Initial magnetic susceptibility of ferrocolloids: The influence of chain aggregates

The behavior of a system of noninteracting chain aggregates of ferroparticles suspended in a liquid matrix and subjected to a weak uniform permanent magnetic field was studied based on the free energy functional of a monodisperse low-concentration magnetic colloid. The orientational response of a flexible chain to a weak external field showed that the model of rigid rods is applicable only for short chains (doublets and triplets) of large ferroparticles characterized by a strong magnetodipole interaction. The calculated values of the initial magnetic susceptibility of aggregated ferrofluid agree well with the computer simulation data.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 756–765.Original Russian Text Copyright © 2004 by Ivanov.     

Low-temperature susceptibility of concentrated magnetic fluids

The initial susceptibility of concentrated magnetic fluids (ferrocolloids) has been experimentally investigated at low temperatures. The results obtained indicate that the interparticle dipole-dipole interactions can increase the susceptibility by several times as compared to the Langevin value. It is shown that good agreement between recent theoretical models and experimental observations can be achieved by introducing a correction for coefficients in the series expansion of susceptibility in powers of density and aggregation parameter. A modified equation for equilibrium susceptibility is offered to sum over corrections made by Kalikmanov (Statistical Physics of Fluids, Springer-Verlag, Berlin, 2001) and by B. Huke and M. Lucke (Phys. Rev. E 67, 051403, 2003). The equation gives good quantitative agreement with the experimental data in the wide range of temperature and magnetic particles concentration. It has been found that in some cases the magnetic fluid solidification occurs at temperature several tens of kelvins higher than the crystallization temperature of the carrier liquid. The solidification temperature of magnetic fluids is independent of particle concentration (i.e., magneto-dipole interparticle interactions) and dependent on the surfactant type and carrier liquid. This finding allows us to suggest that molecular interactions and generation of some large-scale structure from colloidal particles in magnetic fluids are responsible for magnetic fluid solidification. If the magnetic fluid contains the particles with the Brownian relaxation mechanism of the magnetic moment, the solidification manifests itself as the peak on the "susceptibility-temperature" curve. This fact proves the dynamic nature of the observed peak: it arises from blocking the Brownian mechanism of the magnetization relaxation.     

Magnetic susceptibility of FeOCl

Magnetic susceptibility (χ) measurements on FeOCl are reported. The data were obtained on single crystals over the temperature range 7–400 K. A small but distinct singularity at the Neel temperature T_N ≈ 84 K was observed; extensive short-range ordering above T_N was also evident from these data. The present work suggests that the intrinsic χ(T) of FeOCl was masked by magnetic impurities in the two previous χ studies of this compound.     

The aggregation of ferrocolloids in a magnetic field

The structure of chain aggregates forming in a low-concentration monodisperse ferrocolloid in a uniform permanent magnetic field was studied. The configuration integral of a flexible fluctuating chain of arbitrary length was calculated using the matrix method of coordinate system rotation with allowance made for the dipole-dipole interaction of the magnetic moments of nearest ferroparticles. The equilibrium distribution over aggregate chain lengths was determined from the condition of minimum of the free energy functional. It was shown that, even for infinitely diluted ferrofluids, an increase in the strength of external magnetic field led to a noticeable lengthening of chain aggregates at strong interparticle magnetodipole interaction. This results in faster (compared to Langevins law) growth of the magnetization of a system in weak and intermediate fields.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 766–774.Original Russian Text Copyright © 2004 by Ivanov.     

Magnetic susceptibility of the BH molecule

The coupled Hartree-Fock (CHF) perturbation approach and its extension to multiconfiguration wavefunctions (MC CHF scheme) were used to calculate the magnetic susceptibility of the BH molecule. The results obtained for an SCF and two pair-excitation MC SCF functions confirm the paramagnetism of the BH molecule and indicate a rather weak dependence of the computed molecular susceptibility on the correlation effects. Sponsored by the Institute of Low Temperatures and Structure Research, Polish Academy of Sciences.     

Magnetic susceptibility of liquid gold-indium-zinc alloys

Summary Alloys of the ternary system gold-indium-zinc with a variable content of goldx _Au at a constant ratio ofx _In:x _Zn=1:1 have been investigated by susceptibility measurements from 300 K up to 1500 K with emphasis on the liquid state. The magnetic susceptibilities of the homogeneous liquids were found to vary in a non-linear manner with temperature and composition. At temperatures close to the liquid-solid transition, the susceptibilities pass through a characteristic minimum which tends to disappear with increasing temperature. The anomaly located atx _Au 0.6 seems to be related to a particular valence electron concentration ofe/a 1.6.

