M. Buchmeier

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.

Exchange coupling of molecular-beam-epitaxy-grown Fe/Al/Fe trilayers by dynamic techniques

Network analyzer ferromagnetic resonance (NA-FMR) spectroscopy and Brillouin light-scattering (BLS) techniques are used to probe, in frequency domain, the interlayer exchange coupling (bilinear and biquadratic) of ultrathin Fe/Al/Fe trilayer films epitaxially grown by molecular-beam epitaxy (MBE). These two dynamic techniques provide the possibility to determine the magnetic excitation frequency f (acoustic and optic modes) as well as the frequency linewidth Δf without perturbing the magnetization orientation. A large advantage of the NA-FMR technique compared to conventional FMR is that it can detect the complex magnetic signal at a fixed magnetic field so that the magnetic state of the exchange-coupled trilayer is not perturbed during the frequency sweep. As the spin-wave frequencies are very sensitive to the strength of the static magnetic field and the orientation of the magnetizations, their field dependence reflects the reorientation transitions taking place in a coupled trilayer system. The exchang...

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 Fe films and Fe∕Si∕Fe trilayers grown on GaAs(001) and MgO(001) by ion-beam sputter epitaxy

We grow monocrystalline Fe(001) films and Fe∕Si∕Fe(001) trilayers by ion-beam sputter epitaxy on GaAs(001) and MgO(001) substrates. Ion-beam sputtering parameters such as substrate presputtering time, substrate temperature, beam voltage, and target angle are optimized for 10-nm-thick Fe(001) films with respect to epitaxial growth and magnetic properties. In situ low-energy electron diffraction patterns confirm the epitaxial and monocrystalline nature of the sputtered films, surprisingly even on untreated and thus oxidized substrates. The magneto-optical Kerr effect and ferromagnetic resonance are employed to investigate the magnetic properties, and the structural properties are characterized by atomic force microscopy and x-ray reflectivity measurements. Using the optimized set of parameters that yields the best magnetic properties for single Fe films on GaAs, we deposit epitaxial Fe∕Si∕Fe(001) structures and observe antiferromagnetic interlayer exchange coupling for epitaxially sputtered Fe∕Si∕Fe(001) tr...

Rotating-field magnetoresistance of exchange-biased spin valves

We investigate the magnetoresistance (MR) of spin valves by (i) varying the strength of the field applied in a fixed direction and (ii) rotating the field with fixed strength. The latter data reflect in general a mixture of giant and anisotropic magnetoresistance (GMR and AMR). We present an experimental procedure to suppress the AMR contributions of all ferromagnetic layers in the spin valve without disturbing the GMR response. The resulting angular MR curves are fitted with a single-domain model to determine with high precision the exchange bias field, the uniaxial anisotropies, the GMR ratio, and the interlayer coupling field. The application of the method to differently prepared Ta(5.0nm)∕NiFe(3.0nm)∕FeMn(15.5nm)∕NiFe(3.0nm)∕Co(2.0nm)∕Cu(3.5nm)∕Co(2.0nm)∕NiFe(7.0nm) spin valves with GMR ratios of 1.8% and 4% demonstrates the sensitivity and reveals differences of the order of a few percents of the exchange bias field for the uniaxial anisotropy fields of the free and pinned layer as well as for the in...

Intensity of Brillouin light scattering from spin waves in magnetic multilayers with noncollinear spin configurations : Theory and experiment

The scattering of photons from spin waves (Brillouin light scattering -- BLS) is a well-established technique for the study of layered magnetic systems. The information about the magnetic state and properties of the sample is contained in the frequency position, width, and intensity of the BLS peaks. Previously [Phys. Rev. B 67, 184404 (2003)], we have shown that spin wave frequencies can be conveniently calculated within the ultrathin film approach, treating the intralayer exchange as an effective bilinear interlayer coupling between thin virtual sheets of the ferromagnetic layers. Here we give the consequent extension of this approach to the calculation of the Brillouin light scattering (BLS) peak intensities. Given the very close relation of the BLS cross-section to the magneto-optic Kerr effect (MOKE), the depth-resolved longitudinal and polar MOKE coefficients calculated numerically via the usual magneto-optic formalism can be employed in combination with the spin wave precessional amplitudes to calculate full BLS spectra for a given magnetic system. This approach allows an easy calculation of BLS intensities even for noncollinear spin configurations including the exchange modes. The formalism is applied to a Fe/Cr/Fe/Ag/Fe trilayer system with one antiferromagnetically coupling spacer (Cr). Good agreement with the experimental spectra is found for a wide variety of spin configurations.

Magnetization reversal via symmetric rotation of layers in exchange biased multilayers

We have investigated the magnetization reversal for exchange coupled polycrystalline [IrMn∕CoFe]N multilayers. Polarized neutron reflectivity (PNR) data indicate a simultaneous coherent rotation of all ferromagnetic layers for a sample with N=10 and angles of 45° and 90° between the applied field and the exchange bias direction. On the other hand, magneto-optic Kerr effect (MOKE) measurements, which are sensitive mainly to the two topmost bilayers, reveal a variation of the strength of the exchange bias and the uniaxial anisotropy as a function of N for multilayers with N=1 up to 10. The MOKE data thus indicate the direction of the magnetization to vary from layer to layer for intermediate fields. PNR was found to be insensitive to this variation as the deviation of the layer magnetization directions from its mean value is relatively small (≈10°). These studies demonstrate how the complementary techniques PNR and MOKE can be used to obtain a layer-by-layer vector magnetometry of multilayer stacks.

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.

Interlayer Exchange Coupling of Ferromagnetic Films Across Semiconducting Interlayers

We review our observations of surprisingly strong antiferromagnetic interlayer exchange coupling across Si-rich Fe 1−x Si x spacers, which becomes stronger with increasing Si content, x, in the spacer. We show that the nominally pure (x = 1) spacers that mediate the strongest coupling act at the same time as a tunneling barrier for electric transport in current-perpendicular-to-plane geometry by verifying the validity of the necessary and sufficient Rowell criteria for inelastic tunneling. Moreover, we present first data on the coupling across spacers that contain Ge as an alternative semiconductor material.

Antiferromagnetic exchange coupling in epitaxial Fe/Si/Ge/Si/Fe structures

In this paper, we grow nominally pure Ge spacers by electron-beam evaporation in order to obtain epitaxial growth of the whole structure, a necessary condition for strong AFC.