Enhancement of AC-losses of magnetic nanoparticles for heating applications

Aqueous ferrofluids of maghemite nanoparticles coated with carboxydextran were investigated with respect to their specific loss power (SLP) in dependence on frequency and field amplitude of magnetic AC-fields. In order to elucidate the effect of the size distribution on SLP fluid fractions with different mean particle core size were prepared by a magnetic separation procedure from the original ferrofluid. Structural characterisation by means of TEM and XRD as well as reconstruction of core size distributions from magnetisation curves reveals that the narrow size distributions of the fractions cover a range of mean core sizes from about 8 up to 20 nm. Spectra of the complex susceptibility were measured for a frequency range of 20 Hz to 1 MHz. From the imaginary part of the susceptibility the specific loss power is calculated in dependence on frequency. The results are compared with calorimetrical measurements performed in dependence on field amplitude up to 11 kA/m at 410 kHz. A very high specific loss power in the order of 400 W per gram maghemite was found at 410 kHz and 11 kA/m for the fluid fraction having the largest mean core diameter. A deviation from linear response behaviour is found for this sample showing a power law field dependence of the specific loss power SLPH^2.5. In addition to liquid suspensions measurements were performed with particles immobilised in mannitol or gel in order to elucidate the role of Brownian relaxation. The experimentally found dependence of SLP on the mean particle core diameter may be understood in the frame of the Debye dispersion model. Results are discussed with respect to applications of ferrofluids in RF-magnetic hyperthermia.     

Relaxation behavior measuring of transmitted light through ferrofluids film

In this paper, relaxation behavior of transmitted light through thin ferrofluid film under an applied magnetic field is measured. The results show that the intensity of transmitted light through a ferrofluid film increases quickly as soon as an external magnetic field is applied then weakens with time. If uniformity of the field is poor, the transmission of light continuously decreases in a measured duration. Otherwise, the transmission of light will tend increasingly towards a stable value after it decreases to a minimum value while the gradient of the field is low. The relaxation time would increase to an order of some hundreds seconds magnitude and is dependent on the strength of magnetic field and viscosity of the ferrofluids. The field-induced relaxation behaviors of transmitted light through ferrofluids correspond to anisotropic microstructure of the ferrofluids under applied magnetic field. PACS 75.50.Mm; 78.20.Ls     

Magnetic Behaviour and Heating Effect of Fe3O4 Ferrofluids Composed of Monodisperse Nanoparticles

Fe3O4 ferrofluids containing monodisperse Fe3O4 nanoparticles with different diameters of 8, 12, 16 and 18nm are prepared by using high-temperature solution phase reaction. The particles have single crystal structures with narrow size distributions. At room temperature, the 8-nm ferrofluid shows superparamagnetic behaviour, whereas the others display hysteresis properties and the coercivity increases with the increasing particle size. The spin glass-like behaviour and cusps near 190K are observed on all ferrofluids according to the temperature variation of field-cooled （FC） and zero-field-cooled （ZFC） magnetization measurements. The cusps are found to be associated with the freezing point of the solvent. As a comparison, the ferrofluids are dried and the FC and ZFC magnetization curves of powdery samples are also investigated. It is found that the blocking temperatures for the powdery samples are higher than those for their corresponding ferrofluids. Moreover, the size dependent heating effect of the ferrofluids is also investigated in ac magnetic field with a frequency of 55 kHz and amplitude of 200 Oe.     

Evidence for diffusion-induced convection in ferrofluids from small-angle neutron scattering

Isothermal convection in ferrofluids has been induced by a gradient in particle concentration antiparallel to a magnetic field gradient. The deviation of local particle concentration from its equilibrium value produced by the convective motion of the whole fluid gives rise to a corresponding spatial variation of magnetization. This variation has been observed by magnetic neutron scattering in good agreement with expectations based on flow measurements with an anemometric method.     

The onset of thermomagnetic convection in stratified ferrofluids

A non-uniform magnetic field causes an inhomogeneous distribution of magnetic grains in colloidal magnetics (so-called ferrofluids). The rate of concentration equilibrium settling is very low owing to the smallness of the particle diffusion coefficient. Therefore, if the equilibrium does not have enough time to settle, a ferrofluid behaves like a pure fluid, so that stationary convection occurs and no other. In the opposite case, that is when some non-uniform concentration profile has been formed, an oscillatory instability arises. The latter can be effectively excited under the action of a low-amplitude time-periodic magnetic field of an appropriate frequency.     

Polydispersity effects on the magnetization of diluted ferrofluids: a lognormal analysis

Based on a lognormal particle size distribution, this paper makes a model analysis on the polydispersity effects on the magnetization behaviour of diluted ferrofluids. Using a modified Langevin relationship for the lognormal dispersion, it first performs reduced calculations without material parameters. From the results, it is extrapolated that for the ferrofluid of lognormal polydispersion, in comparison with the corresponding monodispersion, the saturation magnetization is enhanced higher by the particle size distribution. It also indicates that in an equivalent magnetic field, the lognormally polydispersed ferrofluid is magnetically saturated faster than the corresponding monodispersion. Along the theoretical extrapolations, the polydispersity effects are evaluated for a typical ferrofluid of magnetite, with a dispersity of σ =0.20. The results indicate that the lognormal polydispersity leads to a slight increase of the saturation magnetization, but a noticeable increase of the speed to reach the saturation value in an equivalent magnetic field.     

