Single-domain Magnetic Particles Research Articles

Magnetic properties of chemical coprecipitated Sr0.8La0.2Fe11.8Co0.2O19 powders

For this work, Sr0.8La0.2Fe11.8Co0.2O19 hexaferrite powders were manufactured, using the chemical co-precipitation method. Major and minor magnetization loops recorded at room temperature show that the optima hard magnetic properties are achieved when the calcination temperature is 1000°C. SEM, TEM and XRD measurements show that the 1000°C calcined powder consists of single hexaferrite phase fine particles with a mean diameter of 90nm. From remanence magnetization, coercivity, magnetic viscosity and irreversible susceptibility measurements there can be concluded that the powder consists of single domain magnetic particles of platelet shape.

Synthesis and characterisation of magnetic iron sulfide nanocrystals

Fe1−xS and Fe3S4 nanocrystals with a variety of morphologies and average sizes were synthesised by the reaction of iron(II) acetylacetonate (Fe(acac)2) and elemental sulfur in oleylamine. Reaction at 200°C for 240min produced extremely thin Fe3S4 sheets, which displayed low coercivities (14kAm−1) suggestive of pseudosingle-domain or multidomain particles. Reaction temperatures ≥300°C for 30min produced 70nm Fe1−xS nanocrystals with hexagonal plate and hexagonal prism morphologies, which displayed high magnetic coercivities (110kAm−1) characteristic of single magnetic domain particles. Rapid injection of sulfur solution at 280°C followed by immediate cooling produced a mixture of Fe1−xS nanocrystals and spherical, polydisperse ∼5nm Fe3S4 nanocrystals, which displayed superparamagnetism above an average blocking temperature of 55K.

Coercivity control in finite arrays of magnetic particles

Micromagnetic simulation has been performed for two-dimensional arrays of single-domain magnetic particles using the Landau-Lifshitz equation of motion and an energy minimization method. Effects of array anisotropy and spin positional disorder on the hysteresis loop and coercivity of the particle systems are investigated. Simulation results show that the hysteresis loop can be largely modified by breaking geometric symmetry of square arrays, and coercivity in general is found to increase with array disorder. Magnetic hysteresis is strongly affected by disorder when the array contains only a few particles.

Noncollinear states in a chain of single-domain magnetic particles

The ground state of a chain of single-domain magnetic particles has been theoretically analyzed. The conditions under which this state corresponds to a noncollinear structure in zero external magnetic field are determined. Such noncollinear states are due to the features of the long-range magnetostatic interactions in the systems without the center of inversion.

Transient electromagnetic field in magneto-viscous medium

Abstract When using Transient Geo-Electromagnetics (TEM) unusually slow decaying transients are sometimes observed in the absence of well-conductive objects. Many researchers explain this phenomenon by presence of material with magneto-viscous or superparamagnetic (SPM) properties. The analysis of electromagnetic (EM) field equations in conductive SPM medium is suggested in the present paper. It is shown that in a simplest case of small size single-domain magnetic particles with relative higher potential barrier energy, the SPM-medium is characterized by a very slow decaying exponential transient magnetic field, the level of which is much more as compared to that of due to non-magnetic medium of the same conductivity. This situation is traced up to the late transient time like 0.1 s and more. SPM and Induction fields are in the near zone additive. They depend in a different way on the superparamagnetic and electric properties of the SPM-medium. They also depend in different ways on the transient time and on the size of the transmitter–receiver configuration. These characteristics make it possible to recognize the presence of SPM-effect and to separate these fields.

Phase diagram of a two-dimensional square lattice of magnetic particles with perpendicular anisotropy

The ground state of an array of small single-domain magnetic particles having perpendicular anisotropy and forming a square two-dimensional lattice is studied in the presence of a magnetic field. The stability of some basic states with respect to nonuniform perturbations is analyzed in a linear approximation, and analytical model calculations and numerical simulation are used for an analysis. The entire set of states at various anisotropy constants and magnetic fields is considered when a field is normal to the array plane. Two main classes of states are possible for an infinite system, namely, collinear and noncollinear states. For collinear states, the magnetic moments of all particles are normal to the array plane. At a sufficiently high anisotropy, a wide class of collinear states exists. At low fields, a staggered antiferromagnetic order of magnetic moments takes place. An increase in the magnetic field causes an unsaturated state, and this state transforms into a saturated (ferromagnetic) state with a parallel orientation of the magnetic moments of all particles at a sufficiently high field. At a lower anisotropy, the ground state of the system is represented by noncollinear states, which include a complex four-sublattice structure for the components of the magnetic moments in the array plane and a nonzero projection of the magnetic moments of the particles onto the field direction. A phase diagram is plotted for the states of an array of anisotropic magnetic particles in the anisotropy constant-magnetic field coordinates. For a finite array of particles, sample boundaries are shown to play a significant role, which is particularly important for noncollinear states. As a result of the effect of the boundaries at a moderate field or anisotropy, substantially heterogeneous noncollinear states with a heterogeneity size comparable with the sample size can appear in the system.

