Element-specific characterization of heterogeneous magnetism in (Ga,Fe)N films

We employ x-ray spectroscopy to characterize the distribution and magnetism of particular alloy constituents in (Ga,Fe)N films grown by metal organic vapor phase epitaxy. Furthermore, photoelectron microscopy gives direct evidence for the aggregation of Fe ions, leading to the formation of Fe-rich nanoregions adjacent to the samples surface. A sizable x-ray magnetic circular dichroism (XMCD) signal at the Fe $L$ edges in remanence and at moderate magnetic fields at 300 K links the high-temperature ferromagnetism with the Fe(3$d$) states. A sizable value of the orbital moment measured in remanence points to a large magnetic anisotropy and to a correspondingly high blocking temperature, which can stabilize the ferromagnetic-like response of the Fe-rich nanocrystals at 300 K. The XMCD response at the N $K$ edge highlights that the N(2$p$) states, due to exchange coupling to Fe ions, carry considerable spin polarization. These results, supported by ab initio studies of XMCD spectra for various FeN${}_{\delta }$ nanocrystals, demonstrate that nanocrystals with $\delta >0.25$ account for the ferromagnetic behavior of (Ga,Fe)N films.

Figure 1

(a), (b) In situ low-energy electron diffraction patterns obtained from a typical surface of GaN during the XAS and XMCD experiments, as described in the text. The energy of the electron beam is set to 272 eV in (a) and 310 eV in (b). (c) XAS spectra taken in situ on a test sample during the sample preparation using soft Ar${}^{+}$ ion sputtering. These data are taken at an angle of x-ray incidence of 45${}^{\circ }$ to the surface plane. It is observed that, in particular, the high-energy continuum, proportional to the N atom content, is close to constant, indicating no N deficiency in the near-surface region.

Figure 2

Left panel: PEEM micrograph with a 2-$\mu$m field of view for sample $s$300. The photon energy is set at the Fe $L$${}_3$ white line maximum. The bright spots denote areas with a high Fe concentration. The remaining regions are not entirely dark, indicating that Fe is also dissolved in the lattice. Right panel: Spatially resolved XAS, corresponding to small areas within the micrograph, with differing shadowing.

Figure 3

XAS and XMCD spectra in photocurrent (TEY) mode versus photon energy. The XAS spectra are shown with 1.0 subtracted. The in-plane magnetic field of $H=\pm 800$ Oe serves to reverse the magnetization direction for the evaluation of the XMCD magnitude at 300 K. For XMCD measurements, the magnetic field and x-ray beam direction are parallel. Upper panel: The XAS spectra (left scale) are obtained at $H=\pm 800$ Oe (solid and dotted lines) for samples $s$300 and $s$100 with nearly circularly polarized x rays and a 45${}^{\circ }$ x-ray incidence angle. Upper inset: Fe $L$${}_3$ white line for $s$300 together with the data for bcc Fe. Middle panel: The XMCD difference spectra for (Ga,Fe)N samples $s$300 and $s$100 (scaled by a factor of 5; left scale). Lower panel: XMCD spectra (right scale) for $s$300 and bcc Fe (scaled by a factor of 0.019). Lower inset: The dichroic signal of the Fe (4$s$) states for sample $s$300 is stronger comparing to bcc Fe.

Figure 4

N $K$ edge for the (Ga,Fe)N sample $s$300 (left scale), with 1.0 subtracted, taken with linear x rays at normal (90${}^{\circ }$) and grazing (20${}^{\circ }$) x-ray incidence $versus$ the film plane. The corresponding difference spectra are shown: negative peaks (left scale) indicate $s$ $\to$ $p_z$ states, positive peaks $s$ $\to$ $p_x,p_y$. Also shown is the XMCD difference, taken with circularly polarized x rays at normal and grazing x-ray incidence (right scale) under an applied field of $\pm 800$ Oe. The second derivative of the N XAS spectrum at grazing incidence is shown, as it reproduces the oscillatory part of the XMCD signal, pointing to some itinerant character of the final states.

Figure 5

Upper part: Calculated N $K$-edge XAS spectra versus photon energy using the feff code for GaN in the wurzite crystal structure. In the upper part, theoretical spectra are shown assuming full linear x-ray polarization with the electric field vector along the $c$ axis ($E$ along [001]) or in the hexagonal basal plane ($E$ along [100]). Lower part: The corresponding difference spectrum to the spectra shown in the upper part gives the angle intensity variation calculated with linear light. For completion, referring to the experimental data, the second derivative for a spectrum taken with circular light for the light propagation along the $c$ axis is also shown.

Figure 6

Calculated N $K$-edge XAS and XMCD spectra versus photon energy using the feff code for $\gamma$-Fe${}_4$N, $\epsilon$-Fe${}_3$N, and $\zeta$-Fe${}_2$N. The XAS spectra (full line and dotted lines, left scale) correspond to full circular x-ray polarization. The corresponding XMCD difference spectrum is shown in each case (right scale). For $\gamma$-Fe${}_4$N, $\epsilon$-Fe${}_3$N, the XMCD difference to the stable ferromagnetic state is calculated, and for $\zeta$-Fe${}_2$N a ferromagnetic state is assumed (full lines for the XMCD, right scale). The second derivatives of the spectra taken as the half sum of the theoretical plus and minus spectra are also shown (right scale).