Single-domain Magnetic Particles Research Articles

The electromagnetic properties of nanoparticle colloids at radio and microwave frequencies

The aim of this study is to investigate the electromagnetic properties of nanoparticle colloids in the frequency range 1 MHz–20 GHz, with focus on the electromagnetic absorption mechanisms at microwave frequencies. A broad range of magnetic and dielectric properties are investigated and a number of mechanisms are highlighted for tailoring the electromagnetic performance. Results from the CoxNi1−x series, with particle sizes ranging from 25 to 200 nm, show particle size-related dielectric and magnetic properties, which aid the optimization of the resulting properties, in addition to conventional mechanisms, which are also demonstrated in colloidal form. A further reduction in particle size to below 20 nm leads to single magnetic domain particles, which also exhibit enhanced electromagnetic properties, as demonstrated with broadband magnetic performance achieved for Co ferrofluid with an average particle size of 5 nm.

Special Issue on the Magnetic and Electric Field Effects at the Micro- and Nano-Scale Systems and Flows in Material Processing

Special Issue on the Magnetic and Electric Field Effects at the Micro- and Nano-Scale Systems and Flows in Material Processing

Perpendicular magnetic anisotropy of epitaxially grown L1-FePdCu nanoparticles with preferential c-axis orientation

Oriented and well-isolated 14-nm-sized Fe41Pd52Cu7 ternary alloy nanoparticles with the L10-type ordered structure have been fabricated by the sequential deposition of Pd, Cu, and Fe on NaCl (001) substrate followed by postdeposition annealing. The annealing temperature required to obtain a high coercivity decreased by at least 50K upon the addition of a small amount of Cu. Furthermore, it was revealed that a strong preferential c-axis orientation along the film normal direction was achieved by the addition of Cu, which resulted in a strong perpendicular magnetic anisotropy. The population of the nanoparticles with their c-axis oriented normal to the film plane was 74%. The alloy composition was independent of the particle size, as determined by energy dispersive x-ray spectroscopy using nanoprobe electrons. Nanobeam electron diffraction revealed that the axial ratio is constant for FePdCu nanoparticles with sizes between 10 and 25nm. Interparticle magnetostatic and exchange interactions played an insignificant role in the isolated FePdCu nanoparticles. The correlation between their preferential c-axis orientation and magnetic properties is discussed based on the rotation magnetization of single magnetic domain particles.

Self-assembled magnetic nanostructures: Epitaxial Ni nanodots on TiN/Si (001) surface

Systems containing single domain magnetic particles are of great interest in view of their possible applications in ultrahigh-density data storage and magnetoelectronic devices. The focus of this work is plan-view STEM Z-contrast imaging study of the self-assembly growth of magnetic nickel nanostructures by domain matching epitaxy under Volmer–Weber (V–W) mode. The growth was carried out using pulsed laser deposition (PLD) technique with epitaxial titanium nitride film as the template, which was in turn grown on silicon (001) substrate via domain matching epitaxy. Our results show that the base of nickel islands is rectangular with the two principal edges parallel to two orthogonal 〈110〉 directions, which is [110] and [ $$\bar{1}10$$ ] for [001] oriented growth. The size distribution of the islands is relatively narrow, comparable to that obtained from self-assembled islands grown under Stranski–Krastanow (S–K) mode. A certain degree of self-organization was also found in the lateral distribution of islands: island chains were observed along the directions close to 〈011〉, which are also the edge directions. The interaction between neighboring islands through the island edge-induced strain field is believed to be responsible for the size uniformity and the lateral ordering.

Time-dependent solution of the N-state Stoner-Wohlfarth model

A set of N coupled differential equations has been developed for calculating the history-dependent magnetization of an ensemble of single domain magnetic particles. These equations, which are a generalization of a previously reported two-state model, have been solved for the case of hysteresis loops for various numbers of states, N, and several field sweep rates. It has been found that the two-state model significantly misestimates the coercivity and that satisfactory convergence may be achieved by N=16. Coercivity was observed to increase logarithmically with field sweep rate.

Magnetic properties of polypyrrole-coated iron oxide nanoparticles

Iron oxide nanoparticles were prepared using the sol–gel process. In situ polymerization of a pyrrole monomer in the presence of oxygen in an iron oxide–ethanol suspension resulted in an iron oxide polypyrrole nanocomposite. The structure and magnetic properties of the nanocomposites with varying pyrrole concentrations are investigated. X-ray diffraction studies indicate the presence of the γ-Fe2O3 phase for the concentrations investigated. FTIR studies confirm the presence of polypyrrole. TEM studies show agglomeration in uncoated samples and in samples with a lower concentration of polypyrrole. Agglomeration is greatly reduced for samples coated with a higher concentration of polypyrrole. The ac susceptibility measurements performed in the temperature range 77–300 K show the presence of blocking, indicating the superparamagnetic phase. The blocking temperature is found to depend on the pyrrole concentration. Monte Carlo studies for an array of polydispersed single domain magnetic particles, based on an interacting random anisotropy model, were also carried out, and the blocking temperatures obtained from the simulation of the field cooled–zero field cooled magnetization compare favourably with experimental results.

