M. Fraune

RWTH Aachen University, Aachen, North Rhine-Westphalia, Germany

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Publications (12)34.35 Total impact

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    ABSTRACT: The coercive fields of Fe/FeF2(110) bilayers above the Néel temperature of FeF2 have been studied by means of the magneto-optic Kerr effect (MOKE). To distinguish the anisotropies, the longitudinal as well as the transverse magnetization components were measured and simulated. The latter allow for a conclusion about the different magnetization reversal mechanisms above and below the Néel temperature of FeF2, i.e. for unbiased and exchange-biased bilayers, leading to a modified description of the magnetization reversal processes at high temperatures with explicitly forbidden local energy minima.
    Journal of Magnetism and Magnetic Materials 01/2009; 321(18):2932-2935. · 2.00 Impact Factor
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    ABSTRACT: Fully epitaxial Fe(110)/MgO(111)/Fe(110) magnetic tunnel junctions (MTJs) have been tested with respect to symmetry-enforced spin filtering. The Fe(110) electrodes exhibit Σ1↑ and Σ1↓ spin states, both crossing the Fermi level, but with a group velocity about 50% smaller for the minority states compared to the majority ones. These epitaxial but symmetry-mismatched MTJs yield tunneling magnetoresistance (TMR) values of 54% at 1.5 K and 28% at room temperature. The TMR value and the estimated tunneling spin polarization are consistent with a partial spin filtering due to the Σ1↑ states partially compensated by the Σ1↓ states.
    Applied Physics Letters 08/2008; 93(8):083512-083512-3. · 3.52 Impact Factor
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    ABSTRACT: The magnetic anisotropies of epitaxially grown exchange bias CoO∕Co bilayers with (111), (110), and (100) crystallographic orientations have been investigated by Brillouin light scattering (BLS) by means of spin waves. Experiments were carried out at 293 and 140 K, i.e., above and below, respectively, the Néel temperature of CoO. The unidirectional, twofold, and fourfold anisotropy constants, bulk and surface, were obtained by fitting the angular dependence of the spin wave frequencies. For all samples, we observed an isotropic up-shift of the spin wave frequencies. We conclude that all observed magnetic anisotropies are related to the crystallographic symmetry of the ferromagnetic and antiferromagnetic layers, depending on the lattice mismatch between layers, the sequence of layers upon deposition, and the quality of the ferromagnetic/antiferromagnetic interface.
    Physical Review B 01/2007; 75(5):054425. · 3.66 Impact Factor
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    ABSTRACT: We report on tunnelling magnetoresistance (TMR), current–voltage (IV) characteristics and low-frequency noise in epitaxially grown Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) magnetic tunnel junctions (MTJs) with dimensions from 2×2 to 20×20 μm2. The evaluated MgO energy barrier (0.50±0.08 eV), the barrier width (13.1±0.5 Å) as well as the resistance times area product (7±1 MΩ μm2) show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. At low temperatures (T<10 K) inelastic electron tunneling spectroscopy (IETS) shows anomalies related to phonons (symmetric structures below 100 meV) and asymmetric features above 200 meV. We explain the asymmetric features in IETS as due to generation of electron standing waves in one of the Fe electrodes. The noise power, though exhibiting a large variation, was observed to be roughly anti-correlated with the TMR. Surprisingly, for the largest junctions we observed a strong enhancement of the normalized low-frequency noise in the antiparallel magnetic configuration. This behavior could be related to the influence of magnetostriction on the characteristics of the insulating barrier through changes in local barrier defects structure.
    Journal of Magnetism and Magnetic Materials 05/2006; 300(1):132–135. · 2.00 Impact Factor
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    ABSTRACT: Fe(110) films were epitaxially grown on sapphire substrates using a Mo(110) buffer layer in an ultrahigh-vacuum molecular-beam epitaxy system. The magnetic properties were examined ex situ by Brillouin light scattering and superconducting quantum interference device magnetometry. To determine the magnetic anisotropy constants the frequency of the Damon-Eshbach [J. Phys. Chem. Solids 19, 308 (1961)] surface spin-wave mode was measured as a function of the in-plane angle between the external magnetic field and the Fe[001] crystal axis. The angle-dependent frequency was fitted by a spin-wave model. We found that the easy axis of the cubic magnetocrystalline anisotropy K<sub>1</sub> and an additional uniaxial in-plane anisotropy K<sub>||</sub><sup>(2)</sup> are aligned parallel to the in-plane Fe[001] axis for Fe-layer thicknesses from 0.8 to 37 nm , with K<sub>1</sub> increasing and K<sub>||</sub><sup>(2)</sup> decreasing with increasing Fe thickness. Possible origins of the observed uniaxial anisotropy are discussed.
    Journal of Applied Physics 03/2006; · 2.21 Impact Factor
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    ABSTRACT: Focused-ion-beam milling is used to fabricate nanostencil masks suitable for the fabrication of magnetic nanostructures relevant for spin transfer torque studies. Nanostencil masks are used to define the device dimensions prior to the growth of the thin film stack. They consist of a wet etch resistant top layer and an insulator on top of a pre-patterned bottom electrode. The insulator supports a hard mask and gives rise to an undercut by its selective etching. The approach is demonstrated by fabricating current perpendicular to the plane Co/Cu/Co nanopillar junctions, which exhibit current-induced magnetization dynamics.
    Journal of Applied Physics 01/2006; · 2.21 Impact Factor
