Search

Driving immobilized, single-domain magnetic nanoparticles at high frequency by square wave fields instead of sinusoidal waveforms leads to qualitative and quantitative improvements in their performance both as point-like heat sources for magnetic hyperthermia and as sensing elements in frequency-resolved techniques such as magnetic particle imaging and magnetic particle spectroscopy. The time evolution and the frequency spectrum of the cyclic magnetization of magnetite nanoparticles with random easy axes are obtained by means of a rate-equation method able to describe time-dependent effects for the particle sizes and frequencies of interest in most applications to biomedicine. In the presence of a high-frequency square-wave field, the rate equations are shown to admit an analytical solution and the periodic magnetization can be therefore described with accuracy, allowing one to single out effects which take place on different timescales. Magnetic hysteresis effects arising from the specific features of the square-wave driving field results in a breakthrough improvement of both the magnetic power released as heat to an environment in magnetic hyperthermia treatments and the magnitude of the third harmonic of the frequency spectrum of the magnetization, which plays a central role in magnetic particle imaging. [ABSTRACT FROM AUTHOR]

Benzyl alcohol route, starting form Fe( III) acetylacetonate, was applied to obtain crystalline magnetite nanoparticles. In particular, the effect of synthesis parameters (time and temperature) and use of SOCl2 on the physical and magnetic properties of the so-obtained powders were evaluated. By means of this approach, monocrystalline magnetite particles with size ranging from 5 to 15 nm were obtained. [ABSTRACT FROM AUTHOR]

Magnetite nanoparticles were prepared by a non-hydrolytic sol-gel (NHSG) process in the presence of benzyl alcohol. The obtained magnetite suspensions were mixed with an aliphatic epoxy resin and the formulations were photo-polymerized to achieve composite materials with magnetic properties. The prepared magnetite nanoparticles and epoxy composites were fully characterized in terms of their magnetic properties. [ABSTRACT FROM AUTHOR]

Isothermal magnetization and initial dc susceptibility of spheroidal, nearly monodisperse magnetite nanoparticles (typical diameter: 8 nm) prepared by a standard thermo-chemical route have been measured between 10 and 300 K. The samples contained magnetite nanoparticles in the form of either a dried powder (each nanoparticle being surrounded by a stable oleic acid shell as a result of the preparation procedure) or a solid dispersion in PEGDA-600 polymer; different nanoparticle (NP) concentrations in the polymer were studied. In all samples the NPs were not tightly agglomerated nor their ferromagnetic cores were directly touching. The high-temperature inverse magnetic susceptibility is always found to follow a linear law as a function of T, crossing the horizontal axis at negative temperatures ranging from 175 to about 1,000 K. The deviation from the standard superparamagnetic behavior is related to dipolar interaction among NPs; however, a careful analysis makes it hard to conclude that such a behavior originates from a dominant antiferromagnetic character of the interaction. The results are well explained considering that the studied samples are in the interacting superparamagnetic (ISP) regime. The ISP model is basically a mean field theory which allows one to straightforwardly account for the role of magnetic dipolar interaction in a NP system. The model predicts the existence of specific scaling laws for the reduced magnetization which have been confirmed in all studied samples. The interaction of each magnetic dipole moment with the local, random dipolar field produced by the other dipoles results in the presence of a large fluctuating energy term whose magnitude is comparable to the static barrier for magnetization reversal/rotation related to magnetic anisotropy. On the basis of the existing theories on thermal crossing of a barrier whose height randomly fluctuates in time it is predicted that the rate of barrier crossing is substantially driven by the rate of barrier fluctuations, which is fast (10-10 Hz) and almost independent of temperature. As a consequence, the standard picture of superparamagnetic NPs which undergo single-particle blocking by a static barrier below the blocking temperature should be substantially revised, at least in the present materials. The ISP model is perfectly matching with the view of activated magnetization rotation whose kinetics is significantly modified by barrier height fluctuations. [ABSTRACT FROM AUTHOR]

Abstract: Slow fluctuations of the electrical resistance R have been systematically observed in three glass-ceramics containing aggregates of magnetite nanoparticles, in measurements where R was studied as a function of time at the magnetic remanence. These fluctuations are shown to be an intrinsic (i.e., not instrumental or spurious) feature of these materials. The associated electrical noise has been obtained by performing a numerical FFT transform of different segments of the R(t) curves after proper baseline subtraction. In this way, the spectral density has been investigated in the very low-frequency region (1×10−3

Abstract: Magnetite nanoparticles are synthesized by thermal decomposition of Fe(acac)3 and subsequently coated with a silica shell exploiting a water-in-oil synthetic procedure. The as-produced nanopowder is mixed with a photocurable hyperbranched resin and the polymerization process is studied by means of real-time FTIR (RT-FTIR). Owing to the presence of the silica shell, the photocuring ability of the systems containing core–shell structures is highly improved compared with that of the formulations filled with bare magnetite nanoparticles, allowing the efficient polymerization of a 100 μm-thick film loaded with an unprecedented 8 vol% of magnetic filler. [Copyright &y& Elsevier]