Thermal and particle size distribution effects on the ferromagnetic resonance in magnetic fluids

Thermal and particle size distribution effects on the ferromagnetic resonance of magnetic fluids were theoretically investigated, assuming negligible interparticle interactions and neglecting the viscosity of the carrier liquid. The model is based on the usual approach for the ferromagnetic resonance description of single-domain magnetic particle systems, which was amended in order to take into account the finite particle size effect, the particle size distribution and the orientation mobility of the particles within the magnetic fluid. Under these circumstances the shape of the resonance line, the resonance field and the line width are found to be strongly affected by the temperature and by the particle size distribution of magnetic fluids.

Switching probabilities for single-domain magnetic particles

Solving the stochastic Landau-Lifshitz equation numerically, we compute as a function of time $t$ the probability per unit time, ${P}_{s}(t)$, that a classical, single-domain magnetic particle with an easy uniaxial anisotropy and a collinear applied magnetic field will reverse its magnetization (``switch'') via thermal activation over the energy barrier. The ${P}_{s}(t)$ curves increase with $t$ for small $t$, achieving a maximum at some time ${\ensuremath{\tau}}_{P}$ before decaying exponentially with time constant ${\ensuremath{\tau}}_{D}$ at long time, as per the standard Neel-Brown picture. Both ${\ensuremath{\tau}}_{P}$ and ${\ensuremath{\tau}}_{D}$ increase (the latter exponentially) with increasing barrier height; ${\ensuremath{\tau}}_{P}$ grows logarithmically with ${\ensuremath{\tau}}_{D}$, consistent with a recent phenomenological ``energy-ladder'' model, and experiments on submicron-sized magnetic thin films.

Rapid magnetization reversal in magnetic small particles induced by non-static bias field

Magnetization reversal processes by a switching field in single-domain nano-sized magnetic particles in the presence of a small transverse non-static bias field are studied. Applying an oscillating bias field instead of a static field, the reversal time becomes much shorter when the switching field is slightly stronger than the effective anisotropy field. A pulsed bias field of a suitably chosen duration in the nanosecond scale is found to induce a rapid switching, even when the switching field is smaller than the anisotropy field. The dependence of the reversal time on the frequency of an oscillating bias field and the duration of a pulsed bias field are studied. The present work thus complement the earlier studies on switching in the presence of a static bias field.

Thermal fluctuations and longitudinal relaxation of single-domain magnetic particles at elevated temperatures

We present numerical and analytical results for the swiching times of magnetic nanoparticles with uniaxial anisotropy at elevated temperatures, including the vicinity of T_c. The consideration is based in the Landau-Lifshitz-Bloch equation that includes the relaxation of the magnetization magnitude M. The resulting switching times are shorter than those following from the naive Landau-Lifshitz equation due to (i) additional barrier lowering because of the reduction of M at the barrier and (ii) critical divergence of the damping parameters.

Mechanism of the magnetic susceptibility enhancements of the Chinese loess

Chinese loess/paleosol sequences have been regarded as excellent continental archives for encoding continuous paleoclimatic variations over the past 2.5 Myr. However, the mechanism for magnetic enhancements (especially the low‐field mass‐specific magnetic susceptibility, χ) of Chinese paleosols is still not completely resolved. This study quantifies contributions of aeolian and pedogenic magnetic particles to the bulk magnetic properties of the Chinese loess/paleosols by using a magnetic extraction technique. Magnetic properties of magnetic separates (extractable) and the corresponding residues (nonextractable) for five characteristic samples covering both loesses and paleosols were comprehensively investigated by hysteresis loops, frequency and low‐temperature dependence of magnetic susceptibility, and interparametric ratios. Results show that (1) with moderate degrees of pedogenesis (χ < 10 × 10−7 m3 kg−1), χ is enhanced more by increased concentration of stable single domain (SD) magnetic particles than by viscous superparamagnetic (SP) particles. For more mature paleosols with χ > (10–12) × 10−7 m3 kg−1, contributions of pedogenically related fine‐grained pseudosingle‐domain (PSD, ∼100 nm to several microns) particles become significant; (2) pedogenic particles have a narrow grain size distribution concentrated above the SP/SD threshold; and (3) anhysteretic remanent magnetization (ARM) is carried dominantly by SD grains. Moreover, we propose that only the nonextractable fraction of χ, saturation magnetization (Ms) and remanent magnetization (Mrs) show a strong relationship with the degree of pedogenesis. This new interpretation of magnetic enhancements helps us to retrieve more accurate and quantitative paleoclimatic signals recorded by the Chinese loess/paleosol sequences.

