C. Viddal

Nanoparticles of Fe in SiO2: A superparamagnet with a collective state?

We present a detailed experimental characterization of the irreversible response of a thin film of nanodimensional Fe particles embedded in SiO2, which includes measurements of the field and temperature dependence of the field-cooled moment, the zero-field-cooled moment, the thermoremanent moment, and the isothermal remanent moment, as well as measurements of the relaxation of the moment with time following various field application and removal protocols. The data are analyzed within the framework of a Preisach ensemble of temperature-dependent, thermally activated, asymmetric double-well subsystems. The analysis allows us to estimate the thermal fluctuation field, and to reconstruct the Preisach spectrum of double-well dissipation fields and bias fields, and its evolution with temperature. Relaxation isotherms exhibit an age dependence which suggests that the free-energy landscape of the thermally blocked state may possess an organization similar to that which characterizes the collectively frozen state ...

Thermal fluctuations in a titanomagnetite mineral : A two-level subsystem approach

We present measurements of the field and temperature dependences of the relaxation of the magnetic moment of a magnetic mineral consisting of nanodimensional particles of titanomagnetite Fe3−xTixO4, with x≅0.1, embedded in a volcanic glass, over an observation time window of 10s⩽t⩽104s, following recoil from a positive saturating field to negative holding fields in the vicinity of the coercive field. The relaxation data are analyzed within the framework of a model which decomposes the free energy landscape into an ensemble of double well potentials, and the thermal fluctuation field Hf for this system is shown to exhibit a maximum as a function of temperature. Numerical simulations of viscosity isotherms based on the double well formalism are presented, which show that this maximum is a natural feature of the formalism, related to the distribution of characteristic energies of the double wells, and thus does not contradict the assumption of Arrhenius relaxation dynamics based on Maxwell-Boltzmann statistics.

Critical behavior and irreversibility in La0.5Sr0.5CoO3

We present detailed measurements of 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. In particular, we present a detailed, quantitative analysis of both the critical behavior in the vicinity of the ferromagnetic ordering temperature TC and the technical/regular magnetic response below TC, with particular emphasis on trying to establish features which are not present in conventional ferromagnets, that is, ferromagnets which do not exhibit spontaneous electronic phase separation. No such features could be identified through the analysis of these macroscopic quantities.

Interpreting viscosity isotherms in materials with significant structural disorder

Numerical simulations of viscosity isotherms are presented for a model system with substantial structural disorder. The system consists of an ensemble of metastable two-level subsystems, characterized individually by an elementary activation moment μ and a double well potential with a dissipation barrier Wd=μHd and a level splitting Ws=2μHs, and collectively by a distribution of equivalent fields p(Hd,Hs)=(2πσd2Hd2)−1∕2exp{−[ln(Hd∕Hdm)]2∕2σd2}×(2πσs2)−1∕2exp(−Hs2∕2σs2). The disorder is modeled with broad dispersions σd=σs∕Hdm=1.0. Under these conditions, a typical experimental observation window 10s⩽t⩽104s reveals only a short quasilogarithmic fragment of the complete relaxation isotherm. In the current investigation, we present a prescription for evaluating the thermal fluctuation field Hf=kT∕μ based on a plot of Tln(tr∕τ0) versus Ha, where tr is the “reversal time” at which an isotherm changes sign (or passes through m=0), and we discuss the merits of this approach with respect to scaling strategies.