This chapter is addressing the physical impact of ferromagnetic Heusler entities when approaching the nanoscale, e.g. as nanoparticles or as very small grains in magnetic shape Heusler alloys, on resulting magnetic as well as microstructural properties. Based on the soft magnetic behavior of Co2FeGa and Co2FeSi as two representatives of the full Heusler family their superparamagnetic potential is projected to applications in biotechnology. These applications can now be pictured due to the progress which has been made in synthesizing Heusler nanoparticles. Taken Co2FeGa as a candidate the chemical preparation avenue to achieve nanoparticles with reliable physical properties is demonstrated leading to a nanoparticular GMR-effect. It is shown that magnetic nanoparticles can be embedded in agarose as a biogel when employing external magnetic fields so as to configure the nanoparticle arrangements for optimizing the GMR-effect. Possible consequences in case of a nanoparticular TMR-effect are pictured. The very small grain size in magnetic shape Heusler alloys is determining the austenite-martensite transformation in ultra-thin films which might play a major role for spintronic applications also bridging two research field in addition. The principle microstructural influences on the austenite-martensite transformation in thin films are discussed in terms of epitaxial growth, phase compatibility, crystal quality and size scale effects. Thereafter, details concerning the martensitic transformation in a film thickness range from 10 to 100nm are discussed for two off-stoichiometric NiMnSn Heusler compositions.The triumphant advance of Heusler compounds can mainly be attributed to their unique band structure enabling the realization of different physical properties such as ferromagnetism, semi- and superconductivity in one material class [1]. Moreover, the theoretical prediction of 100% spin polarization in an entire class of materials, the half-Heusler XYZ [2-5] as well as the full-Heusler X2YZ [6-9] alloys (X being a transition metal element such as Fe, Ru, Co, Rh, Ni, Pd, Pt, or Cu, Y being another transition metal element such as Ti, Zr, V, Nb, Cr, or Mn, and Z being a group III, IV, or V element such as Al, Ga, In, Si, Ge, Sn, As, or Sb) are currently the driving force for spintronic applications in form of thin film devices such as magnetic tunneling junctions (MTJ). A Heusler-MTJ generally is a layered thin film structure with a tunneling barrier, e.g. Al2O3 [10], MgO [11, 12] or BaO[13], sandwiched in between two ferromagnetic electrodes whereby at least one electrode is made of a Heusler compound. The interface quality between the ferromagnetic electrodes and the tunneling barrier as well as the crystallinity of the barrier critically determine the resulting tunneling magneto resistance (TMR) effect amplitude. The thickness of the ferromagnetic electrodes has not a marked impact on the TMR effect amplitude.However, the central question of this chapter is aiming at small ferromagnetic Heusler entities approaching the nanoscale such as nanoparticles or grains where size effects as well as constrains by the surroundings are drastically influencing the physical properties.

We present results of Monte Carlo and stochastic spin dynamics simulations of a magnetic nanoparticle model system based on experimentally produced samples. Thermodynamic investigations as well as spin dynamics studies show characteristic features, both resembling magnetic dipole glass behaviour. While spin dynamics studies at T = 0 yield a multitude of low energy configurations, thermodynamic simulations show a clear transition between a paramagnetic and a frozen magnetic state. Moreover, we demonstrate the application of experimentally inspired demagnetization protocols to compute low energy configurations of the systems under consideration efficiently.

The formation of magnetic bead or nanoparticle superstructures due to magnetic dipole dipole interactions can be used as configurable matter in order to realize low-cost magnetoresistive sensors with very high GMR-effect amplitudes. Experimentally, this can be realized by immersing magnetic beads or nanoparticles in conductive liquid gels and rearranging them by applying suitable external magnetic fields. After gelatinization of the gel matrix the bead or nanoparticle positions are fixed and the resulting system can be used as a magnetoresistive sensor. In order to optimize such sensor structures we have developed a simulation tool chain that allows us not only to study the structuring process in the liquid state but also to rigorously calculate the magnetoresistive characteristic curves for arbitrary nanoparticle arrangements. As an application, we discuss the role of magnetoresistive sensors in finding answers to molecular recognition.

