R. R. Gareev

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.

The role of weak interlayer coupling in the spin-reorientation of perpendicular ultrathin Co-Fe-B/MgO-based heterostructures

Ultrathin magnetic tunneling structures implicate fundamental interlayer exchange coupling between magnetic layers. Here, we describe its important role in the spin-reorientation transition of weakly coupled perpendicular ultrathin Ta/Co-Fe-B/MgO/Co-Fe-B/Ta heterostructures. Near the spin-reorientation, the domain structure is quite sensitive to weak interlayer exchange coupling. Antiferromagnetic coupling stabilizes homogeneous perpendicular magnetization at the remanence, whereas ferromagnetic coupling favors in-/out-of-plane stripe domains. Close to the spin-reorientation transition, even the subtle changes of interlayer exchange coupling can lead to reversible switching between stable in- and out-of-plane states. Our results suggest that this multi-stability caused by the interplay of perpendicular anisotropy and weak interlayer coupling can be utilized in perpendicular spin torque devices operating under reduced spin currents.

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.

Antiferromagnetic coupling across silicon regulated by tunneling currents

We report on the enhancement of antiferromagnetic coupling in epitaxial Fe/Si/Fe structures by voltage-driven spin-polarized tunneling currents. Using the ballistic electron magnetic microscopy, we established that the hot-electron collector current reflects magnetization alignment and the magnetocurrent exceeds 200% at room temperature. The saturation magnetic field for the collector current corresponding to the parallel alignment of magnetizations rises up with the tunneling current, thus demonstrating stabilization of the antiparallel alignment and increasing antiferromagnetic coupling. We connect the enhancement of antiferromagnetic coupling with local dynamic spin torques mediated by spin-polarized tunneling electrons.

Antiferromagnetic Coupling in Combined Fe/Si/MgO/Fe Structures with Controlled Interface Diffusion

We study antiferromagnetic coupling and interface diffusion in Fe/Si/MgO/Fe structures grown by molecular beam epitaxy. The Fe/Si/Fe samples with a 1.2-nm-thick Si spacer demonstrate antiferromagnetic coupling J1~-1.5 mJ/m2 and prevailing interdiffusion at the top Si/Fe interface, as revealed by conversion electron Mossbauer spectroscopy. For combined Si/MgO spacers with 0.9-nm-thick Si, interdiffusion continuously reduces upon changing the MgO thickness from 0.3 to 0.5 nm accompanied by a decrease of antiferromagnetic coupling from |J1|~1 mJ/m2 to |J1|~0.002 mJ/m2. We emphasize that monolayer-scaled engineering of insulating spacers is a promising tool for the precise control of antiferromagnetic coupling and interface diffusion.

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.