Melting of mesoscopic lattices of magnetic fluid thin film subjected to perpendicular fields

Applying a magnetic field on the magnetic fluid thin film perpendicularly, leads a phase separation that is concentrated in particles separating from a dilute phase. The concentrated phase forms cylindrical columns that construct two-dimensional lattices. This kind of artificial lattice is a novel mesoscopic system and has been explored with optical microscope, CCD, and digital imaging analysis. We explore the ordering evolution of the two-dimensional extraordinary lattice by varying the applied field. The ordering of these lattices is analyzed in terms of translational and bond-orientation correlation functions to address the two-dimensional melting.     

Mössbauer investigation of a nepheline substrate used for preparing a water treatment reagent

The aim of this work is to reveal the results of the reaction of nepheline mineral with concentrated sulfuric acid. The valence states of iron atoms are determined in the nepheline and the reacted nepheline. The existence of iron containing magnetic particles in the sediment is shown. The differences in the lineshape are discussed also and are associated with a transfer from one phase to another of the magnetic particles.     

Thermodynamic and magnetic properties in two artificial frustrated lattices

With the Monte Carlo simulation, we investigate the thermodynamics and magnetic properties of the artificial frustrated square and honeycomb lattices. The results from the Ising-like dipolar model show that there occurs one magnetic order transition for the square lattice while the honeycomb lattice exhibits two magnetic order phase transitions. When the magnetic field is applied perpendicular to one of sublattices, a sharp field-independent peak in the specific heat curves appears at a very low temperature for both frustrated lattices due to the occurrence of a long-range ordered state induced by the magnetic field. For the square lattice, the coercive field slightly increases with the angle of field relative to the vertical axis. For both frustrated lattices, the magnetic reversal is achieved mostly via flipping a chain of the nearest neighbor spins.     

Lattices of nonspherical ferromagnetic grains with magnetic dipole interaction: Theory and experimental examples

The magnetic properties (ground state, magnetic phase diagram, and phase transitions in a magnetic field) of two-and three-dimensional lattices of ferromagnetic grains with the intergrain dipole interaction are studied. The main attention is paid to the lattices formed by nonspherical grains (prolate and oblate ellipsoids of revolution) and their extreme forms (rodlike and disc-shaped grains). An analysis shows that the conclusions of the theory are in good agreement with the results of experiments.     

Quantum phase transition and entanglement in Heisenberg XX spin chain with impurity

In this paper,we study the quantum phase transition and the effect of impurity on the thermal entanglement between any two lattices in three-qubit Heisenberg XX chain in a uniform magnetic field.We show that the quantum phase transition always appears when impurity parameter is an arbitrary constant and unequal to zero,the external magnetic field and impurity parameters have a great effect on it.Also,there exists a relation between the quantum phase transition and the entanglement.By modulating the temperature,magnetic field and the impurity parameters,the entanglement between any two lattices can exhibit platform-like behaviour,which can be used to realize entanglement switch.     

Extremely small domain in the lattice of magnetic dipoles

Lattices of magnetic dipoles with 1–4 rows are investigated. Numerical analysis reveals the smallest stationary domains formed in the lattices, necessary conditions for the formation and destruction of such domains are obtained, and the change in the magnetic moment of the lattices during domain formation is considered. It is shown that the action of an external field on one of the dipoles forming a domain is sufficient for its breaking. The lattices in which the orientational phase transition appears upon perturbation of several dipoles and propagates over the entire system are revealed.     

Construction of “resonant” magneto-optical lattices with controlled momentum compaction factor

On the basis of the theory of “resonant” magneto-optical lattices for synchrotrons with complex transition energy developed in [1], methods for construction of such lattices with application to various accelerators are proposed. Apart from allowing elimination of transition energy crossing by accelerated particles, these lattices should meet a number of important requirements. In particular, they must have dispersion-free straight sections intended for accommodation of RF cavities, Siberian snakes and detectors, and a large enough dynamic aperture for minimizing the effect of magnetic optics nonlinearity on the beam parameters after chromaticity correction by sextupoles.     

Ground states for lattices of ferromagnetic granules with dipolar magnetic interaction

The magnetic phase diagrams of 2D and 3D regular lattices formed by nonspherical single-domain ferromagnetic granules featuring a dipolar magnetic interaction are studied. The energy of a magnetic state of such systems is calculated using an approximate expression for the pair interaction of nonspherical granules. The character of the magnetic ground state of the system is determined by three geometric parameters: (i) the eccentricity of granules; (ii) the ratio of periods of the rectangular (2D) or tetragonal (3D) lattice; and (iii) the ratio of a lattice period to a granule size. In contrast to the case of lattices formed by point (or spherical) magnetic moments, in which the ground state is always antiferromagnetic or frustrated (for triangular lattices), the ground state of a 2D lattice composed of nonspherical granules can be ferromagnetic. The magnetic phase diagrams of the systems studied are constructed in the space of the above geometric parameters.     