Magnetic properties of substituted strontium ferrite nanoparticles and thin films

SrFe_12−x(Zr_0.5Mg_0.5)_xO₁₉ nanoparticles and thin films with x=0-2.5 were synthesized by a sol-gel method on thermally oxidized silicon wafer (Si/SiO₂). Structural and magnetic characteristics of synthesized samples were studied employing x-rays diffraction (XRD), transmission electron microscopy (TEM), magnetic susceptometer, atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM). TEM micrographs display that the narrow size distribution of ferrite nanoparticles with average particle size of 50 nm were fabricated. Fitting obtained data of effective magnetic susceptibility by Vogel-Fulcher law confirms the existence of strong magnetic interaction among fine particles. XRD patterns and FE-SEM micrographs demonstrated that single phase c-axis hexagonal ferrite films with rather narrow grain size distribution were obtained. AFM micrographs exhibited that the surface roughness increases with an increase in Zr-Mg content. It was found from the VSM graphs that with an increase in substitution contents the coercivity decreases, while the saturation of magnetization increases. The Henkle plots confirms the existence of exchange coupling among nano-grain in ferrite thin films.     

Synthesis and magnetic properties of NiFe2−xAlxO4 nanoparticles

Nanocrystalline Al-doped nickel ferrite powders have been synthesized by sol–gel auto-ignition method and the effect of non-magnetic aluminum content on the structural and magnetic properties has been studied. The X-ray diffraction (XRD) revealed that the powders obtained are single phase with inverse spinel structure. The calculated grain sizes from XRD data have been verified using transmission electron microscopy (TEM). TEM photographs show that the powders consist of nanometer-sized grains. It was observed that the characteristic grain size decreases from 29 to 6 nm as the non-magnetic Al content increases, which was attributed to the influence of non-magnetic Al concentration on the grain size. Magnetic hysteresis loops were measured at room temperature with a maximum applied magnetic field of ≈1 T. As aluminum content increases, the measured magnetic hysteresis curves become more and more narrow and the saturation magnetization and remanent magnetization both decreased. The reduction of magnetization compared to bulk is a consequence of spin non-collinearity. Further reduction of magnetization with increase of aluminum content is caused by non-magnetic Al³⁺ ions and weakened interaction between sublattices. This, as well as the decrease in hysteresis was understood in terms of the decrease in particle size.     

Maghemite nanoparticles with very high AC-losses for application in RF-magnetic hyperthermia

Maghemite nanoparticles covalently coated with polyethylene glycol are investigated with respect to different loss processes in magnetic AC-fields. Transmission electron microscopy reveals a narrow size distribution which may be well approximated by a normal distribution (mean diameter 15.3 nm and distribution width 4.9 nm). Aqueous ferrofluids were characterised by DC-magnetometry, by measuring susceptibility spectra for a frequency range 20 Hz to 1 MHz and by calorimetric measurements of specific loss power (SLP) at 330 and 410 kHz for field amplitudes up to 11.7 kA/m. Extremely high values of SLP in the order of 600 W/g result for 400 kHz and 11 kA/m. In addition to liquid ferrofluids measurements were performed with suspensions in gel in order to elucidate the role of Brownian relaxation. The measured susceptibility spectra may be well reproduced by a model using a superposition of Néel and Brown loss processes under consideration of the observed narrow normal size distribution. In this way the observed very high specific heating power may be well understood. Results are discussed with respect to further optimisation of SLP for medical as well as technical RF-heating applications.     

Magnetic properties and magneto-birefringence of magnetic fluids

Static magnetic properties of a large variety of magnetic fluids with magnetite particles is studied. A qualitative study of magnetization curves was performed to establish the influence of interactions or the presence of agglomerations in each sample. Improved equations for magneto-granulometric analysis, for ideal ferrofluids, were proposed. Better results for the mean magnetic diameter than in the case of using the known equations were obtained. A quantitative study using several models for ideal and interacting particles was performed to select the best method and dimensional distribution function for magneto-granulometric analysis as well as for accurately determining macroscopic quantities of samples (initial susceptibility, saturation magnetization, particle number density or magnetic volume fraction) and properties of nanoparticles (mean magnetic diameter, thickness of the nonmagnetic layer and particle distribution). A new model for magneto-birefringence was proposed and discussed as well as applied for diluted and concentrated ferrofluids. The Langevin behaviour of samples was investigated and compared with the investigation based on magnetic properties. Nanoparticles parameters like mean “magneto-optical” diameter, effective anisotropy constant, Shliomis diameter and the real part of the electrical permittivity of particles were accurately determined. Received 18 July 1999 and Received in final form 13 January 2000