Iron silicide nanoparticles in a SiC/C matrix from organometallic polymers: characterization and magnetic properties

Thermolysis of poly(ferrocenylsilane) at 1000 °C or poly[{(dimethylsilyl)ferrocenyl}diacetylene] at 850 °C resulted in ferromagnetic Fe3Si nanoparticles, well distributed in a SiC/C matrix. The average size of the formed nanoparticles, their size distribution and their agglomeration behaviour depend on the choice of polymer and the thermolysis conditions. The size-dependent magnetic properties of the nanoparticles were analyzed by SQUID measurements. The smallest Fe3Si nanoparticles (10–30 nm) are single magnetic domain particles with superparamagnetic behaviour.

Study on the Ferrofluid Lubrication with an External Magnetic Field

Ferrofluids (FFs) are stable colloidal systems consisting of single-domain magnetic particles with a diameter of approximately 10 nm coated with surfactants and dispersed in a carrier liquid. By applying an external magnetic field, these fluids can be confined, positioned, shaped and, controlled at desired places. The load capacity of a lubricant film of FF can also be increased with an appropriate magnetic field. In this paper, Fe3O4-based FFs with different saturation magnetizations (M s) were prepared by the co-precipitation technique. The tribological experiments of FFs under different magnets distributions were conducted on a ring-on-cylinder tribometer. The results show that the magnetic field intensity distributions on the rubbing surface have a significant influence on the tribological properties of FFs. The experimental results also indicate that FFs have a good friction-reduction performance in the presence of an external magnetic field compared with the carrier liquid and that its lifetime of friction can be greatly improved.

Generation of wear elements and origin of tribomagnetization phenomenon

Experiments showed that a generation of wear elements, that means elemental debris of wear particles, induced a spontaneous magnetization of the sliding surface ( i.e. tribomagnetization phenomenon) when the ferromagnetic materials iron, nickel and cobalt were subjected to sliding friction in the absence of an external magnetic field. Origin of the tribomagnetization phenomenon that arose through the generation of wear elements with a size of about 15 nm to a few tens of nanometer, a size that was of the same order as that of a single magnetic domain particle of ferromagnetic materials, was revealed by using a magnetic force microscope (MFM). The magnetic flux density that varied in both strength and direction over the entire sliding surface was identified by using a Tesla meter with a three-axis or single-axis probe. From experimental results the relation between a generation of wear elements in tribological process and the original process in mechanism of the tribomagnetization phenomenon was discussed.

Magnetic properties of iron nanoparticle

Magnetic properties of Fe nanoparticles with different sizes synthesized by a physical deposition technique have been investigated experimentally. We have used a high pressure sputtering technique to deposit iron nanoparticles on a silicon substrate. The nanoparticles are then analyzed using atomic force microscopy (AFM), transmission electron microscopy (TEM), and superconducting quantum interference device techniques. TEM and AFM data show that the particle size could be tuned by adjusting the deposition conditions. The magnetic properties have been investigated from temperature dependent magnetization M(T) and field dependent magnetization M(H) measurements. The results show that two phases including both ferromagnetic and superparamagnetic particles are present in our system. From these data we extracted the superparamagnetic critical size to be 9 nm for our samples. Ferromagnetic particles are single magnetic domain particles and the magnetic properties can be explained by the Stoner and Wohlfarth model. For the superparamagnetic phase, the effective anisotropy constant, Keff, decreases as the particle size increases.

Relaxation Mössbauer spectra of polymer composites with magnetic nanoparticles

It is shown that the shape of Mossbauer spectra of composites based on single-domain magnetic particles is defined to a greater degree by interparticle interactions rather than the superparamagnetic properties of individual particles. The characteristic features of the spectra of such systems are described and shown to be adequately described within a three-level relaxation model.