Cylindrical domain structure in metamagnets and its stability conditions

The possibility of the existence of a cylindrical magnetic domain structure near the first-order phase transition line is investigated theoretically in metamagnets, compounds with anisotropy energy substantially exceeding the exchange interaction energy. The domain structure of such compounds is related to the kinetics of the transition from the paramagnetic to the antiferromagnetic state. The static properties of isolated cylindrical magnetic domains and their equilibrium lattices are studied using an energetic approach. It is shown that for a low-temperature metamagnetic phase transition domains of the existence of a cylindrical domain structure, lattices and isolated cylindrical magnetic domains, abut, on the domain of plane-parallel domain structure existence from both sides. The features of their behavior are determined as a function of the magnitude of the external magnetic fields.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 84–88, March, 1988.The author is grateful to Yu. I. Gorobets for supporting the research and for useful remarks, and also to D. A. Yablonskii and I. M. Vitebskii for fruitful discussions.     

Ratchet transport of overdamped particles in superimposed driven lattices

Ratchet transport of overdamped particles is investigated in superimposed driven lattices using Langevin dynamics simulations. It is found that noise can strongly affect the transport of the particles. When lattices driving dominates the transport, the noise acts as a disturbance of the directed transport and slows down the average velocity of the particles.When the driving phase has less impact on particle transport, Gaussian white noise can play a positive role. By simply modulating these two parameters, we can control efficiency and the direction of the directed currents.     

Quasi-2D Monte Carlo simulations of a colloidal dispersion composed of magnetic plate-like particles with magnetic moment normal to the particle axis

We have investigated aggregation phenomena of a colloidal dispersion composed of magnetic plate-like particles by means of Monte Carlo simulations. Such plate-like particles have been modelled as disk-like particles with magnetic moment normal to the particle axis at the particle centre, with the section shape of a spherocylinder. The main objective of the present study is to clarify the influences of the magnetic field strength and magnetic interactions between particles on particle aggregation phenomena. We have concentrated our attention on a quasi-2D system from an application point of view such as the development of surface quality changing technology using such magnetic plate-like particles. A magnetic field is applied along the direction perpendicular to the plane of the monolayer. Internal structures of particle aggregates are discussed quantitatively in terms of radial distribution and orientational pair correlation functions. For the case of strong magnetic interactions between particles, particles form long column-like clusters with their magnetic moments alternating in direction between the neighbouring particles. These tendencies appear under circumstances of a weak applied magnetic field. However, as the magnetic field strength increases, particles incline towards the magnetic field direction, so that particles do not form such clusters.     

Cluster structure and the first-order phase transition in dipolar systems Monte Carlo simulation

: The Monte Carlo technique is used to simulate a 3D dipolar hard-sphere system. The spatial and magnetic structure of clusters formed by magnetic dipolar interactions in zero applied field is investigated. It is shown that the many-particle clusters are characterized by a quasi-spherical shape, extremely small magnetic moments, and a fractal dimension close to three. These clusters are regarded as nuclei of a new concentrated isotropic phase. The numerical simulation of the first-order phase transition has been realized which allows us to find the interface between two coexisting phases. It has been found that the dipole-dipole and steric interactions are sufficient to separate the system into two phases with low and high concentrations of particles. The introduction of any additional attraction potential is not required. The phase diagram of dipolar system in zero applied field has been obtained. The simulation results are in qualitative agreement with the predictions of some analytical models.     

Equilibrium configurations and phase transitions in dipole lattices

Two-dimensional square and hexagonal lattices of magnetic dipoles with the number of rows 1–4 have been studied. Based on the numerical analysis, equilibrium stable domain configurations, including the minimum number of lattice dipoles, have been revealed; the conditions for the creation and destruction of domains have been determined; and their associated changes in the magnetic moment of the lattice and in the energy of the dipole interaction have been found. The conditions for the occurrence of phase transitions that change the configuration of the lattices have been investigated and the conditions for unidirectional propagation of the front of the phase transition have been established. A comparative analysis of different square and hexagonal lattices has been performed in terms of the specific features of the formed domains and the observed orientation phase transitions.     

Planar magnetic colloidal crystals

We report a novel form of planar magnetic colloidal crystals formed by coated magnetic microspheres floating on a liquid meniscus. Under an external magnetic field, the balance between the repulsive magnetic interaction and the "attractive" interaction, due to the weight of the particles projected along the surface tangent, yields not only the triangular lattice with a variable lattice constant, but also all the other planar crystal symmetries such as the oblique, centered-rectangular, rectangular, and square lattices. By using two different sized magnetic particles, local formations of 2D quasi-crystallites with fivefold symmetry are also observed.     

