Formation Of Magnetic Domains Research Articles

The Diamagnetic Phase Transition of Dense Electron Gas: Astrophysical Applications

Neutron stars are ideal astrophysical laboratories for testing theories of the de Haas–van Alphen effect and diamagnetic phase transition which is associated with magnetic domain formation. The “magnetic interaction” between delocalized magnetic moments of electrons (the Shoenberg effect), can result in an effect of the diamagnetic phase transition into domains of alternating magnetization (Condon's domains). Associated with the domain formation are prominent magnetic field oscillation and anisotropic magnetic stress which may be large enough to fracture the crust of magnetar with a super-strong field. Even if the fracture is impossible as in “low-field” magnetar, the depinning phase transition of domain wall (DW) motion driven by low field rate (mainly due to the Hall effect) in the randomly perturbed crust can result in a catastrophically variation of magnetic field. This intermittent motion, similar to the avalanche process, makes the Hall effect be dissipative. These qualitative consequences about magnetized electron gas are consistent with observations of magnetar emission, and especially the threshold critical dynamics of driven DW can partially overcome the difficulties of “low-field” magnetar bursts and the heating mechanism of transient, or “outbursting” magnetar.

Influence of radial and tangential anisotropy components in single wall magnetic nanotubes. A Monte Carlo approach

Abstract Magnetic behaviour of nanotubes with square cell has been studied by the Monte Carlo Method, under the Metropolis algorithm and Heisenberg model. The Hamiltonian used includes nearest neighbour exchange interaction and radial and tangential direction for uniaxial anisotropy. Periodic boundary conditions were implemented at the sample’s edges. Simulations were carried out varying the nanotube’s diameter by changing the number of magnetic moments per ring and anisotropy values. Two transition temperatures were identified corresponding to states where moments were aligned as either ferromagnetic type or anisotropy direction. At low temperatures and low anisotropy values, the system exhibited a ferromagnetic alignment; as the anisotropy was increased, and continued in the low temperature range, spins were aligned in the anisotropy (radial or tangential) direction. As the temperature was increased, spins were reoriented in the ferromagnetic direction, generating a radial (tangential) anisotropy to ferromagnetic transition temperature. When the temperature continued increasing, the system transited toward the paramagnetic phase, appearing a ferromagnetic to paramagnetic transition phase temperature. In several cases studied here, between these two transition temperatures (anisotropy to ferromagnetic and ferromagnetic to paramagnetic transition phases), the magnetization of the system exhibited instabilities. These instabilities are caused because of the influence of the anisotropy values and the diameter of the nanotubes on the magnetic domains formation. As a consequence, there exist anisotropy values and diameters where metastable states were formed.

Domain size criterion for the observation of all-optical helicity-dependent switching in magnetic thin films

To understand the necessary condition for the observation of all-optical helicity-dependent switching (AO-HDS) of magnetization in thin films, we investigated ferromagnetic Co/Pt and Co/Ni multilayers as well as ferrimagnetic TbCo alloys as a function of magnetic layer compositions and thicknesses. We show that both ferro-and ferrimagnets with high saturation magnetization show AO-HDS if their magnetic thickness is strongly reduced below a material-dependent threshold thickness. By taking into account the demagnetizing energy and the domain wall energy, we are able to define a criterion to predict whether AO-HDS or thermal demagnetization (TD) will be observed. This criterion for the observation of AO-HDS is that the equilibrium size of magnetic domains forming during the cooling process should be larger than the laser spot size. From these results we anticipate that more magnetic materials are expected to show AO-HDS. However, the effect of the optical pulses' helicity is hidden by the formation of small magnetic domains during the cooling process.

Birdsong dialect patterns explained using magnetic domains.

