Annihilation Of Domain Walls Research Articles

Magnetic Barkhausen Noise: Aspects of Generation and Modeling

The Magnetic Barkhausen noise (MBN) techniquemeasures discrete changes in magnetization, known as“Barkhausen jumps”, whi ch can be seen as steps i n the hysteresis curve. When a ferro-magnetic material issubjected to an external varying magnetic field, themagnetization of the material wil l change in a non-linearway. These changes in magnetization are described by thehysteresis curve and are the result of modifications of domain structure of the material. Changes in magnetizationare caused by domain wall creation, domain wallannihilation, domain wall motion and rotation. In order toimprove the interpretation of Barkhausen noise, models areneeded which describe the dependence of magnetization on material properties.

Collapsing domain wall networks: impact on pulsar timing arrays and primordial black holes

Unstable domain wall (DW) networks in the early universe are cosmologically viable and can emit a large amount of gravitational waves (GW) before annihilating. As such, they provide an interpretation for the recent signal reported by Pulsar Timing Array (PTA) collaborations. A related important question is whether such a scenario also leads to significant production of Primordial Black Holes (PBH).We investigate both GW and PBH production using 3D numerical simulations in an expanding background, with box sizes up to N = 3240,including the annihilation phase. We find that:i) the network decays exponentially, i.e. the false vacuum volume drops as ∼ exp(-η 3), with η the conformal time;ii) the GW spectrum is larger than traditional estimates by more than one order of magnitude, due to a delay between DW annihilation and the sourcing of GWs. We then present a novel semi-analytical method to estimate the PBH abundances: rare false vacuum pockets of super-Hubble size collapse to PBHs if their energy density becomes comparable to the background when they cross the Hubble scale. Smaller (but more abundant) pockets will instead collapse only if they are close to spherical. This introduces very large uncertainties in the final PBH abundance. The first phenomenological implication is that the DW interpretation of the PTA signal is compatible with observational constraints on PBHs, within the uncertainties. Second, in a different parameter region, the dark matter can be entirely in the form of asteroid-mass PBHs from the DW collapse. Remarkably, this would also lead to a GW background in the observable range of LIGO-Virgo-KAGRA and future interferometers, such as LISA and Einstein Telescope.

Axion domain walls, small instantons, and non-invertible symmetry breaking

Non-invertible global symmetry often predicts degeneracy in axion potentials and carries important information about the global form of the gauge group. When these symmetries are spontaneously broken they can lead to the formation of stable axion domain wall networks which support topological degrees of freedom on their worldvolume. Such non-invertible symmetries can be broken by embedding into appropriate larger UV gauge groups where small instanton contributions lift the vacuum degeneracy, and provide a possible solution to the domain wall problem. We explain these ideas in simple illustrative examples and then apply them to the Standard Model, whose gauge algebra and matter content are consistent with several possible global structures. Each possible global structure leads to different selection rules on the axion couplings, and various UV completions of the Standard Model lead to more specific relations. As a proof of principle, we also present an example of a UV embedding of the Standard Model which can solve the axion domain wall problem. The formation and annihilation of the long-lived axion domain walls can lead to observables, such as gravitational wave signals. Observing such signals, in combination with the axion coupling measurements, can provide valuable insight into the global structure of the Standard Model, as well as its UV completion.

Gravitational waves from domain wall collapse, and application to nanohertz signals with QCD-coupled axions

We study for the first time the gravitational waves generated during the collapse of domain walls, incorporating the potential bias in the lattice simulations. The final stages of domain wall collapse are crucial for the production of gravitational waves, but have remained unexplored due to computational difficulties. As a significant application of this new result, we show that the observed NANOGrav, EPTA, PPTA, and CPTA data, which indicate stochastic gravitational waves in the nanohertz regime, can be attributed to axion domain walls coupled to QCD. In our model, non-perturbative effects of QCD induce a temperature-dependent bias around the QCD crossover, inducing the rapid collapse of the domain walls. We use sophisticated lattice simulations that account for the temperature-dependent bias to measure the gravitational waves resulting from the domain wall annihilation. We also discuss the future prospects for accelerator-based searches for the axion and the potential for the formation and detection of primordial black holes.

