Single-domain Ferromagnetic Particles Research Articles

The intrinsic switching field of single domain ferromagnetic particles

The intrinsic switching field defined as the minimum field to induce spontaneous magnetic reversal is calculated at finite temperatures for a uniaxially anisotropic single domain particle using a formalism based on the generalized free energy function of the system. For a fine ferromagnetic particle coupled to a constant temperature heat bath, the switching field is smaller than the field predicted by the Stoner-Wohlfarth (SW) theory and the difference is dependent on the direction of the applied field resulting in breaking of the symmetry of the SW astroid.

Dynamic susceptibility of soft ferrogels. Effect of interparticle interaction.

We present the results of theoretical analysis of the dynamic susceptibility of soft elastic-viscous ferrogels with embedded single-domain ferromagnetic particles chaotically distributed in the host medium. The magnetic anisotropy of the particle is supposed to be strong. The effect of magnetic interparticle interaction is a focus of our attention. A differential equation for the statistically averaged (measured) magnetic moment of the particle is derived. Our analysis shows that in the case of a weak applied field, the interparticle interaction increases the composite magnetization and decreases the rate of its remagnetization.

To the theory of remagnetization kinetics of ferromagnetic nanoparticles. Non linear effects

The paper deals with theoretical study of remagnetization kinetics of a single-domain ferromagnetic particles in an alternating magnetic field, arbitrary directed with respect to the particle axis of easy magnetization. Period of the field is not supposed large as compared with the characteristic time of the particle Neel remagnetization. The nonlinear magnetization of the particle is considered. At the assumption of the particle strong magnetic anisotropy, a new equation for its average magnetic moment is derived.

Resistive Switching in Individual Ferromagnetic Filaments in ZrO-=SUB=-2-=/SUB=-(Y)/Ni Based Memristive Stacks

The resistive switching effect in individual ferromagnetic filaments in memristive stacks based on ZrO2(Y)/Ni functional layers was studied experimentally. A conductive probe of an atomic force microscope played a role of a movable top electrode of a virtual memristive stack. The features of bipolar-type resistive switching found were related to the rapture and restoring of the filaments containing Ni atoms in the ZrO2(Y) dielectric films and are probably caused by different degree of metallization of the filaments. The filaments fromed were manifested in the images obtained by magnetic force microscopy as single-domain ferromagnetic particles. Keywords: memristor, resistive switching, atomic force microscopy, ferromagnetic filaments.

Theory of static magnetization of magnetopolymer composites: The second virial approximation.

This paper deals with a theoretical study of the static magnetization of a composite, consisting of nanodisperse single-domain ferromagnetic particles immobilized in a nonmagnetic medium. This situation is typical for magnetopolymer systems-smart materials, which found applications in high industrial and biomedical applications. It is supposed that the composite was polymerized without an external magnetic field; after curing the particles retain the spatial distribution and orientation of their axes of easy magnetization that they had before the host medium polymerization. Our results demonstrate that, in contrast to ferrofluids with a liquid host medium, a magnetic interparticle interaction provokes a decrease of the macroscopic magnetization of the system.

Формирование наноразмерных ферромагнитных филаментов Ni в пленках ZrO-=SUB=-2-=/SUB=-(Y)

In thin ZrO2 (Y) / Ni films, with used of an atomic force microscope (AFM) probe, conductive ferromagnetic filaments of nanometer sizes, consisting of Ni atoms, are formed. Memristor structures based on such films, the upper electrode of which was the AFM probe, demonstrated bipolar-type resistive switching (RP) associated with the destruction and reduction of Ni filaments. The area where the conducting filament emerges on the surface of the ZrO2 (Y) film manifested itself in the magnetic force image as a single-domain ferromagnetic particle.

To the theory of magnetic hyperthermia in viscoelastic media: effect of interparticle interaction

We propose a theoretical model of magnetic hyperthermia produced by a system of single-domain ferromagnetic particles in elasto-viscous and rigid media. Both mechanisms of the energy absorption, the internal remagnetization of the particles and their rotation under the field action are taken into account. Effects of magnetic interparticle interaction are in focus of our attention. Our analysis shows that the interaction can significantly enhance the heat production. The thermal effects in the elasto-viscous and rigid media are compared.

On the theory of magnetic hyperthermia: clusterization of nanoparticles.

