Saturation Magnetization and Law of Approach to Saturation for Self-formed Ionic Ferrofluids Based on MnFe2O2 Nanoparticles

测量了MnFe₂O₄纳米微粒及其磁性液体在室温下的磁化曲线.微粒的中值粒径为13.67 nm. 磁性液体的比饱和磁化强度小于理论值.在高场范围(5～10 kOe)下,磁性液体趋于饱和时,其体积分数越大,磁化曲线的斜率越大. 这种饱和磁化强度性质和趋饱和律分别源自于无场时的环状自组装团聚体和场致团聚体. 场致团聚体是耗散结构,以致于其趋饱和磁化律不同于顺磁理论所描述的趋饱和律. 磁性液体中的大微粒导致了表观磁滞现象.     

The modulation of coupling in the relaxation behavior of light transmitted through binary ferrofluids

With an external magnetic field, a relaxation process is observed when the light transmit through binary ferrofluids composed of ferrimagnetic CoFe₂O₄ and paramagnetic p-NiFe₂O₄ nanoparticles similar to ferrofluids consisting only of CoFe₂O₄. Since only the ferrimagnetic nanoparticles are able to form field-induced chainlike structures for such binary ferrofluids by magnetic interaction between the particles, so the relaxation behavior of the transmitted light is caused mainly by the ferrimagnetic system. In the binary ferrofluids, the paramagnetic nanoparticles, regarded as magnetically polarized gas molecules, are restrained to occupy the space between the ferrimagnetic chains and distribute following the CoFe₂O₄ particle chains covering and diverging, producing a modulation effect on the relaxation behavior of the transmitted light. The modulation effect can be characterized by range and time parameters that describe the relationship of the relaxation behavior of the transmitted light to the properties of the binary ferrofluids and single CoFe₂O₄ ferrofluids.     

Magnetooptical light concentration by regular domain structures

Expressions are obtained for the diffracted field in the Fresnel and Fraunhoffer diffraction domains. It is shown that the field in the diffraction pattern is the superposition of two orthogonally polarized fields. The distribution of a field with polarization orthogonal to the incident radiation depends substantially on the domain structure parameters. Thus, for zeros of the function f ()=J₀((/)) assigned as radii of the annular domains, a magnetooptical light concentration in an annular image is possible, while for f()=cos(²/2R ₁ ² ) the light is concentrated at a point, i.e., the annular domain structure plays the part of a lens. The effects of light concentration are examined for both the case of annular systems and strip domains.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 31–38, October, 1978.     

On the time evolution of transmittivity in magnetic fluids

When a magnetic fluid is subjected to a magnetic field, a part of the magnetic particles in the fluid agglomerates to form chains. Thus, the ferrofluid becomes optically anisotropic. In this work we describe optically observed patterns in some magnetic fluid films in applied parallel magnetic fields and optical effects of these, especially the optical transmittance. The most interesting experimental observation is that concerning the time dependence of relative transmittivity . For kerosene base ferrofluids relax rapidly at coupling and decoupling magnetic field, but for a transformer-oil magnetic fluid the relaxation times can attain (5–10) minutes, depending on the intensity of applied magnetic field.     

Spin dynamics in the spin glass phase of Y(Mn1−xAlx)2

Zero field SR has been used to probe spin dynamics in the concentrated spin glass Y(Mn_0.9Al_0.1)₂. The spectra follow a stretched exponential form, G_z(t)=exp(–(t)), with reaching 1/3 as the freezing temperature, T_g=60K, is approached. The evolution of G_z(t) with temperature is suggestive of the behaviour of the spin correlation function found by Ogielski in Monte Carlo simulations of the dynamics of the Ising spin glass model. The SR results are correlated with results previously obtained by inelastic neutron scattering.     

