Theory Of Magnetic Relaxation Research Articles

Quantitation method of loss powers using commercial magnetic nanoparticles based on superparamagnetic behavior influenced by anisotropy for hyperthermia

Local hyperthermia using magnetic nanoparticles has attracted attention as a less invasive cancer therapy. Accurate estimation of heat dissipation of magnetic nanoparticles is important for hyperthermia treatment. In this study, the magnetization properties and heat dissipation of Feraheme®, Resovist®, and Synomag®-D commercial magnetic nanoparticles was measured. The effective core diameter and anisotropy constant were estimated by the Langevin function taking into consideration the effect of anisotropy. The required frequency and intensity of an applied magnetic field with respect to the weight and concentration of magnetic nanoparticles in an assumed spherical tumor were estimated based on measurement and theory of magnetic relaxation. A method to quantitate realistic heat dissipation of magnetic nanoparticles in tumor tissue by measurement of their magnetization properties is introduced. Because the physical rotation of magnetic nanoparticles is inhibited in tumor tissue, the realistic heat dissipation is estimated by solidifying a mass of nanoparticles in an experimental sample. The specific loss power and intrinsic loss power as the heat dissipations of magnetic nanoparticles were estimated from the area of the magnetization curves. The results indicated that only the linear component of magnetization was associated with the loss powers. Our method to quantitate loss powers and the index for applied magnetic field will facilitate the clinical application of hyperthermia treatment using magnetic nanoparticles.

Relaxation rates of magnetization in LiYF4:Ho3+ crystals

Abstract Theory of magnetization relaxation in a diluted paramagnet based on calculations of relaxation rates due to electron–phonon coupling and magnetic dipole–dipole interactions is developed. Assuming fast establishment of equilibrium in the spin system and the Gaussian line shape for the spectral density of the dipole–dipole reservoir, we have derived a master equation for a paramagnetic ion. The developed approach is used to interpret dynamic magnetic measurements in LiYF 4 :Ho 3+ single crystals. We show specific variations of low temperature magnetization dynamics at the magnetic field values corresponding to (anti-)crossings of electron-nuclear sublevels of the Ho 3+ ground state. The measured dependences of the ac-susceptibility on frequency, holmium concentration and the applied magnetic field are described when taking into account the phonon bottleneck effect and the cross-relaxation. Evolution of the magnetization of the LiYF 4 :Ho 3+ in the reversed field is considered.

Magnetic relaxation in rare-earth oxide pyrochlores

A theory of magnetic relaxation is developed for geometrically frustrated three-dimensional magnets that can be described by an antiferromagnetic Ising model. These magnetic materials are exemplified by some of the recently synthesized rare-earth oxide pyrochlores, such as Dy2Ti2O7, Ho2Ti2O7, or Yb2Ti2O7. A model based on an analogy between the spin ordering in Ising magnets and proton ordering in ice is proposed. In this model, magnetic point defects treated as noninteracting quasiparticles characterized by well-defined energies, mobilities, and effective magnetic charges play a fundamental role analogous to that of ion defects in the physics of ice or by electrons and holes in semiconductors. The proposed model is used to derive expressions for magnetic susceptibility as a function of frequency and temperature.

An empirical correlation for E(J, T ) of a melt-cast-processed BSCCO-2212 superconductor under self field

An empirical correlation is developed for the electrical field strength E(J, T) of a melt-cast processed BSCCO-2212 superconductor. The empirical correlation is based, in part, on the theory of magnetic relaxation and on experimental data at 77 and 87 K. It is developed for temperatures in the range between 77 and 92 K, which is the range of interest for practical devices such as the superconducting fault current limiters. The general form of the correlation may be applicable to other high-T/sub c/ superconductors.

Analysis of the magnetic relaxation in BiPbSrCaCuO high-TC superconductors by means of an interaction model

The magnetic relaxation of cylindrical polycrystalline samples of BiPbSrCaCuO high-TC superconductors has been measured using an inductive technique near the transition temperature. The relaxation data are analyzed by means of an interaction model based on the general theory of magnetic relaxation. This model assumes the existence of an effective energy barrier (Ueff) and considers thermal activation of the vortices over it. The pinning energy (U0) is reduced to the effective barrier Ueff=U0−Uint by the energy associated to the force of repulsion between vortices (Uint). In this work a linear dependence between the interaction energy and the susceptibility has been used, but other dependencies can easily be introduced to account for the observed nonlinearities of the U(J) curve. The model has been used to extract information of the relaxation of BiPbSrCaCuO samples at several magnetic fields and temperatures after switching off the field. A comparison with the results obtained by the Anderson–Kim model is also provided.

