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The effect of finite temperature T and observation time t on the Henkel plots of ac and thermally demagnetized systems has been investigated within the framework of a generalized Preisach model, in which it is assumed that thermally activated hopping will occur over all energy barriers W < [W.sup.*] = [k.sub.B]T ln(t/[[Tau].sub.0]), where [[Tau].sub.0] is a microscopic time, and will systematically drive the Preisach plane towards equilibrium. The Preisach distribution function is assumed to be a factorized product of a Gaussian coercive field distribution, with mean value [Mathematical Expression Omitted] and dispersion [[Sigma].sub.c], and a Gaussian interaction field distribution, with a self-consistent mean-field average [Mathematical Expresssion Omitted] and dispersion [[Sigma].sub.s]. Increases in temperature or observation time cause a progressive collapse of the hysteresis cycle, as expected, and also enhance demagnetizing-like curvature in Henkel plots, at least for ac demagnetized systems. An exception is a thermally demagnetized system with k = 0, which has a linear Henkel plot independent of [W.sup.*]. Varying the effective time for thermal relaxation of the magnetization from branch to branch of the hysteresis cycle can have the effect of imitating mean field interactions of both magnetizing-like and demagnetizing-like sign in systems with k = 0, and can even lead to Henkel plots which violate the lower boundary [i.sub.d] = -[i.sub.r].

The implications of thermal demagnetization with respect to the observation of interaction effects in Henkel plots are discussed within the framework of a scalar moving Preisach model. The Preisach distribution is assumed to be a product of a Gaussian coercive field distribution and a Gaussian interaction field distribution. Numerical calculations of the magnetizing and demagnetizing remanences show that, by contrast with other demagnetizing procedures, thermal demagnetization yields Henkel plots whose direction of curvature is uniquely related to the sign of the mean interaction field. In particular, a distribution of interaction fields which is symmetric about the origin yields the same linear Wohlfarth relation as a completely noninteracting system, while demagnetizing (magnetizing)-like mean fields always curve the Henkel plot below (above) the Wohlfarth line. The simple linearity of the Henkel plot in the absence of mean field effects was exploited to evaluate the effectiveness of a recent proposal for experimentally suppressing shape demagnetizing effects in perpendicular recording media.

Measurements of magnetic relaxation isotherms were performed as a function of magnetic field and temperature on an exchange-bond-disordered ferromagnetic perovskite [Ca.sub.0.2] [Sr.sub.0.8]Ru[O.sub.3], over an observation time window 10 s [less than or equal to] t [less than or equal to] [10.sup.4] s, and over a wide range of temperatures which spanned the entire irreversible phase. A plot of Tln([t.sub.r]/[[tau].sub.0]) as a function of holding field--[H.sub.a] was constructed from the times [t.sub.r] at which the measured relaxation isotherms reversed sign. The plot was interpreted with the aid of numerical simulations based on a model collection of two-level subsystems, which showed that the temperature dependence of the magnetic response of this perovskite was shaped by the coexistence of thermal fluctuations and the explicit evolution of the free energy landscape with temperature, and which yielded estimates for the thermal fluctuation field and for the mean intrinsic dissipation barrier as functions of temperature. Index Terms--Ferromagnet, hysteresis, relaxation, thermal fluctuation field.

Measurements of the field and temperature dependence of the field cooled moment, the zero field cooled moment, the isothermal remanent moment (IRM), and the thermoremanent moment (TRM), are presented for a thin film of nanodimensional particles of Fe embedded in alumina ([Al.sub.2][O.sub.3]). At each measurement temperature, the TRM approaches saturation much more rapidly than the corresponding IRM, and warming accelerates the approach to saturation for both TRM and IRM isotherms. Comparisons of numerical simulations of the IRM and TRM, based on the Preisach formalism, which decomposes the metastable state excitation spectrum into an ensemble of double wells with dissipation fields Ha and bias fields [H.sub.s], with the measured isotherms, yields the distribution of characteristic fields p([H.sub.d], [H.sub.s]) and its variation with temperature. Index Terms--Hysteresis, nanoparticles, Preisach models.

The field and temperature dependence of the ac susceptibility, magnetic hysteresis isotherms and the zero-field-cooled and field-cooled moments of La0.5Sr0.5CoO3 are examined. A typical grain size of about 1 mum.ac susceptibility is observed.

It has been possible to calculate the major hysteresis loop and the temperature dependence of two ferromagnetic perovskites La0.95Mg0.05MnO3 and La0.5Sr0.5CoO3. The model presented makes it possible to define the extent to which intrinsic anisotropy effects and thermal fluctuation effects each influence the temperature dependence of the macroscopic response of these materials.

A study was conducted to analyze a modified version of the scalar Preisach model that can replicate experimental systematics. Numerical computations based on the modified Preisach model were determined. The moment reversal along the demagnetizing branch was characterized by two distinct sets of energy barriers. In addition, the initial wall motion was described by a reversible nonlinearity.

The principal magnetizing and demagnetizing remanences of Cr[O.sub.2] audio tape have been measured as a function of applied field, at various temperatures between room temperature and the ferromagnetic Curie temperature, under both ac and thermally demagnetized initial conditions. The Henkel plots of the thermally demagnetized data are all essentially linear and obey the Wohlfarth relation, while those for the ac demagnetized data all exhibit positive curvature, below the Wohlfarth line, which becomes progressively weaker with increasing temperature. Fits to the scalar Preisach model show that the interaction field distribution has no mean field component, and that thermal demagnetization produces a random initial state which is insensitive to fluctuations in the local interaction fields due to disorder, as long as there are no long range interactions Index Terms - Henkel plot, interactions, Preisach model, thermal demagnetization.

