnetically void regions appear that confine the magnetic domains and clusters, similar to segregated granular magnetic recording media.

Thermodynamics of free domain wall fermions

Studying various thermodynamic quantities for the free domain wall fermions for both finite and infinite fifth dimensional extent ${N}_{5}$, we find that the lattice corrections are minimal for ${N}_{T}\ensuremath{\ge}10$ for both energy density and susceptibility, for its irrelevant parameter $M$ in the range 1.45--1.50. The correction terms are, however, quite large for small lattice sizes of ${N}_{T}\ensuremath{\le}8$. We propose modifications of the domain wall operator, as well as the overlap operator, to reduce the finite cutoff effects to within 10% of the continuum results of the thermodynamic quantities for the currently used ${N}_{T}=6--8$ lattices. Incorporating the chemical potential, we show that divergences proportional to ${\ensuremath{\mu}}^{2}/{a}^{2}$ are absent for a large class of such domain wall fermion actions although the chiral symmetry is broken for $\ensuremath{\mu}\ensuremath{\ne}0$ at any finite lattice spacing.

Towards QCD thermodynamics using exact chiral symmetry on a lattice

The thermodynamics of massless ideal gas of overlap quarks has been investigated numerically for both zero and nonzero baryon chemical potential μ. While the parameter M has been shown to be irrelevant in the continuum limit, it is shown numerically that the continuum limit can be reached with relatively coarser lattices for certain ranges of M. Numerical limitation of the existing method of introduction of chemical potential in the overlap formalism is discussed. We have also studied the energy density of free domain wall fermions in the absence of μ and estimated the extent of lattice in the fifth dimension L5 for which the overlap results are recovered. Interestingly, this value of L5 is also minimum for the same range of M found in the overlap case.

Free Boundary Domain Wall Pinning Model for the Magnetization Reversal in Magnetic Thin Films

A free boundary domain wall pinning model for the magnetization reversal in magnetic thin films was proposed based on the experimental observations. The dependence of the switching field on the defect width, the direction of the external magnetic field, and the magnetic parameters (including magnetization, anisotropy, and exchange stiffness) of the host material and the defect was studied. In particular, the switching field, domain wall angle, and even the whole hysteresis loops of the strongly exchange-coupled NiFe/SmFe/NiFe trilayered film, which were found experimentally to depend on the direction of the external magnetic field, were well reconstructed by the free boundary domain wall pinning model using the experimental average magnetic parameters of the film.

Chiral gauge theory on the lattice with domain wall fermions

We investigate a $U(1)$ lattice chiral gauge theory with domain wall fermions and compact gauge fixing. In the reduced model limit, our perturbative and numerical investigations show that there exist no extra mirror chiral modes. The longitudinal gauge degrees of freedom have no effect on the free domain wall fermion spectrum consisting of opposite chiral modes at the domain wall and at the anti-domain wall which have an exponentially damped overlap.

Thermodynamics of free domain wall and overlap fermions

Studies of non-interacting lattice fermions give an estimate of the size of discretization errors and finite size effects for more interesting problems like finite temperature QCD. We present a calculation of the thermodynamic equation of state for free domain wall and overlap fermions.

U(1) chiral gauge theory on lattice with gauge-fixed domain wall fermions

We investigate a U(1) lattice chiral gauge theory ( Lξ GT) with domain wall fermions and gauge fixing. In the reduced model limit, our perturbative and numerical investigations at Yukawa coupling y = 1 show that there are no extra mirror chiral modes. The longitudinal gauge degrees of freedom have no effect on the free domain wall fermion spectrum consisting of opposite chiral modes at the domain wall and the anti-domain wall which have an exponentially damped overlap. Our numerical investigation at small Yukawa couplings ( y ⪡ 1) also leads to similar conclusions as above.

Macroscopic quantum dynamics of a free domain wall in a ferromagnet

We study macroscopic quantum dynamics of a free domain wall in a quasi-one-dimensional ferromagnet by use of the spin-coherent-state path integral in discrete-time formalism. Transition amplitudes between typical states are quantitatively discussed by use of a stationary-action approximation with respect to collective degrees of freedom representing the center position and the chirality of the domain wall. It is shown that the chirality may be loosely said to be canonically conjugate to the center position; the latter moves with a speed depending on the former. It is clarified under what condition the center position can be regarded as an effective free-particle position, which exhibits the phenomenon of wave-packet spreading. We demonstrate, however, that in some cases the nonlinear character of the spin leads to such a dramatic phenomenon of a nonspreading wave packet as to completely invalidate the free-particle analogy. In the course of the discussion, we also point out various difficulties associated with the continuous-time formalism.

