Magnetostatic response and field-controlled haloing in binary superparamagnetic mixtures

The work is devoted to the theoretical and numerical analysis of a two-component superparamagnetic system. Namely, of a rigid superparamagnetic cluster embedded in a superparamagnetic medium and subjected to a uniform magnetic field. Both cluster and the medium contain single-domain nanoparticles of the same diameter and magnetic moment. But the concentration of nanoparticles within the cluster is higher than that in the surrounding medium. Equilibrium magnetic response of the system in wide ranges of concentrations and interaction energies is calculated using Langevin dynamics simulations. Corresponding theoretical predictions are obtained within the analytical framework, previously developed for ferrofluid emulsions. The framework is proven to be accurate in the case when nanoparticles of the medium are immobilized. However, if particles are subjected to a translational Brownian motion, the applied field causes their local redistribution in the cluster vicinity. This behavior is reminiscent of the so-called ``haloing'' effect previously observed experimentally in bimodal magnetorheological fluids. It is shown that the haloing can lead to an anomalous increase of the system magnetization at large enough applied fields.Comment: 11 pages, 7 figure

Exploring Neuronal Bistability at the Depolarization Block

Many neurons display bistability - coexistence of two firing modes such as bursting and tonic spiking or tonic spiking and silence. Bistability has been proposed to endow neurons with richer forms of information processing in general and to be involved in short-term memory in particular by allowing a brief signal to elicit long-lasting changes in firing. In this paper, we focus on bistability that allows for a choice between tonic spiking and depolarization block in a wide range of the depolarization levels. We consider the spike-producing currents in two neurons, models of which differ by the parameter values. Our dopaminergic neuron model displays bistability in a wide range of applied currents at the depolarization block. The Hodgkin-Huxley model of the squid giant axon shows no bistability. We varied parameter values for the model to analyze transitions between the two parameter sets. We show that bistability primarily characterizes the inactivation of the Na+ current. Our study suggests a connection between the amount of the Na+ window current and the length of the bistability range. For the dopaminergic neuron we hypothesize that bistability can be linked to a prolonged action of antipsychotic drugs.Comment: 26 pages, 8 figures, accepted to PLoS ON