Fe-doped In₂O₃ Room Temperature Ferromagnetic Semiconductor

Since Dietl et al. predicted that p-type ZnO doped with 5% of Mn would be ferromagnetic with Curie temperature above 300K in the year of 2000, world-wide zest on oxide-based diluted magnetic semiconductors have been arisen. The mostly researched systems are magnetic transition element doped ZnO, TiO₂, SnO₂, and In₂O₃. To integrate electronics, magnetics and photonics for next generation multifunctional devices, it will be important to search for a magnetic semiconductor with tunable carrier density, high mobility and magnetic moment as well as optical transparency.Here we report the observation of room temperature ferromagnetic behavior in Fe-doped In₂O₃ that may be expected to meet the above criteria.In₂O₃ is a transparent semiconductor material with a direct band gap of 3.75eV and cubic bixbyite crystal structure with a lattice constant of 10.12 A. It can be an n-type semiconductor with high conductivity by introducing oxygen vacancies or Sn-doping,.The carrier density and mobility can be easily changed by doping or changing oxygen pressures during deposition. Such favorable properties make it widely used in many practical applications, such as gas-sensing materials, transparent electrodes, flat planar displays and other optoelectronic devices. Compared with the extensively studied ZnO and TiO₂-based magnetic semiconductor systems, research on In₂O₃-based magnetic semiconductors is just at its beginning. Only a few papers have reported about ferromagnetism in transition metal doped In₂O₃ systems , among which In₂O₃ with Fe doping is more attractive. That's mainly because the most probable valence states of iron and indium are the same as +3, which makes it easier for iron to dissolve in the In₂O₃ lattice. However, in order to determine whether the room temperature ferromagnetism observed in Fe-doped InO₃ films is intrinsic, more research should be carried out.Fe-doped In₂O₃ films were deposited on R-cut sapphire substrates by pulsed laser deposition.The (In_1-xFe_x)₂O₃ (x=0.01, 0.03, 0.05, 0.07, 0.09) ceramic targets were prepared using conventional solid-state reaction method. The 99.99% In₂O₃ and 99.99% Fe₂O₃ powders were first mixed and ground for 2 hours, then pressed disk with a diameter of 40 mm and sintered in air at 1300℃for 10 hours to form a ceramic target.Fe-doped InO₃ films were deposited on R-cut sapphire substrates by pulsed laser deposition at different temperatures. The targets were ablated by a KrF excimer laser operating at 248 nm and 5 Hz. The energy density on the targets was 1.8 J/cm². The deposition chamber was firstly evacuated to a base pressure of 1.0×10^-5 Pa. High pure oxygen was introduced during deposition with pressure in the range of 0～1.0×10^-3 Pa. The target-to-substrate distance was 10 cm. After deposition the samples were cooled in the same pressure at a rate of 20℃/min.The microstructure analysis and phase identification were carried out by X-ray diffraction (XRD), giving out two kinds of structures. One shows five diffraction peaks of bcc In₂O₃ crystal structure with the (440) peak much higher than the other four peaks, which indicates that the films have a strong oriented texture along (440). The other shows only (222) peak that indicates the films have a strong oriented texture of (222) plane. No traces of any known Fe clusters, iron oxides or binary indium-iron phases were observed in the detection limit of the X-ray machine.X-ray Photoelectron Spectroscopy was carried out to determine the valence of Fe element. The peak of Fe2p^3/2 locates at 711 eV, which excludes the formation of iron metal clusters since the binding energy of Fe2p^3/2 for iron metal is 707eV. Both the peak position and the energy difference between Fe2p^3/2 and Fe2p^1/2 indicate that Fe element in our sample exists in the ion form, with the valence of +2 and/or +3.Considering the XRD and XPS results together, we believe that Fe element incorporates into the indium oxide lattice by substituting the position of indium atoms.Magnetic properties were measured using an Alternating Gradient Magnetometer (AGM), a Vibrating Sample Magnetometer (VSM) and a Superconducting Quantum Interference Device (SQUID).In order to study the effect of the oxygen vacancies on the magnetic properties of Fe-doped InO₃ films, oxygen with different pressures was introduced into the vacuum chamber during deposition. From the M-H curves, one can see that robust room-temperature ferromagnetism is found as the oxygen pressure is less than 6×10^-4 Pa, and that the magnetization does not vary monotonically with the oxygen pressure. The magnetic moment increases dramatically as the oxygen partial pressure increases from 0 to 4×10^-4 Pa. For the film deposited at the oxygen pressure of 4×10^-4 Pa, the saturate magnetic moment could reach 2.5μb/Fe The ferromagnetism decreases with further increase of the oxygen pressure and almost disappears when the oxygen pressure is higher than 8×10^-4 Pa.The hysteresis loops were measured at room temperature with the applied magnetic field parallel and perpendicular to the film plane for the sample deposited without introduced oxygen. It is clear that the room-temperature ferromagnetism has distinct magnetic anisotropy. When the applied magnetic field is perpendicular to the film plane, the sample gets saturate much easier with a saturate magnetization of 1.35μB /Fe and a coercivity of 680 Oe, contrasting with 1.0μB /Fe and 380 Oe in the case of parallel applied field. We don't see magnetic anisotropy of the film which oriented along with [111] direction.Temperature-dependent magnetization measurements were carried out by SQUID (below 300 K) and VSM (above 300K) with the applied field of 1T in the film plane. The results indicate the saturate magnetization decreases slowly in the temperature range from 2K to 300K. While the temperature increases further, the magnetization decreases rapidly. From the M-T curve, the Curie temperature is estimated to be as high as 927K, which is much higher that ofγ-Fe₂O₃ and Fe₃O₄.In order to further check the effects of the oxygen vacancies on the ferromagnetism in Fe-doped InO₃ magnetic semiconductor films, the as-grown films were alternately annealed in high vacuum to produce more oxygen vacancies, and in air to deduce oxygen vacancies. The result shows the reversible transforms in ferromagnetism by alternately annealing for the sample deposited with the oxygen pressure of 2×10^-4 Pa. Ferromagnetism was enhanced by annealing in vacuum and weakened by annealing in air, with the saturate magnetization appearing a reversible oscillation. When the magnetic field was applied perpendicular to the film plane, the saturate magnetization oscillated between 2.12μb/Fe and 3.0μb/Fe. Moreover, the magnetic anisotropy didn't disappear after repeatedly annealing, still with the easy axis of magnetization perpendicular to the film plane.