A Surface Effect on a One-Dimensional Competitive System: Free Domain-Wall Structures Induced by a Free-Surface

Some one-dimensional classical systems with competitive interactions show quite complex ground-state phase-diagrams, in which infinitely many phases exist. A free-surface effect on the ground-state structures of one of such systems is discussed. A direct numerical method is applied to evaluate the ground-state structure of the semi-infinite system. We pay attention to the area near the phase boundary between a non-uniform modulated phase and a uniform phase. The free-surface induces domain-wall structures. The domain-wall may be free from the free surface in some parameter region although it is usually bound near the surface.

Susceptibility and magnetization processes in Sm2Fe17N3 powders

Low field ac susceptibility and magnetization curves of milled Sm2Fe17N3 powders are measured for isotropic and aligned samples with average particle sizes ranging from 3.0 to 52 μm. Most of the particles are in a multidomain state before application of a magnetic field. The transverse ac susceptibility (χ⊥ac∼0.08) due to magnetization rotation against the anisotropy field is independent of particle size. The parallel ac susceptibility χ∥ac and the susceptibility of isotropic powders are dominated by almost free domain wall movement in multidomain particles for which 〈χ〉=1.35. In very low fields (<0.5 mT) χ∥ac is small (∼0.1), but it increases toward a constant value of order 1 when the field exceeds a value of order 1 mT which is inversely proportional to the particle size. There is a corresponding increase in loss angle which is attributed to weak pinning of domain walls at surface defects. The volume fraction of single-domain and multidomain powder are deduced as a function of average particle size both in the unmagnetized and remanent states, from an analysis of high-field magnetization curves on isotropic powder. For example, about 87% of the volume of powder with an average size of 6.7 μm is multidomain in the unmagnetized state, but only 43% of the volume is multidomain in the remanent state.

Low temperature spin wave and domain wall thermodynamics and form factors for the classical easy-plane ferromagnetic chain

Using the steepest descent approach, equivalent to the static limit of the ideal gas phenomenology, we derive low temperature expressions for the free energy and the static spin correlation functions of the classical easy-plane ferromagnetic chain in an in-plane field. At low field the system forms a stable gas spin waves and domain walls. Approaching the critical field for domain wall transition, the domain wall free energy, the longitudinal and transverse in-plane domain wall form factors increase in strength. The transverse in-plane spin wave correlation display damped oscillations and the out-of-plane spin correlations decay exponentially with a growing correlation length. Correspondingly, the out-of-plane form factor increases in intensity. We propose to look for this effect in CsNiF 3 by means of (polarized) neutrons.

Domain wall phenomena in bubble propagation layers

The role of domain walls in bubble propagation layers is reviewed. Conventional permalloy overlay structures of typically 0.4–0.5 μm thickness show stray field free domain wall structures. These walls are very wide and mobile. It is concluded that wall pinning and irreversibilities are primarily connected with Bloch lines and other “singularities” which cannot be stray field free. In ion-implanted bubble memories domain walls play a more important role. Generally one expects symmetric Néel walls. By a partial stress relaxation “charged” walls are produced which are used for bubble transportation. The behaviour of these charged walls in connection with the details of the magnetoelastic interaction is investigated.

Magnetic bubble and stripe domain wall velocity in amorphous GdCoMo films

The free bubble domain wall radial velocity and mobility in GdCoMo sputtered amorphous films has been measured and compared to stripe wall motion in the same sample. A sampling optical microscope with a sampling time of 10 nsec and a single exposure electronic recording TV system was used to observe the transient bubble size and stripe width. Wall velocities up to 400 m/sec were observed for applied fields up to 350 Oe. The velocity for some samples was linear with respect to drive although nonlinear behavior was also observed as well as domain distortion on the very temperature-sensitive samples. For the linear samples, a mobility of 1.95 m/sec Oe was measured in the effective drive field range 0–200 Oe.