A magnetic field-dependent modulation effect tends to stabilize light transmission through binary ferrofluids

In binary ferrofluids composed of ferromagnetic γ?Fe₂O₃/Ni₂O₃ composite nanoparticles (A particles) and noncrystalline Fe₂O₃ nanoparticles (B particles), the A particles alone will form chain-like aggregates upon application of a magnetic field. Due to both the long-range ‘magnetic convergent force’ (F_C) and the short-range ‘magnetic divergent force’ (F_D), the A-particle chains immersed in the B-particle ‘sea’ will move in a manner similar to the process of vibrational damping. The apparent damping of the ferrofluids will vary from weak to overdamping according to the motion of the chains, so that the intensity of light transmitted through a ferrofluid film along the direction of the field would tend to stabilize after a period of rapid decrements and increments. In binary ferrofluids, the B-particle system can produce a modulation effect on both the damping and the driving force, further stabilizing the behavior of the transmitted light. At low fields (e.g., 500 Gs, 900 Gs) only the modulation of the viscosity drag force (F_v) is considerable, so that overdamping increases linearly with B-particle volume fraction (Ф_B), and the variation in the transmitted light is much slower during the process tending towards stability as Ф_B increases. However, at high fields (e.g., 1300 Gs) the polarization of the B-particle ‘sea’ is enhanced, so that F_D is modulated as well as F_v (i.e., both the practical damping and driving forces are modulated simultaneously). Thus, the apparent overdamping of the binary ferrofluids system will vary non-linearly as Ф_B increases, and the transmitted light will tend to stabilize faster for ferrofluids with high Φ_B than for those with low Ф_B at an applied magnetic field of 1300 Gs.     

Proton Transfer of Phenanthridone in the Lowest Excited Singlet State in Moderately Concentrated Mineral Acid Solutions

A modified form of the steady state treatment for the determination of excited state proton transfer rate constants was successfully applied to a variety of oxygen and nitrogen center aromatic excited acids. These compounds shared the characteristic of being more acidic in the lowest excited singlet state (S₁) than in the ground electronic state (S₀) and of requiring a concentrated mineral acid medium for protonation. This treatment was extended to phenanthridone, the lactam of 6-hydroxyphenanthridine, which is a weak enough base in the ground and the excited states to require moderately concentrated mineral acids for protonation, and becomes a stronger base in the excited state than it is in the ground state. Phenanthridone exists as an -lactam and is a weaker base in the ground and excited states than the -lactams derived from 2-hydroxypyridine, 2-hydroxyquinoline, and 1-hydroxyisoquinoline. It is also a much weaker base than the vinylogous -lactams. The reasons for this are discussed here.     

Electric field effects on persistent spectral holes: Perylene in the polar polymer polyvinylbutyral

We investigated the effects of an electric field on a spectral hole burned in the inhomogeneously broadened S ₀–S ₁ transition of perylene in different samples of the polar polymer polyvinylbutyral (PVB) and in cellulose nitrate. The spectral hole is broadened and reduced in depth by the electric field. It was checked experimentally for perylene in PVB that the hole area remains constant when an electric field is applied. We determined the effective matrix-induced electric dipole moment differences * for perylene in different PVB samples and in cellulose nitrate. Within experimental accuracy the value of * is approximately independent of the composition of PVB and its water content. For perylene in cellulose nitrate the value of * is larger by a factor of 1.5 than in PVB. The results are discussed on the basis of a simple model for the electric field effect.     

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.     

Field screening in insulating crystals: The quasistatic case

A study is made of a metal-dielectric-semiconductor-dielectric-metal structure subjected to a linearly increasing external voltage. On the assumption of quasistatic screening conditions, the carrier-density and field distribution in the near-anode and nearcathode regions of the semiconductor are determined. This distribution proves to be nonuniform not only in the region of the surface space charge induced by the external field, but also within the interior of the semiconductor to a depth E₀·t_M/₀, where is the lifetime and the mobility of the carriers, E₀/₀ is the rise rate of the external field, and t_M is the Maxwellian relaxation time. The character of the distribution for opposite faces of the semiconductor is different, since in the near-cathode region for n-type semiconductors allowance must be made for the effect of the uncompensated space charge of the current carriers on the conductivity.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 7–10, February, 1973.We are indebted to G. M. Guro and V. B. Sandomirskii for a discussion of the work.     