On the theory of phase transitions in magnetic fluids

Particles of magnetic fluids (ferrofluids), as is known from experiments, can condense to bulk dense phases at low temperatures (that are close to room temperature) in response to an external magnetic field. It is also known that a uniform external magnetic field increases the threshold temperature of the observed condensation, thus stimulating the condensation process. Within the framework of early theories, this phenomenon is interpreted as a classical gas-liquid phase transition in a system of individual particles involved in a dipole-dipole interaction. However, subsequent investigations have revealed that, before the onset of a bulk phase transition, particles can combine to form a chain cluster or, possibly, a topologically more complex heterogeneous cluster. In an infinitely strong magnetic field, the formation of chains apparently suppresses the onset of a gas-liquid phase transition and the condensation of magnetic particles most likely proceeds according to the scenario of a gas-solid phase transition with a wide gap between spinodal branches. This paper reports on the results of investigations into the specific features of the condensation of particles in the absence of an external magnetic field. An analysis demonstrates that, despite the formation of chains, the condensation of particles in this case can proceed according to the scenario of a gas-liquid phase transition with a critical point in the continuous binodal. Consequently, a uniform magnetic field not only can stimulate the condensation phase transition in a system of magnetic particles but also can be responsible for a qualitative change in the scenario of the phase transition. This inference raises the problem regarding a threshold magnetic field in which there occurs a change in the scenario of the phase transition.     

Symmetric behavior of vortex states of superconducting networks in a magnetic field

We present magnetic field dependence of phase transition temperature and vortex configuration of superconducting networks based on theoretical study. The applied magnetic field is called “filling field” that is defined by applied magnetic flux (in unit of the flux quantum) per unit loop of the superconducting network. If a superconducting network is composed of very thin wires whose thicknesses are less than coherence length, the de Gennes–Alexander (dGA) theory is applicable. We have already shown that field dependences of transition temperature curves have symmetric behavior about the filling field of 1/2 by solving the dGA equation numerically in square lattices, honeycomb lattices, cubic lattices and those with randomly lack of wires networks. Many experimental studies also show the symmetric behavior. In this paper, we make an explicit theoretical explanation of symmetric behaviors of superconducting network respect to the applied field.     

Monte-Carlo study of spin-glass behaviour in concentrated binary magnetic systems

Monte Carlo calculations on a stochastic Ising model of a concentrated binary magnetic system with very strong, quenched bond disorder were done on square lattices. The specific heat and static susceptibility show rounded peaks at a temperature below which field-cooling effects and long time relaxation, typical of spin-glasses, start appearing.     

Local Magnetic Nanoparticle Delivery in Microvasculature

The transport and capture of therapeutic magnetic nanoparticles in human microvasculature is studied numerically. The nanoparticles are injected into a vascular system upstream from malignant tissue, and are captured at the tumour site with the aid of a local applied magnetic field positioned outside the body. Taking into account the dominant magnetic and fluidic forces on the particles, our study shows that the nanoparticles can be directed to and concentrated at the desired zone that is within a few centimetres from the surface of the body. In addition, influence of the particles size, average blood flow velocity and the diameter of the blood vessel on the captured efficiency are parametrically analysed.     

Site-directed research of magnetic nanoparticles in magnetic drug targeting

The targeting of ferrofluids composed of 20 nm magnetic particles was studied through simulation and animal experiment. The results showed that some magnetic particles were concentrated in the target area depending on the applied magnetic field. Through theoretical analysis, the retention of the magnetic nanoparticles in a target area is due to large magnetic liquid beads formed by the magnetic field.     

Effect of anisotropy and dipole interaction on long-range order magnetic structures generated by Dzyaloshinskii–Moriya interaction

Self-organized long-range order structures, such as stripe domains and magnetic skyrmion lattices, are formed by the competition between ferromagnetic exchange interaction and Dzyaloshinskii–Moriya (DM) interaction. We investigated the properties of the magnetic structures generated by a DM interaction under the influence of anisotropy or magnetic dipole interaction, by performing Monte-Carlo simulated annealing. We constructed phase maps in external-field and anisotropy space to study the effect of anisotropy or dipole interaction on the phase boundaries between the magnetic structures. The simulation results show that the phase boundaries are sensitive to perpendicular anisotropy and that the skyrmion lattice region in phase space is extended under easy-plane anisotropy. The effect of the long-range dipole interaction was studied and was found to stabilize the skyrmion lattice phase and reduce the size of the magnetic structures.