Kh. Zakeri

Asymmetric Spin-Wave Dispersion on Fe(110): Direct Evidence of the Dzyaloshinskii-Moriya Interaction

The influence of the Dzyaloshinskii-Moriya interaction on the spin-wave dispersion in an Fe double layer grown on W(110) is measured for the first time. It is demonstrated that the Dzyaloshinskii-Moriya interaction breaks the degeneracy of spin waves and leads to an asymmetric spin-wave dispersion relation. An extended Heisenberg spin Hamiltonian is employed to obtain the longitudinal component of the Dzyaloshinskii-Moriya vectors from the experimentally measured energy asymmetry.

Magnetic anisotropy in nanoscaled materials probed by ferromagnetic resonance

Ferromagnetic resonance measurements probe the dynamical response of magnetic systems due to an excitation within the microwave regime. Offering high sensitivity and energy resolution in the μeV range of ferromagnetic resonance this technique is well suited for the investigation of magnetic anisotropy in nanoscale systems. Ferromagnetic Resonance experiments give direct and quantitative access to magnetic anisotropy based on an analysis that uses the Landau--Lifshitz equation of motion. This will be demonstrated for the case of ultrathin magnetic 5--20ML thick Fe films on {4×6}GaAs(001) (2D system) which have been grown and measured in situ in ultra high vacuum, magnetic MnAs stripes (1D system) grown on GaAs(001) as well as for arrays of highly monodisperse FePt nanoparticles (quasi 0D system).

Magnon excitations in ultrathin Fe layers: The influence of the Dzyaloshinskii-Moriya interaction

High wave-vector magnon excitations in ferromagnetic Fe monolayer and double-layer grown on W(110) are investigated using spin-polarized electron energy loss spectroscopy. The magnon dispersion relation is obtained up to the Brillouin zone boundary. A direct comparison among different systems shows that the magnons in the Fe monolayer are extremely soft and are even softer than the acoustic surface mode of Fe(110). By measuring the spectra in both energy loss and gain regions on a double-layer Fe film at room temperature and by reversing the sample magnetization, it is demonstrated that the magnon dispersion is asymmetric with respect to the sign of the wave-vector. The asymmetric dispersion relation is attributed to the degeneracy breaking of the magnons due to the presence of the Dzyaloshinskii-Moriya interaction.

Thickness-dependent reorientation of the magnetization of Fe monolayers on InP(001)

Fe films were grown by molecular beam epitaxy on {2×4}InP(001) at room temperature in the thickness range of 5–21 monolayer equivalent (ML). The magnetic anisotropy and magnetization were studied using in situ ferromagnetic resonance and in situ SQUID magnetometry, respectively. For the whole thickness range the magnetization was found to favor an in-plane alignment. The easy axis of magnetization is parallel to the []-direction below 7 ML, which rotates by 45° towards the [100]-direction for thicker Fe layers. The magnetic anisotropy energy of the system has been quantitatively determined as a function of film thickness. The perpendicular magnetic anisotropy is strongly thickness dependent with a large surface-interface contribution which is comparable to that of Fe grown on GaAs. The cubic anisotropy K4≈1×104 J/m3, however, is small compared to that of bulk Fe over the whole thickness regime and almost thickness independent. The in-plane uniaxial anisotropy is strongly thickness dependent and originates at the Fe/InP interface.