The songs and calls of many bird species, like human speech, form distinct regional dialects. We suggest that the process of dialect formation is analogous to the physical process of magnetic domain formation. We take the coastal breeding grounds of the Puget Sound white crowned sparrow as an example. Previous field studies suggest that birds of this species learn multiple songs early in life, and when establishing a territory for the first time, retain one of these dialects in order to match the majority of their neighbors. We introduce a simple lattice model of the process, showing that this matching behavior can produce single dialect domains provided the death rate of adult birds is sufficiently low. We relate death rate to thermodynamic temperature in magnetic materials, and calculate the critical death rate by analogy with the Ising model. Using parameters consistent with the known behavior of these birds we show that coastal dialect domain shapes may be explained by viewing them as low-temperature "stripe states."

Phase-field simulations of partial pseudoelastic stress-strain behavior and microstructure evolution of Ni-Mn-Ga

The partial pseudoelastic stress-strain behavior and the underlying microstructure evolution of Ni-Mn-Ga ferromagnetic shape memory alloy under different values of constant (assisted) magnetic field H and demagnetization factor N are studied by phase-field simulations. The results show that both H and Nfield (component of N parallel to H) have significant influence on the partial strain recovery in the compressive stress loading-unloading cycle, while Nstress (component of N parallel to the stress) only affects the formation of magnetic domains in the stress-preferred variant and has no influence on the strain recovery behavior. The recovered strain can be increased by raising H or reducing Nfield. Moreover, the range of H for the partial strain recovery increases with Nfield. It is shown that Ni-Mn-Ga exhibits the partial pseudoelasticity at moderate/high H under a large Nfield rather than just at low H as observed in existing experiments, demonstrating the significant influence of Nfield on the strain recovery behavior. The work reveals that both H and Nfield affect the strain recovery behavior through the effective magnetic field *Hfield (component of the internal magnetic field parallel to H) during the stress cycle. *Hfield decreases with the twin boundary motion during the reverse martensite reorientation and eventually becomes insufficient to trigger the twin boundary motion, which is responsible for the partial strain recovery.

Kitaev anisotropy induces mesoscopicZ2vortex crystals in frustrated hexagonal antiferromagnets

The triangular-lattice Heisenberg antiferromagnet (HAF) is known to carry topological Z_2 vortex excitations which form a gas at finite temperatures. Here we show that the spin-orbit interaction, introduced via a Kitaev term in the exchange Hamiltonian, condenses these vortices into a triangular $Z_2$ vortex crystal at zero temperature. The cores of the Z_2 vortices show abrupt, soliton-like magnetization modulations and arise by a special intertwining of three honeycomb superstructures of ferromagnetic domains, one for each of the three sublattices of the 120-degree state of the pure HAF. This is a new example of a nucleation transition, analogous to the spontaneous formation of magnetic domains, Abrikosov vortices in type-II syperconductors, blue phases in cholesteric liquid crystals, and skyrmions in chiral helimagnets. As the mechanism relies on the interplay of geometric frustration and spin-orbital anisotropies, such vortex mesophases can materialize as a ground-state property in spin-orbit coupled correlated systems with nearly hexagonal topology, as in triangular or strongly frustrated honeycomb iridates.

Formation of tree-like and vortex magnetic domains of nanocrystalline α-(Fe,Si) in La–Fe–Si ribbons during rapid solidification and subsequent annealing

The characteristic magnetic domains at the surfaces of melt-spun La–Fe–Si ribbons under different thermal processing conditions are explored using room-temperature Lorentz force microscopy and electron holography. In as-quenched ribbons, the magnetic domain structure is found to change from a tree-like morphology on the surface far from the copper wheel during quenching to a vortex-type domain structure on the surface in contact with the wheel. In the initial stages of annealing, domains on both surfaces developed a vortex behavior. Detailed microstructural observations demonstrated nanocrystalline α-(Fe,Si) regions embedded in the majority La(Fe,Si)13 phase due to a partially completed peritectoid reaction between α-(Fe,Si) and LaFeSi. The room temperature magnetic domains disappeared along with the residual α-(Fe,Si) upon extended heat treatment, yielding a homogeneous La(Fe,Si)13 material. Magnetometry measurements and analysis reveal the absence of strong coupling between the α-(Fe,Si) and La(Fe,Si)13 phases, so that the presence of α-(Fe,Si) nanocrystallites with a vortex-type structure does not favor an enhanced magnetocaloric response in the ribbons.