Structural studies of ferrofluids by small-angle neutron scattering

The aim of this paper is to present method to investigate the properties of magnetic fluids by means of small angle neutron scattering. Ferrofluids are dispersions of small, single-domain magnetic particles suspended in a fluid carrier. The neutron scattering methods have been largely used the last t wo d ecades for the determination of structural properties of magnetic liquids at microscopic level. There can be investigated the structure of the particle, the aggregation phenomena, the magnetic liquid dynamics, p article-surfactant i nteraction, surfactant li quid-base interaction and structure, magnetic behavior of the samples. Experiments on small angle neutron scattering were ca rried ou t on SANS instrument YuMO in function at IBR-2 h igh-pulsed reactor at the Frank Laboratory of

Dipolar interaction energy for a system of magnetic nanoparticles

AbstractThe long range dipolar interaction energy of a system of single domain magnetic particles with randomly oriented spins and easy axes is calculated by a direct space summation method based on the Lekner formalism. The method is useful for the numerical simulations of positionally disordered nanomagnetic arrays which are being widely used for these systems and is easily adapted for any geometry of the simulation cell. The summations are rapidly convergent, requiring only a few terms. This method is an alternative to the commonly used Ewald summation methods and can also be used for the quasi two – dimensional (2D + h) systems where the Ewald summation method does not give correct results. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Exact activation energy of magnetic single domain particles

I present the exact analytical expression for the activation energy as a function of externally applied magnetic fields for a single-domain magnetic particle with uniaxial anisotropy (Stoner–Wohlfarth model), and investigate the scaling behavior of the activation energy close to the switching boundary.

Single domain magnetic arrays: role of disorder and interactions

The hysteresis of an array of interacting single domain magnetic particles is studied, where the particles interact via exchange and dipolar interactions. The dependence of the magnetic properties on the distribution of grain sizes, the density of the grains, the anisotropy energy and the exchange interactions in the array is investigated through Monte Carlo simulations for \(\gamma- {\rm Fe_2 O}_3\) nanoparticle systems. We also present some experimental results on the \(\gamma- {\rm Fe_2 O}_3\)-polypyrrole nanocomposite system which agree with the trends observed in our simulations.

A variational approach to the relaxation of single domain magnetic particles based on Brown's model

Brown's model for the relaxation of the magnetization of a single domain ferromagnetic particle is considered. This model results in the Fokker–Planck equation of the process. The solution of this equation in the cases of most interest is non-trivial. The probability density of orientations of the magnetization in the Fokker–Planck equation can be expanded in terms of an infinite set of eigenfunctions and their corresponding eigenvalues where these obey a Sturm–Liouville type equation. A variational principle is applied to the solution of this equation in the case of an axially symmetric potential. The first (non-zero) eigenvalue, corresponding to the largest time constant, is considered. From this we obtain two new results. Firstly, an approximate minimising trial function is obtained which allows calculation of a rigorous upper bound. Secondly, a new upper bound formula is derived based on the Euler–Lagrange condition. This leads to very accurate calculation of the eigenvalue but also, interestingly, from this, use of the simplest trial function yields an equivalent result to the correlation time of Coffey et al. and the integral relaxation time of Garanin.

Giant single-molecule magnets: a [Mn84] torus and its supramolecular nanotubes.

The discovery a decade ago that individual molecules can function as magnetizable magnets provided a new, 'bottom-up' approach to nanoscale magnetic materials, and such molecules have since been called single-molecule magnets (SMMs). Each molecule functions as a nanoscale, single-domain magnetic particle that, below its blocking temperature (TB), exhibits the classical macroscale property of a magnet, namely magnetization hysteresis. In addition, they straddle the classical/quantum interface in also displaying quantum tunnelling of magnetization (QTM) and quantum phase interference, the properties of the atomic or microscale. SMMs have a number of potential applications, including very high density information storage, where each bit would be stored as the magnetization orientation of an individual molecule, and as quantum bits for quantum computing, taking advantage of the quantum superposition of states provided by the QTM. To facilitate development of techniques for addressing individual SMMs for such applications, and other reasons, it has been of interest to us to explore whether very large examples (by molecular standards) could be synthesized. In effect, can the molecular (or 'bottom-up') approach reach the size regime of the classical (or 'top-down') approach to nanoscale magnetic materials? Indeed, here we sure that a giant Mn84 SMM can be prepared with a torus structure of 4 nm dimensions, that it displays both magnetization hysteresis and QTM, and that it crystallizes as highly ordered, supramolecular nanotubes.

