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.

An austenite-martensite transition was observed in a 100-nm-thick Ni51.6Mn32.9Sn15.5 film by temperature-dependent resistivity and magnetization measurements, revealing a martensite starting temperature of M-S approximate to 260 K. The influence of the structural phase transition on the electronic structure and the magnetic properties was studied element specifically employing temperature-dependent x-ray-absorption spectroscopy and x-ray magnetic circular dichroism. In addition, density functional theory calculations have been performed to study the electronic and magnetic properties of both phases. It is shown that off-stoichiometric Ni-Mn-Sn alloys can exhibit a substantial magnetocrystalline anisotropy energy in the martensite phase. For Mn a change of the electronic structure and a strong increase of the ratio of orbital to spin magnetic moment m(l)/m(S) can be observed, whereas for Ni nearly no changes occur. Applying an external magnetic field of B = 3 T reverses the change of the electronic structure of Mn and reduces the ratio of m(l)/m(S) from 13.5 to approximate to 1 % indicating a field-induced reverse martensitic transition.

Two series of epitaxial Ni–Mn–Sn thin films of different thickness are investigated for the thickness and composition dependence of the martensitic transformation. Thin films ranging in thickness from 20 to 200nm (series A) and 10 to 100nm (series B) were prepared by magnetron co-sputtering and deposited on heated MgO(001) substrates. The structural characterization was done by temperature-dependent X-ray diffraction measurements. Magnetization and resistivity measurements were performed to investigate the transformation characteristics. We find a strong influence of the film thickness on the relative amount of material undergoing the martensitic transformation, the temperature range of the transformation, and the transformation temperatures. The main contribution originates from the rigid substrate which delays the transformation of the Ni–Mn–Sn near the interface and even leads to a layer of residual austenite at low temperatures. Another issue are size effects which presumably broaden the martensitic transformation and decrease the transformation temperatures. By variation of the thin film composition we find changes of the substrate influence due to a different mismatch between the lattice of MgO and austenite. A better phase compatibility between martensite and austenite, denoted by λ2, not only results in a smaller hysteresis but is also beneficial for the transformation of material close to the substrate.

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.

Epitaxial Ni-Mn-Sn thin films of 200 nm thickness were prepared by magnetron sputtering and deposited onto MgO(100) substrate. They reveal an inverse magnetocaloric effect with a martensitic phase transition around 260 K. The resulting magnetocaloric properties of these films have been determined performing magnetization measurements in the temperature range between 10 and 330 K applying different magnetic fields. The maximum values of entropy change and relative cooling power are 1.6 J kg−1 K−1 and 36.5 J kg−1 for cooling and 1.5 J kg−1 K−1 and 33.9 J kg−1 for heating in μ0ΔH=1 T, respectively. These data are comparable with bulk values of Ni-Mn-Sn Heusler alloys.

A thickness dependent exchange bias in the low temperature martensitic state of epitaxial Ni-Mn-Sn thin films is found. The effect can be retained down to very small thicknesses. For a Ni50Mn32Sn18 thin film, which does not undergo a martensitic transformation, no exchange bias is observed. Our results suggest that a significant interplay between ferromagnetic and antiferromagnetic regions, which is the origin for exchange bias, is only present in the martensite. The finding is supported by ab initio calculations showing that the antiferromagnetic order is stabilized in the phase.

We report site-specific energy loss magnetic dichroism measurements of the technologically interesting Heusler alloy Ni2MnSn. In addition, we confirm the theoretical prediction that under certain conditions, two different atoms on inequivalent lattice sites give dichroic signals with opposite signs. With this, it is possible to distinguish the magnetic moments of atomic columns that are merely 1.5Å apart using a conventional transmission electron microscope without the need for aberration corrections.

In this chapter the theoretical basics important to understand all aspects of this work are presented. Theoretical concepts that are import for single chapters only are presented accordingly. In the first section, fundamental properties of the martensitic transformation are introduced. The concept of phase compatibility is introduced in the following section. In the last section, the influences of defects and the thin film geometry on the martensitic transformation are discussed. Note that not all details that are required for understanding can be presented in this thesis. It is assumed that the reader is accustomed to basic linear elasticity and continuum mechanics. A good introduction about both topics can be found in e.g. [16]. Further reading about martensitic transformations can be found in e.g. [8] and [17].

We investigate the influence of the film thickness on the martensitic transformation for the example of Ni-Mn-Sn thin films. Epitaxial films with thicknesses ranging from 100 nm down to 10 nm were deposited on MgO by co-sputtering on heated substrates. The martensitic transformation is investigated using temperature dependent x-ray diffraction, magnetization and resistivity measurements. X-ray diffraction and transmission electron microscopy is used to study the growth and the martensitic structure of the films. We find that the martensitic transformation temperatures reduce and the transformation range increases with decreasing film thickness. We show that the transformation is still possible down to a film thickness of 10 nm. A systematic study on the resistance change caused by the martensitic transformation implies that the transformation is suppressed close to the interfaces to the MgO.

