Influence of an axial field on the movement of the boundary between circular domains in stress annealed Co-Fe-Si-B amorphous wire

The influence of an axial field on the velocity of a single boundary between two circular domains (CDW) was studied in a stress annealed amorphous Co_68.2 Fe_4.3 Si_12.5 B₁₅ wire. The velocity of the CDW depends on the magnitude and orientation of the axial field and it is also different for parallel and anti-parallel direction with respect to the current flow. At lower currents the observed behavior can be explained as a combined effect of helical anisotropy (with a very small deviation of the easy axis from the ideal circular direction) and of non-diagonal components of the resistivity tensor (the so called “domain drag effect”).     

Mobility of the boundary between circular domains in stress annealed CoFeSiB amorphous wire

Dynamics of a single boundary (circular domain wall: CDW) between two circular domains was studied in stress annealed amorphous Co_68.2Fe_4.3Si_12.5B₁₅ wire. The experimental procedure, in which the displacement of the single CDW is measured after sending a pulse of a constant current, was proposed. The linear dependence of the velocity of the CDW versus circular field was observed. Eddy currents around a single CDW moving at constant velocity were calculated and a proportionality factor (CDW mobility) between velocity and circular field was derived. A good agreement between experimental and theoretical value of the CDW mobility was obtained.     

Evaluation of helical magnetoelastic anisotropy in Fe-based amorphous wire from the decomposed susceptibility spectra

A novel method is introduced here to evaluate the intrinsic magnetic anisotropy from susceptibility spectra. The combination of two techniques namely, the decomposition of susceptibility spectra together with its dependence on applied torsion is employed to determine quantitatively the intrinsic helical anisotropy in an amorphous wire. The susceptibility spectra of Fe_77.5Si_7.5B₁₅ amorphous wires have been experimentally measured as a function of torsion. The reversible susceptibility is ascribed to wall motion of axial domains within the core, and to magnetization rotation within radial domains in the shell. The relaxation frequencies of these magnetization processes are evaluated to be 0.36 and 1.82 MHz, respectively. The static rotational susceptibility shows an asymmetric behavior with regards to positive and negative torsion angles, and is maximum at the torsion angle of 30°, which counterbalances the internal stress. Present results indicate the existence of an intrinsic helical anisotropy corresponding to an average helical stress of 33 MPa.     

DC magnetization processes in bistable glass-coated ferromagnetic microwires

Interaction of a single magnetic domain wall with an inhomogeneous magnetic field and distribution of local nucleation fields along glass-coated Fe_77.5−xNi_xB₁₅Si_7.5 (x=0, 27.9, 34.9) microwires were experimentally studied. It was shown that the wall separating two axial domains and moving along the wire can be stopped by an inhomogeneous magnetic field. The wall remains stable and trapped in a local potential minimum after external fields are switched off. Wall coercivity increases with Ni content. For all samples the minimum of critical field for axial magnetization reversal was observed near the end of the wire. For samples with non-zero Ni content a distribution of nucleation fields lower than 950 A/m was observed in regions far enough from the wire ends. In Ni-free samples the nucleation fields were higher than 950 A/m.     

Stress dependence of the domain wall dynamics in the adiabatic regime

In this work, we determine the domain wall velocity in the low field region and study the domain dynamics in as-cast and annealed bi-stable amorphous glass-covered Fe_77.5Si_7.5B₁₅ microwires. In particular, from the relation between the domain wall velocity and magnetic field in the adiabatic regime, the power-law critical exponent β, the critical field H₀ and the domain wall damping η were obtained. It has been verified that the main source of domain wall damping is the eddy current and spin relaxation, both with a strong relation with the magnetoelastic energy. This energy term is changed by the axial applied stress, which, by its time, modifies the damping mechanisms. It was also verified that the domain wall damping terms present different behavior at low (mainly eddy currents) and high applied stress (spin relaxation).     

