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

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

Magnetic hysteresis in granular CuCo alloys

Room-temperature hysteresis loops of granular Cu100-xCox alloys (5 less than or equal to x less than or equal to 15) obtained by planar flow casting in air and submitted to proper annealing treatments have been measured up to a field of 10 kOe by means of a vibrating sample magnetometer. In major loops (\H-vert\ = 10 kOe), the reduced remanence-to-saturation ratio m(r) = M-r/M-s and the coercivity H-c measured on all studied materials appear to be related by an almost linear law of the type m(r) approximate to 1/3 (mu H-c/kT), mu being the average magnetic moment on Co particles. A similar relation is also observed on minor symmetrical loops (100 Oe less than or equal to\H-vert\ less than or equal to 9 kOe). The observed results are accounted for by a model which considers the hysteresis as originating by magnetic interactions among nearly superparamagnetic Co particles. (C) 1999 American Institute of Physics. [S0021-8979(99)51408-4]

Thermally evaporated Cu-Co top spin valve with random exchange bias

A cobalt-copper top spin valve was prepared by thermal evaporation of a stack of ferromagnetic thin films separated by thin layers of the diamagnetic metal, with a cap layer containing an antiferromagnetic AFM exchange-biasing material. A nonconventional top AFM layer was used, in order to optimize the multilayer roughness and to avoid electrical interference with metallic layers; it consists of a composite material easily processed by means of optical lithography, basically a polymeric matrix composite with a dispersion of nickel oxide microparticles. Magnetization and magnetoresistance measurements were performed from 4 to 300 K. The measurements of both quantities indicate random pinning action of the top AFM layer, resulting in a small exchange-bias field and in asymmetric magnetization and magnetoresistance curves. A simple model explains the observed physical effect

Magnetic properties and giant magnetoresistance in melt‐spun Co‐Cu alloys

Magnetic, structural, and transport properties of as‐quenched and annealed Co10Cu90 samples have been investigated using x‐ray diffraction and a SQUID magnetometer. The largest value of MR change was observed for the as‐quenched sample annealed at 450 °C for 30 min. The magnetic and transport properties closely correlate with the microstructure, mainly with Co magnetic particle size and its distribution. For thermal annealing the as‐quenched samples below 600 °C, the Co particle diameters increase from 4.0 to 6.0 nm with a magnetoresistance (MR) drop from 33.0% to 5.0% at 10 K. Comparison with the theory indicates that the interfacial electron spin‐dependent scattering mechanism correlates with GMR for Co particle diameters up to about 6.0 nm