Short Magnetic Field Pulse Research Articles

Dynamics of Pulsed Remagnetization of Magnetically Uniaxial Nanoparticles

The response of the magnetic moment of a magnetically uniaxial nanoparticle and a planar array of such nanoparticles to a short Gaussian magnetic field pulse is studied in the presence and absence of its modulation. The periodic dependence of the response duration and the final orientation of magnetic moments on the pulse duration and its peak value are revealed and analyzed. The effect of the weak bias magnetic field and the pulse field deviation from the transverse orientation on remagnetization processes is studied. It was shown that the effect of the dipole–dipole interaction leads to modulation of the response to the pulse exposure.

Metal plate deformation under magnetic field pulse of complex shape

A dynamic deformation of sheet magnetic and nonmagnetic metal workpieces under a combination of a slowly varying primary magnetic field and a sequence of short magnetic field pulses with the opposite polarity is studied. This regime corresponds to backward electromagnetic forming. A fast motion of the workpiece induces an additional electromotive force in a discharge circuit and substantially alters a shape of the current pulse in an inductor. During the short opposite pulses, the Lorentz and magnetization components of the magnetic pressure on the magnetic workpiece demonstrate an unusual behavior, that is, the Lorentz pressure is attractive and the magnetization one is predominantly repulsive relative to the inductor. This phenomenon is discussed in terms of a two-dimensional axisymmetric problem.

The response of the magnetic nanoparticles lattice to a Gaussian magnetic field pulse

The paper investigates the response of homogeneous and binary planar lattices of magnetic nanoparticles with uniaxial anisotropy to the action of a short Gaussian magnetic field pulse. It is shown that the influence of the dipole–dipole interaction and the presence of two types of nanoparticles leads to modulation of the response amplitude and its decrease. A complex periodic dependence of the amplitude and duration of the system response on the duration of the pulse has been revealed. The research shows the possibility of a pulse-induced magnetization reversal of the entire lattice or its individual parts upon defining the lattice composition and the parameters of the bias field and the effective pulse. • The response of a plane lattice of uniaxial magnetic nanoparticles to a Gaussian magnetic field pulse. • The dynamics of lattices with two alternating types of nanoparticles with different magnetic moment. • The reversal magnetization of various parts of the magnetic nanoparticles lattice.

Stochastic Dynamics of a Ferromagnetic Vortex Revealed by Single-Spin Magnetometry

Fast manipulation of nanoscale magnetic textures, such as vortices and skyrmions, underlies devices proposed for next-generation magnetic memory, logic, and sensing. Understanding the switching processes in these structures requires a probe with both nanometer spatial resolution and nanosecond time resolution. This study combines time-resolved magneto-optical microscopy and single-spin magnetometry to unveil the switching of a magnetic vortex between pinning sites, in response to a short magnetic field pulse. The complex dependence of switching probability on pulse duration is explained, revealing a path toward the fast, reliable control required for future nanomagnetic technology.

Dynamic Imaging of the Delay- and Tilt-Free Motion of Néel Domain Walls in Perpendicularly Magnetized Superlattices.

We report on the time-resolved investigation of current- and field-induced domain wall motion in the flow regime in perpendicularly magnetized microwires exhibiting antisymmetric exchange interaction by means of scanning transmission X-ray microscopy with a 200 ps time step. The sub-ns time step of the dynamical images allowed us to observe the absence of incubation times for the motion of the domain wall within an uncertainty of 200 ps, together with indications for a negligible inertia of the domain wall. Furthermore, we observed that, for short current and magnetic field pulses, the magnetic domain walls do not exhibit a tilting during their motion, providing a mechanism for the fast, tilt-free, current-induced motion of magnetic domain walls.