Velocity and profile of the boundary between circular domains in cylindrical ferromagnetic samples

The velocity of the boundary between circular domains driven by DC current in a ferromagnetic cylindrical sample (wire) and its profile were calculated up to the first order approximation. The formula for critical current above which the curvature of the CDW can no longer compensate field variations was derived in this approximation. Its value is not more than a few milliamperes for typical stress annealed amorphous wires. Taking into account wall distortion the velocity versus current (circular field) dependence deviates from the linear one and for higher currents (i.e. for higher degree of wall distortion) velocity is higher than for planar CDW.     

Linear intermode conversion of electromagnetic wave energy in a magnetically soft gyrotropic material

The excitation of axial radio-frequency (rf) magnetic induction by an axial rf current is observed in a conductor with circular magnetic anisotropy when a weak magnetizing field is applied. The conductor is an amorphous cobalt-based wire, which exhibits azimuthal magnetic anisotropy. It serves as the central conductor in a coaxial line. The axial rf magnetic induction produces an emf in an induction coil coaxial to the conductor. The induction coil is part of a matched receiving circuit. The power conversion coefficient is as high as tens of percent. The measurements demonstrate the high sensitivity of the conversion coefficient to an external field. The theory of ferromagnetic resonance faithfully describes the results of the observations. Zh. Tekh. Fiz. 69, 58–63 (March 1999)     

Controlled domain wall motion by current into patterned-U Ni80Fe20 wires

The current-induced domain wall motion was observed experimentally in the case of the domain wall trapped at the semicircular arc within the U shape Ni₈₀Fe₂₀ wire. The measurement of the current-induced domain wall motion was achieved by adding a biased field before switching field and a critical current density was measured. We found two magnetic domain structures in the U pattern. At zero fields, the vortex domain wall nucleated at the semicircular arc of the U pattern. Continuous magnetic state without wall was investigated in near-switching field.     

The effect of domain wall motion on the nondiagonal impedance component in amorphous wires with circular magnetic anisotropy

The effect of the domain wall motion on the nondiagonal impedance component in amorphous ferromagnetic wires with circular anisotropy was theoretically studied. The frequency spectrum of the electromotive force (emf) induced in the pick-up coil wound around the wire is analyzed. For sufficiently small amplitudes of the ac current passing in the wire, the emf frequency equals doubled frequency of the current. For the ac current amplitudes exceeding certain threshold value, all even harmonics appear in the frequency spectrum of the signal. The emf strongly depends on the longitudinal component of the applied magnetic field at any value of the ac current amplitude. These results can be important for developing frequency transducers controlled by magnetic field.     

The Analysis of Large Barkhausen Effect in the FeSiB Amorphous Wire

Amorphous FeSiB wires with positive magnetostriction are very perspective soft magnetic materials for many applications, e.g. torque, field or current sensors, pulse generators and highly sensitive magnetometers. The appearance of the Large Barkhausen Effect (LBE) during slow magnetization of FeSiB wires is described by means of the core-shell model assuming a residual radial tensile stresses in the as-cast state. In this work, the LBE during magnetization reversal of Fe_77.5Si_7.5B₁₅ amorphous wire in the as-cast state was analysed. We have studied the kinetics of the reverse domain in the core region of the wire by means of Sixtus-Tonks method of two small pick-up coils placed in an asymmetric way with respect to the ends of the wire. We estimated the velocity of the reverse domain wall and the core region volume of the wire. It was found that the residual radial tensile stress distribution of the shell region strongly influences the magnetization reversal in the FeSiB wire.     

Thickness dependence of magnetic domain formation in La0.6Sr0.4MnO3 epitaxial thin films studied by XMCD-PEEM

We have used photoelectron emission microscopy (PEEM) and X-ray magnetic circular dichroism (XMCD) to study the effect of thin film thickness on the magnetic domain formation in La_0.6Sr_0.4MnO₃ samples that were epitaxially grown on stepped SrTiO₃ (0 0 1) substrates. The magnetic image exhibited a stripe structure elongated along the step direction, irrespective of film thickness, suggesting that uniaxial magnetic anisotropy induced by step-and-terrace structures plays an important role in the magnetic domain formation. Additional domains evolved gradually with increasing film thickness. In these domains, the direction of magnetization differed from the step direction due to biaxial magneto-crystalline anisotropy. The evolution of additional magnetic domains with increasing film thickness implies that a competition exists between the two anisotropies in LSMO films.     

