Magnetic fluids based on Ba hexaferrite as well as iron oxide particles with enhanced anisotropy barriers show heating effects in ac magnetic fields which may be useful in technical processes as well as medical applications (magnetic hyperthermia). Such particles also allow the detection of biological binding reactions through an enhanced Neel relaxation time above the Brown relaxation. The loss processes and the relaxation times depend strongly on the mean particle size and the size distribution width. To influence and improve the mean size as well as the size distribution, new approaches to the preparation are promising, where nucleation and growth of the particles can be influenced independently or where further growth is possible on small given particles without further nucleation. We used a glass crystallization method for preparation of nanocrystalline Ba hexaferrite as well as magnetic iron oxide and a cyclic growth method based on 'conventional' precipitation for iron oxide preparation. Properties of the powders prepared, as well as water based ferrofluids, were analysed using x-ray diffraction, transmission electron microscopy and magnetic methods. Values of the specific loss power of the order of 100 W/g maghemite may be achieved with the option of further increase by improving the core size distribution.

/pdf/nanocrystalline-iron-oxide-and-ba-ferrite-particles-in-the-33nop6odyj.pdf

Nanocrystalline iron oxide and Ba ferrite particles in the superparamagnetism-ferromagnetism transition range with ferrofluid applications

Advanced materials for drug delivery and biosensors based on magnetic label detection

Patterned monolayers of n-octadecyltrichlorosilane have been used as templates for site-selective derivatization using Fe nanoparticles (see picture). The templates are fabricated by an electrochemical oxidation process that is mediated by a bias voltage applied with a conductive SFM tip. The surface terminal CH3 groups of the monolayer are locally converted into COOH groups under and in close vicinity of the tip.

Fabrication via Electrochemical Oxidation of Self‐Assembled Monolayers and Site‐Selective Derivatization of Surface Templates

Since 2006, fully scalable matrix-based magnetoresistive biochips have been proposed. This integration was initially achieved with thin film switching devices and moved to complementary metal-oxide-semiconductor (CMOS) switching devices and electronics. In this paper, a new microfabrication process is proposed to integrate magnetoresistive sensors on a small CMOS chip (4 mm2). This chip includes a current generator, multiplexers, and a diode in series with a spin valve as matrix element. In this configuration, it is shown that the fabricated spin-valves have similar magnetic characteristics when compared to standalone spin valves. This validates the successfulness of the developed microfabrication process. The noise of each matrix element is further characterized and compared to the noise of a standalone spin valve and a portable electronic platform designed to perform biological assays. Although the noise is still higher, the spin valve integrated on the CMOS chip enables an increase in density and compactness of the measuring electronics.

/pdf/integration-of-magnetoresistive-biochips-on-a-cmos-circuit-46jueqobsq.pdf

Integration of Magnetoresistive Biochips on a CMOS Circuit

The detection and manipulation of biomolecules on a common platform is of considerable interest not only for application in devices such as diagnostic tools but also for basic research in biological and medical systems. A promising approach is the utilisation of magnetic particles as markers and carriers for biomolecules. The principle functionality of this approach is demonstrated by the authors. Magnetic particles used as markers can be detected by highly sensitive magnetoresistive sensors resulting in a purely electronic signal. A direct comparison with the standard fluorescence method reveals the advantages of using the magnetic particles. In addition, magnetic particles used as carriers can be manipulated on-chip via currents running through especially designed line patterns. Some current drawbacks and future aspects are discussed. The combination of sensing and manipulating magnetic particles is a promising choice for future integrated lab-on-a-chip systems.

Magnetic particles as markers and carriers of biomolecules

Magnetoresistive logic and biochip

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Magnetoresistive logic and biochip