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    ABSTRACT: We report on tunneling magnetoresistance (TMR), current–voltage (I–V) characteristics and low-frequency noise in epitaxially grown Fe(110)/MgO(111)/Fe(110) magnetic tunnel junctions (MTJs) with dimensions from 2×2 to 20×20 μm2. The evaluated MgO energy barrier (0.50±0.08 eV), the barrier width (13.1±0.5 Å) as well as the resistance times area product (7±1 MΩ μm2) show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. The noise power, though exhibiting large variation, was observed to be roughly anticorrelated with the TMR. Surprisingly, for the largest junctions we observed a strong enhancement of the normalized low-frequency noise in the antiparallel magnetic configuration. This behavior could be related to an interplay between the magnetic state and the local barrier defects structure of the epitaxial MTJs.
    Applied Physics Letters 07/2005; 87(4). · 3.52 Impact Factor
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    ABSTRACT: Exchange bias effects have been induced along the perpendicular-to-film direction in nanostructures prepared by electron beam lithography, consisting of a ferromagnetic [Pt/Co] multilayer exchange coupled to an antiferromagnet (FeMn). As a general trend, the exchange bias field and the blocking temperature decrease, whereas the coercivity increases, as the size of the nanostructures is reduced. © 2004 American Institute of Physics.
    Applied Physics Letters 04/2004; 84(18):3696-3698. · 3.52 Impact Factor
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    ABSTRACT: We report a combined experimental-computational investigation of the electronic structure of CrO2. We have measured the magneto-optical Kerr spectra of CrO2 at 10 K and 300 K. At 10 K the Kerr signal is significantly enhanced over that obtained at 300 K. We compare the measured Kerr spectra to first-principles theoretical spectra, which we computed using three different approximations to the exchange-correlation functional, i.e., the local spin-density approximation (LSDA), generalized gradient approximation (GGA), and LSDA+U. The experimental low-temperature magneto-optical Kerr spectra are best explained by calculations employing the GGA functional. The addition of an on-site Coulomb correlation U does not lead to reasonable Kerr spectra.
    Physical review. B, Condensed matter 04/2002; 65(16). · 3.77 Impact Factor
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    ABSTRACT: Highly a-axis-textured CrO2 films have been deposited on Al2O3(0001) and on isostructural TiO2(100) substrates by a chemical vapour deposition technique. For Al2O3 substrates a columnar growth of CrO2(010) on an initial Cr2O3(0001) layer has been found in transmission electron microscopy as well as in x-ray diffraction investigations. The sixfold in-plane symmetry of a (0001)-oriented Cr2O3 initial layer leads to three equivalent in-plane orientations of the CrO2 unit cell as confirmed by electron diffraction and scanning electron microscopy. The growth can be understood by a simple model of the in-plane symmetries of the Al2O3(0001), Cr2O3(0001), and CrO2(010) lattices. The growth on TiO2(100) substrates leads to (100)-oriented CrO2 films of higher crystalline quality than the ones grown on Al2O3(0001). Transmission electron microscope images show growth of CrO2(100) directly on the TiO2(100) substrates and no significant Cr2O3 inclusions within the CrO2(100) layer. All contributions to the magnetoresistance (MR) due to anisotropic MR, Lorentz MR, spin disorder, and intergrain tunnelling MR have been determined and partly correlated with the crystalline properties of the samples investigated. For films of both types the intrinsic linear contribution to the high-field MR does not depend on the crystalline quality of the films and supports the suggested intrinsic double-exchange mechanism for CrO2.
    Journal of Physics Condensed Matter 12/2001; · 2.22 Impact Factor
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    ABSTRACT: The growth of (010)-oriented CrO2 thin films on Al2O3(0001) substrates leads to a higher grain boundary density than the growth of (100)-oriented CrO2 thin films on isostructural TiO2(100) substrates. For both types of films an intrinsic linear contribution to the high field magnetoresistance (MR) due to spin disorder has been determined at T=300 K. This contribution does not depend on the crystalline quality of the films and supports the suggested intrinsic double exchange mechanism for CrO2. At low temperature (T=10 K) intergrain tunneling MR and Lorentz MR appear, which strongly depend on the crystalline properties of the CrO2 films. © 2001 American Institute of Physics.
    Journal of Applied Physics 05/2001; 89(11):7699-7701. · 2.21 Impact Factor
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    ABSTRACT: NiO/Ni wires have been investigated as a function of their width in order to investigate the size dependence of exchange bias. The samples have been prepared by e-beam lithography and ion milling of ion beam sputtered thin films. For NiO/Ni wires narrower than 3 μm, the exchange bias field significantly depends on the wire width. A NiO/Ni film shows an exchange bias field of −78 Oe whereas the exchange bias field of wires narrower than 200 nm is reduced to approximately −40 Oe. The coercive field of the NiO/Ni film is 28 Oe and increases to 210 Oe for the narrowest wires. The decrease of the exchange bias field for the narrowest wires is consistent with a recent microscopic model of exchange bias where the appearance of a unidirectional anisotropy in ferromagnet/antiferromagnet bilayers has been attributed to the presence of antiferromagnetic domains in the bulk of the antiferromagnet. A possible onset of a transition from a multidomain to a single-domain state of the antiferromagnet as a function of the NiO/Ni wire width seems to be the origin for the observed decrease of the exchange bias field for narrow wires. © 2000 American Institute of Physics.
    Applied Physics Letters 12/2000; 77(23):3815-3817. · 3.52 Impact Factor