This paper highlights recent advances in synthesis and magnetotransport properties of magnetic Co nanopartides. It is shown that magnetic Co nanoparticles self-assembled in nanoparticular monolayers revealing giant magnetoresistance similar to granular systems but with additional features resulting from dipolar interactions between small domains of nanoparticles. A spin-valve with one magnetic Co nanoparticular electrode is employed as a model to demonstrate that individual magnetic moments of Co nanoparticles can be coupled to a magnetic Co layer which in turn offers tailoring of the resulting giant magnetoresistance characteristics. In addition, it is demonstrated that combining a magnetic on-off ratchet with magnetic tunneling junctions integrated in the ratchet introduces a new biosensor concept enabling: (1) simultaneous transporting and separating biomolecules, (2) dynamical biomolecule detection when passing magnetic tunneling junctions in a 1D arrangement. It is projected that this biosensor concept could be applied for viruses as well as for bacteria.

Magnetic particles diluted in liquid agglomerate in rod-like particle arrays if an external homogeneous magnetic field is applied. This work introduces a method to specifically exploit particle–particle interaction to obtain flow control of magnetic particles without changing the motion state of the carrier liquid. Experiments show the possibility to uncouple the particle flux from the motion state of liquid. We show how this method may be applied to design a microfluidic geometry in which the particle flow in a specific direction is either enabled or suppressed by the relative orientation of the fluid velocity and the external field.

Using magnetron sputtering, we have prepared Co-Fe-B/tunnel barrier/Co-Fe-B magnetic tunnel junctions with tunnel barriers consisting of alumina, magnesia, and magnesia-alumina bilayer systems. The highest tunnel magnetoresistance ratios we found were 73% for alumina and 323% for magnesia-based tunnel junctions. Additionally, tunnel junctions with a unified layer stack were prepared for the three different barriers. In these systems, the tunnel magnetoresistance ratios at optimum annealing temperatures were found to be 65% for alumina, 173% for magnesia, and 78% for the composite tunnel barriers. The similar tunnel magnetoresistance ratios of the tunnel junctions containing alumina provide evidence that coherent tunneling is suppressed by the alumina layer in the composite tunnel barrier.

The authors present a method for flow guidance of magnetic particles in microfluidic devices. Using dipolar particle interactions, the flow of magnetic particles can be manipulated without changing the flow of the carrier liquid. The proposed system operates without any external magnetic gradient fields. Therefore, no electromagnetic components on the microscale are necessary. The method used for the manipulation is deduced from theoretical simulations and tested experimentally. Two different applications are introduced: a particle diverter designed for particle guidance along channel cross-sections and a device for generating a discretized particle flow.

The oxidation behavior of metallic nanoparticles is investigated in respect to ligand influences. The nanoparticle oxidation is modeled in a shell-core approach. The shell represents oxidation of surface atoms modeled by Johnson–Mehl–Avrami–Kolmogorov equations for isothermal growth. The oxidation of the nanoparticle core is described by a model introduced by Cabrera and Mott [Rep. Prog. Phys. 12, 163 (1949)]. In order to investigate the ligand influence one single parameter is introduced for both surface and bulk oxidation. The growth of the oxide layer is simulated in a level set framework via finite element methods. The theoretical results are compared to experimental findings of Kanninen et al. [J. Coll. Interf. Sci. 318, 88 (2008)].

The detection capability of magnetic beads and nanoparticles by tunneling magnetoresistance sensors is analyzed in a finite element framework. The limitations for single particle detection and the determination of the particle position are investigated. It will be shown how varying the geometrical sensor design may readily be employed to adjust the setup to a specific measurement task. Especially, we show up strategies increasing the sensitivity by introducing magnetically soft areas. Further, number sensitive detections are discussed and the influence of dipolar particle coupling on the measured signal is calculated.

This article reviews recent developments on magnetoresistive detection of magnetic beads or nanoparticles by nanoscale sized sensors. Sensors are analyzed from an experimental and a numerical point of view in respect to their capability to either localize the position of a single magnetic particle or to detect the number of particles in a certain range. Guidelines are shown up on how to extend single sensors to sensor arrays with very high spatial resolution and how to modify the sensor shape in order to provide long distance measurements. Further, sensors in biological lab-on-a-chip environments are discussed. The magnetic ratchet and a gravitation based microfluidic component are reviewed as important tools to position and, therefore, detect biological components in continuous-flow devices.

In this work, different approaches in order to enhance the sensitivity of tunnel magnetoresistive sensors are discussed by means of finite element simulations. Several sensor layouts consisting of a free CoFeB sensing layer and a pinned bottom electrode are investigated. A decrease in the detection threshold is predicted by introducing magnetic areas at the sensor boundaries which can be easily switched due to a combined interaction of exchange contribution and stray field coupling of the layers.

Cobalt nanoparticles have been prepared with TOPO, subsequently a ligand exchange was carried out. Samples have been prepared by dropping particle solution on Si-wafer, which lead to samples which consist of multilayers partially. The samples were studied with respect to the 3D order to gain information about the influence of the ligand on the 3D structure of the particle array and the oxidation process in multilayered particles.

In this work a device for separating small magnetic particles in continuous flow is introduced, consisting of two microfluidic channels that are connected by a junction channel. Applying two different flow rates, particles can be separated combining hydrodynamic and magnetophoretic effects. The two different flow rates introduce an additional degree of freedom that enables the microfluidic geometry to act as a hydrodynamic switch that can overcome diffusive contributions making the device applicable for particles of the size scale below 100 nm. Theoretical predictions based on finite element methods are compared to experimental observations.