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Magnetotransport measurements on randomly distributed and of regularly arranged MnAs nanoclusters

Project summary

Granular hybrid structures, consisting of ferromagnetic MnAs nanoclusters embedded in a paramagnetic GaAs matrix, show magnetoresistance effects similar to the giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR). Moreover, they combine ferromagnetic and semiconducting properties in one single material system and thus offer an interesting alternative to dilute magnetic semiconductors. A technological utilisation of conventionally synthesised hybrid systems in the area of magneto-electronic devices is currently mainly restricted by a random cluster distribution, which results in statistical fluctuations of the component parameters. These fluctuations increase with further miniaturisation.

The problem of a random cluster distribution can be overcome by the method of selective-area metal-organic vapor phase epitaxy (SA-MOVPE). With this growth method, it is possible to control the growth of ferromagnetic MnAs clusters on GaAs (111) B substrates. For the controlled positioning of the nanoclusters a SiO2 layer is deposited on the substrate by sputtering and then patterned using electron beam lithography and wet chemical etching or reactive ion etching. The SiO2 layer is removed at the places at which the MnAs nanoclusters are to be situated. SA-MOVPE therefore offers the possibility to control the cluster arrangement, the size, and the cluster shape. The specific arrangement of nanoclusters on the substrate thus opens new perspectives for the use of such structures in innovative planar magneto-electronics.

Investigations of the magnetic properties using magnetic force microscopy (MFM) and ferromagnetic resonance (FMR) reveal that the shape of the clusters strongly influences the orientation of the cluster magnetisation (Fig. 1). By controlling the cluster shape, the magnetisation is set along the cluster direction, which provides the basis for planar magneto-electronic devices.

MFM-Aufnahme einer Clusteranordnung
Fig. 1: MFM-image of a cluster arrangement consisting of one hexagon-shaped and two elongated nanoclusters. Although a small external magnetic field is applied to the arrangement, the magnetisation orientation (blue arrow) is determined by the shape of the nanocluster.

In our group, the transport properties of such a MnAs/GaAs:Mn hybrids with random and regular distributions of nanoclusters are studied. This includes the transport through the hybrid system as a whole as well as the transport through single clusters and cluster arrangements in dependence on the orientation of an external magnetic field. For this purpose, different arrangements of MnAs nanoclusters were grown by SA-MOVPE and contacted in an additional structuring step (fig. 2).

SEM-Aufnahmen verschiedener kontaktierter MnAs–Nanocluster-Anordnungen
Fig. 2: SEM images of different contacted MnAs nanocluster arrangements

The aim of the research is to gain insight into the physical fundamentals and the underlying microscopic mechanisms of transport in such systems, as well as the potential to examine these structures as possible magneto-electronic devices.

Current issues include:

  • Influence of the cluster properties on the transport behavior through the matrix
  • Switching behavior of single nanoclusters and cluster arrangements in an external magnetic field
  • Influence of the domain structure and magnetization orientation of the clusters on the transport properties of the matrix.
  • Transport through the nanoclusters and the influence of a change of the relative magnetization orientation on the nanoclusters’ resistance.

Publications on this topic

  • M. T. Elm, P. J. Klar, W. Heimbrodt, U. Wurstbauer, M. Reinwald, W. Wegscheider, “Annealing-induced transition from a (311)A-oriented Ga0.98Mn0.02As alloy to a GaMnAs/MnAs hybrid structure studied by angle-dependent magnetotransport”, Journal of Applied Physics, 103 093710 (2008), DOI: 10.1063/1.2917414 (6 pages)
  • C. Michel, M. T. Elm, B. Goldlücke, S. D. Baranovskii, P. Thomas, W. Heimbrodt and P. J. Klar, “Tailoring the magnetoresistance of MnAs/GaAs:Mn granular hybrid nanostructures”, Applied Physics Letters, 92 223119 (2008), DOI: 10.1063/1.2937128 (3 pages)
  • M. T. Elm, C. Michel, J. Stehr, D. M. Hofmann, P. J. Klar, S. Ito, S. Hara, and H.-A. Krug von Nidda, “Comparison of the magnetic properties of GaInAs/MnAs and GaAs/MnAs hybrids with random and ordered arrangements of MnAs nanoclusters”, Journal of Applied Physics, 107 013701, (2010), DOI: 10.1063/1.3275427 (5 pages)
  • Heiliger, M. Czerner, P. J. Klar, S. Hara, “Magnetic sensor devices based on ordered planar arrangements of MnAs nanocluster”, IEEE Trans Magn, 46 1702-1704 (2010), DOI: 10.1109/TMAG.2010.2041194
  • M. T. Elm, P. J. Klar, S. Ito, and S. Hara, “Influence of ordered arrangements of cluster chains on the hopping transport in GaAs:Mn/MnAs hybrids at low temperatures”, Phys. Rev. B, 83 235305 (2011), DOI: 10.1103/PhysRevB.83.235305 (6 pages)
  • M. T. Elm, P. J. Klar, S. Ito, S. Hara and H.-A. Krug von Nidda, “Effect of the cluster magnetization on the magnetotransport at low temperatures in ordered arrays of MnAs nanoclusters on (111)B GaAs”, Phys. Rev. B, 84 035309 (2011), DOI: 10.1103/PhysRevB.84.035309 (8 pages)

Group members working on this topic

MSc Martin Fischer

Dr. Matthias T. Elm

Collaborators

Prof. Dr. Shinjiro Hara, Hokkaido University, Sapporo, Japan

Dr. Hans-Albrecht Krug von Nidda, Universität Augsburg

Prof. Dr. Christian Heiliger, Universität Gießen