M. Bahiana

Magnetostatic interactions between magnetic nanotubes

The investigation of interactions between magnetic nanotubes is complex and often involves substantial simplifications. In this letter an analytical expression for the magnetostatic interaction, taking into account the geometry of the tubes, has been obtained. This expression allows for the definition of a critical vertical separation for relative magnetization between nanotubes and can be used for tailoring barcode-type nanostructures with prospective applications such as biological separation and transport.

Reversal modes in arrays of interacting magnetic Ni nanowires: Monte Carlo simulations and scaling technique

The effect of dipolar interactions in hexagonal arrays of Ni nanowires has been investigated by means of Monte Carlo simulations combined with a scaling technique, which allows the investigation of the internal structure of the wires. A strong dependence of the coercivity and remanence on the distance between wires has been observed. At intermediate packing densities the coercivity exhibits a maximum, higher than the non-interacting value. This behavior, experimentally observed, has been explained on grounds of the interwire dipolar interactions. Also, different reversal modes of the magnetization have been identified.

Stability of magnetic configurations in nanorings

The relative stability of the vortex, onion, and ferromagnetic phases in nanorings is examined as a function of the ring geometry. Total energy calculations are carried out analytically, based on simple models for each configuration. Results are summarized by phase diagrams, which might be used as a guide to the production of rings with specific magnetic properties.

Domain wall control in wire-tube nanoelements

The possibility of a three-state nanoelement, composed by a wire and a tube, is investigated by means of Monte Carlo simulations. The desired behavior may be identified by a step or plateau in the hysteresis curve, corresponding to a partial pinning of the domain wall at the interface between wire and tube sections. This step may be augmented in segmented nanoelements with large coercivity difference between the sections. Different possibilities, such as geometry and choice of materials, are explored.

Magnetostatic bias in multilayer microwires : Theory and experiments

The hysteresis curves of multilayer microwires consisting of a soft magnetic nucleus, intermediate nonmagnetic layers, and an external hard magnetic layer are investigated. The magnetostatic interaction between magnetic layers is proved to give rise to an antiferromagneticlike coupling resulting in a magnetostatic bias in the hysteresis curves of the soft nucleus. This magnetostatic biasing effect is investigated in terms of the microwire geometry. The experimental results are interpreted considering an analytical model taking into account the magnetostatic interaction between the magnetic layers.

Magnetic properties of layered nanorings

The magnetic structure of nanorings consisting of alternate layers of magnetic and nonmagnetic materials is investigated as a function of their geometry. Phase diagrams giving the relative stability of characteristic internal magnetic configurations of the rings are obtained. Attention is focused on the condition for occurrence of the vortex configurations, in which case the layered structure might be used to produce magnetoresistive random access memories.

Asymmetric hysteresis loop in magnetostatic-biased multilayer nanowires.

The hysteresis of multilayer nanowires composed by a soft magnetic cylindrical wire, a non-magnetic spacer layer and an external hard magnetic shell is investigated. The external magnetic shell originates a non-homogeneous magnetic field on the inner wire, which is responsible for a displacement and a change of the width of the hysteresis curve of the wire. Moreover, different reversal modes occur at each branch of the hysteresis loop, which can be understood by analyzing the interaction magnetostatic field along the wire. Our results open the possibility of controlling two parameters of the hysteresis loop, the coercivity and the bias, providing an interesting system to be investigated.

Magnetic properties of La0.67Sr0.33MnO3/BiFeO3(001) heterojunctions: Chemically abrupt vs. atomic intermixed interface

Using first-principles density-functional calculations, we address the magnetic properties of the ferromagnet/antiferromagnet La0.67Sr0.33MnO3/BiFeO3(001) heterojunctions, and investigate possible driving mechanisms for a ferromagnetic (FM) interfacial ordering of the Fe spins recently observed experimentally. We find that the chemically abrupt defect-free La0.67Sr0.33MnO3/BiFeO3(001) heterojunction displays, as ground state, an ordering with compensated Fe spins. Cation Fe/Mn intermixing at the interface tends to favour, instead, a FM interfacial order of the Fe spins, coupled antiferromagnetically to the bulk La0.67Sr0.33MnO3 spins, as observed experimentally. Such trends are understood based on a model description of the energetics of the exchange interactions.

Vortex core size in interacting cylindrical nanodot arrays

The effect of dipolar interactions among cylindrical nanodots, with a vortex–core magnetic configuration, is analyzed by means of analytical calculations. The cylinders are placed in a N × N square array in two configurations—cores oriented parallel to each other and with antiparallel alignment between nearest neighbors. Results comprise the variation in the core radius with the number of interacting dots, the distance between them and dot height. The dipolar interdot coupling leads to a decrease (increase) of the core radius for parallel (antiparallel) arrays.

Stability of magnetic nanoparticles inside ferromagnetic nanotubes

During the last years great attention has been given to the encapsulation of magnetic nanoparticles. In this work we investigated the stability of small magnetic particles inside magnetic nanotubes. Multisegmented geometries were tested in order to optimize the stability of the particle inside the nanotubes. Our results evidenced that multisegmented nanotubes are more efficient to entrap the particles at temperatures up to hundreds of kelvins.