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Magnetische Suszeptibilität von flüssigen Gold-Indium-Zink-Legierungen

Zusammenfassung Legierungen des ternären Systems Gold-Indium-Zink mit einem variablen Goldgehaltx _Au bei konstantem Indium-Zink-Verhältnis (x _In:x _Zn=1:1) wurden zwischen 300 und 1500 K auf ihre magnetische Suszeptibilität untersucht. Das Hauptaugenmerk wurde dabei auf den flüssigen Zustand gelegt. Die magnetischen Suszeptibilitäten der homogenen Schmelzen variieren nichtlinear mit Temperatur und Zusammensetzung. Bei Temperaturen nahe dem übergang flüssig-fest tritt ein charakteristisches Minimum der Suszeptibilität auf, das mit steigender Temperatur verschwindet. Die Anomalie bei einemx _Au vonca. 0.6 scheint mit einer speziellen Valenzelektronenkonzentration (e/a) von etwa 1.6 verknüpft zu sein.

     

Magnetic susceptibility of rubidium superoxide and ozonide

Conclusions We were the first to determine the magnetic susceptibility of rubidium ozonide on preparations that contained 93–97% of the ozonide. The magnetic susceptibility, determined for rubidium superoxide, is in agreement with the literature data.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 446–448, February, 1971.     

Magnetic susceptibility of MnIn2S4

Recent magnetic resonance results suggest that MnIn₂S₄ is antiferromagnetic with a Néel point near liquid helium temperature, in disagreement with previously reported dc (direct current) susceptibility measurements. The resonance data suggest that these susceptibility measurements were made at an applied dc field comparable to or greater than the critical field. This would tend to obscure observation of a magnetic transition, since some of the spins would be “flopped.” The susceptibility measurements have been repeated at a much lower applied field. From these new measurements, a Néel temperature of 4.9°K is obtained, in agreement with the resonance data, and an exchange parameter of 1430 is computed.     

Magnetogranulometric analysis of ferrocolloids: Second-order modified mean field theory

A statistical model of highly concentrated ferrocolloids is proposed. The model is based on the use of the mean field theory for the single-particle orientational distribution function of the magnetic moments of ferroparticles. The expression for the effective mean field is derived with the method of perturbation theory of the second order with respect to the concentration and intensity of magnetodipole interparticle interaction. It is proposed to apply the magnetogranulometric analysis for describing magnetization curves for highly concentrated ferrocolloids. The high efficiency of the modified mean field theory is demonstrated during the determination of the parameters of disperse composition of ferrocolloids by the magnetogranulometric analysis.     

The influence of chain aggregates on the magnetic properties of ferrocolloids

Static magnetic properties of monodisperse low-concentrated aggregated ferrocolloid are studied within the framework of the model of flexible chain aggregates. For the account of long-range magnetic dipoledipole interaction between all ferroparticles in a system, it was proposed to use the modified mean field theory. It is shown that the model of short flexible chains, in combination with the modified mean field theory, quantitatively explains anomalously high magnetic characteristics of aggregated ferrocolloids obtained in computer experiments.     

Density-functional study of model bidisperse ferrocolloids in an external magnetic field

We present phase diagrams of a model bidisperse ferrocolloid consisting of a binary mixture of dipolar hard spheres (DHSs) under the influence of an external magnetic field. The dipole moments of the particles are chosen proportional to the particle volume to mimic real ferrocolloids, and we focus on dipole-dominated systems where isotropic attractive interactions are absent. Our results are based on density-functional theory in the modified mean-field (MMF) approximation. For one-component DHS fluids in external fields, and for corresponding mixtures dominated by one of the components, MMF theory predicts the tricritical point of the transition between an isotropic gas and a ferromagnetic liquid occurring at zero field to be changed into a critical point separating two magnetically ordered phases of different density. The corresponding critical temperature displays a nonmonotonic dependence on the field strength. Completely different behavior is found for the critical temperature related to the demixing phase transitions appearing in strongly asymmetric mixtures [G. M. Range and S. H. L. Klapp, Phys. Rev. E 70, 061407 (2004)]. For such systems, we find a monotonic decrease of the demixing critical temperature with increasing field. The field strength dependence of the critical temperature can therefore be tuned between nonmonotonic and monotonic behaviors just by changing the composition of the mixture--e.g., by adjusting the chemical potentials. This allows us to efficiently control the influence of external magnetic fields on the phase behavior over a large temperature interval.