Dynamically controlled energy dissipation for fast magnetic vortex switching

Manipulation of vortex states in magnetic media provides new routes towards information storage and processing technology. The typical slow relaxation times (∼100 ns) of magnetic vortex dynamics may present an obstacle to the realization of these applications. Here, we investigate how a vortex state in a ferromagnetic microdisk can be manipulated in a way that translates the vortex core while enhancing energy dissipation to rapidly damp the vortex dynamics. We use time-resolved differential magneto-optical Kerr effect microscopy to measure the motion of the vortex core in response to applied magnetic fields. We first map out how the vortex core becomes sequentially trapped by pinning sites as it translates across the disk. After applying a fast magnetic field step to translate the vortex from one pinning site to another, we observe long-lived dynamics of the vortex as it settles to the new equilibrium. We then demonstrate how the addition of a short (<10 ns) magnetic field pulse can induce additional energy dissipation, strongly damping the long-lived dynamics. A model of the vortex dynamics using the Thiele equation of motion explains the mechanism behind this effect.

Development of a 50-T pulsed magnetic field facility by using an 1.5-MJ capacitor bank

Because DC magnets consume a huge amount of electricity (resistive DC magnet) or liquid helium (superconducting magnet), a capacitor-bank-driven pulsed magnet is known to be a cost-effective way of generating high magnetic fields. This type of pulsed magnet is normally operated at liquid nitrogen temperature and consumes little electric power to generate over 50 tesla (T) during a short transient time of less than 50 millisecond (ms). With modern fast data acquisition systems, almost all kinds of physical quantities, such as photoluminescence, magnetization or resistance can be measured during a short magnetic field pulse. We report a recently home-built capacitor-bankdriven pulsed magnetic field facility, in which a capacitor bank of 1.5-MJ maximum stored energy is utilized to generate pulsed magnetic fields up to 50 T with transient pulse time of 22 ms.

Controlling the spins angular momentum in ferromagnets with sequences of picosecond acoustic pulses.

Controlling the angular momentum of spins with very short external perturbations is a key issue in modern magnetism. For example it allows manipulating the magnetization for recording purposes or for inducing high frequency spin torque oscillations. Towards that purpose it is essential to modify and control the angular momentum of the magnetization which precesses around the resultant effective magnetic field. That can be achieved with very short external magnetic field pulses or using intrinsically coupled magnetic structures, resulting in a transfer of spin torque. Here we show that using picosecond acoustic pulses is a versatile and efficient way of controlling the spin angular momentum in ferromagnets. Two or three acoustic pulses, generated by femtosecond laser pulses, allow suppressing or enhancing the magnetic precession at any arbitrary time by precisely controlling the delays and amplitudes of the optical pulses. A formal analogy with a two dimensional pendulum allows us explaining the complex trajectory of the magnetic vector perturbed by the acoustic pulses.

Micromagnetic simulation of energy consumption and excited eigenmodes in elliptical nanomagnetic switches

Sub-200nm patterned magnetic dots are key elements for the design of magnetic switches, memory cells or elementary units of nanomagnetic logic circuits. In this paper, we analyse by micromagnetic simulations the magnetization reversal, the dissipated energy and the excited spin eigenmodes in bistable magnetic switches, consisting of elliptical nanodots with 100×60nm lateral dimensions. Two different strategies for reversal are considered and the relative results compared: (i) the irreversible switching obtained by the application of an external field along the easy axis, in the direction opposite to the initial magnetization; (ii) the precessional switching accomplished by the application of a short magnetic field pulse, oriented perpendicular to the initial magnetization direction. The obtained results are discussed in terms of deviation from the macrospin behavior, energy dissipation and characteristics of the spectrum of spin eigenmodes excited during the magnetization reversal process.

Nonadiabatic spin-dependent transitions in iron clusters as a possible cause of the magnetoplastic effect in NaCl : Fe

A model of nonadiabatic transitions initiated by elastic fields of stresses of moving dislocation in Fe2+ ion pairs has been considered. The transitions are spin-forbidden in the absence of magnetic field. The forbidding is removed in magnetic field. Nonmonotonic dependence of dislocation mean free paths under action of a short (100 μs) magnetic field pulse on the pulse amplitude in NaCl : Fe crystals is explained in the framework of the model of nonadiabatic transitions in Fe2+-Fe2+ pairs.

