Ordered arrangements of selective-area grown MnAs clusters as components for planar magneto-electronic devices: Experiment and theory
Final Report Abstract
The main scientific objective of the project was to investigate the possibilities of using single or arrangements of MnAs nanoclusters as building blocks of planar magnetoelectronic devices. For this purpose the magnetotransport properties of single clusters, ordered cluster arrangements in a matrix as well as coupled nanoclusters were investigated experimentally as well as theoretically. For the experimental studies different arrangements of MnAs nanoclusters were prepared in the laboratory of our collaboration partner Prof. Hara. The arrangements were grown using selective-area metal-organic vapor phase epitaxy, which offers the possibility to actively tune position, size and shape of the nanoclusters. A planar nanocluster device was fabricated, which consists of two single domain nanoclusters connected by a gold layer. Such a structure is comparable to a planar giant magnetoresistance (GMR) structure. Measurements of the transport through this structure showed jumps in the magnetoresistance at certain magnetic field strengths, which could be attributed to switching of the magnetization of the nanoclusters. At a temperature of 120 K the arrangement showed a switching behavior comparable to a spin-valve device for a magnetic field orientation parallel to the elongation direction of the nanoclusters. These results clearly demonstrate that arrangements of single MnAs nanoclusters have the potential to serve as building blocks for planar magnetoelectronic devices structure. Investigations of the transport through coupled MnAs nanoclusters, which had merged during the growth process revealed in a certain temperature range magnetic random telegraph noise (RTN), i.e. jumps of the resistance between discrete resistance levels. These jumps could be attributed to thermally activated changes in the magnetic structure of the cluster arrangements. Further investigations of the magnetic RTN are of high interest not only to evaluate their impact on the device performance, but also to obtain a detailed understanding of magnetization reversal processes in nanoscaled systems. For this purpose, MnAs nanoclusters are an ideal model system as the clusters are of high structural quality and their size and shape can be actively controlled during the growth. For the theoretical part of this project we addressed the magnetotransport properties of the clusters by mainly focusing on the cluster arrangements designed to form GMR-like structures. In that case two clusters are separated by a metal layer. In order to perform theoretical transport calculations through the GMR layer system exact knowledge of the atomic structure of the layer is needed, which is presently not known from experiment. Therefore, we combined two methods to obtain realistic atomic positions for the metal layer, which are Molecular Dynamics (MD) and Force Matching. Force Matching is a crucial step before doing MD as it provides the effective potentials needed for MD simulations. With these simulations we were able to investigate the deposition of the gold metal layer (Au) and analyze the structure of the grown layer. In the process of Force Matching we conducted several ab initio calculations of so-called reference structures, including bulk and surface structures of MnAs and gold. A specific potential model was fitted to those reference structures to reproduce the physical situation at hand. The resulting effective potential for the MnAs/Au system was validated by comparing it to ab initio calculation not included in the fit. The latter is a very important process as this truly defines the quality of the potential. With the developed potential we investigated different surface facets and terminations to find the most stable surface structures for the deposition simulations, which are also not known from experiment. We did static calculations to obtain the surface energies as well as dynamic simulations to investigate possible surface reconstructions. The deposition simulation of gold on a stable MnAs surface revealed a growth mode known as the Stranski-Krastanov growth or ‘layer-plus-island growth’ respectively. After forming a wetting layer of one monolayer thickness the deposited gold grows mainly in a fcc structure with several stacking faults. From the analysis of the grown gold layers we were able to deduce the orientation of the fcc structure and, with that, constructing a GMR layer system (MnAs/Au/MnAs), which will be used for ab initio transport calculations.
Publications
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Transport Properties of Hybrids with Ferromagnetic MnAs Nanoclusters and Their Potential for New Magnetoelectronic Devices. Advanced Materials. 2014;26:8079-95
Elm MT, Hara S
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Magnetoresistance effects and spin-valve like behavior of an arrangement of two MnAs nanoclusters. Applied Physics Letters. 2015;106:032401
Fischer M, Elm MT, Sakita S, Hara S, Klar PJ