Experiments show that clusters consisting of nano-sized ferromagnetic particles strongly affect the intensity of heat production during magnetic hyperthermia. In this paper, a theoretical study and mathematical modelling of the heat production by clusters of single-domain ferromagnetic particles, immobilized in a host medium, are presented. Two situations of strong and weak magnetic anisotropy of the particles are considered. Our results show that, in the case of strong anisotropy, the clusterization weakens the thermal effect, whereas in the case of weak anisotropy it enhances it. This article is part of the theme issue 'Patterns in soft and biological matters'.

To the theory of magnetic hyperthermia in a system of single-domain ferromagnetic particles

The work deals with theoretical study of magnetic hyperthermia produced by a system of immobilized single-domain ferromagnetic particles under the action of an external oscillating magnetic field. New equation of dynamics of the particles mean magnetic moment is derived by using Fokker–Planck equation for a function of distribution over the moment orientations. Intensity of the heat production by the particle is calculated based on this equation. The results show that the traditionally used phenomenological Debye equations for the moment dynamics can lead to serious deviations of the calculated heat production from that, determined by using the detailed statistical approach.

To the Intrinsic Magnetism of the Bi108Sn0.02Sb0.9Te2S Topological Insulator

Using Electron Spin Resonance spectroscopy together with the Superconducting Quantum Interference Device magnetometry, we found that the intrinsic magnetic moments, originating from the nonmagnetic structural defects of Bi1.08Sn0.02Sb0.9Te2S topological insulator form the superparamagnetic state. It represents an array of nanoscale single domain ferromagnets randomly distributed in the nonmagnetic media. Their net magnetic polarization in the absence of external magnetic field is completely averaged out. Single domain ferromagnetic particles at elevated temperatures behave magnetically in a manner analogous to the Langevin paramagnetism of moment bearing atoms. The main distinction is that the moment of the particle may be 102—103 times the atomic moment.

Effect of internal chain-like structures on magnetic hyperthermia in non-liquid media.

This paper deals with a theoretical study of the effect of chain-like aggregates on magnetic hyperthermia in systems of single-domain ferromagnetic particles immobilized in a non-magnetic medium. We assume that the particles form linear chain-like aggregates and the characteristic time of the Néel remagnetization is much longer than the time of medium heating (time of process observation). This is applicable to magnetite particles when the particle diameter exceeds 20-25 nm. Our results show that the appearance of the chains significantly decreases the intensity of heat production. This article is part of the theme issue 'Heterogeneous materials: metastable and non-ergodic internal structures'.

Temperature effects on drift of suspended single-domain particles induced by the Magnus force.

We study the temperature dependence of the drift velocity of single-domain ferromagnetic particles induced by the Magnus force in a dilute suspension. A set of stochastic equations describing the translational and rotational dynamics of particles is derived, and the particle drift velocity that depends on components of the average particle magnetization is introduced. The Fokker-Planck equation for the probability density of magnetization orientations is solved analytically in the limit of strong thermal fluctuations for both the planar rotor and general models. Using these solutions, we calculate the drift velocity and show that the out-of-plane fluctuations of magnetization, which are not accounted for in the planar rotor model, play an important role. In the general case of arbitrary fluctuations, we investigate the temperature dependence of the drift velocity by numerically simulating a set of effective stochastic differential equations for the magnetization dynamics.

Exactly solvable model for drift of suspended ferromagnetic particles induced by the Magnus force

The phenomenon of drift motion of single-domain ferromagnetic particles induced by the Magnus force in a viscous fluid is studied analytically. We use a minimal set of equations to describe the translational and rotational motions of these particles subjected to a harmonic force and a non-uniformly rotating magnetic field. Assuming that the azimuthal angle of the magnetic field is a periodic triangular function, we analytically solve the rotational equation of motion in the steady state and calculate the drift velocity of particles. We study in detail the dependence of this velocity on the model parameters, discuss the applicability of the drift phenomenon for separation of particles in suspensions, and verify numerically the analytical predictions.

Strong nonequilibrium effects in spin-torque systems

We consider a problem of persistent magnetization precession in a single domain ferromagnetic nano particle under the driving by the spin-transfer torque. We find that the adjustment of the electronic distribution function in the particle renders this state unstable. Instead, abrupt switching of the spin orientation is predicted upon increase of the spin-transfer torque current. On the technical level, we derive an effective action of the type of Ambegaokar-Eckern-Sch\"on action for the coupled dynamics of magnetization (gauge group $SU(2)$) and voltage (gauge group $U(1)$).

