title: Transport Properties and Magnetoresistance of Cluster-Assembled Fe-Ge and Fe-Ag Nanocomposites
Creators: name: Gack, Nicolas Sebastian
Publisher: name: UNSPECIFIED
Year: 2023

description: Granular nanocomposites are composite materials in which grain-like particles with dimensions on the order of nanometers form one of the phases. These nanoparticles are embedded in a second phase, the matrix. Such granular nanocomposites constitute a very promising class of materials with great potential for novel and tailorable properties, making granular nanocomposites especially interesting for scientific endeavor. In the simplest case, granular nanocomposites are synthesized via co-deposition of two immiscible chemical elements. In this approach, nanoparticles grow via incorporation of diffusing atoms of one of the elements forming the prototype material; the remaining atoms of the other element constitute the matrix. This phase segregation process may be assisted by thermal annealing. Another approach used to form granular nanocomposite prototype materials is to ion-implant nanoparticle-type atoms into already grown films or wafer surfaces. However, since these two approaches utilize the immiscibility of the combined materials, they can be applied to such immiscible material systems only. Furthermore, the range of achievable elemental compositions and particle sizes is limited. An interesting alternative strategy to synthesize granular nanocomposites is to deposit the matrix material simultaneously with preformed, spherical nanoparticles. In this approach, the nanoparticles are embedded into the matrix in a direct fashion. The preformed, spherical nanoparticles are called clusters, correspondingly, the created nanomaterials are called cluster-assembled nanocomposites. The great advantage of this special co-deposition approach is that it allows for the creation of nanocomposites out of elements that are at least partially miscible or that can form crystallographic mixed phases—that is, for the creation of so-called nonequilibrium compositions. Embedding the nanoparticles as preformed constituents instead of letting them segregate during the deposition process also increases the degree of control over the deposition process. An ultimate degree of control over the composition is achieved when the clusters are size-selected prior to deposition. This is the strategy pursued in the present thesis. Here, a cluster ion beam deposition system that features a narrow cluster size distribution of ±10% is used to synthesize films of cluster-assembled nanocomposites. Two different nanocomposites are prepared and examined: nanocomposites made of Fe-clusters embedded in Ge-matrices and nanocomposites of Fe-clusters embedded in Ag-matrices. The created Fe-clusters are only a few nanometers in size and, therefore, of superparamagnetic kind. The study of the physical properties of the prepared nanocomposites as a function of cluster size and cluster concentration, in particular, of their transport and magnetoresistive properties, is the central aim of this thesis. First, the Fe-Ge nanocomposites are examined. In this course, also the process of sample preparation and the various performed measurements are discussed. Embedding magnetic Fe nanoparticles into a semiconductor aims for a synthesis of the magnetic and the semiconducting properties, that is, for the creation of so-called magnetic semiconductors. Magnetic semiconductors define a class of materials whose properties can be controlled by means of a magnetic field in addition to—or even instead of—an electric field. For this reason, magnetic semiconductors represent an essential component for the emerging field of spintronics. Two series of Fe-Ge nanocomposites are prepared: one with clusters consisting of 500 ± 50 Fe atoms and one with clusters consisting of 1000 ± 100 Fe atoms. In the course of the analysis, Ge is found to grow in an amorphous structure under the conditions of the co-deposition experiments. A co-deposition sample layout that consists of a co-deposition mask and a complementing sample chip layout is developed. The deposited nanocomposite samples are studied by means of resistance and magnetoresistance measurements in a cryostat, by means of scanning electron microscopy including energy-dispersive X-ray spectroscopy, and by means of SQUID magnetometry. Besides tunneling magnetoresistance, which is negative, of saturating kind, and observed with a magnitude on the order of 1% here, at least one other effect not saturating within the examined magnetic field range of |µ0 H| ≤ 6 T is observed. Several effects that may explain the observed non-saturating behavior are discussed, however, the origin remains unsolved. Furthermore, the resistivity of the Fe-Ge nanocomposites as well as the tunneling magnetoresistance are each found to be a function of the average distance between the surfaces of neighboring clusters rather than the average distance between their centers of mass. Finally, some of the Fe-Ge nanocomposite samples are thermally annealed in vacuum, under the presence of hydrogen gas, and at two different temperatures in various steps. Thermal annealing alters the structure of the as-deposited nanocomposites, which is reflected by changes in the measured physical properties. These changes are identified and discussed. Secondly, the Fe-Ag nanocomposites are examined. In comparison to the Fe-Ge system, the Fe-Ag system is represented in the literature rather well. In particular, it is well-known that the giant magnetoresistance effect can occur in layered as well as in granular Fe-Ag structures. Here, the aim is to confirm that the applied methods give results comparable to those found in the literature and to perhaps even improve upon existing data. Again, two series of nanocomposite samples with clusters consisting of 500 and 1000 Fe atoms, respectively, are fabricated. In addition, a third series of Fe-Ag nanocomposite samples with clusters consisting of 1500 ± 150 Fe atoms is prepared. Giant magnetoresistance of maximum −6% is observed. The giant magnetoresistance effect increases in magnitude with decreasing size of the embedded clusters. Furthermore, an optimum composition of clusters and matrix material for a maximum magnitude of the giant magnetoresistance effect seems to exist. However, no clear dependence of the measured properties on neither the Fe concentration nor the average distance between the surfaces of neighboring clusters is observed. Besides the examination of Fe-Ge and Fe-Ag nanocomposites, a setup that combines laser ablation and inert gas condensation is designed and assembled. In contrast to other techniques, laser ablation features a large fraction of uncharged output particles. Further, laser ablation also allows for the creation of nanoparticles made of electrically insulating materials. Accordingly, the original application considered for the setup lies in the field of matter-wave diffraction experiments. In principle, the setup may be used for the deposition of cluster-assembled materials as well. However, it has never been used for experiments in any of these fields. Nevertheless, the present state of the setup as well as its principle of operation are reviewed. The review is completed with a brief analysis of a test sample of collected Ag clusters prepared with the setup.