Current-induced domain wall motion with adiabatic and nonadiabatic spin torques in magnetic nanowires

We investigate current-driven domain wall (DW) propagation in magnetic nanowires in the framework of the modified Landau-Lifshitz-Gilbert equation with both adiabatic and nonadiabatic spin torque (AST and NAST) terms. By employing a simple analytical model, we can demonstrate the essential physics that any small current density can drive the DW motion along a uniaxial anisotropy nanowire even in absence of NAST, while a critical current density threshold is required due to intrinsic anisotropy pinning in a biaxial nanowire without NAST. The DW motion along the uniaxial wire corresponds to the asymptotical DW oscillation solution under high field/current in the biaxial wire case. The current-driven DW velocity weakly depends on the NAST parameter β in a uniaxial wire and it is similar to the β = α case (α: damping) in the biaxial wire. Apart from that, we discuss the rigid DW motion from both the energy and angular momentum viewpoints and point out some physical relations in between. We also propose an experimental scheme to measure the spin current polarization by combining both field- and current-driven DW motion in a usual flat (biaxial) nanowire.     

Fast domain wall dynamics in amorphous and nanocrystalline magnetic microwires

We have studied the effect of thermal treatment on the domain wall dynamics of FeSiB and FeCoMoB microwires. It was shown that annealing in transversal magnetic field increases the domain wall mobility as well as the domain wall velocity. Annealing under the tensile stress hinders the appearance of the monodomain structure but application of tensile stress leads to the magnetic bistability having the domain wall mobility twice higher that in as-cast state. Further increase of the tensile stress reduces the domain wall mobility but the domain wall velocity increases as a result of the decrease of critical propagation field. Annealing of the FeCoMoB microwire by Joule heating leads to introduction of the circular anisotropy that favors the vortex domain wall. Such treatment increases the domain wall mobility as well as the maximum domain wall velocity.     

The defects influence on domain wall propagation in bistable glass-coated microwires

We studied the domain wall (DW) dynamics of magnetically bistable amorphous glass-coated Fe₇₄B₁₃Si₁₁C₂ microwires. In according to our experimental results magnetic field dependences of DW velocity of studied microwires can be divided into two groups: with uniform or uniformly accelerated DW propagation along the microwire. Strong correlation between the type of the magnetic field dependence of domain wall velocity, v(H), and the distribution of the local nucleation fields has been observed.Moreover, we observed abrupt increasing of DW velocity (jump) on the magnetic field dependences of the domain wall velocity, v(H), for the both types of the v(H) dependences. At the same time usual linear increasing of the domain wall velocity with magnetic field persists below these jumps. It was found that the jump height correlates with the location of nucleation place of the new domain wall. We have measured local nucleation field distribution in all the microwires. From local nucleation field distribution we have obtained the DW nucleation locations and estimated the jump height     

Effect of a pulsed magnetic field on the nonlinear dynamics of vortexlike domain walls in magnetic films

The Landau-Lifshitz equation is numerically solved to study the nonlinear dynamic behavior of domain walls with the 2D vortexlike magnetization distribution in magnetically uniaxial films that have in-plane anisotropy and are exposed to a pulsed magnetic field. It is shown that a pulsed magnetic field H _p may induce transitions between various steady wall motions that differ in magnetization distribution. Solitary rectangular pulses, as well as a regular train of rectangular pulses, may be used to control the period of nonlinear dynamic transformations of the wall internal structure and the related period of variation of the wall velocity.     