Spin paramagnetic deformation of a neutron star

Quantum mechanical corrections to the hydromagnetic force balance equation, derived from the microscopic Schr\"{o}dinger-Pauli theory of quantum plasmas, modify the equilibrium structure and hence the mass quadrupole moment of a neutron star. It is shown here that the dominant effect --- spin paramagnetism --- is most significant in a magnetar, where one typically has $\mu_{B}|\boldsymbol{B}|\gtrsim k_B T_e$, where $\mu_{B}$ is the Bohr magneton, $\boldsymbol{B}$ is the magnetic field, and $T_e$ is the electron temperature. The spin paramagnetic deformation of a nonbarotropic magnetar with a linked poloidal-toroidal magnetic field is calculated to be up to ${{\sim 10}}$ times greater than the deformation caused solely by the Lorentz force. It depends on the degree of Pauli blocking by conduction electrons and the propensity to form magnetic domains, processes which are incompletely modelled at magnetar field strengths. The star becomes more oblate, as the toroidal field component strengthens. The result implies that existing classical predictions underestimate the maximum strength of the gravitational wave signal from rapidly spinning magnetars at birth. Turning the argument around, future gravitational-wave upper limits of increasing sensitivity will place ever-stricter constraints on the physics of Pauli blocking and magnetic domain formation under magnetar conditions.

Evolution of magnetic domain structure formed by ion-irradiation of B2-Fe0.6Al0.4.

Magnetic domains and magnetization reversal in 40 nm thick films of Fe0.6Al0.4, have been studied by longitudinal magneto-optical Kerr effect. By varying the Ne(+) ion-energy E between 2 and 30 keV (keeping a constant fluence), we varied the depth-penetration of the ions, and thereby influenced the homogeneity of the induced saturation magnetization M(s). The dependence of coercivity on ion energy shows maximum for 5 keV Ne(+). Considerable differences in the magnetic domain formation and magnetization reversal processes were observed: at low E (≤ 5keV), the reversal process is dominated by domain nucleation mechanism (high density of domain nucleation sites), consistent with the occurrence of an inhomogeneous M(s). Films irradiated with E > 5keV ions exhibit significantly low domain nucleation density, and the reversal is dominated by domain propagation mechanism, suggesting homogeneity in induced M(s). These results demonstrate the tunability of magnetization reversal behavior in materials possessing disorder induced magnetic phase transitions.

Perpendicular magnetisation from in-plane fields in nano-scaled antidot lattices

Investigations of geometric frustrations in magnetic antidot lattices have led to the observation of interesting phenomena like spin-ice and magnetic monopoles. By using highly focused magneto-optical Kerr effect measurements and x-ray microscopy with magnetic contrast we deduce that geometrical frustration in these nanostructured thin film systems also leads to an out-of-plane magnetization from a purely in-plane applied magnetic field. For certain orientations of the antidot lattice, formation of perpendicular magnetic domains has been found with a size of several μm that may be used for an in-plane/out-of-plane transducer.

Performance Boost of Spiral Inductors With Thickness-Controlled Domain-Patterned Permalloy

In CMOS radio frequency integrated circuits, inductors are a barrier in achieving better monolithic solutions because of the large area occupied and the associated parasitics. For better performance of on-chip inductors, soft magnetic materials are incorporated with them. In an earlier work, it was shown that by tailoring the shape and size of the permalloy patterns, formation of magnetic domains can be controlled. When such domain patterns are incorporated with inductors, they have varied effects on inductor performance. In this paper, it is demonstrated that by depositing 60 nm thick domain-controlled permalloy patterns, better performance of inductors can be obtained at higher frequencies (>5 GHz). About 2× increment in inductance and 7× increment in quality factor is obtained at frequencies >5 GHz by incorporating appropriate domain-controlled permalloy patterns with inductors.