Domain wall annihilation: A QFT perspective

Domain wall annihilation: A QFT perspective

Meso-inflationary QCD axion

We study the possibility that the axion Peccei-Quinn symmetry is spontaneously broken after the beginning of inflation. This scenario interpolates between preinflationary and postinflationary axion cosmology with significant phenomenological differences from both. Since the axion is not present at the early stages of inflation large inflationary fluctuations are produced only at scales not constrained by cosmic microwave background (CMB), avoiding the strongest isocurvature constraints. The energy density in isocurvature perturbations at short scales however can be comparable with the adiabatic contribution from misalignment. These large overdensities can lead to the formation of axion miniclusters and also to constraints from Lyman-$\ensuremath{\alpha}$ forest and future CMB spectral distortion measurements. If Peccei-Quinn symmetry is broken during the first O(25) $e$-foldings of inflation no axions are produced from the string network but contributions from the annihilation of domain walls can further boost the abundance. This scenario is minimally realized if the Hubble scale during inflation drops below the Peccei-Quinn symmetry breaking scale but other realizations are possible.

Field-Dependent Magnetic Domain Behavior in van der Waals Fe3GeTe2

Two-dimensional magnetic van der Waals (vdW) materials can show a variety of topological nontrivial spin textures, such as Bloch- or Néel-type stripe, skyrmion, or bubble domains under certain external stimuli. It is critical to understand the magnetic domain behavior in vdW materials in order to control their size and density in response to external stimuli, such as electric and magnetic fields. We examine the magnetic field dependence of topologically non-trivial magnetization spin textures in vdW Fe3GeTe2. Néel-type stripe domains and skyrmions are formed depending on the magnetic field-cooling protocol used during in situ Lorentz transmission electron microscopy (LTEM) experiments. Use of quantitative reconstruction of magnetic induction maps and micromagnetic simulations allow for the understanding of the LTEM results of Néel-type stripe domains as well as skyrmions. In addition, the deformation of skyrmion contrast is observed as a result of the introduction of an in-plane magnetic field. We demonstrate the stability of the stripe domains and skyrmions in response to an externally applied magnetic field due to an energy barrier for domain wall annihilation. Our results establish an understanding of the energy landscape that governs the behavior of the topologically non-trivial spin textures in vdW materials which can be harnessed for spintronic applications.

Domain wall nucleation, propagation and annihilation in coupled bent ferromagnetic nanofibers with rotating local input fields

Magnetic nanofibers can be used for data transport and storage, especially related to the emerging field of neuromorphic computing. Domain walls in bent nanofibers can nucleate, e.g., due to rotating local magnetic fields. Their propagation through bent nanofibers, however, depends on the bending direction in correlation to the rotational orientation of the magnetic field, making such nanofibers suitable for semi-deterministic logic operations. Here we report on domain wall nucleation, propagation and annihilation in bent nanowire networks with multiple data inputs and outputs. Our results show the influence of the bending radii on domain wall nucleation and propagation, leading to suggestions for possible realization of multi-level systems for logic operations.

Testing clockwork axion with gravitational waves

We investigate the gravitational waves (GWs) produced from the Peccei-Quinn (PQ) phase transition associated with the clockwork axion. The PQ phase transition can be first-order when the dimension-6 operator is included into the scalar potential. The GWs from the PQ phase transition at scale in the range of 103-106 GeV are detectable for the BBO and ALIA interferometers. The LISA and Taiji interferometers can probe the GWs from the PQ scale f ≤ 104 GeV, while the GW signals from the scale f ≥ 105 GeV can be detected by the ground-based GW observatories ET and CE. We find that the parameter space κm ∼ 0.06-0.001, κl ∼ 0.04-0.001, and ε ∼ 0.1-0.01 at the scale f = 105 GeV and most of the parameter regions at the scale f = 106 GeV have been excluded by the LIGO O2 run. The LIGO O3 and design phases can further probe the remaining parameter space. We show that the GWs from the annihilation of domain walls with a PQ scale f ≃ 2 × 105 GeV can induce the stochastic signals with the right amplitude for the NANOGrav 12.5-year observations, but having a steeper spectral slope than the observations.The LIGO O3 run has the opportunity of detecting the GW signals from the first-order PQ phase transition around this scale.

Annihilation and Control of Chiral Domain Walls with Magnetic Fields.

The control of domain walls is central to nearly all magnetic technologies, particularly for information storage and spintronics. Creative attempts to increase storage density need to overcome volatility due to thermal fluctuations of nanoscopic domains and heating limitations. Topological defects, such as solitons, skyrmions, and merons, may be much less susceptible to fluctuations, owing to topological constraints, while also being controllable with low current densities. Here, we present the first evidence for soliton/soliton and soliton/antisoliton domain walls in the hexagonal chiral magnet Mn1/3NbS2 that respond asymmetrically to magnetic fields and exhibit pair-annihilation. This is important because it suggests the possibility of controlling the occurrence of soliton pairs and the use of small fields or small currents to control nanoscopic magnetic domains. Specifically, our data suggest that either soliton/soliton or soliton/antisoliton pairs can be stabilized by tuning the balance between intrinsic exchange interactions and long-range magnetostatics in restricted geometries.