Mössbauer spectra of hematite and magnetite nanoparticles in polymer composites

It is experimentally demonstrated that the shape of Mössbauer spectra of polymer composites based on single-domain magnetic particles depends more on an inter-particle interaction than on superparamagnetic properties of a separate particle. The characteristic features of the spectra of such systems with strong inter-particle interactions are separated, such as an asymmetric shape of lines with sharp outer and smeared inward sides and presence of central anomalously splitted doublet. It is shown that the three-level relaxation model, which takes into account the precession and diffusion of magnetization of single-domain particles between two local energy minima and one absolute energy maximum, allows to describe all characteristic features of the spectra of such systems.

Role of anisotropy and interactions in magnetic nanoparticle systems

Magnetic nanoparticle systems are characterized by several competing effects like anisotropy, an inherent disorder, the long range dipolar and the short range exchange interactions due to clustering effects. The sensitivity of the observed static and dynamical properties of these systems like the blocking temperature, the hysteresis and the susceptibility, to the methods of preparation, annealing and the resulting morphology is a manifestation of this. However, given the complexity of the system, it is often difficult to isolate the effects which might be dominant in a particular sample, which has a direct bearing on the desired applications. In this paper we report the effects of anisotropy, interactions and particle concentration on the temperature dependent remanence and coercivity through a numerical simulation on an array of single domain magnetic particles which incorporates all the above mentioned factors. Our results show that it is possible to distinguish between purely anisotropic systems and interacting systems with these measurements. In confirmation of our simulation results we also present the experimental results on the remanence and coercivity of nanomagnetic nickel ferrite composites.

Anthropogenic magnetic particles and heavy metals in the road dust: Magnetic identification and its implications

Magnetic properties of road dusts in the East Lake area in Wuhan, China, were measured and compared with the results of heavy metal analyses in order to delineate the sources of pollutants. A total of ninety-seven dust samples were collected spatially from four segments with different traffic density and field settings from the roads encircling the lake. Thermomagnetic and hysteresis measurements revealed that the dominant magnetic carrier is coarse-grained magnetite. Correlations between magnetic parameters and element concentrations with traffic density and distances to the industrial region revealed that elements Cu, Ni and Fe mainly originate from vehicle traffic, which is also the major source of coarser magnetic particles (e.g., pseudo-single-domain/multi-domain (PSD/MD) grains), while element Pb and the smaller grains such as single-domain (SD) magnetic particles mainly originate from industrial emissions. The ratio between anhysteretic remanent magnetization and low-field magnetic susceptibility (ARM/ χ lf) can be employed as an indirect indicator for Cu, Fe and Ni emissions resulting from vehicle traffic. Due to the intermixture of elements from different sources, the element concentrations are not conclusive about the pollution source. A linear correlation between magnetic concentration-related parameters (e.g., ARM and saturation isothermal remanent magnetization, SIRM) and the concentrations of major elements (e.g., Cu, Co, Fe, Mn, Ni and V) suggests that they can be used as a proxy for heavy metal pollution. Road dusts in four segments show different magnetic characteristics, indicating various influxes of anthropogenic magnetic materials from vehicle traffic and industrial plants due to the different traffic loads and field settings. These results suggest that magnetic measurements can serve as an efficient complementary tool for the routinely employed geochemical methods to map the heavy metal pollution and trace the sources of pollutants in the road dust.

Topological defects and magnetization processes for the triangular lattice of ising particles with an antiferromagnetic interaction

For the frustrated triangular lattice of Ising magnetic moments with an antiferromagnetic interaction, which is in a state with two sublattices, a new type of topological defects with zero energy in the approximation of the interaction between only the nearest-neighbors has been found. These defects have a nonzero magnetic moment, and the magnetization in a low field occurs via the formation of a system of such defects. These properties are valid for a 2D superstructure in the form of a triangular lattice of single-domain magnetic particles with perpendicular anisotropy and dipole coupling.

Mechanism of surface magnetization by friction of ferromagnetic materials

Experiments showed that in the absence of an external magnetic field, the ferromagnetic materials Ni and Co undergo spontaneous magnetization of their frictional sliding surfaces as a result of tribological actions such as friction and wear. The magnetization phenomenon on the sliding surface was confirmed by measurements of the magnetic flux density and measurements of magnetic forces with a magnetic force microscopy. The mechanism of spontaneous magnetization by tribological action (tribomagnetization) is discussed in relation to the formation of elemental wear debris with a size of a few tens of nanometers, a size that is of the same order as that of a single magnetic domain particle, and of transfer particles that are formed by accumulation of debris adhering to the sliding surface in tribological processes.

Characterization of bacterial magnetotactic behaviors by using a magnetospectrophotometry assay.