Theory of ground-state switching in an array of magnetic nanodots by application of a short external magnetic field pulse

A theory of a ground-state switching in an array of axially magnetized cylindrical magnetic dots arranged in a square lattice is developed. An array can be switched into a quasiregular chessboard-antiferromagnetic state by the application of a short pulse of external in-plane magnetic field having a sufficiently long trailing front. The statistical properties of an array magnetization in its final (after switching) state are determined at the linear stage of growth of unstable collective spin-wave modes of the array under the action of a time-dependent magnetic field, and depend critically on the rate of the field decrease: the slower this decrease, the more regular is the final magnetization state. An analytical procedure is presented that allows one to relate the statistical properties of the final demagnetized state of the array and the linewidth of the array's microwave absorption to the parameters of the external switching pulse. The comparison of the developed analytic theory with the results of numerical simulations is presented and demonstrates good agreement between analytical and numerical results.

Magnetic Vortex Core Dynamics in Correlated Double Plates of Permalloy

In this work, we theoretically investigate the dynamics of magnetic vortex in a double-plate nanostructure, in which two square permalloy plates are joined together by imposing closely on a thin permalloy film. It is found that magnetic vortices have been formed in the two square permalloy plates. Due to the involving of the joint, the out-of-plane magnetization can also be observed at the sidewalls between the square plates and the joint area. The ground states with the vortex polarities (1,1) and (1,-1) are obtained when the thickness of the joint area is varied. By applying a short in-plane magnetic field pulse, the vortices deviate from their equilibrium positions and gyrate with damping radius. Interestingly, the vortex polarity can be switched if the strength of magnetic field approaches the threshold of switching field. Particularly by changing the strength of magnetic field, the two vortices can be switched alternatively in the double-plate nanostructure. The investigations may provide potential applications in magnetic vortex memories.

Elasticity Imaging and Sensing Using Targeted Motion: From Macro to Nano

The methods used to estimate tissue elasticity based on the motion of local inhomogeneities such as laser-induced gas microbubbles, magnetic nanoparticles and other targets under an externally applied force are reviewed. The theoretical bases of the motion of a target in a viscoelastic medium are described. Given various targets differing in size, these methods allow for elasticity measurements at different levels of spatial resolution. Short acoustic radiation force and magnetic field pulses were used to initiate the motion of various test objects (solid spheres, laser-induced microbubbles and magnetic nanoparticles) with sizes ranging from several millimeters to tens of nanometers. The induced motion was monitored by conventional pulse-echo ultrasound systems adapted for these measurements. The elasticity of the medium was evaluated based on a theoretical model describing the motion of a particle in a viscoelastic medium. Shear elasticity, evaluated using the developed approach, agrees well with independent measurements of mechanical properties of the medium. In this paper we review our recent theoretical and experimental results and discuss the applications of elasticity imaging and sensing using targeted motion, i.e., motion induced via the external forces acting on local inhomogeneities.

Signal transfer in a chain of stray-field coupled ferromagnetic squares

We study the vortex-core dynamics in a chain of three stray-field coupled permalloy squares. Time-resolved scanning transmission x-ray microscopy is employed to image the out-of-plane magnetization of the cores. After exciting the first element via a short in-plane magnetic field pulse, the excitation can be transferred through the chain via dipolar interaction. The transfer efficiency of the gyrotropic vortex motion strongly depends on the configuration of the core polarizations. For alternating polarizations, a transfer efficiency of about 56% to the third square is achieved. The chain can be switched back and forth between the transmitting and a locking state.

Coupled Vortex Oscillations in Spatially Separated Permalloy Squares

We experimentally study the magnetization dynamics of pairs of micron-sized permalloy squares coupled via their stray fields. The trajectories of the vortex cores in the Landau-domain patterns of the squares are mapped in real space using time-resolved scanning transmission x-ray microscopy. After excitation of one of the vortex cores with a short magnetic-field pulse, the system behaves like coupled harmonic oscillators. The coupling strength depends on the separation between the squares and the configuration of the vortex-core polarizations. Considering the excitation via a rotating in-plane magnetic field, it can be understood that only a weak response of the second vortex core is observed for equal core polarizations.