Combinatorial relaxation of magnetic fields

Abstract We present a simple combinatorial theorem in the theory of weighted, framed links which is, broadly speaking, consistent with J. B. Taylor's theory of magnetic relaxation (Taylor, 1986). In this theory magnetic helicity plays a preferred role, decaying on the diffusive timescale whereas other topological invariants and the magnetic energy are presumed to decay much more quickly. Upper limits can be placed on the helicity dissipated during a fixed time interval in an isolated magnetised plasma (Berger, 1984). These upper limits vary as η½ (η = resistivity) and hence vanish in the limit η→0. On the other hand, finite energy dissipation may well be possible in that limit; processes which liberate energy on dynamic rather than resistive timescales are known as rapid (or fast) reconnection (Forbes and Priest, 1987). Solar flares, for example, would be difficult to explain without invoking rapid reconnection.

On the quasithermodynamic theory of magnetic relaxation

Abstract The problem of the choice of the independent internal thermodynamic parameters for the description of nonequilibrium systems is discussed on the basis of Bogolubov's idea concerning the relaxation time hierarchy as well as successive reductions in the description of nonequilibrium states of the system when it tends to equilibrium. The set of parameters is shown to be specified, in principle, by the explicit form of the hamiltonian of the system. General conclusions are used to analyse a number of difficulties in the phenomenological theory of magnetic relaxation in solids. Special attention is paid to a good choice of macroscopic parameters. The problem concerning the question why in some cases the application of Bloch equations is incorrect is discussed.

Green's Function Theory of Magnetic Relaxation. IV

The behaviour of paramagnetic resonance, in particular its width and shape, in the neigh­ bourhood of a magnetic transition is calculated on the basis of a Green's function method. The calculation presented in our previous paper is re-examined and refined. New results are discussed in physical terms and compared with existing experimental data. A quantitative agreement is found in the case of MnF 2 without any adjustable parameter. Temperature change of line shape is predicted for a large value of anisotropy.

Theory of Magnetic Relaxation in a Near-Ising System: DyES

A theory of magnetic relaxation in an Ising system is developed, and the results are compared with the experiments of Cooke, Edmonds, Finn, and Wolf on dysprosium ethyl sulfate (DyES), believed to be an Ising dipolar ferromagnet below 0.13\ifmmode^\circ\else\textdegree\fi{}K. Small spin-spin perturbations are postulated to give rise to relaxation within the spin system without having to invoke spin-lattice coupling. The methods of Kubo and Tomita are used to express the relaxation rate in terms of time correlation functions. These can be evaluated far more easily in an Ising system than in a Heisenberg one, and what appear to be reliable estimates are made for the DyES lattice. The resulting relaxation rate is strongly dependent on the spontaneous magnetization, which is calculated from a phenomenological model similar to the one employed by Cooke et al. (nearest-neighbor Ising linear chain plus molecular field). Good agreement is then obtained with experiment for temperature dependence of the ferromagnetic relaxation time. Numerical agreement requires either a strain-induced ${g}_{\ensuremath{\perp}}$ of about 0.05 or a next-nearest-neighbor spin-spin interaction of about 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. These are shown to be plausible numbers by investigating effects of rhombohedral lattice distortion and electric quadrupole-quadrupole interaction.

An experimental evaluation of single domain theories

The Néel theory of magnetization relaxation has been systematically and quantitatively tested for six samples containing synthetic and natural single domain grains. The results of a wide range of experiments involving time, temperature, and direct and alternating magnetic fields were compared with theoretical results predicted by using the Néel theory and the experimental grain size‐coercivity distribution of each sample. Good agreement was found for many experiments involving small field remanences, where grain interaction effects are negligible. Most of the discordant results could be explained by using either the Preisach theory or an extended form of the Néel theory, which takes account of interactions. The two methods are shown to be equivalent, but the Néel theory is more useful in analyzing thermal processes that are intractable by the Preisach method. The similarity of the behavior of synthetic and natural samples suggests that monodomain material is important in many rocks, but experiments on a wide variety of rock types are needed for verification.

A mechanism for the quadrupole relaxation of nuclear spins in liquid metals

The Kubo-Tomita magnetic relaxation theory is used to study the quadrupole mechanism for the relaxation of nuclear spins which was recently proposed by Borsa and Rigamonti to explain experimental relaxation rates.

Green's Function Theory of Magnetic Relaxation. II

As an application of the general theory, line width in the paramagnetic range of temperature is discussed in terms of the spatial correlations of spin deviations. It is shown that the width decreases monotonically with decreasing temperature and that it decreases more rapidly with decreasing temperature than that predicted by previous theories based on the second moment. This is considered as a consequence of the fact that the net effect of modulation due to the exchange interaction will be enhanced in the neighborhood of the critical point. The effect of including the non-secular interaction is also examined. It is shown that the temperature dependence of width is not affected very much by them; however, its magnitude becomes 10/3 times that of the secular width in the limit of high temperature, which is consistent with the results obtained by other theories.

Investigations of solutions of electrolytes in dipole liquids by the method of magnetic resonance. Theory of magnetic relaxation

Investigations of solutions of electrolytes in dipole liquids by the method of magnetic resonance. Theory of magnetic relaxation

Theory of Magnetic Relaxation

Theory of Magnetic Relaxation