Magnetic relaxation in a random system of interacting rodlike nanoparticles

It is shown in the framework of the generalized mean-field approximation taking into account spatial fluctuations of the local magnetic field that the collective effect of dipole interaction in a random 3D system of identical (rodlike) magnetic nanoparticles with parallel easy magnetization axes shifts the relaxation magnetization curves towards shorter times (i.e., accelerates the relaxation process). In addition, the course of this process depends (via the demagnetizing field) on the sample shape. The interaction between nanograins affects the magnetization relaxation of a random 2D system only when the magnetic moments of the grains are perpendicular to the plane of the system.     

Propriétés magnétiques et relaxation magnétique nucléaire de Be9 dans des composés intermétalliques de terres rares: GdBe13, TbBe13 et DyBe13

Be⁹ nuclear magnetic relaxation, magnetic susceptibilities and magnetic moments measurements of intermetallic compounds GdBe₁₃, TbBe₁₃ and DyBe₁₃ have been performed. The relaxation rate is mainly due to a dipolar interaction between the nuclear moments and the paramagnetic ions moments. Antiferromagnetic ordering was observed for all compounds. Curie-Weiss behaviour was observed at high temperature. The effective moments are comparable to that expected from the three positive ions.     

Complex magnetic susceptibility of uniaxial superparamagnetic particles in a strong static magnetic field

The magnetic relaxation of a system of single-domain ferromagnetic particles in the presence of a strong static magnetic field directed at an arbitrary angle relative to the particle anisotropy axis is investigated. A system of linear difference-differential equations for the moments (averaged spherical harmonics) is derived without recourse to the Fokker-Planck equation by averaging Gilbert’s equations with a fluctuating field. An exact solution (in terms of matrix continuous fractions) is found for this system. The relaxation times and spectra of the complex magnetic susceptibility are calculated. Fiz. Tverd. Tela (St. Petersburg) 40, 1642–1649 (September 1998)     

Nuclear spin-lattice relaxation in finely dispersed carbonizate powders

Finely dispersed carbonizate powders were studied with the aim of revealing their suitability for producing hyperpolarized noble gases. In the temperature and frequency dependences obtained over a wide range of temperatures and magnetic fields for the spin-lattice relaxation times of the magnetic moments of ³He, ¹H, and ¹³C nuclei, anomalous features caused by the suppression of the exchange between surface paramagnetic centers in a magnetic field were observed. It is shown that the interaction with magnetic moments of the ¹H nuclei situated near the paramagnetic centers is the main polarization-leakage channel for the noble-gas nuclear spins.     

Magnetic resonance of paramagnetic ion-nuclei in metals and superconductors

The magnetic resonance lineshape of paramagnetic ion-nuclei in metals is calculated using the temperature Green functions method and is analyzed for limiting cases of fast and slow spin lattice relaxation of localized moments. The longitudinal spin lattice relaxation rate for paramagnetic ion-nuclei in type II superconductors due to the hyperfine coupling with local moments is calculated. The influence of the fluctuation coupling of electrons on relaxation of paramagnetic ion-nuclei in “dirty” type II superconductors is investigated in magnetic field slightly above the upper critical field H_c2.     

Magneto-optical study of the orientation confinement of particles in ferrolyotropic systems

Ferrosmectics and ferrohexagonals are magnetic liquid crystals which consist of a lamellar and a hexagonal phase in which magnetic nanoparticles are incorporated. The magnetic field variation and the relaxation of the linear optical birefringence of ferrosmectics and ferrohexagonals and its time relaxation are measured in different geometries and compared with that of a ferrofluid sample. We interpret our data by the existence of a mean orientation of the magnetic particles in the lyotropic structure, which appears to be non-random in zero field. We conclude that the magnetic moments of the particles are preferentially aligned in the plane of the ferrosmectic layers and along the axis of the ferrohexagonal cylinders, respectively. To account for this preferred alignment, we propose that some of the particles in the ferrosmectic are adsorbed on the surfactant layers with their moment aligned in the lamellar plane, while the orientation of the moments in the ferrohexagonal is restricted to a cone around the cylinder axis. Received 31 December 1998 and Received in final form 22 September 1999     