Magnetic hyperthermia in solid magnetic colloids

Abstract We present results of theoretical study of magnetic hyperthermia in systems of single-domain ferromagnetic particles homogeneously distributed in a solid matrix. The heat effect is induced by linearly polarized alternating magnetic field. The effect of magnetic interaction between the particles as well as influence of orientation of the particles magnetic axes are in a focus of our consideration. Analysis shows that the interparticle interaction increases intensity of the heat production. The thermal effect in the systems with parallel orientation of the particles axes of easy magnetization is significantly higher than that in the case of random orientation of these axes.

The Magnetic States and Processes of Magnetization Reversal of One-Dimensional Arrays of Ferromagnetic Nanoparticles

The properties of a one-dimensional array of single-domain ferromagnetic particles are analyzed using the original simulation method. Despite the simplicity of the system, it is shown that its properties are quite diverse. Magnetic configuration and magnetization curves are studied in the whole range of fields of interaction including distances wherein the interaction could be still considered as a dipole-dipole. Along with a purely academic interest, the results may be of practical value in the field of creation of new instruments based on magneto-resistance effect. A comparison with experiments showed that the model adequately reflects the properties of real systems.

Novel positive exchange bias effect in compacted Ni3Al/NiO core/shell nanoparticles

We have observed an unusual type of positive exchange bias (EB) in polycrystalline Ni3Al–NiO core-shell nanoparticle system in that the M–H loop center shifts in the same direction as that of the cooling magnetic field, Hcool, regardless of the sign and magnitude of Hcool. However, for weak (≲ 50 Oe) negative cooling fields, as the temperature is raised, a crossover from positive to negative EB occurs at a temperature, which depends on the cooling temperature and Hcool. This kind of positive EB is not accompanied by the training effect but by the vertical shift in the hysteresis loops, ME. A simple model of single-domain ferromagnetic particles embedded in a disordered antiferromagnetic matrix is shown to provide a straightforward explanation for the direct relation observed between the EB field, HE, and −ME.

Effect of interparticle interaction on magnetic hyperthermia in ferrofluids

The work deals with the theoretical study of effect of magnetic interaction between single-domain ferromagnetic particles on the hyperthermia effect produced by these particles under the action of oscillating magnetic field. We consider a homogeneous (without heterogeneous aggregates) ferrofluid consisting of identical spherical Brownian particles. Effects of the particles diameter and their magnetic properties on the intensity of the heat production are studied.

Use of supercomputer for modeling coherent processes in magnetic nano-structures

Abstract Multi-scale spin dynamics of systems of nanomagnets is investigated by numerical simulation using parallel algorithms. A FORTRAN program was developed using an application programming interface OpenMP. The parallel code provides following areas of research: study of the possibility of regulation time of switching of magnetization of the nanostructure; study of the role of nanocrystal geometry of coherent relaxation of 1-, 2- and 3-dimensional objects; study of magnetodynamics of spin system coupled with the passive resonator (radiation damping (RD)); application of RD to ultra-fast relaxation in an assembly of single-domain ferromagnetic particles; study of the role of long distant dipole–dipole fields as the origin of the extremely random behavior in hyperpolarized NMR maser, etc. Estimates of speedup and efficiency of implemented algorithms in comparison with sequential algorithms have been obtained. It is shown that the use of supercomputing technology for study of spin dynamics provides simulation power for spin systems which include thousands of magnetic voxels.

Role of geometrical symmetry in thermally activated processes in clusters of interacting dipolar moments

Thermally activated magnetization decay is studied in ensembles of clusters of interacting dipolar moments by applying the master-equation formalism, as a model of thermal relaxation in systems of interacting single-domain ferromagnetic particles. Solving the associated master-equation reveals a breakdown of the energy barrier picture depending on the geometrical symmetry of structures. Deviations are most pronounced for reduced symmetry and result in a strong interaction dependence of relaxation rates on the memory of system initialization. A simple two-state system description of an ensemble of clusters is developed which accounts for the observed anomalies. These results follow from a semi-analytical treatment, and are fully supported by kinetic Monte-Carlo simulations.