Reinert Schreiber

Thickness dependence of linear and quadratic magneto-optical Kerr effects in ultrathin Fe(001) films

Magneto-optical Kerr effect (MOKE) magnetometry is one of the most widely employed techniques for the characterization of ferromagnetic thin-film samples. Some information, such as coercive fields or anisotropy strengths can be obtained without any knowledge of the optical and magneto-optical (MO) properties of the material. On the other hand, a quantitative analysis, which requires a precise knowledge of the materials index of refraction n and the MO coupling constants K and G is often desirable, for instance for the comparison of samples, which are different with respect to ferromagnetic layer thicknesses, substrates, or capping layers. While the values of the parameters n and the linear MO coupling parameter K reported by different authors usually vary considerably, the relevant quadratic MO coupling parameters G of Fe are completely unknown. Here, we report on measurements of the thickness dependence (0-60nm) of the linear and quadratic MOKE in epitaxial bcc-Fe(001) wedge-type samples performed at a commonly used laser wavelength of 670nm. By fitting the thickness dependence we are able to extract a complete set of parameters n, K, (G11 - G12), and G44 for the quantitative description of the MOKE of bcc-Fe(001). We find sizable different n, K, and G parameters for films thinner than about 10nm as compared to thicker films, which is indicative of a thickness dependence of the electronic properties or of surface contributions to the MOKE. The effect size of the quadratic MOKE is found to be about a third of the record values recently reported for Co2FeSi.

Carrier-induced ferromagnetism in Ge(Mn,Fe) magnetic semiconductor thin-film structures

We report on the carrier-induced ferromagnetism in Ge(Mn,Fe) magnetic semiconductor insulating-type thin-film structures prepared using sequential deposition at Tg=520K with subsequent annealing at Tg. In the resulting films Mn and Fe are diffused in the Ge matrix without compromising the epitaxial structure. The anomalous Hall effect serves as a manifestation of the carrier-induced magnetism, with p-type conductivity and the Curie temperature TC=209K. The additional doping with Fe stabilizes epitaxial growth and carrier-mediated magnetism at levels of magnetic doping exceeding 10%. We conclude that indirect ferromagnetic exchange is mediated by localized holes with concentration n∼1020cm−3 and mobility μ∼10cm2∕(Vs).

Depth-resolved soft x-ray photoelectron emission microscopy in nanostructures via standing-wave excited photoemission

Depth-resolved soft x-ray photoelectron emission microscopy in nanostructures via standing-wave excited photoemission F. Kronast 1 , R. Ovsyannikov 1 , A. Kaiser 2 , C. Wiemann 2 , S.-H. Yang 3 , A. Locatelli 4 , D. E. Burgler 3 , R. Schreiber 3 , F. Salmassi 5 , P. Fischer 5, H.A. Dűrr 1 , C. M. Schneider 2 ,W. Eberhardt 1 and C. S. Fadley 2,5,6 BESSY mbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany Forschungzentrum Julich GmbH, IFF-9, 52425 Julich, Germany Almaden Research Center, San Jose, CA 95120 USA Elettra, Sincrotrone Trieste S.C.p.A., 34012 Basovizza, Trieste, Italy Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA University of California, Davis, CA 95616, USA Corresponding author: Florian Kronast Address: BESSY mbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany e-mail : kronast@bessy.de Phone: (+49) 30 6392 4620 Fax: (+49) 30 63924980 Abstract We present an extension of conventional laterally resolved soft x-ray photoelectron emission microscopy. A depth resolution along the surface normal down to a few A can be achieved by setting up standing x-ray wave fields in a multilayer substrate. The sample is an Ag/Co/Au trilayer, whose first layer has a wedge profile, grown on a Si/MoSi2 multilayer mirror. Tuning the incident x-ray to the mirror Bragg angle we set up standing x-ray wave fields. We

Enhanced antiferromagnetic exchange coupling in Fe/Si/Fe epitaxial trilayers with Fe0.5Si0.5 boundary layers

Epitaxial Fe/Fe0.5Si0.5/Si-wedge/Fe0.5Si0.5/Fe structures are prepared by thermal evaporation with Fe0.5Si0.5 boundary layers grown by coevaporation at 200 °C. Magnetic properties are examined with Brillouin light scattering and longitudinal magneto-optic Kerr effect hysteresis. The interlayer coupling is found to increase in excess of 8 mJ/m2 by introducing a boundary layer at the bottom interface. The coupling maximum shifts from 7 to 3 A nominal Si thickness. This effect is related to reduced interdiffusion with the formation of an epitaxial, pinhole-free spacer at smaller thickness. Together with the strong increase of the coupling for decreasing spacer thickness, this results in an enhancement of the coupling.

