Magnetic states at the surface of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ thin films doped with Ti, Zn, or Sn

The spin states at the surface of epitaxial thin films of hematite, both undoped and doped with 1% Ti, Sn, or Zn, respectively, were probed with x-ray magnetic linear dichroism (XMLD) spectroscopy. Morin transitions were observed for the undoped $(T_M\approx 200$ K) and Sn-doped $(T_M\approx 300$ K) cases, while Zn- and Ti-doped samples were always in the high- and low-temperature phases, respectively. In contrast to what has been reported for bulk hematite doped with the tetravalent ions ${\mathrm{Sn}}^{4+}$ and ${\mathrm{Ti}}^{4+}$, for which $T_M$ dramatically decreases, these dopants substantially increase $T_M$ in thin films, far exceeding the bulk values. The normalized Fe $L_{II}$-edge dichroism for $T<T_M$ does not strongly depend on doping or temperature, except for an apparent increase of the peak amplitudes for $T<100$ K. We observed magnetic field-induced inversions of the dichroism peaks. By applying a magnetic field of 6.5 T on the Ti-doped sample, a transition into the $T>T_M$ state was achieved. The temperature dependence of the critical field for the Sn-doped sample was characterized in detail. It was demonstrated the sample-to-sample variations of the Fe $L_{III}$-edge spectra were, for the most part, determined solely by the spin orientation state. Calculations of the polarization-dependent spectra based on a spin-multiplet model were in reasonable agreement with the experiment and showed a mixed excitation character of the peak structures.

Figure 1

The spin ordering in hematite for the magnetic state for $T<T_M$ (left) and $T>T_M$ (right). The iron atoms are represented by the large blue spheres, and arrows pointing in their spin directions, and the small red spheres are oxygen atoms separating the Fe planes. Within each plane, the spin directions are the same, as indicated by the stacks of arrows on each side. For $T>T_M$, the spins cant out of the basal plane with an angle $\delta$, as shown. Made with the help of freely available balls and sticks software [13].

Figure 2

(a) $\theta \text{−}2\theta$ x-ray diffraction pattern showing $c$-axis orientation of hematite thin film grown on $c$-plane sapphire. (b) High-resolution x-ray diffraction pattern of hematite (0006) reflection revealing the existence of Laue oscillations. (c) Rocking curve of (006) peak having a width of $0.{05}^{\circ }$. (d) Pole figure of the (1 0 4) reflection showing heteroepitaxial in-plane alignment of hematite films on sapphire substrate with small in-plane mosaic spread.

Figure 3

(a) The Fe $L_{II}$ and $L_{III}$ x-ray absorption spectra for the electric field of the incident x ray approximately parallel and perpendicular to the $c$ axis of the 1% Ti-doped sample at $T$ = 300 K. The linear polarizations of the near-grazing incident x ray with respect to the $c$ axis of the sample is indicated. (b) The linear dichroism spectrum, defined as the difference between the two curves in (a). (c) The room temperature dichroism of the Zn-doped sample.

Figure 4

The dichroism spectra at the Fe $L_{II}$ edge, measured at the indicated temperatures for the (a) undoped, (b) 1% Zn-doped, (c) 1% Ti-doped, and (d) 1% Sn-doped samples.

Figure 5

The difference of peak height values between the first and second main peaks of the Fe $L_{II}$ edge dichroism spectra, determined relative to zero as shown in the inset (below zero is considered as a negative peak height). They are plotted as a function of temperature for the four samples.

Figure 6

Effect of applying high magnetic fields $({20}^{\circ }$ to the $c$ axis) on the dichroism spectra of each of the samples, at $T$ = 295 K. For clarity the spectra of each sample are displaced along the vertical axis. The fields are as indicated, with solid curves corresponding to $B$ = 0.05 T (approximately zero), and the circles corresponding to the high magnetic fields. The inset shows a typical example of the progression of the spectra with increasing field in the 1% Sn-doped sample, with a magnetic phase transition induced at $B$ = 4 T, for $T$ = 278 K.

Figure 7

Critical field as a function of temperature for the 1% Sn-doped sample (large circles), fit to square-root function (solid line). The result of Foner and Shapira for bulk hematite [41], shifted along the temperature axis by 35 K, which is the difference of $T_M$'s between the two samples (small squares).

Figure 8

Absorption spectra at the Fe $L_{III}$ edge, for polarizations approximately parallel and perpendicular to the crystallographic $c$ axis, for the four samples with dopants as indicated, for (a) $T$ = 5 K and (b) $T$ = 300 K. The spectra for each sample are offset along the vertical axis for clarity. The spin state, as determined from the analysis of the $L_{II}$ edge, is indicated by the labels of temperature relative to the Morin transition. The bottom curves [identical for (a) and (b)] are from calculations using the program ctm4xas as described in the text.