Magnetic properties and magnon excitations in Fe(001) films grown on Ir(001)

We present a comprehensive study of magnetic properties and high wave-vector magnon excitations in epitaxial Fe(001) films grown on Ir(001) substrate. The magnetic properties are investigated by magneto-optical Kerr effect for various thicknesses of Fe film from 4 up to 27 monolayers. The magnon dispersion relation is obtained by means of spin-polarized electron energy loss spectroscopy. By comparing the experimental and theoretical results, we comment on the effect of the strain induced tetragonal distortion in the Fe film and the electronic hybridizations between the film and the substrate on the exchange interaction and magnon dispersion relation. The thickness-dependent measurements show that the magnon dispersion relation does not change significantly while changing the film thickness in the regime, where the film is uniformly strained. In addition, the measurements are also performed on thicker Fe films, where the films are relaxed into a bulklike structure. The magnon energies close to the zone boundary differ from the ones measured on thick Fe(110) films on W(110). However, the magnon dispersion relation measured for both systems can be well described within the Heisenberg model by using the same effective exchange parameters.

Figure 1

(a) Schematic representation of the bcc-Fe(001) lattice on fcc-Ir(001). Selection of LEED patterns of (b) Ir(001) with ($5\times 1+1\times 5$) surface reconstruction, (c) 4 ML, (d) 6 ML, (e) 10 ML, (f) 19 ML, (g) 23 ML, and (h) 27 ML Fe films grown on Ir(001). All images are taken at a primary electron energy of 100 eV. Before recording the LEED patterns, Fe films are annealed at 900 K.

Figure 2

Longitudinal MOKE measurements on Fe films grown on Ir(001) with different thicknesses at 300 K. The hysteresis loops are obtained on (a) 4.2 ML Fe, (b) 5 ML Fe, (c) 6 ML Fe, (d) 10 ML Fe, (e) 13 ML Fe, and (f) 23 ML Fe. All loops are obtained after annealing the sample at about 900 K. The magnetic field is applied along the [0$\overline{1}$0] direction. (g) Kerr ellipticity as a function of the Fe thickness in saturation (Ms) (black squares) and in remanence (Mr) (red circles). The large difference between Ms and Mr in the region of 10–19 ML is due to the structural relaxation taking place in this region (see the text). Inset shows an enlargement at the scale of thicknesses from 2 to 8 ML.

Figure 3

(a) Scattering geometry used for the SPEELS measurements. SPEEL spectra $I_{\downarrow }$ (red), $I_{\uparrow }$ (blue), and their difference for (b) 4.2 ML, (c) 4.8 ML, and (d) 5 ML Fe grown on Ir(001) at an in-plane wave-vector transfer of $\Delta K_{\parallel }=0.8{\AA }^{-1}$. The experiments are performed at 300 K. The scattering plane is parallel to the Fe[100] direction. All data are obtained using the incident electron energy of 6 eV with a total energy resolution of about 15 meV. The hysteresis loops obtained by the longitudinal MOKE measurements with the field applied along the [0$\overline{1}$0] direction are shown in the insets. The y axis of all insets is the Kerr ellipticity in the units of microrad. (e) The difference ($I_{\downarrow }-I_{\uparrow }$) spectra at $\Delta K_{\parallel }=0.8{\AA }^{-1}$ for different film thicknesses are plotted together for comparison.

Figure 4

(a) Typical SPEEL spectra recorded on 6 ML Fe/Ir(001) at an in-plane wave-vector transfer of $\Delta K_{\parallel }=0.7{\AA }^{-1}$ probed along the Fe[100] direction. The intensity spectrum $I_{\downarrow }$ ($I_{\uparrow }$) is obtained using the incident electrons with the spin orientation antiparallel (parallel) to the spin of majority electrons in the Fe film. The experiments are performed at 300 K. (b) A series of difference spectra ($I_{\downarrow }-I_{\uparrow }$) at in-plane wave-vector transfers from $0.5$ to $0.8{\AA }^{-1}$ probed along the Fe[100]-($\overline{\Gamma }$–$\overline{\mathrm{X}}$) direction.

Figure 5

(a) Experimental (open diamonds) and theoretical (solid curve) magnon dispersion relation of 6 ML Fe/Ir(001). (b) The calculated magnon dispersion relation for four different cases: (i) a pseudomorphically grown Fe film on Ir(001) with the vertical lattice constant obtained by IV-LEED measurements [25] (green solid line); (ii) a pseudomorphically grown Fe film on Ir(001) with the vertical lattice constant of the bulk Fe (blue dashed line); (iii) a free-standing Fe film with the experimental lattice constants (light green dash-dotted line); (iv) a free-standing Fe film with the Fe bulk lattice constants (gray dotted line).

Figure 6

Calculated site-resolved interlayer, $J_{\perp }$, and intralayer, $J_{\parallel }$, exchange constants for the atoms located in the interface layer (b),(d) and in the surface layer (c),(e) [the geometry is shown in (a)] for the case of (i) a pseudomorphically grown Fe film on Ir(001) with the vertical lattice constant obtained by IV-LEED measurements [25] (red solid line), (ii) a pseudomorphically grown Fe film on Ir(001) with the vertical lattice constant of bulk Fe (blue dashed line), and (iii) a free-standing Fe film with the experimental lattice constants (green dash-dotted line).

Figure 7

(a) Spin- and layer-resolved density of states of Fe atoms sitting in the surface layer (dashed curve), in the interior of the Fe film (dotted curve), and in the interface Fe layer (solid curve). (b) Spin- and layer-resolved density of states of Ir atoms sitting in the bulk Ir (dotted curve) and in the interface Ir layer (solid curve).

Figure 8

Magnon dispersion relation of 6 to 9 ML Fe grown on Ir(001) obtained at room temperature. The symbols represent the experimental data. The solid lines show the results of calculations.

Figure 9

(a) Calculated (solid diamonds) and experimental (open circles) magnon energy at $\Delta K_{\parallel }=0.8$ ${\AA }^{-1}$ versus Fe thickness. (b) Blue solid circle: asymmetry [($I_{\downarrow }-I_{\uparrow })/(I_{\downarrow }+I_{\uparrow }$)] at the magnon energy as a function of Fe thickness; red open symbols: Kerr ellipticity at remanence divided by the Fe thickness. The dashed line indicates the onset of the ferromagnetic order at room temperature, which is at 5 ML.

Figure 10

Magnon dispersion relation measured along the $\overline{\Gamma }$–$\overline{\mathrm{X}}$ direction on thick Fe ($>10$ ML) films grown on Ir(001) and compared to the one of 6 ML. The zone boundary of the relaxed film is at 1.10 ${\AA }^{-1}$ shown as the dotted line, and the one of the 6 ML film is at 1.16 ${\AA }^{-1}$ shown as the dashed line.

Figure 11

Magnon dispersion relation obtained on thick Fe films grown on different substrates with different surface orientations. The red circles are the experimental data measured on 24 ML Fe(110) grown on W(110) [8]. The black squares are the results of 27 ML Fe(001) on Ir(001). The error bars in energy are typically around 5 meV from 0.4 to 0.8 ${\AA }^{-1}$, and around 10 meV above 0.8 ${\AA }^{-1}$. The dispersion relation could be well described by the Heisenberg model using the same exchange parameters. The fit parameters are $J_{N}S=6.8$ meV and $J_{NN}S=4.1$ meV.