Spin-wave modes and line broadening in strongly coupled epitaxial Fe'Al'Fe and Fe'Si'Fe trilayers observed by Brillouin light scattering

We report on the spin-wave excitation frequencies and the broadening of the mode linewidths in Stokes and anti-Stokes Brillouin light scattering spectra of strongly exchange coupled (2–6.5 mJ/m2), epitaxial trilayers with the following structures: Fe(80 A)/Si wedge/Fe(100 A) and Fe(50 A)/Al wedge/Fe(70 A). Both spacer materials yield qualitatively similar spectra which evolve in the same way when the spacer thickness increases. We determine the type of interlayer coupling and quantify its strength as a function of the spacer thickness by comparing and fitting the mode positions to a model calculation. Furthermore, we observe clearly different behaviors of the mode linewidths as a function of spacer thickness for the optic and acoustic modes. There are also strong differences between the Stokes and anti-Stokes side of the spectra. The largest linewidths of up to 8 GHz occur at spacer thicknesses between 5 and 8 A for both spacer materials. Lateral averaging and two-magnon scattering are qualitatively discu...

Magnetic properties of EuS spin filter tunnel contacts to silicon

We investigate the magnetic properties of the ferromagnetic insulator EuS in view of its potential in spin-filter tunnel contacts to silicon. We prepared thin polycrystalline EuS films directly on (001) oriented Si substrates that show well-defined magnetic properties down to the monolayer regime. Addressing the question of magnetic coupling between a EuS magnetic tunnel barrier and a CoO/Co magnetic electrode, we succeeded in realizing an independent magnetic switching behavior in this spin-valve-type system. These results are important prerequisites for future spin-dependent transport experiments.

Resonant tunneling magnetoresistance in antiferromagnetically coupled Fe-based structures with multilayered Si/Ge spacers

We report on the experimental evidence of the tunneling magnetoresistance (TMR) effect near 3% and its inversion in strongly antiferromagnetically coupled Fe(001)∕([Si(0.2nm)∕Ge(0.2nm)]*5)∕Fe epitaxial structures with diffused interfaces. We explain the inversion of TMR with biasing voltage by resonant tunneling across impurity states with weak spin split ΔE∼10meV and spin-dependent filtering in the spacer layer. The resonant tunneling is manifested in spin-dependent resonances close to zero biasing voltages related to antiferromagnetic coupling across impurity states.

Antiferromagnetic interlayer exchange coupling across epitaxial, Ge-containing spacers

boundary layers or Si‐Ge-multilayers. The coupling strengths are of the order of 1 mJ/m 2 and decay on a length scale below 2 A as determined from magneto-optic Kerr effect and Brillouin light scattering. The coupling evolves with the spacer thickness from ferromagnetic to prevailing 90° or antiferromagnetic for Ge wedges and Si‐Ge multilayers, respectively. The bilinear coupling is comparable in both cases, but the biquadratic contribution is suppressed for Si‐Ge-multilayer spacers. Thus, Si‐Ge-multilayer spacers give rise to perfect antiparallel alignment of the Fe film magnetizations.

Controlling Magnetic Properties of EuS-Based Spin Valve Structures on Si(001)

We report on the growth and magnetic characterization of the ferromagnetic insulator Europium Sulfide (EuS) on (001)-oriented Silicon substrates. The influence of the EuS film thickness on the coercive field and Curie temperature was systematically investigated with regard to a potential application of thin EuS films as spin filter tunnel barriers. In a further step, we fabricated EuS/Si(001) spin valve structures with both ferromagnetic Gd and exchange-biased CoO/Co counter electrodes. An independent magnetic switching of the EuS barrier and the ferromagnetic layers was accomplished by eliminating intermediate magnetic exchange couplings. Our results clearly demonstrate the feasibility to employ thin EuS ferromagnetic insulator films as spin filter tunnel barriers on Silicon(001) in spin valve structures for future magnetotransport experiments.

Antiferromagnetic Interlayer Exchange Coupling Across Epitaxial Si Spacers

We report on sizable antiferromagnetic interlayer exchange coupling (AFC) of Fe(001) layers across epitaxial Si spacers, for which epitaxial growth of a pseudomorphic phase stabilized by the interface is confirmed by low-energy electron diffraction and high-resolution transmission electron microscopy. The coupling strength decays with spacer thickness on a length scale of a few A and shows a negative temperature coefficient. Transport measurements of lithographically structured junctions in current-perpendicular-to-plane geometry show the validity of the three “Rowell criteria” for tunneling: (i) exponential increase of resistance R with thickness of the barrier, (ii) parabolic dI/dV-V curves, and (iii) slight decrease of R with increasing temperature. Therefore, AFC is mediated by non-conductive spacers, which in transport experiments act as tunneling barriers with a barrier height of several tenths of an eV. We discuss our data — in particular the strength, thickness and temperature dependence — in the context of two previously proposed models for AFC across non-conducting spacers. We find that neither the molecular-orbital model for heat-induced effective exchange coupling nor the quantum interference model extended to insulator spacers by introducing complex Fermi surfaces can account for the strong AFC across epitaxial Si spacers and its negative temperature coefficient. The recently proposed defect-assisted interlayer exchange coupling model, however, yields qualitative agreement with the enhanced AFC and the temperature dependence.