Influence of magnetocrystalline anisotropy on the magnetization dynamics of magneticmicrostructures

The study of magnetodynamics using stroboscopic time-resolved x-ray photoemissionelectron microscopy (TR-XPEEM) involves an intrinsic timescale provided by the pulsestructure of the synchrotron radiation. In the usual multi-bunch operation mode, the timespan between two subsequent light pulses is too short to allow a relaxation of thesystem into the ground state before the next pump–probe cycle starts. Usinga deflection gating mechanism described in this paper we are able to pick thephotoemission signal resulting from selected light pulses. Thus, PEEM measurements canbe carried out in a flexible timing scheme with longer delays between two lightpulses. Using this technique, the magnetodynamics of both Permalloy and ironstructures have been investigated. The differences in the dynamic response on a shortmagnetic field pulse are discussed with respect to the magnetocrystalline anisotropy.

Nonequilibrium Magnetic Domain Structures as a Function of Speed of Switching Process in Ni$_{80}$Fe$_{20}$ Thin-Film Element

The influence of the speed of the switching process on the nonequilibrium domain structures in magnetic thin-film elements undergoing large angle rotations was studied with time-resolved scanning Kerr microscopy and numerical modelings. In particular, the systematic evolution of the spatiotemporal nature of the reversal of a small Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> element to progressively faster switching fields has been investigated. Generally, the magnetization evolves into a complex domain structure in nonequilibrium state when a magnetic element is excited by applying a short magnetic field pulse. The degree of complexity of the nonequilibrium domain structure is found to be a strong function of the speed of the switching process

Field induced low-spin high-spin transition

We have investigated the temperature and magnetic field induced high-spin low-spin transition of [ μ-Tris(1,4-bis(tetrazol-1-yl)butane- N4, N4′)iron(II)] Bis(hexafluorophosphate) in transient megagauss fields using optical reflection measurements at λ=0.632 and 0.541 μm radiation. The experimental setup uses plastic optical fiber (POF) in an environment between 140 and 200 K. For sample temperature within this transition range magnetic field induced small reflection changes with strong hysteresis were detected. Evidently the strong (160 T) but short (5 μs) magnetic field pulse cannot drive the transition into saturation. Using magnetic field pulses with different time constants the dynamic process could be studied. For [ μ-Tris(1,4-bis(tetrazol-1-yl)butane- N4, N4′)iron(II)] Bis(hexafluorophosphate) it was found that the solvent used in the actual production process of the material had a strong influence on the experimental results.

Magnetic domain structure and dynamics in interacting ferromagnetic stacks with perpendicular anisotropy

The time and field dependence of the magnetic domain structure at magnetization reversal were investigated by Kerr microscopy in a structure consisting of a hard and a soft ferromagnetic Co∕Pt multilayer stack with perpendicular anisotropy, separated by a thicker nonmagnetic Pt spacer layer. Large local inhomogeneous magnetostatic stray fields appear as soon as a nonuniform magnetic area exists within one of the stacks and induce a correlated domain structure within the other. The long range nature of this magnetostatic interaction gives rise to ultraslow dynamics even in zero applied field, i.e., it affects the long time domain stability. Due to this additional interaction field, the magnetization reversal under short magnetic field pulses differs markedly from the well-known slow dynamic behavior. Namely, in high field, the magnetization of the coupled harder layer has been observed to reverse more rapidly by domain wall motion than the softer layer alone.

Precessional dynamics of elemental moments in a ferromagnetic alloy

We demonstrate an element-specific measurement of magnetization precession in a metallic ferromagnetic alloy, separating Ni and Fe moment motion in Ni81Fe19. Pump-probe X-ray magnetic circular dichroism (XMCD), synchronized with short magnetic field pulses, is used to measure free magnetization oscillations up to 2.6 GHz with elemental specificity and a rotational resolution of < 2 deg. Magnetic moments residing on Ni sites and Fe sites in a Ni81Fe19(50nm) thin film are found to precess together at all frequencies, coupled in phase within instrumental resolution of 90 ps.