Imaging of dynamic magnetic fields with spin-polarized neutron beams

Precession of neutron spin in a magnetic field can be used for mapping of a magnetic field distribution, as demonstrated previously for static magnetic fields at neutron beamline facilities. The fringing in the observed neutron images depends on both the magnetic field strength and the neutron energy. In this paper we demonstrate the feasibility of imaging periodic dynamic magnetic fields using a spin-polarized cold neutron beam. Our position-sensitive neutron counting detector, providing with high precision both the arrival time and position for each detected neutron, enables simultaneous imaging of multiple phases of a periodic dynamic process with microsecond timing resolution. The magnetic fields produced by 5- and 15-loop solenoid coils of 1 cm diameter, are imaged in our experiments with ∼100 μm resolution for both dc and 3 kHz ac currents. Our measurements agree well with theoretical predictions of fringe patterns formed by neutron spin precession. We also discuss the wavelength dependence and magnetic field quantification options using a pulsed neutron beamline. The ability to remotely map dynamic magnetic fields combined with the unique capability of neutrons to penetrate various materials (e.g., metals), enables studies of fast periodically changing magnetic processes, such as formation of magnetic domains within metals due to the presence of ac magnetic fields.

Magnetotransport Properties of Perpendicular [Pt/Co]/Cu/[Co/Pt] Pseudo-Spin-Valves

We studied the giant magnetoresistance (GMR) effect in [Co/Pt]4/Co/Cu/Co/[Co/Pt]4 pseudo-spin-valves with perpendicular magnetic anisotropy (PMA) and analyzed the impact of the Cu spacer layer thickness as well as the Co layer thickness at the Cu/Co interface. The magnetotransport measurements were carried out by a four-point probe method in current-in-plane geometry at room temperature and additionally by the van der Pauw method at low temperatures. The GMR ratio at room temperature can be almost doubled to $\sim 1.5$ % by increasing the Co layer thickness to 10 A at each side of the Cu spacer layer, while keeping all other thicknesses constant. Due to this relatively large single Co layer thickness, which reduces the perpendicular magnetic anisotropy, the magnetic reversal is driven by magnetostatic interactions, leading to the formation of vertically correlated magnetic domains. In addition, by tuning the PMA of both [Co/Pt] multilayers, the formation of magnetic domains in the soft layer can be achieved without affecting the hard layer, which stays uniformly magnetized, resulting in a GMR ratio of up to 1.6% at room temperature. Upon formation of vertically correlated domains, the GMR ratio will be reduced again.

The Formation of Magnetic Flux Domain in the Type-II Superconductors with Ferromagnetic Defects

The magnetization of 2D high-temperature superconductors with internal ferromagnetic defects under application of transport current and external dc magnetic field was studied by using Monte Carlo method. The current-voltage characteristics were obtained. The S-type nonlinearity of current-voltage characteristics of the superconductor/ferromagnet system in external magnetic field due to the local reversal magnetization of magnetic particles by the field of vortices was analyzed in detail. The formation of magnetic flux domains and the wave of defects magnetization during vortex flow were demonstrated.

Co/Pd]磁性細線における磁区の形成・駆動・検出

Co/Pd]磁性細線における磁区の形成・駆動・検出

Origin of hysteretic magnetoelastic behavior in magnetoelectric 2-2 composites

The local magnetization behavior of the magnetostrictive phase of ferromagnetic/piezoelectric magnetoelectric composites is compared to the hysteretic response using advanced magneto-optical imaging. Local magnetoelastic relaxation leads to the formation of magnetization modulated branched domain structures in the magnetic phase. This results in a complex field response governed by interlocking domain processes. An interrelation of magnetic domain formation and the piezomagnetic response is derived, revealing the origin of the hysteretic magnetoelectric response. As a result, domain wall induced effects lead to a reduction of magnetoelectric signal. Controlling the magnetic domain formation processes is the foundation for reversible magnetoelectric behavior.

