Magnetic Nanoparticle Imaging: Insight on the Effects of Magnetic Interactions and Hysteresis of Tracers

The dynamic properties of magnetite nanoparticles are investigated by rate equations with the aim of clarifying the factors affecting their performance as tracers in magnetic particle imaging (MPI). It is shown that size-dependent effects such as magnetic hysteresis and dipole-dipole interaction may have a great impact on the behavior of MPI tracers. Usually, magnetic imaging exploits the higher-order harmonics of the magnetization waveform without considering either intraparticle hysteresis or interparticle interactions. These assumptions may result in an incorrect estimate (either by excess or by defect) of the nanoparticle concentration, which is the ultimate aim of MPI. The mismatch between real and estimated values is apparent for concentrations typical of some therapeutic applications of magnetic nanoparticles or reached by effect of particle accumulation in organs because of slow clearance processes. We show that this difficulty can be removed by measuring not only the magnitude of the third harmonic of the signal but also the phase shift with respect to the driving field. The proposed technique of signal adjustment makes use of the settings of present-day MPI operating devices. The validity of the adjustment procedure is checked by a proof of concept using nonuniform nanoparticle concentrations

Influence of lattice defects on the ferromagnetic resonance behaviour of 2D magnonic crystals

This paper studies, from a modelling point of view, the influence of randomly distributed lattice defects (non-patterned areas and variable hole size) on the ferromagnetic resonance behaviour and spin wave mode profiles of 2D magnonic crystals based on Ni80Fe20 antidot arrays with hexagonal lattice. A reference sample is first defined via the comparison of experimental and simulated hysteresis loops and magnetoresistive curves of patterned films, prepared by self-assembly of polystyrene nanospheres. Second, a parametric analysis of the dynamic response is performed, investigating how edge, quasi-uniform and localized modes are affected by alterations of the lattice geometry and bias field amplitude. Finally, some results about the possible use of magnetic antidot arrays in frequency-based sensors for magnetic bead detection are presented, highlighting the need for an accurate control of microstructural features

Growth of room temperature ferromagnetic Ge1-xMnx quantum dots on hydrogen passivated Si (100) surfaces

A method for the synthesis of room-temperature ferromagnetic dilute semiconductor Ge1-xMnx (5 % < x < 8 %) quantum dots by molecular beam epitaxy by selective growth on hydrogen terminated silicon (100) surface is presented. The functionalized substrates, as well as the nanostructures, were characterized in situ by reflection high-energy electron diffraction. The quantum dots density and equivalent radius were extracted from field emission scanning electron microscope pictures, obtained ex-situ. Magnetic characterizations were performed by superconducting quantum interference device vibrating sample magnetometry revealing that ferromagnetic order is maintained up to room temperature: two different ferromagnetic phases were identified by the analysis of the field cooled – zero field cooled measurements

Measurement of thin film magnetostriction using field-dependent atomic force microscopy

Measurement of thin film magnetostriction is a challenging task, as magnetostrictive material deformations in parts per million, in conjunction with materials at small dimensions, require high precision, often with dedicated set-ups, for reproducible results. We have developed a novel approach employing a commercial atomic force microscope (AFM) with attached electromagnets. Magnetostriction measurements are demonstrated on 50 - 500 nm thick Fe81Al19 films sputter deposited directly on high aspect ratio commercial AFM micro-cantilevers. A magnetostrictive deflection of the cantilever bimorph translates into a deflection force acting in a contact mode measurement, which is interpreted and recorded as a change in height. For determination of the magnetostriction coefficient, we have developed a modified version of the equation for the magnetostrictive deflection of a cantilever bimorph by Guerrero and Wetherhold, taking into account long-range attractive forces acting during contact mode AFM measurements in air. The sub-atomic precision of the AFM, combined with the widespread availability of all components and the simple set-up, makes the measurement of magnetostriction on films of just a few tens of nanometers thickness easily accessible.H2020-MSCA-ITN-2014 SELECTA (grant agreement no. 642642 of the European Commission

Influence of shape, size and magnetostatic interactions on the hyperthermia properties of permalloy nanostructures

We present a detailed study of permalloy (Ni80Fe20) nanostructures with variable shape (disk, cylinder and sphere) for magnetic hyperthermia application, exploiting hysteresis losses for heat release. The study is performed modifying nanostructure aspect ratio and size (up to some hundreds of nanometres), to find the optimal conditions for the maximization of specific heating capabilities. The parameters are also tuned to guarantee negligible magnetic remanence and fulfilment of biophysical limits on applied field amplitude and frequency product, to avoid aggregation phenomena and intolerable resistive heating, respectively. The attention is first focused on disk-shaped nanostructures, with a comparison between micromagnetic simulations and experimental results, obtained on nanodisks still attached on the lithography substrate (2D array form) as well as dispersed in ethanol solution (free-standing). This analysis enables us to investigate the role of magnetostatic interactions between nanodisks and to individuate an optimal concentration for the maximization of heating capabilities. Finally, we study magnetization reversal process and hysteresis properties of nanocylinders (diameter between 150 nm and 600 nm, thickness from 30 nm up to 150 nm) and nanospheres (size between 100 nm and 300 nm), to give instructions on the best combination of geometrical parameters for the design of novel hyperthermia mediators