Entanglement entropy of fermions from Wigner functions: Excited states and open quantum systems

We formulate a new ``Wigner characteristics''-based method to calculate entanglement entropies of subsystems of Fermions using Keldysh field theory. This bypasses the requirements of working with complicated manifolds for calculating R\'enyi entropies for many-body systems. We provide an exact analytic formula for R\'enyi and von Neumann entanglement entropies of noninteracting open quantum systems, which are initialized in arbitrary Fock states. We use this formalism to look at entanglement entropies of momentum Fock states of one-dimensional Fermions. We show that the entanglement entropy of a Fock state can scale either logarithmically or linearly with subsystem size, depending on whether the number of discontinuities in the momentum distribution is smaller or larger than the subsystem size. We also use this formalism to describe entanglement dynamics of an open quantum system starting with a single domain wall at the center of the system. Using entanglement entropy and mutual information, we understand the dynamics in terms of coherent motion of the domain wall wavefronts, creation and annihilation of domain walls, and incoherent exchange of particles with the bath.

Study on the characteristics of magneto acoustic emission for mild steel fatigue.

Fatigue life of materials or structures can be classified into three stages: fatigue hardening or softening, crack initiation and crack propagation, which includes two stages. Current mature non-destructive testing (NDT) methods can only detect macro or visible cracks in stage II crack propagation. In order to detect and evaluate the fatigue damage occurring before stage II crack propagation quickly and effectively, magneto acoustic emission (MAE) measurement was carried out on laboratory specimens with different numbers of fatigue cycles. With the accumulation of fatigue damage, the RMS of MAE decrease steadily on the whole, making MAE a promising non-destructive method for evaluating fatigue damage. To make MAE applicable in noisy environments, square waveform voltage were selected to excite magnetic fields, and 'T' type MAE signals with higher amplitude were produced. The variation of MAE with number of fatigue cycles at different excitation intensity indicated that the defects associated with fatigue damage have greater effects on the creation and annihilation of domain walls. The point where the MAE amplitude begins to increase instead of decrease with fatigue can be an indicator for the onset of stage II crack propagation. This article is part of the theme issue 'Advanced electromagnetic non-destructive evaluation and smart monitoring'.

Gravitational waves from cosmic domain walls: a mini-review

Domain walls are sheet-like topological defects produced when a discrete symmetry is spontaneously broken in the early universe. Although the existence of stable domain walls is disfavored by cosmological considerations, it is possible to consider unstable domain walls which disappear early enough not to lead cosmological disasters. In this contribution, we discuss the possibility that a significant amount of gravitational waves is produced by annihilation of such unstable domain walls in the early universe. After reviewing cosmological evolution of domain walls, we give an estimate of the expected gravitational wave signal based on the results of numerical simulations. In addition, we briefly review some well-motivated particle physics models that predict the formation of unstable domain walls. The detectability of predicted signals is also discussed in prospect of planned gravitational wave observatories.

Magnetoresistance Effects in the Metallic Antiferromagnet Mn2Au

In antiferromagnetic spintronics, it is essential to separate the resistance modifications of purely magnetic origin from other effects generated by current pulses intended to switch the N\'eel vector. We investigate the magnetoresistance effects resulting from magnetic field induced reorientations of the staggered magnetization of epitaxial antiferromagnetic Mn2Au(001) thin films. The samples were exposed to 60 T magnetic field pulses along different crystallographic in-plane directions of Mn2Au(001), while their resistance was measured. For the staggered magnetization aligned via a spin-flop transition parallel to the easy [110]-direction, an ansiotropic magnetoresistance of -0.15 % was measured. In the case of a forced alignment of the staggered magnetization parallel to the hard [100]-direction, evidence for a larger anisotropic magnetoresistance effect was found. Furthermore, transient resistance reductions of about 1 % were observed, which we associate with the annihilation of antiferromagnetic domain walls by the magnetic field pulses.

Facilitating domain wall injection in magnetic nanowires by electrical means

We investigate how to facilitate the injection of domain walls in chiral ferromagnetic nanowires by electrical means. We calculate the critical current density above which domain walls are injected into the nanowire depending on the material parameters and the source of interaction including spin-transfer torques as well as spin-orbit torques. We demonstrate that the Dzyaloshinskii-Moriya interaction can significantly reduce the required critical current to inject the types of domain walls favored by the Dzyaloshinskii-Moriya interaction. We find that in chiral magnets it is only possible to shed a single domain wall by means of spin-orbit torques, as they modify the ground state orientation of the system. In contrast, for spin-transfer-torque induced shedding of domain walls, we show that there exist two different critical current densities for the two different domain wall chiralities, respectively. Additionally, for the consecutive creation of domain walls by means of spin-transfer torques, we find that the interaction between the domain walls cannot be neglected and even may lead to the pairwise annihilation of consecutive domain walls with opposite chiralities.