Magneto-optical and micromagnetic simulation study of the current-driven domain wall motion in ferromagnetic (Ga,Mn)As

We have studied current-driven domain wall motion in modified Ga_0.95Mn_0.05As Hall bar structures with perpendicular anisotropy by using spatially resolved polar magneto-optical Kerr effect microscopy and micromagnetic simulation. Regardless of the initial magnetic configuration, the domain wall propagates in the opposite direction to the current with critical current of 1-2×10⁵ A/cm². Considering the spin-transfer torque term as well as various effective magnetic field terms, the micromagnetic simulation results are consistent with the experimental results. Our simulated and experimental results suggest that the spin-torque rather than Oersted field is the reason for current-driven domain wall motion in this material.     

Domin structure of (111) crystal plates of ferrite-garnets with uniaxial anisotropy

The influence of the magnetic field, thickness, and magnetic prehistory on the general form and quantitative parameters of domain structure (DS) of the (111) single-crystal plates of the ferrite-garnet (EuEr)₃(FeGa)₅O₁₂ is investigated. The field interval of stability of circular (conical) and annular domains is determined. It is shown that the DS of ferrite-garnet structures with induced uniaxial anisotropy reveals a series of features which may be explained by the influence of cubic anisotropy.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 57–61, March, 1982.     

Current distribution and giant magnetoimpedance in composite wires with helical magnetic anisotropy

The giant magnetoimpedance effect in composite wires consisting of a non-magnetic inner core and soft magnetic shell is studied theoretically. It is assumed that the magnetic shell has a helical anisotropy. The current and field distributions in the composite wire are found by means of a simultaneous solution of Maxwell equations and the Landau–Lifshitz equation. The expressions for the diagonal and off-diagonal impedance are obtained for low and high frequencies. The dependences of the impedance on the anisotropy axis angle and the shell thickness are analyzed. Maximum field sensitivity is shown to correspond to the case of the circular anisotropy in the magnetic shell. It is demonstrated that the optimum shell thickness to obtain maximum impedance ratio is equal to the effective skin depth in the magnetic material.     

Micromagnetic study of magnetic domain structure and magnetization reversal in amorphous wires with circular anisotropy

In this work we present a detailed numerical investigation on the magnetic domain formation and magnetization reversal mechanism in sub-millimeter amorphous wires with negative magnetostriction by means of micromagnetic calculations. The formation of circular magnetic domains surrounding a multidomain axially oriented central nucleus was observed for the micromagnetic model representing the amorphous wire. The magnetization reversal explained by micromagnetic computations for the M-H curve is described in terms of a combined nucleation-propagation−rotational mechanism after the saturated state. Results are interpreted in terms of the effective magnetic anisotropy.     

Domain structure and magnetization processes in permalloy propagation elements

A review of domain structures and magnetization processes in permalloy overlays is given, together with some new results. The simplest domain structure for a given element consists of a loop of magnetic flux, but in elements with irregular geometry the circulating flux is not constant. More complex structures arise when an element contains internal closure domains. In-plane anisotropy in permalloy affects the distribution of closure domains but with decreasing bar width the influence of anisotropy is reduced. Reversible wall motion in weak fields gives way to hysteresis effects when the applied field exceeds a certain level, H_s. In particular magnetization buckling may occur. Some details of buckling in asymmetric chevrons and half-discs are given and compared with the behaviour in an I-bar. The proximity of a bubble medium containing stripe domains is shown to reduce considerably the applied fields needed for buckling in overlay components. Following saturation, changes in the demagnetized state are usually apparent. On a simple level, the spin structure and polarity of Bloch walls is altered. More noticeably the wall pattern itself can change when closure domains are created or annihilated in pairs. The significance of these fluctuations for bubble propagation is assessed by considering the intrinsic stray field profile of a Bloch wall segment. A simple wall model is employed. It is demonstrated that a curved domain wall provides a reasonable basis for modelling the field of a magnetized bar up to saturation. Calculated values of H_s agree qualitatively with experiment. The external field of the bar is rather insensitive to the exact distribution of free-pole density. Together with the observed complexities of domain behabiour this reaffirms the validity of the continuum approach to modelling.