Volume 220, Issue 2

In the past, magnetic nanoparticles have found increasing interest in different biomedical applications, e.g. the magnetic hyperthermia of tumor cells or the remote controlled drug delivery to the gut. These applications are based on a magnetically induced heating effect caused by different magnetic loss mechanisms in the nanoparticles. To advance the present state of the art of these methods, it is important to use particles with a higher specific heating power (SHP) at lower magnetic field amplitudes. To this aim, several iron oxide nanoparticle powders, consisting of particles in the diameter range from 10 nm up to 100 nm, were prepared by two different chemical methods and magnetically as well as morphologically characterized. The magnetic characterization was done by using a vibrating sample magnetometer and the calorimetrical determination of SHP. The dependence of the magnetic losses on the morphological properties was investigated. Magnetic characterization showed that several suitable iron oxide absorbers can be utilized. With decreasing particle size, hysteresis loss underestimates SHP at higher frequencies as measured calorimetrically. The effect of measurement frequency on the hysteresis losses is shown experimentally. Experimental results are discussed in the frame of known theoretical models of nanoparticle magnetism.

A recently introduced shear-flow-free method for measuring the rotational viscosity of a resonantly forced torsional pendulum is used to determine the transverse magnetic relaxation time in magnetite and cobalt-based ferrofluids. From these data the average size of the ferromagnetic grains and their hydrodynamic diameter (core plus surfactant coating) are deduced under in-situ conditions, i.e. without diluting the sample. The reliability of the method is demonstrated by comparing the results with those of the complementary techniques of magneto-granulometry, X-ray diffraction, electron microscopy, and photon-correlation spectroscopy.

Ni- and Co-nanoparticles of average sizes of about 4 to 10 nm were produced by “physical” means with the cluster source LUCAS based upon laser evaporation into seeding gas and adiabatic expansion into an UHV experimentation chamber. They were deposited and subsequently oxidized in situ at about 400 °C. The structures of the nanos were characterized by HRTEM and chemically analyzed by EFTEM. Clear core/shell structures were established with crystalline metallic core and crystalline oxide shell of final thickness of about 2 nm. In one sample it might be that the shell is amorphous. An analysis of magnetic properties by SQUID was performed at temperatures between 5 and 300 K and for three different states of oxidation. Surprisingly strong influences of magnetic defects in the nominally (but probably incompletely saturated) antiferromagnetic oxide layers were observed which result in additional exchange anisotropy, in drastic exchange bias shifts of the hystereses and, in one sample, in a second hysteresis loop contribution. In all samples we found suppression of the superparamagnetic state. The special sample with two loop contributions was numerically evaluated (by co-author H. Krenn). In the final chapter a method is proposed how to transform these “physically” produced magnetic nanoparticles into a ferrofluid of well known composition.

Colloids made from ferrite nanoparticles are a good example of how the properties of materials change when scaled down to the nanometre scale: the continuous spectrum of magnetic spin wave excitations breaks up to a set of discrete absorption lines. In order to exploit this phenomenon, the chemical composition of the ferrite as well as the preparation method for the suspensions are essential: in nickel zinc ferrites, the magnetic anisotropy field and the life time of magnetic resonance states can be fine tuned by the zinc content to achieve optimal microwave absorption. This makes such colloids interesting for high-power microwave applications, such as bond/disbond-on-command, that are difficult to obtain with polycrystalline materials.

A horizontal ferrofluid layer is submitted to a lateral heating and to a strong oblique magnetic field. For a thin enough layer, the steady state solution is the product of the classical solution corresponding to the usual Newtonian fluid submitted to a lateral gradient of temperature, times a modulating factor. This last accounts for the inclination and for the ratio Ra m / Ra , where Ra is the Rayleigh number due to buoyancy and Ra m the magnetic Rayleigh number due to the Kelvin magnetic forces. Physically, fundamental modifications of the temperature and velocity profile intervene when Ra m / Ra >> 1, since, as function of the inclination of the magnetic field, the direction of motion in the infinitely elongated cells can become inverted.

Three different thermally induced phoresis of magnetic colloids are indentified using thermodynamics of irreversible processes. Has been used an exact and original implementation of the mechanical balance that conciliates it with the usual hydrodynamic description of phoresis used to quantify the transport coefficients. Here, only the “translational” approximation has been developed. Are so distinguished: thermomagneto chemophoresis, reversibly coupled to chemophoresis (diffusion), and irreversibly coupled to them, the thermophoresis and magnetophoresis that occurs because the system is a polarized one. The two latter are quantified respectively by the “heat of transport” and the here introduced “magnetisation of transport”. The effects are better compared to thermophoresis, that can be measured individually, and reference measurements of them are mandatory to understand the experimental results.

We consider a ferrofluid cylinder, that is rotating with constant rotation frequency Ω = Ω e z as a rigid body. A homogeneous magnetic field H 0 = H 0 e x is applied perpendicular to the cylinder axis e z . This causes a nonequilibrium situation. Therein the magnetization M and the internal magnetic field H are constant in time and homogeneous within the ferrofluid. According to the Maxwell equations they are related to each other via H = H 0 − M /2. However, H and M are not parallel to each other and their directions differ from that of the applied field H 0 . We have analyzed several different theoretical models that provide equations for the magnetization in such a situation. The magnetization M is determined for each model as a function of Ω and H 0 in a wide range of frequencies and fields. Comparisons are made of the different model results and the differences in particular of the predictions for the perpendicular components H y = − M y /2 of the fields are analyzed.

Rotational Brownian motion of a colloidal magnetic particle in a ferrofluid under the influence of an external magnetic field may give rise to a noise induced rotation of the particle due to the rectification of thermal fluctuations. Via viscous coupling the associated angular momentum is transferred to the carrier liquid and can be measured as a macroscopic torque. In the present contribution we study the case of a rotating ferrofluid and elucidate the interplay between the ratchet effect and the non-zero vorticity of the flow.

Magnetic drug targeting (MDT) is a new approach in local cancer therapy. Starch coated magnetite nanoparticles bound to the chemotherapeutic agent mitoxantrone can be enriched in the tumor region of a VX2 squamous cell carcinoma in rabbits. Using fluorescence-microscopy we showed enrichment of the chemotherapeutic agent in its site of action, the DNA.

This paper describes the preparation, characterization and compare of three different modes for coupling biologically active substances to magnetic particles: (i) covalently bound proteins and enzymes to freshly prepared magnetite in the presence of carbodiimide (CDI), (ii) the encapsulation of magnetic nanoparticles and drugs in lipid mixture dipalmitoylphosphatidylcholine (DPPC) and (iii) the encapsulation of magnetic nanoparticles and drugs in biodegradable poly D,L-lactide polymer (PLA).

The experiments have been performed on the stability of buoyancy-driven flows of a ferrofluid for the cases of an inclined and vertical orientation of thin cylindrical encloser. Visualization of flow patterns is provided by a temperature-sensitive liquid crystal sheet which serves as the lower surface of a transparent heat exchanger. The local temperature sensor is used for measurement of heat transport across the fluid layer. The results indicate that with the help of an external homogeneous transversal magnetic field it is possible to control the stability of thermogravitational shear flow under vertical orientation of the layer, i.e. induce the thermomagnetic mode of instability with vertical orientation of longitudinal convection rolls. As numerous experimental investigations show the origin of concentration density gradients due to gravity sedimentation of magnetic particles and their aggregates results in traveling wave regimes of Rayleigh and thermomagnetic convection flows.