Intensity saturation and polarization rotation of magnetically induced second-harmonic radiation

We have observed saturation of the intensity of magnetically induced second-harmonic radiation in atomic sodium vapor and rotation of the polarization angle of the second-harmonic radiation from that of the fundamental field at the magnetic field of about 1 kG. These phenomena have been fully analyzed, and explained by the Hanle effect of the second-harmonic quadrupole moments observed in the forward direction. A possibility of measuring the transverse relaxation times of the E2 allowed transition is suggested.     

On line nuclear orientation: A rapidly developing technique

Nuclear orientation facilities are operating fully on-line to acclerators/isotope separators capable of producing wide ranges of radioisotopes. The lowest on-line base temperatures are close to 7 mK and the shortest half-life yet studied is 0·9 s. Experimental results yield magnetic dipole interaction strengths, nuclear spin-lattice relaxation times, directional distributions of alpha, beta and gamma emissions and gamma ray linear polarization. Nuclear magnetic resonance has been observed on-line.The broad range of the technique in opening up systematic measurements of nuclear magnetic moments and excited state structures is stressed, illustrated by examples from current work.     

Variational master equation approach to dynamics of magnetic moments

Non-equilibrium properties of a model system comprised of a subsystem of magnetic moments strongly coupled to a selected Bose field mode and weakly coupled to a heat bath made of a plurality of Bose field modes was studied on the basis of non-equilibrium master equation approach combined with the approximating Hamiltonian method. A variational master equation derived within this approach is tractable numerically and can be readily used to derive a set of ordinary differential equations for various relevant physical variables belonging to the subsystem of magnetic moments. Upon further analysis of the thus obtained variational master equation, an influence of the macroscopic filling of the selected Bose field mode at low enough temperatures on the relaxation dynamics of magnetic moments was revealed.     

Model of the Linearized Collision Operator for Plasmas with Temperature Anisotropy

A model kinetic operator is proposed, describing the relaxation of the first sixteen hydrodynamic moments of a two-component plasma with temperature anisotropy with respect to an external magnetic field. The analytical expression given for the model, taking into account only density, velocity, and temperature relaxation, is more exact than a corresponding Bhatnagar-Gross-Krook model.     

Applied field Mössbauer studies of the iron storage proteins ferritin and haemosiderin

Applied field Mössbauer spectra obtained from ferritin and haemosiderin have been fitted using a spin-Hamiltonian model, to determine the effect of the applied field on the direction of the iron magnetic moments. The results of these fits indicate that the ordering of magnetic moments within the ferrihydrite core of these proteins is antiferromagnetic. The values of the anisotropy field obtained from these fits correspond to much higher values of the superparamagnetic blocking temperatures than those actually observed. This anomaly has implications for our understanding of the superparamagnetic relaxation processes in these materials.     

On the distribution of potential wells and its relation to collision-induced dipole moments in nonpolar solvents. - Part I.

The relaxation times of some rigid polar molecules in three non-polar solvents of different molecular geometries (and different collision induced dipole moments) are studied. There is some relationship to be observed between the arbritary distribution of surrounding potential wells and the dipole moments measured in the various solvents.     

Low-temperature dynamics of local-moment systems investigated by thermal cycling methods

We report studies of the low-temperature dynamics of several dilute alloy systems in which local moments are present, using various thermal-cycling nuclear orientation methods. Determinations of nuclear spin-lattice relaxation over a wide concentration range in the giant-moment alloys CoPd illustrate the various types of magnetic behavior in this system. Experiments on dilute TbAu show evidence of clustering and impurity-impurity interactions. Strong local moments are formed on Mn in the weak ferromagnet ZrZn₂, but the relaxation behaves as in a paramagnetic alloy.     