A crossover in the presence of reentrant transition was observed when low-field thermoremanent relaxation in a reentrant was measurement. The crossover involved the reentrant's transition from equilibrium dynamics in the ferromagnetic phase to nonequilibrium dynamics. The transition was age-dependent in the reentrant phase.

Low field static magnetization and thermoremanent decay of a Cr-17 at. % Fe spin glass and a re-entrant Cr-22 at. % Fe ferromagnet were studied. The results showed that thermoremanent relaxation during the pure spin glass phase featured nonequilibrium effects and could be characterized by the product of a power law and a stretched exponential superposed on a constant term. A crossover between two dynamic regimes was observed in the ferromagnet.

Numerical simulations of viscosity isotherms are described for the model system with significant structural disorder. The simulations have suggested an alternative strategy for recovering the spectrum of intrinsic metastable state excitation barriers and the thermal fluctuation field.

Measurements of the field and temperature dependence of the relaxation of the magnetic moment of a magnetic mineral consisting of nanodimensional particles of titanomagnetite are presented. The relaxation data are studied within the framework of a model, which decomposes the free energy landscape into an ensemble of double well potentials.

A detailed experimental characterization of the irreversible response of a thin film of nanodimensional Fe particles embedded in SiO2 is presented. The data are analyzed within the framework of a Preisach ensemble of temperature-dependent, thermally activated, asymmetric double-well subsystems.

The formalism is applied to the analysis of irreversible response of a ferromagnetic phase of La0.7Sro.3MnO3. Latter condition is fulfilled in the La0.7Sr0.3MnO3 system, and accounts for the systematic change in the curvature of the TRM as the field is increased.

Measurements of the initial magnetizing curve, the magnetizing remanence and the demagnetizing remanence are performed on a reentrant ferromagnet Fe(sub 0.65)Ni(sub 0.23)Cr(sub 0.12) at several temperatures in the vicinity of the postulated ferromagnetic-spin glass 'reentrant transition'. Magnetic materials with structural disorder exhibit a variety of magnetically ordered phases including sin glass, cluster glass, antiferromagnetism and ferromagnetism.

We present an analysis and comparison of magnetization and remanence data from two particulate systems: commercial Cr[O.sub.2] audiotape and a frozen suspension of magnetoferritin particles. The measurements were performed over a range of temperatures, 25 [degrees] C [less than or equal to] T [less than or equal to] 120 [degrees] C for Cr[O.sub.2] and 5 K [less than or equal to] T [less than or equal to] 10 K for magnetoferritin, and over a range of fields sufficient to saturate the remanence at each temperature. The Cr[O.sub.2] tape was prepared by both ac demagnetization and by thermal demagnetization from above the Curie temperature, while the magnetoferritin was thermally demagnetized from above its highest blocking temperature. The data were analyzed within the framework of a scalar Preisach model that included thermally activated relaxation and a reversible nonlinearity. In the case of Cr[O.sub.2], our fits showed that changes in hysteresis with temperature were due primarily to thermal variations in the spontaneous moment and the anisotropy of the particles, and in their magnetostatic interaction fields, and hence to changes in the free-energy landscape. In magnetoferritin, the temperature dependence of the magnetic response was due almost exclusively to changes in the relative populations of blocked and unblocked particles which relax by thermal activation over a fixed distribution of energy barriers. Our analysis of interaction effects was based on the Wohlfarth - Henkel parametric plot of the principal magnetizing and demagnetizing remanences. These plots showed curvature indicative of demagnetizing-like interactions in both thermally demagnetized magneloferritin and ac-demagnetized Cr[O.sub.2], but essentially no curvature in thermally demagnetized Cr[O.sub.2]. Our analysis revealed that there were no long-range, mean field interactions in either system, but that thermal demagnetization of Cr[O.sub.2] produced a random initial state that was insensitive to fluctuations in the interaction field due to disorder, provided that mean field effects were absent, while thermal demagnetization of magnetoferritin produced a highly asymmetric initial state resembling that induced by ac demagnetization, which was sensitive to these fluctuations. Index Terms - Fine particles, hysteresis, interactions, Preisach model, thermal relaxation.

Measurements of the field and temperature dependence of the frequency-dependent susceptibility χ(ω) the thermoremanent magnetization (TRM) and the isothermal remanent magnetization (IRM) have been performed on a series of mixed ruthenates with chemical formula CaxSr1−xRuO3. The susceptibility data are analysed within the framework of a static scaling theory. Estimates for the critical exponents γ, β, and δ are consistent with a second-order transition from a paramagnetic to a ferromagnetic state, with exchange bond disorder. The remanence data are analysed within the framework of a Preisach model, which incorporates an explicit reference to the critical temperature TC. The analysis shows that, under certain conditions, the TRM and the IRM yield independent scans of the Preisach distribution along two orthogonal directions in the Preisach plane, the TRM along the bias field axis Hs and the IRM along the dissipation field axis Hd. [Copyright &y& Elsevier]