Direct observation of oxygen induced room temperature ferromagnetism in MoO2 thin films by x-ray magnetic circular dichroism characterizations

Abstract

We report the element specific polarized near edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments on well characterized undoped MoO2 thin films that show ferromagnetism at room temperature. The polarization dependent of O K edge NEXAFS spectra indicate a strong hybridization of O 2p-4d Mo orbitals followed by a strong anisotropy in the electronic properties. An unquenched orbital magnetic moment within the O 2p shell is clearly evident from the XMCD O K edge, which is ferromagnetically coupled to the neighboring Mo moments as confirmed by Mo M3,2 edge XMCD experiment.

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

“…In undoped MoO 2 , it was explained by a charge transfer process from ligand O 2p orbitals to the Mo 4d orbitals, resulting in a net magnetic polarization of O 2p orbitals [5]. The absence of XMCD signal at the O K-and Mo M-edges for the Fe-doped thin films indicate that more defects are added to the system and magnetism is completely ruled by Fe 2+/3+ ions substituted at the Mo site.…”

“…For undoped MoO 2 , the NEXAFS spectrum displays several prominent spectral features. It has been shown previously [5] that the energy region between 529 and 538 eV (marked by A 1 -D 1 ) is due to O 2p hybridization with Mo 4d states which form the bottom of the conduction band, while the region between 539 and 550 eV (marked by E 1 ) can be assigned to O 2p-Mo 5sp hybridized states and above 550 eV, the spectrum arises due to the O 2p states that extend to Mo higher orbitals. Since Mo ions exist in Mo 4+ (4d 2 ) and Mo 5+ /Mo 6+ (4d 1 /4d 0 ) oxidation states and the Mo 4+ component increases at the cost of Mo 5+/6+ ions after Fe-doping [9], the observed pre-edge features A 1 -D 1 are due to the 4d-2p(O) hybridization, which is influenced strongly by distortion of MoO 6 octahedra.…”

“…All the samples were characterized by x-ray diffraction, bulk magnetization, and resistivity measurements. The details of sample preparation and characterization are described elsewhere [5,9]. The NEXAFS and XMCD measurements were performed at the ESRF's ID08 beamline, which uses an APPLE II type undulator giving ∼100% circular polarization.…”

“…Interestingly, unexpected room temperature (RT) ferromagnetism has also been observed in undoped wideband-gap semiconducting or insulating thin films and nanoparticles such as TiO 2 , HfO 2 , In 2 O 3 , ZnO, CeO 2 , SnO 2 , Al 2 O 3 , and MgO [1][2][3][4]. Surprisingly, we have observed recently that the highly a-axis oriented undoped thin film of MoO 2 shows ferromagnetism at RT by near edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments [5]. These results have been explained on the basis of strong hybridization of O 2p-Mo 4d orbitals and a large orbital magnetic moment of O 2p states, which are further confirmed theoretically by different groups [6,7].…”

Van der Waals epitaxy of functional MoO2 film on mica for flexible electronics

“…Recently, several studies pointed out the existence of ferromagnetism in MoO 2 thin films due to oxygen vacancies. 40 Because the deposition of our films was carried in high vacuum, it is interesting to explore the magnetic properties of the MoO 2 film on muscovite. Therefore, we performed the resistance measurements at 2 K under applied magnetic fields.…”