On-line nuclear orientation of the deformed neutron-deficient Eu,Sm and Pm isotopes

Low-temperature nuclear orientation measurements made on-line at the SERC Daresbury Laboratory on^142m Eu,^141m Sm, and¹⁴¹Pm, with known magnetic dipole moments, have yielded the magnitude of the hyperfine fields of these isotopes in an iron host lattice. Thus measurements for the isotopes^{139, 138}Eu,^139m Sm, and¹³⁸Pm yielded values for the respective magnetic moments. Limits on the thermal relaxation times of Eu and Sm isotopes in Fe were also deduced. The results for¹³⁸Eu appear to contradict the earlier πh_11/2⊗νh_11/2 ground-state configuration assignment.     

Dynamics of field-induced ordering in magnetic colloids studied by new time-resolved small-angle neutron-scattering techniques

The reversal of magnetic moments of nanoparticles in concentrated Co ferrofluids was monitored in an oscillating magnetic field by new time-resolved stroboscopic small-angle neutron-scattering techniques. Time resolution in the micros range was achieved by using a pulsed beam technique, TISANE, while in continuous mode resolution was limited by the wavelength spread to about 1 ms. The frequency dependence of anisotropic scattering patterns has been modeled using Langevin dynamics. The dynamics follows a two step mechanism: field-induced ordering is governed by fast Brownian rotation of nanoparticles with a characteristic time of about 160 micros. Magnetic relaxation of locally ordered domains of about 100 nm in size takes place within a few seconds by Brownian rotation or by Néel type rotation of magnetic moments.     

Relaxation of Magnetic Nanoparticle Chain without Applied Field

The relaxation of a one-dimensional magnetic nanoparticle linear chain with lattice constant $a$ is investigated in absence of applied field. There is an equilibrium state (or steady state) where all magnetic moments of particles lie along the chain (x-axis), back to which the magnetic nanoparticle chain at other state will relax. It is found that the relaxation time T_x is determined by T_x=10^β× a³. This relaxationis compared with that of single magnetic nanoparticle system.     

Magnetic properties of Fe and Co nanoclusters embedded in the first Cu(100) surface layer

The magnetic anisotropy energies, as well as spin and orbital magnetic moments of the atoms involved in the simplest nanostructures formed due to the self-organization within the first Cu(100) surface layer, are calculated in the framework of the density functional theory. The critical role of the surface relaxation, which leads to the rotation of the easy magnetization axis in iron nanoclusters, is demonstrated in the calculations of magnetic anisotropy.     

Long-time relaxation of an ordered magnetic structure

In a Mössbauer investigation of the parameters of the hyperfine interactions on Fe⁵⁷ nuclei in the ferromagnetic Tb_0.8Y_0.2Fe₂, we have made the first observation of long-time (days, months) relaxation of an ordered magnetic structure. In this effect the orientation of some moments in a sample changes under the action of a pulsed magnetic field (up to 250 kOe) and then slowly reverts to the initial state by means of gradual decomposition of clusters of the reoriented moments. A theoretical estimate of the relaxation time is made.     

Spin relaxation times in disordered graphene

We consider two mechanisms of spin relaxation in disordered graphene. i) Spin relaxation due to curvature spin orbit coupling caused by ripples. ii) Spin relaxation due to the interaction of the electronic spin with localized magnetic moments at the edges. We obtain analytical expressions for the spin relaxation times τ_SO and τ_J due to both mechanisms and estimate their values for realistic parameters of graphene samples. We obtain that spin relaxation originating from these mechanisms is very weak and spin coherence is expected in disordered graphene up to samples of length .     

Magnetic field dependence of the 31P longitudinal nuclear-spin relaxation time in GdP at high temperatures

The magnetic field dependence of the longitudinal nuclear-spin relaxation time for ³¹P nuclei in paramagnetic GdP is expressed in terms of a wavevector frequency-dependent relaxation-shape function. A Gaussian representation of the memory function associated with the spin-velocity relaxation function is chosen that satisfies the first six frequency moments. The results are compared with the experimental data of Myers and Narath.