Giant Single‐Molecule Magnets: A {Mn84} Torus and Its Supramolecular Nanotubes

The discovery a decade ago that individual molecules can function as magnetizable magnets provided a new, 'bottom-up' approach to nanoscale magnetic materials, and such molecules have since been called single-molecule magnets (SMMs). Each molecule functions as a nanoscale, single-domain magnetic particle that, below its blocking temperature (TB), exhibits the classical macroscale property of a magnet, namely magnetization hysteresis. In addition, they straddle the classical/quantum interface in also displaying quantum tunnelling of magnetization (QTM) and quantum phase interference, the properties of the atomic or microscale. SMMs have a number of potential applications, including very high density information storage, where each bit would be stored as the magnetization orientation of an individual molecule, and as quantum bits for quantum computing, taking advantage of the quantum superposition of states provided by the QTM. To facilitate development of techniques for addressing individual SMMs for such applications, and other reasons, it has been of interest to us to explore whether very large examples (by molecular standards) could be synthesized. In effect, can the molecular (or 'bottom-up') approach reach the size regime of the classical (or 'top-down') approach to nanoscale magnetic materials? Indeed, here we sure that a giant Mn84 SMM can be prepared with a torus structure of 4 nm dimensions, that it displays both magnetization hysteresis and QTM, and that it crystallizes as highly ordered, supramolecular nanotubes.

Electron spin resonance studies on quantum tunneling in spinel ferrite nanoparticles

The electron spin resonance (ESR) spectrometer, a very sensitive instrument with fast detecting window to explore quantum phase transitions for magnetic nanoparticles, was exploited to study the fascinating interplay between thermal and quantum fluctuations in the vicinity of a quantum critical point. We have measured ESR in ferrofluid samples containing nanosize particles of Fe 2 O 3 . The evolution of the ESR spectrum with temperature suggests that quantum tunneling of spins occurs in single domain magnetic particles in the low temperature regime. The effects of various microwave fields, particle sizes, and temperatures on the magnetic states of single domain spinel ferrite nanoparticles are investigated. We can consistently explain experimental data assuming that, as the temperature decreases, the spectrum changes from superparamagnetic (SPR) to blocked SPR and finally evolves quantum superparamagnetic behaviour as the temperature lowers down further. A nanoparticle system of a highly anisotropic magnetic material can be qualitatively specified by a simple quantum spin model, or by the Heisenberg model with strong easy-plane anisotropy.

Do multiple cycles of aeolian deposit-pedogenesis exist in the reticulate red clay sections in southern China?

The Xiangyang reticulate red clay section was newly sampled in highresolution. The viewpoints different from the previous studies[1-7] are put forward after studying Munsell color values, weathering degrees, magnetic parameters and stable carbon isotopic compositions of the section. They are as follows: (1) The curves of hue, DCB extracted iron (Fed), DCB extracted iron/total iron ratio (Fed/Fet), weathering index (BA) of the section show that the redness is not significantly correlated with the weathering degree of the layers though the uppermost yellow-brown-colored layer is relatively weakly weathered. The variation in hues of the section is possibly attributed to the change of hematite and goethite contents. It is insufficient to determine the existence of the multiple deposit-pedogenesis cycles in the section, like the loess-paleosol sequence in Chinese Loess Plateau, only by the color variation. (2) Magnetic susceptibility (x) values in the non-reticulate and weakly reticulate layers are high; but are quite low in the reticulate red clay layer. The former are more than ten times higher than the latter because of the leaching and collapse of superparamagnetic (SP) and stable single domain (SSD) magnetic particles during the plinthitic processes. Hence,x values in the section are not controlled by weathering and pedogenic processes, and no longer have clear paleoclimatic implication. It is not correct to prove the existence of the multiple deposit-pedogenesis cycles in the section usingx curves. (3) The organic δ13 C curve of the section illustrates the reduction of forests and increasing of C4 plants in the study area since the late stage of the Quaternary period. It could not prove the existence of the multiple deposit-pedogenesis cycles in the section either. (4) As a matter of fact, it is difficult to prove the existence of the multiple deposit-pedogenesis cycles in the Xiangyang section like the loess-paleosol sequence in Chinese Loess Plateau using the evidence available currently.