Magnetic states at the surface of α−Fe2O3 thin films doped with Ti, Zn, or Sn

Abstract

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 (TM ≈ 200 K) and Sn-doped (TM ≈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 Sn 4+ and Ti 4+ , for which TM dramatically decreases, these dopants substantially increase TM in thin films, far exceeding the bulk values. The normalized Fe LII -edge dichroism for T < TM 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 > TM 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 LIII -edge spectra were, for the most part, determined solely by the spin orientation state. Calculations of the polarization-depedent spectra based on a spin-multiplet model were in reasonable agreement with the experiment and showed a mixed excitation character of the peak structures. PACS numbers: 75.25.-j,75.70.-i,78.70.DmHematite (α-Fe 2 O 3 ) is considered to be a promising material for photoelectrochemical (PEC) cells for solar water splitting 1-3 , whereby solar energy can drive hydrogen-production reactions in water 4 . Introducing impurities, or doping, as a source of free charge carriers is a crucial ingredient to making feasible devices 5,6 . The models and tools of semiconductor device physics are being increasingly used in the ongoing effort to develop such cells using hematite photo-anodes 7 . Yet, the electron-electron interactions arising from its Fe 3+ 3dorbitals, mediated by oxygen atoms, make hematite a strongly correlated system, as evidenced by its antiferromagnetism (AF). In addition to the effect of doping on transport properties in the usual way, the doping can also be used to manipulate the magnetic structure. As to which extent this is also important for the device performance has yet to be explored.A magnetic transition, known as the Morin transition, takes place upon raising the temperature past T M (=265 K for bulk, undoped hematite), which changes the spin-ordering from pure AF to a superposition of AF with weak ferromagnetism 8 . Polarized neutron diffraction 9 , magnetic resonance 10,11 and magnetostriction measurements 12 have since confirmed this and revealed that below T M the spins are aligned antiferromagnetically along the c-axis, with stacked c-planes having alternating up and down spins. The spin arrangements are illustrated in fig. 1. Above T M , the spins are rotated ∼90 • to lie in the c-plane, with AF order still maintained along the c direction, but now with re...

Tunable long-distance spin transport in a crystalline antiferromagnetic iron oxide

“…Below T M , the Néel vector is parallel to the DMI vector with no effect on the magnetic moments. Our sample maintains its insulating properties and exhibits no evidence of semiconducting behavior from 300 K down to 4 K. One should note that the Morin transition can be tuned by slightly doping α-Fe 2 O 3 to shift T M to higher (with Ir35 or Sn36 doping) or lower temperatures (with Ti or Ga doping37).…”

Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity

“…After verifying that the TRMC response of the capped films probes bulk properties, epitaxial hematite films with different dopants were prepared. Hematite is often doped to change its magnetic, electronic, or photoelectrochemical properties . Undoped hematite behaves as a weak n‐type semiconductor due to the existence of oxygen vacancies which are compensated by electrons.…”

“…Hematite Film Fabrication : Hematite films were deposited on sapphire c ‐plane (0001) single crystal substrates according to previous reports . The sapphire substrates were thoroughly cleaned with ultrasonic waves, soap, acetone, ethanol, and 1 m KOH.…”

“…Materials Characterization : The crystalline quality of the hematite films was previously examined via X‐ray diffraction (Rigaku Smartlab). The films have (0001) orientation, high degree of crystallinity, and epitaxial alignment to the sapphire substrate, which is described elsewhere in more detail . X‐ray reflectivity measurements were performed in order to extract the thicknesses of the hematite films and are shown in Figure S10 (Supporting Information).…”

Long-distance spin-transport across the Morin phase transition up to room temperature in ultra-low damping single crystals of the antiferromagnet α-Fe2O3

“…Above the Morin temperature (T Morin = 260 K), undoped hematite single crystals undergo a transition from an easy-axis to an easy-plane AFM, due to a change of sign of its anisotropy field H A 19 , with a small sub-lattice canting due to its internal Dzyaloshinskii-Moriya interaction (DMI) 20 . One must notice that the Morin transition can disappear due to size effects in thin films and be recovered through doping 21 . A similar transition towards a canted easy-plane phase can be obtained at lower temperatures for sufficiently high fields in the spin-flop state 20,22 .…”