Ferromagnetism and spin-polarized charge carriers in ${\text{In}}_2{\text{O}}_3$ thin films

We present evidence for spin-polarized charge carriers in ${\text{In}}_2{\text{O}}_3$ films. Both ${\text{In}}_2{\text{O}}_3$ and Cr doped ${\text{In}}_2{\text{O}}_3$ films exhibit room-temperature ferromagnetism after vacuum annealing, with a saturation moment reaching approximately $0.5\text{emu}/{\text{cm}}^3$ for the Cr doped samples. We used point contact Andreev reflection measurements to directly determine the spin polarization, which was found to be approximately $50\%\pm 5\%$ for both compositions. These results are consistent with suggestions that the ferromagnetism observed in certain oxide semiconductors may be carrier mediated.

Figure 1

(a) X-ray diffraction spectrum for a pure ${\text{In}}_2{\text{O}}_3$ sample (lower curve) and a Cr doped to 2 at % ${\text{In}}_2{\text{O}}_3$ thin film (upper curve). (b) HRTEM of the $\text{Cr}:{\text{In}}_2{\text{O}}_3$ thin film, showing the crystalline structure and absence of any impurity phase. (c) HRTEM image of a grain boundary in the film, which shows no evidence for any secondary phase precipitation. (d) Cross-sectional TEM image showing the highly textured thin film.

Figure 2

Raman spectra for the $\text{Cr}:{\text{In}}_2{\text{O}}_3$ thin-film sample (upper curve, black) and bulk ${\text{Cr}}_2{\text{O}}_3$ (bottom curve, gray). The vertical scale for the two curves is different and the curves have been offset for clarity.

Figure 3

(Color online) Representative SEM EDAX area scans of the ${\text{In}}_2{\text{O}}_3$ film for (a) Fe and (b) In. (c) EDAX spectrum for the ${\text{In}}_2{\text{O}}_3$ film. (d) EDAX spectrum for the $\text{Cr}:{\text{In}}_2{\text{O}}_3$ film showing the presence of Cr in this sample.

Figure 4

(Color online) (a) Resistivity vs temperature for the vacuum annealed ${\text{In}}_2{\text{O}}_3$ and $\text{Cr}:{\text{In}}_2{\text{O}}_3$ samples. (b) Magnetization measured as a function of temperature for an ${\text{In}}_2{\text{O}}_3$ film and a $\text{Cr}:{\text{In}}_2{\text{O}}_3$ film, measured in a magnetic field of 1 kOe. The solid line is the fit to a Curie impurity tail, as described in the text. (c) Magnetization vs magnetic field measured at $T=300\text{K}$ for an ${\text{In}}_2{\text{O}}_3$ film and a $\text{Cr}:{\text{In}}_2{\text{O}}_3$ film. Inset: $M\left(H\right)$ for a representative as-prepared ${\text{In}}_2{\text{O}}_3$ film (black circles) and the as-prepared $\text{Cr}:{\text{In}}_2{\text{O}}_3$ film (black triangles) prior to vacuum annealing. The scale for the inset is the same as the scale for the main panel. For all panels, the vacuum annealed ${\text{In}}_2{\text{O}}_3$ data are shown with filled (blue) circles and the vacuum annealed $\text{Cr}:{\text{In}}_2{\text{O}}_3$ data are shown with open (red) triangles.

Figure 5

(Color online) Magnetization curves for $\text{Cr}:{\text{In}}_2{\text{O}}_3$ films having thickness ranging from 200 to 1100 nm, measured at $T=300\text{K}$.

Figure 6

(Color online) (a) and (b) Normalized conductance curves for two different Sn contacts for (a) $\text{Cr}:{\text{In}}_2{\text{O}}_3$ and (b) ${\text{In}}_2{\text{O}}_3$ samples at $T=1.2\text{K}$. One curve in each plot is offset for clarity. Solid lines are the numerical fits obtained with the diffusive model using the BCS gap of bulk Sn, $\Delta =0.57\text{meV}$. (a) Contact resistance $R_c=48\Omega$ (open circles) with fitting parameters: $Z=0.44$ and $P=47\%$; $R_c=35\Omega$ (open triangles) with $Z=0.47$ and $P=52\%$. (b) $R_c=75\Omega$ (open circles) with $Z=0$ and $P=40\%$; $R_c=87\Omega$ (open triangles) with $Z=0$ and $P=42\%$ (c) A single Nb contact with the ${\text{In}}_2{\text{O}}_3$ thin film at different temperatures $\left(R_c=61\Omega \right)$. The BCS temperature dependence of the gap with $\Delta \left(0\right)=1.2\text{mV}$ was used for all the fits. The inset shows the temperature dependence of the resulting spin polarization in ${\text{In}}_2{\text{O}}_3$ with an average $P=56\%$.

Figure 7

(Color online) Temperature dependence of the Nb gap parameter used to fit the PCAR measurements (solid symbols). The dashed line shows the best fit to a BCS temperature dependence.

Figure 8

(Left panel) Magnetization vs temperature for different carrier concentrations. The open triangles are calculated for a carrier concentration of $6.3\times {10}^{19}{\text{cm}}^{-3}$ while the closed circles are for $7.3\times {10}^{19}{\text{cm}}^{-3}$. (Right panel) Room-temperature magnetization vs carrier concentration. Inset: ferromagnetic Curie temperature plotted as a function of carrier concentration.