Magnetic behaviour of γ-Fe2O3 nanoparticles by mössbauer spectroscopy and magnetic measurements

-Fe₂O₃ nanoparticles with varying state of dispersion in a polymer have been investigated by Mössbauer spectroscopy, static magnetic measurements at low applied field, and alternative susceptibility measurements over a large range of frequencies (2×10^–2–10⁴ Hz). The dynamical behaviour was characterized through the variation of the blocking temperature with the characteristic time of the measurement. The Mössbauer blocking temperature was determined according to a procedure described. For quasi-isolated particles an Arrhenius law is demonstrated. Effects of interparticle interactions in concentrated and aggregated systems are satisfactorily explained by the previous model. Dependence of the superparamagnetic susceptibility on the experimental conditions interpreted using the Lorentz or Onsager fields is mentioned.     

NMR in Laves phase alloy system GdPt x —analysis of nonstoichiometry in intermetallic compounds

NMR is applied to ferromagnetic Laves phase compounds Gd_1–x A _x Pt₂ (A=Sc, La) and GdPt _x (2x3). The different hyperfine field contributions are analyzed. Neighbour contributions to the Gd hyperfine field and strength of ferromagnetic coupling are compared. It is found that there is a strong predominance of the nearest neighbour contribution (92%) to the Pt hyperfine field. We show that the Pt NMR allows to derive the deviation in occupation number of Gd atoms on Gd sites in nonstoichiometric compounds with about 4% accuracy.Now with Dürrwächter K. G., Pforzheim     

Hyperfine field calculations: Search for muon stopping sites in Fe3O4

Muon Spin Rotation (SR) results for magnetite (Fe₃O₄) are analyzed and discussed. At room temperature, a SR signal is observed due to the presence of an internal magnetic field (B_int) at the muon site. External transverse field measurements show that B_int is parallel to the magnetic spin direction, the <111> direction in zero applied field. Calculations of the hyperfine field to pinpoint muon stopping sites in magnetite show that the local field contains supertransfer (covalent) and dipolar field contributions. The implanted muons appear to stop at sites structurally similar to those reported for hematite (-Fe₃O₄), where muon-oxygen bond formation was strongly indicated.Research partially supported by US Department of Energy     

Study of the Nonlinear Response of Superconductors in the Microwave Band Using a Local Technique

Using a local technique, we study the microwave radiation power P₃(T,P,H_dc) at the triple frequency of the main signal as a function of the temperature T, the input power P, and the external magnetic field H_dc perpendicular to the superconductor surface for YBa₂Cu₃O₇ (YBCO) films, monocrystals, and polycrystals and for Nb films. The most distinctive feature of the temperature dependence P₃(T) of nonlinear response of superconductors is a maximum of the nonlinearity near the critical temperature T_c. Spatial distributions of the third-harmonic power are obtained for YBCO films at various temperatures. These distributions are indicative of a nonuniform distribution of T_c over the superconductor surface. Additional nonlinearity maxima are discovered in the dependence P₃(T) for YBCO films and monocrystals at temperatures about 2T_c/3. These maxima are probably related to the existence of several superconducting phases with different critical temperatures. For Nb films, the second nonlinearity maximum in the dependence P₃(T) appears only in the presence of an external magnetic field. The experimental data are interpreted within the framework of a two-fluid model of a superconductor, which takes into account the phenomenological nonlinear relationship between the vector potential A and the supercurrent j_s(A). The origin of nonlinearity in the studied superconductors is discussed.     

