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Magnetic Hetero- and Nanostructures (Koch)

Beschreibung: Head of the Magnetic Hetero- and Nanostructures group: Univ. Prof. Dr. Reinhold Koch

Research topics
Selected Publications and Patents
Group members
Equipment
Funding

Research topics:

Integration of ferromagnetic properties into semiconductor devices promises access and control of the spin information of the electron, in addition to the charge information currently utilized in the transistor. Our research aims at the development of half-metallic Heusler compound films on the two technologically leading semiconductor substrates, GaAs(001) and Si(001), with the ultimate goal of realizing (i) efficient spin-injectors and (ii) magnetic tunneling junctions with a high tunneling magnetoresistance (TMR) ratio for application in spin- and magnetoelectronics.

Heusler films on GaAs(001)
Potential electrode materials for spin- and magnetoelectronics applications are half-metallic ferromagnets (e.g., Heusler compounds) that exhibit up to 100% spin-polarization at the Fermi level, thus promising injection of a fully spin-polarized current as well as a high TMR effect.

Interface structure and composition
Recently, we found strong interdiffusion at the interface between the ferromagnetic layers and GaAs(001), which proceeds already at the moderate growth temperatures of 200°C required for the crystallographic ordering and stimulates spin decoherence.

Oxide diffusion barriers
From insertion of a thin insulating diffusion barrier between the Heusler film and the semiconductor we expect both, preventing interdiffusion and providing a tunneling barrier for circumventing the conductivity mismatch.

Stress and film growth
The evolution of the film stress is measured in-situ during growth and post-growth annealing with submonolayer sensitivity by a cantilever beam technique. It provides - additionally to the magnitude of film stress - direct information on the evolution of film structure and morphology.

Magnetization and magnetostriction
The magnetic properties of the Heusler films are determined in-situ as a function of stoichiometry, crystallographic order, and interface composition by using the cantilever beam technique as a sensitive magnetometer.

Magnetologic
Magnetism-based logic gates for a magnetic reconfigurable processors may initiate a paradigm shift in computation. In contrast to conventional CMOS processors with a rigid architecture, the hardware of reconfigurable processors can be reconfigured dynamically by the software providing a much higher level of flexibility and performance.

Selected publications and patents:

Selected publications

R.Koch.
Intrinsic stress of polycrystalline and epitaxial thin metal films.
J. Phys.: Condens. Matter 6, 9519 (1994).

M. Weber, R. Koch, and K. H. Rieder.
UHV cantilever beam technique for quantitative measurements of magnetization, magnetostriction and intrinsic stress of magnetic thin films.
Phys. Rev. Lett. 73, 1166 (1994)

A. Ney, C. Pampuch, R. Koch, and K. H. Ploog.
Programmable computing with a single magnetoresistive element.
Nature, 425, 485 (2003)

R. Koch.
Morphware.
Scientific American Vol. 293, August 2005, pp. 56

R. Koch.
Stress in Evaporated and Sputtered Thin Films – A Comparison,
Surf. Coat. Technol. 204, 1973 (2010)

C. Gusenbauer, T. Ashraf, J. Stangl, G. Hesser, T. Plach, A. Meingast, G. Kothleitner, and R. Koch.
Interdiffusion in Heusler film epitaxy on GaAs(001),
Phys. Rev. B 83, 035319 (2011)

T. Ashraf, C. Gusenbauer, J. Stangl, G. Hesser, M. Wegscheider, and R. Koch.
Stress and interdiffusion during molecular beam epitaxy of Fe on As-rich GaAs(001),
J. Phys.: Condens. Matter 23, 042001 (2011)

S. Wang, A. Sarkar, M. Gruber, and R. Koch.
Epitaxy and stress of MgO/GaAs(001) heterostructures,
J. Appl. Phys. 114, 154511 (2013)

T. Ashraf, C. Gusenbauer, J. Stangl, G. Hesser, and R, Koch.
Growth, structure and morphology of epitaxial Fe (001) films on GaAs(001)c(4×4)
J. Phys.: Condens. Matter 27, 036001 (2015).

A. Sarkar, S. Wang, W. Grafeneder, M. Arndt, and R. Koch.
Ultrathin MgO diffusion barriers for ferromagnetic electrodes on GaAs(001)
Nanotechnol. 26, 165203 (2015).


Granted patents

DE 1031171784, EP 1606879, Magnetische Logikeinrichtung, Logikschaltung, Verfahren zu deren Betrieb und deren Verwendung, inventors: R. Koch, C. Pampuch, A. Ney, A. K. Das

DE10255857, EP1565988, US 7652398: Magnetische Logikeinrichtung, inventors: R. Koch, C. Pampuch, A. Ney, K. H.Ploog

Group members:

Former group members:

  • Amal K. Das (guest professor)
  • Yanfang Hu (postdoc)
  • Tanveer Ashraf (PhD student)
  • Matthias Wegscheider (PhD student)
  • Mohammad Rashidi (PhD student)
  • Hedieh Hosseinzadeh-Kalachi (PhD student)
  • Christian Gusenbauer (diploma student)
  • Thomas Lengauer (diploma student)
  • Michael Fattinger (diploma student)
  • Markus Gruber (diploma student)
  • Shibo Wang (PhD student)

Equipment:

The experiments are performed in two multi-chamber MBE systems. The first is a five-chamber MBE system with separate chambers for the deposition of III/V semiconductors (4 effusion cells) and ferromagnetic materials or oxides (4 electron beam evaporators). The second is a three-chamber MBE system containing 4 electron beam evaporators for the deposition Ge, Si, and ferromagnetic materials.

Each of the two MBE systems is equipped with a CBM for quantitative real-time measurements of the stress, magnetization, and magnetoelastic properties of ferro-magnet/semiconductor hetero- and nanostructures at a maximum magnetic field of 2 T. Furthermore, both systems are equipped with a scanning tunneling microscope (STM) for in situ structural investigations. The crystallographic long-range order can be controlled by in-situ electron diffraction (RHEED in the III/V-system, LEED in the Si/Ge-system).

Funding:


FFG: Subcontractor of Competence Headquarter: Excellence in Electronics Sputtering Target Technology
(2013-2016)

FWF P 24335: Interfaces and Interdiffusion in Heusler-alloy/GaAs(001) Hybrid Structures
(2012-2015)

FWF P 20650: Heusler alloy films on Semiconductor Templates
(2008-2011)