X-ray absorption and magnetic circular dichroism characterization of Mo1–xFexO2 (x = 0–0.05) thin films grown by pulsed laser ablation

Abstract

The electronic and magnetic properties of well characterized Mo 1−x Fe x O 2 (x = 0-0.5) thin films that show ferromagnetism at room temperature (RT) have been investigated by the means of near edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments at the O K-, Fe L-, and Mo M-edges. The NEXAFS spectra at O K-and Mo M 3,2 -edges show a strong hybridization of O 2p-4d Mo orbitals, and Mo ions change their symmetry with the substitution of Fe ions into MoO 2 matrix. The Fe 2p NEXAFS/XMCD spectra exhibit multiple absorption peaks and an appreciable XMCD signal that persists even at RT. These results demonstrate that Fe is in a mixed valence state of Fe 2+ -Fe 3+ , substituting at the Mo site and that the Fe 2+/3+ ions are ferromagnetically polarized.

Low Temperature Nanoscale Oxygen-Ion Intercalation into Epitaxial MoO2 Thin Films

“…We observed the color of 25-nm-thick MoO 2+ x annealed at 250 °C with 400 Torr of oxygen pressure became transparent (Figure S5). In addition, we observed that the increase of prepeak intensity near the 530 eV is prominent and it resembles the O K -edge of MoO 3 previously reported. ,,, The change of prepeak intensity at MoO 2+ x is similar to that of vanadium oxide during the oxidation of VO 2 to V 2 O 5. It is considered as the level of t 2g orbital occupancy varied by the variations of Mo valence states.…”

“…Assignments of the hybridization peaks are based on DFT results , like the case of optical conductivity spectra. There are four distinct hybridization peaks (A–D) in the spectra; A and B are related to the hybridization of O 2p–Mo 4d t 2g and C and D are that of O 2p – Mo 4d e g . − Note that we used two detection modes such as surface-sensitive total electron yield (TEY) shown in Figure a and bulk-sensitive total fluorescent yield (TFY) shown in Figure S3. Each mode has a different penetration depth: several nm for TEY and nearly 100 nm for TFY.…”

Oxygen K-edge X-ray Absorption Spectra

“…MoO 2 has a distorted rutile structure. The oxygen K-edge of MoO 2 , shown in Figure , has peaks related to the empty 4d states, but there is significant variation in the published spectra. ,− Thakur et al compared TEY and TFY spectra with high resolution, and they observed three peaks within the 4d band that they assigned to a distortion of the octahedral site. The three features were also observed by Khyzhun et al Wang et al studied the reduction of MoO 3 to MoO 2 in the electron beam during an EELS experiment .…”

“…Ion yield is an extremely surface sensitive technique and measures only oxygen atoms in the surface layer of the system. 405 Several papers discuss oxygen K-edge spectra of mixed oxides, for example, the (Mo,Fe) mixed oxides in relation to magnetism 421 and the (Mo,V) mixed oxides in relation to catalysis. 423 Technetium Oxides.…”

Nitrogen‐Doped Porous Nickel Molybdenum Phosphide Sheets for Efficient Seawater Splitting

“…It can be observed that the introduction of N and Ni in MoO 3 redistributes the electrons, as evident from the shift in photon energy around C and E and the disappearance of D. Furthermore, the dangling bonds can also be observed in the O-K-edge spectra of MoO 3 , N-MoO 3 , and N-NiMoO 3 represented by Figure S5b where the excitation of these energy states near the Mo 4+ oxidation state stretching mode is in line with the metal-oxygen bond stretching mode for oxide as observed by the Raman spectroscopy results. [23] Figures 3h,i show the NEXAFS Mo L-edge for oxides and phosphides with and without the addition of Ni and N. It can be observed that the edge positions of Mo L-edge have displaced to a higher photon energy after the addition of both Ni and N atoms as compared to the original MoO 3 and Mo 3 P. This shift indicates that Mo in N-NiMoO 3 and N-NiMo 3 P are in a higher oxidation state than that in MoO 3 and Mo 3 P, respectively indicating the higher oxidation state of Mo and changes in the Mo charge density after N and Ni addition. Additionally, the d orbitals of Mo atoms tend to support the fast transfer of electrons with a low energy barrier.…”