Effect of temperature on the formation of dynamic magnetic domains in ferrite-garnet films

Dynamic domain structures in ferrite-garnet films (111) with normal anisotropy are investigated for the temperature range of 77–300 K. The boundaries of the area of the existence of one- and two-armed spiral domains are determined as a function of the temperature, frequency, and amplitude of the vertical field. It is found that the region of the existence of spiral dynamic domains shift toward low frequencies when the temperature is reduced.

Polarization Domain Formation and Domain Dynamics in a Quasi-Isotropic Cavity Fiber Laser

Polarization resolved study on the operation of a quasi-isotropic cavity erbium-doped fiber laser revealed a novel form of periodic fast polarization switching between the two orthogonal linearly polarized eigen-modes of the laser. We show both experimentally and theoretically that the operation of the laser is a result of the polarization domain formation, an effect in analog to the magnetic domain formation in ferromagnetic materials. Features of the polarization domains have been experimentally studied and compared with results of theoretical simulations. It is shown that the polarization domain formation is a general feature of the quasi-isotropic cavity fiber lasers, which could be exploited for controllable ultrafast polarization switching light sources for many photonic applications.

Magnetic Domain Control of Exchange Biased Magneto-Electric Thin Film Composites

Magneto-electric composites constitute a promising class of materials for the application as highly sensitive magnetic field sensors. Their compatibility to miniaturization, their passive nature and their large dynamic range are just some of the numerous advantages of these composites compared to alternative magnetic field sensors, as e.g. flux-gates or magneto-resistive sensors. However, the highest sensitivities are observed in the presence of well-defined magnetic bias fields that are of significant disadvantage in terms of miniaturization and noise performance.Self-biased magneto-electric composites, which are fabricated using the exchange bias effect [1], exhibit an increased total anisotropy in comparison to systems without exchange bias. As a consequence, small exchange bias fields are favourable because of a minor reduction of the magneto-electric voltage coefficient. However, weakly biased magneto-electric composites might loose their self-biasing properties and possibly show an increase of discontinuities in magnetization reversal due to the formation of magnetic domains. By a thickness variation of the ferromagnetic layer a maximum voltage coefficient αME ≈ 430 V/cmOe was found for a magnetostrictive multilayer of 3 x (5 nm Ta / 3 nm Cu / 8 nm Mn-Ir / 333 nm Fe-Co-Si-B). Yet, a stable single domain state indicating a well defined magnetization reversal by coherent magnetization rotation was achieved for layer thicknesses up to 100 nm Fe-Co-Si-B with a slightly reduced magneto-electric cofficient of αME ≈ 340 V/cmOe. This slight reduction is overcompensated by the improved control of the magnetic domain pattern that is highly desirable for magnetic field sensors and of special importance when frequency conversion techniques are applied [2].[1] Lage, E., Kirchhof, C., Hrkac, V., Kienle, L., Jahns, R., Knöchel, R., Quandt, E. & Meyners, D. Exchange biasing of magnetoelec¬tric composites. Nature materials 11, 523–9 (2012).[2] Jahns, R., Greve H., Woltermann, E., Quandt, E. & Knöchel, R. Sensitivity enhancement of mag¬netoelectric sensors through frequency-conversion. Sensors and Actuators A: Physical 183, 16-21 (2012).This work was funded by the German Research Foundation (DFG) as part of the Collaborative Research Center 855 “Magnetoelectric Composites–Future Biomagnetic Interfaces”

Giant magnetic domains in amorphous SmCo thin films

The potential for tuning of magnetic properties and the exceptional uniformity are among the features that make amorphous magnetic materials attractive for technology. Here it is shown that the magnetization reversal in amorphous SmCo thin films takes place through the formation of giant magnetic domains, over a centimeter across. The domain structure is found to be dictated by the direction of the imprinted in-plane easy axis and the film boundaries. This is a consequence of the size of the anisotropy and the structural uniformity of the films, which also allows the movement of millimeter-long domain walls over distances of several millimeters. The results demonstrate the possibility of tailoring the magnetic domain structure in amorphous magnets over a wide range of length scales, up to centimeters. Moreover, they highlight an important consequence of the structural perfection of amorphous films.