of Stochastic Spin Dynamicsand Monte Carlo Methods L. Teich, C. Schröder, C. Müller, A. Patel, J. Meyer and A. Hütten Bielefeld Institute for Applied Materials Research, University of Applied Sciences Bielefeld, Wilhelm-Bertelsmann-Str. 10, 33602 Bielefeld, Germany Department of Engineering Sciences and Mathematics, University of Applied Sciences Bielefeld, Wilhelm-Bertelsmann-Str. 10, 33602 Bielefeld, Germany Department of Physics, Bielefeld University, Universitätsstr. 25, 33615 Bielefeld, Germany

Heusler nanoparticles emerge as a new class of multifunctional materials. In this critical review, the latest progress in studies on Heusler nanoparticles is summarized. The authors discuss their structural and physical properties interesting for research fields such as spintronics and ferromagnetic shape memory alloys. As a young research field, the majority of studies on Heusler nanoparticles focus on their synthesis, structure, and magnetic characterizations. Important issues such as size dependent structure, phase transition, magnetic, and spin-related properties are still open. Further investigations are needed to verify the technical significance of Heusler nanoparticles for practical applications such as data storage, magnetic sensors, and microactuators.

Lab-on-a-chip immuno assays utilizing superparamagnetic beads as labels suffer from the fact that the majority of beads pass the sensing area without contacting the sensor surface. Different solutions, employing magnetic forces, ultrasonic standing waves, or hydrodynamic effects have been found over the past decades. The first category uses magnetic forces, created by on-chip conducting lines to attract beads towards the sensor surface. Modifications of the magnetic landscape allow for additional transport and separation of different bead species. The hydrodynamic approach uses changes in the channel geometry to enhance the capture volume. In acoustofluidics, ultrasonic standing waves force µm-sized particles onto a surface through radiation forces. As these approaches have their disadvantages, a new sensor concept that circumvents these problems is suggested. This concept is based on the granular giant magnetoresistance (GMR) effect that can be found in gels containing magnetic nanoparticles. The proposed design could be realized in the shape of paper-based test strips printed with gel-based GMR sensors.

A suspension of monodisperse Au-particles of either 3 or 6nm were mixed with a dilution of 6nm Co-particles. The resulting mixture was employed for the formation of granular films and the transport properties of these assemblies were analyzed. An increased granular giant magnetoresistive response was observed for samples with a high content of Au-particles. The experimental data were compared to numeric solutions of the Landau–Lifshitz–Gilbert equation for discrete magnetic moments. The alteration of the magnetic properties can be related to the formation of a nanoparticular structure resulting from the minimization of the particle stray fields.

We present transport measurements with magnetoresistance effect amplitudes of up to 260% at room temperature obtained in granular systems consisting of Co nanoparticles embedded in conductive gels as a non-magnetic matrix. In order to gain a better understanding of the transport mechanism in gel during measurement, the granular system was simultaneously monitored by optical microscopy. Gel-like matrices with different conductivities and viscosities were tested and will allow us to realize a highly sensitive granular giant magnetoresistance sensor without the need for lithographic techniques.

Assemblies of highly ordered magnetic nanoparticles dominated by dipolar particle coupling are analyzed in respect to their capability to act as a magnetic field sensor. We demonstrate a wide range of possibilities to tailor the response properties of the sensor to specific demands by either modification of the nanocomponents or the structure as a whole. Additionally, we find that increased detection sensitivity may be bought at the cost of an inherent noise signal. This behavior is contributed to the existence of four distinct operation modes each governed by a different energy contribution within the assembly.

Self-assembled two-dimensional arrays of either 14 nm hcp-Co or 6 nm ε-Co particle components were treated by hydrogen plasma for various exposure times. A change of hysteretic sample behavior depending on the treatment duration is reported, which can be divided in two time scales: oxygen reduction increases the particle magnetization during the first 20 min, which is followed by an alteration of the magnetic response shape. The latter depends on the respective particle species. Based on the Landau-Lifshitz equations for a discrete set of magnetic moments, we propose a model that relates the change of the hysteresis loops to a dipole-driven ordering of the magnetocrystalline easy axes within the particle plane due to the high spatial aspect ratio of the system.