Kondo effect and impurity-impurity interaction in (La,Ce)B6 alloys

Measurements of the magnetic susceptibility between 0.03 and 300 K and of the magnetization between 0.05 and 10K for magnetic fields up to 60kOe have been used to investigate effects from the interaction between the conduction electrons and local magnetic moments in (La_1–x Ce _x )B₆ alloys (0.0007x0.10). For Ce concentrationsx<0.006 the data show Kondo-type single impurity behavior at low temperatures with a transition from a magnetic to a non-magnetic regime of the Ce ions. In the magnetic regime the impurity susceptibility follows a Curie-Weiss law, and in the non-magnetic regime it varies withT ². An external magnetic field gradually restores the free-ion behavior of the Ce impurities. For more concentrated alloys interactions between the impurities are observed. The RKKY interaction strength derived is more than two orders of magnitude smaller than in the Kondo systemCuFe. Values of thes–f exchange integral,J, estimated from both the Kondo effect and the RKKY interactions are in good agreement. The relatively high Kondo temperature, in spite of a smallJ, can be explained by a density-of-states argument. The influence of crystal field excitation on the susceptibility is also discussed.     

On the interpretation of nuclear quadrupole interaction data for rare-earth nuclei at low symmetry sites

Expressions are given for all tensor components of the lattice- and 4f-shell electric field gradient contributions at a rare-earth nucleus and the influence of local symmetry is discussed. To demonstrate that a full tensor analysis is essential for monoclinic symmetry, the quadrupole splitting data for¹⁶⁹Tm in Tm₂O₃ are re-analyzed, resulting in significantly different shielding factorsR _Q and .     

Elastic constants,the ΔE effect,and retarded polarization processes in a ferroelectric niobate ceramic

A resonant method was used to determine the ultrasonic velocity in circular plates of a ferroelectric niobate ceramic having the composition (Pb_0.57Ba_0.43)Nb₂O₆, over the frequency range 3–9 MHz at 19 °C. The average values of the elastic constants are calculated for unpolarized samples. A study is made of the Young's modulus (the E effect) and of the polarization vector P as functions of the polarizing field and the time for which this field is applied. The E effect and P, which increase monotonically but nonlinearly with increasing , approach limiting values as functions of the polarization time in different manners. A comparative analysis of the E and polarization curves shows that the domain structure can be thought of as slightly different from that found from a scheme proposed for its behavior during the polarization process.Translated from Izvestiya VUZ. Fizika, No. 7, pp. 61–65, July, 1970.     

Effects of the rate of evaporation and film thickness on nonuniform drying of film-forming concentrated colloidal suspensions

In this paper, we report on nonuniform distribution of film-forming waterborne colloidal suspensions above the critical concentration _c of the colloidal glass transition during drying. We found that colloidal suspension films dry nonuniformly when the initial rate of evaporation E and/or the initial thickness l₀ are high. We found that a Peclet number Pe, defined as Pe = El₀/D, where D is the diffusion coefficient of the colloids in the diluted suspensions, does not predict uniformity of drying of the concentrated suspensions, contrary to the reported work on drying of diluted suspensions. Since the colloidal particles are crowded and their diffusive motion is restricted in concentrated suspensions, we assumed that above _c water is transported to the drying surface by hydrodynamic flow along the osmotic pressure gradient. The permeability of water through channels between deforming particles is estimated by adapting the theory of foam drainage. We defined a new Peclet number Pe by substituting the transport coefficient of flow (defined as the permeability divided by the viscosity, multiplied by the osmotic pressure gradient) for the diffusion coefficient. This extended Peclet number predicted the nonuniform drying with a criterion of Pe > 1. These results indicate that the mechanism of water transport to the drying surface in concentrated suspensions is water permeation by osmotic pressure, which is faster than mutual diffusion between water and particles --that has been observed in diluted suspensions and discussed by Routh and Russel. The theory fits well the experimental drying curves for various thicknesses and rates of evaporation. The particle distribution in the drying films is also estimated and it is indicated that the latex distribution is nonuniform when Pe > 1.