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In-situ study of ferromagnetic/semiconductor heterostructures

Our main interests in recent years focused on the growth, morphology and magnetic properties of thin ferromagnetic films (e.g. Fe, Co or Permalloy) on semiconductors like GaAs or Ga1-xInxAs (0 ? x ? 1). Additionally we investigated the spin selective transport through oxide insulating barriers (e.g. SiO2, AlOx) sandwiched between these heterostructures. These topics are of great importance for the development of next generation magneto-electronic devices used for example in Magnetic Random Access Memory (MRAM) elements.

The samples were grown and studied in Ultra-High Vacuum (UHV) in our Multiple Technique Chamber, where we can perform in-situ characterization using Low Energy Electron Diffraction (LEED), Relfective High Energy Electron Diffraction (RHEED), Auger Electron Spectrometry (AES), in-situ Magneto-Optical Kerr Effect (MOKE) measurements, in-situ Brillouin Light Scattering (BLS), and Scanning Tunnelling Microscopy (STM). If necessary, the samples can be Au-capped for ex-situ measurements such as BLS, MOKE, Super-Conducting Quantum Interference Device (SQUID), Polarized Neutron Reflectometry (PNR) and X-Ray Diffraction and Reflectometry (XRD, XRR).


Fig. 1(a) RHEED image of GaAs surface prior to growth. (b) A 16.5 x 25 nm2 STM image of 0.3 ML Fe/GaAs(100)-2x6. On the picture two terraces with a (2x6) reconstruction are visible. Most of the Fe islands tend to cluster on the top of the As rows, a few of them are in between the rows. On the upper terrace on the upper right of the image some clusters have already coalesced and formed a bridge in between rows. The image was captured with a current of 2.6 nA and a bias of 2.8 V. The graph in (c) displays the cross-section profile of an Fe cluster.

Our research on Fe/GaAs (100)-4x6 has been helpful to understand the origin of the Uniaxial Magnetic Anisotropy (UMA) [1] present in this system in the thickness range from 4 monolayers (ML) up to approximately 80 ML. The explanation of the UMA has led to a huge debate between scientists all over the world. Recent STM measurements reveal that the growth proceeds via 3D Vollmer-Weber mode and at a thickness of 4 ML the Fe islands coalesce so that long range ferromagnetic ordering develops. In order to understand the interface morphology of the system we have performed a submonolayer study of the growth. Figure 1(a) shows a RHEED image of the GaAs-pseudo 4x6 surface along the [001] axis. Figs 1(b-c) show an STM image and cross-section of 0.3 ML Fe on GaAs, with visible Fe islands almost solely positioned on the top of the As rows of the GaAs-2x6 reconstructed surface.

Fig. 2(a) LEED image of a pseudo-4x6 reconstructed GaAs(001) surface prior to growth, E = 125 eV. (b) Image of a 74 ML (21 nm) thick Fe3Si film showing that the cubic L21 (Heusler) structure has been stabilised, E = 115 eV.




In addition to this we are interested in the growth and magnetic properties of thin films of Heusler alloys (e.g. Fe2MnSi), which should theoretically exhibit half-metallic behaviour. This led us to research the closely related binary compound Fe3Si [2,3], which crystallizes in the cubic L21 (Heusler) structure. Figure 2(a) shows a LEED image of GaAs-4x6 surface prior to deposition, and 2(b) the LEED image of 74 ML (21 nm) of Fe3Si.

Selected References:

[1] G. Wastlbauer et al. Advances in Physics (in press) [2] A. Ionescu et al. J. Magn. Magn. Mater. 286, 72 (2005) [3] A. Ionescu et al. Physical Review B 71, 094401 (2005)

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