Magnetotactic bacteria have the unique capacity of synthesizing intracellular single-domain magnetic particles called magnetosomes. The magnetosomes are usually organized in a chain that allows the bacteria to align and swim along geomagnetic field lines, a behavior called magnetotaxis. Two mechanisms of magnetotaxis have been described. Axial magnetotactic cells swim in both directions along magnetic field lines. In contrast, polar magnetotactic cells swim either parallel to the geomagnetic field lines toward the North Pole (north seeking) or antiparallel toward the South Pole (south seeking). In this study, we used a magnetospectrophotometry (MSP) assay to characterize both the axial magnetotaxis of "Magnetospirillum magneticum" strain AMB-1 and the polar magnetotaxis of magneto-ovoid strain MO-1. Two pairs of Helmholtz coils were mounted onto the cuvette holder of a common laboratory spectrophotometer to generate two mutually perpendicular homogeneous magnetic fields parallel or perpendicular to the light beam. The application of magnetic fields allowed measurements of the change in light scattering resulting from cell alignment in a magnetic field or in absorbance due to bacteria swimming across the light beam. Our results showed that MSP is a powerful tool for the determination of bacterial magnetism and the analysis of alignment and swimming of magnetotactic bacteria in magnetic fields. Moreover, this assay allowed us to characterize south-seeking derivatives and non-magnetosome-bearing strains obtained from north-seeking MO-1 cultures. Our results suggest that oxygen is a determinant factor that controls magnetotactic behavior.

Competing magnetic interactions in nickel ferrite nanoparticle clusters: Role of magnetic interactions

The magnetic behavior of nickel ferrite nanoparticles of different sizes was studied by annealing nickel ferrite powders at temperatures ranging from 300 to 900 °C. Transmission electron microscopy studies show that the average particle sizes change from ∼8 to ∼120 nm with increasing annealing temperatures. The x-ray diffraction patterns of the annealed samples reveal that a single phase is retained. Hysteresis measurements performed up to a field of 10 kOe show a tendency toward saturation. The saturation magnetization is found to increase with annealing temperature (particle size) with the magnetization tending toward the bulk value for powders annealed at 900 °C. Zero field cooled–field cooled measurements performed at 0.5 kOe indicate the presence of a superparamagnetic phase up to an annealing temperature of 700 °C with blocking temperatures in the range of 150–330 K. Numerical simulations are carried out using an interacting model of an array of single domain magnetic particles to explain the change in the magnetic behavior of the samples with annealing temperature and to estimate the anisotropy of the system. Our studies indicate that the observed magnetic behavior can be explained by the changes in the anisotropy of the system and the dominance of the short range interparticle exchange interactions over the long range dipolar interactions with increasing particle sizes. This change in the interaction profile is further confirmed by the Henkel plots for the particles annealed at different temperatures.

Patterned Magnetic Media Made by Self-Assembled Block-Copolymer Lithography

AbstractPatterned magnetic recording media, in which data bits are stored in discrete single-domain magnetic particles, have been proposed for the next generation of recording media. To achieve high densities, features with periodicities on the order of 25 nm and below are required over large areas, which is a challenging task for any lithography process. Block copolymers (BCPs), which phase-separate into ordered periodic nanoscale structures, might provide a path to accomplish such patterning. In this article, we describe BCP lithography and show how the self-assembled patterns can be templated to make large-area arrays of nanoscale structures with long-range order.

Characteristic times of relaxation peaks of magnetic fluids

The use of the Fokker–Planck equation in theoretical studies of relaxation processes in single-domain particle systems is a well-established technique. However this method has a particular disadvantage in that it can give analytical results solely in some limiting cases. An alternative method, which avoids this difficulty, is that of the use of the integral relaxation time, τ int, which is presented in this work. Complex magnetic susceptibility measurements, χ( ω)= χ′( ω)−i χ″( ω), over the frequency range 10–100 MHz, were performed on a commercial kerosene-based magnetic fluid with magnetite particles stabilized with oleic acid. Based on these measurements, the integral relaxation time, τ int, the effective relaxation time, τ eff and the maximum absorption time, τ max =(2 πf max) −1, where f max is the frequency at which χ″( ω) exhibits a maximum, were determined. These characteristic times of a relaxation peak are always different, except for the idealized case of a homogeneous system with a single relaxation time. Due to the fact that τ int can be found analytically in a wide range of parameters, without solving the Fokker–Planck equation of the stochastic process, the presented experimental method can be successfully involved in the investigation of relaxation processes in single-domain magnetic particle systems, enabling a comparison to be made between experimental and theoretical results.