Entanglement and dynamics of diffusion-annihilation processes with Majorana defects

Coupling a many-body system to a thermal environment typically destroys the quantum coherence of its state, leading to an effective classical dynamics at the longest time scales. We show that systems with anyon-like defects can exhibit universal late-time dynamics that is stochastic, but fundamentally non-classical, because some of the quantum information about the state is topologically protected from the environment. Our coarse-grained model describes one-dimensional systems with domain-wall defects carrying Majorana modes. These defects undergo Brownian motion due to coupling with a bath. Since the fermion parity of a given pair of defects is nonlocal, it cannot be measured by the bath until the two defects happen to come into contact. We examine how such a system anneals to zero temperature via the diffusion and pairwise annihilation of Majorana defects, and we characterize the nontrivial entanglement structure that arises in such stochastic processes. Separately, we also investigate simplified "quantum measurement circuits" in one or more dimensions, involving repeated pairwise measurement of fermion parities for a lattice of Majoranas. The dynamics of these circuits can be solved by exact mappings to classical loop models. They yield analytically tractable examples of measurement-induced phase transitions, with critical entanglement structures that are governed by nonunitary conformal fixed points. In the system of diffusing and annihilating Majorana defects, the relaxation to the ground state is analogous to coarsening in a classical 1D Ising model via domain wall annihilation. Here, however, configurations are labeled not only by the defect positions but by a nonlocal entanglement structure. The resulting dynamical process is a new universality class for the coarsening of topological domain walls, whose universal properties can be obtained from an exact mapping.

Lateral domain wall oscillations in IMA/PMA bilayered nano-strips driven by a perpendicular current: A type of domain wall based oscillators

Spin transfer torque-driven dynamic properties of spins in a bilayered nano-strip system composed of a layer with in-plane magnetic anisotropy (IMA) and a layer with perpendicular magnetic anisotropy (PMA) were studied using a micro-magnetic simulation method. It is demonstrated that a domain wall (DW) can nucleate in the IMA layer at the end of the nano-strips and propagate with a large speed toward the other end under a perpendicular current. With DW annihilation and nucleation at either end of the nano-strips, their back and forth motion induces a magnetization oscillation whose amplitude and frequency can be easily manipulated by the current density, the dimensions and material of the nano-strip, and a perpendicularly applied magnetic field. Considering that the oscillation amplitude can be greatly amplified by the simultaneous oscillations of a large number of nano-strips in an array and the simple structure of the each element, this kind of IMA/PMA bilayered nano-strip may have important applications in magnetic nano-oscillators.

Heavy gravitino from dynamical generation of right-handed neutrino mass scale, and gravitational waves

The absence of supersymmetric particles at the weak scale is a puzzle if supersymmetry solves the gauge hierarchy problem. We show that, if the right-handed neutrino masses arise from hidden gaugino condensation via GUT-suppressed operators, successful thermal leptogenesis leads to the lower bound on the gravitino mass, which may explain the little hierarchy problem. If domain walls associated with the gaugino condensation are formed after inflation, they annihilate when H∼m3/2. We address the possibility that observable gravitational wave signals might emerge from the domain wall annihilation.

Electric Control of Superconducting Domains

Researchers have proposed the nucleation and annihilation of chiral domain walls in a topological p-wave superconductor controlled by an electric current.

Energy Losses in Composite Materials Based on Two Ferromagnets

We focused on the low and high induction loss separation in soft magnetic composites (SMC), which determine the regions of different predominant magnetization processes. These are the low induction losses which are related to domain wall displacements and the high induction losses related to magnetization vector rotation, domain wall annihilation, and nucleation. The composite samples prepared by replacing of Somaloy 700 (polycrystalline iron powder covered by insulating layer) with Vitrovac 6155 (amorphous co-based alloy) have been investigated. We found out that the porosity remained constant up to 5 wt.% of Vitrovac fraction, which led to slight improvement of magnetic properties of the resulting SMC-the maximum permeability increased and the magnetization process was confirmed to be realized to a greater extent by the processes of domain wall displacement, up to a higher magnetic induction. The higher inner demagnetizing fields caused by higher porosity in sample with 20 wt.% of Vitrovac resulted in lower mobility of domain walls, exhibiting by higher energy losses and lower maximum permeability.