Research

Expertise
RIXS - resonant inelastic X-ray scattering
XAS - X-ray absorption spectroscopy
XPS - X-ray photoelectron spectroscopy
ARPES - angle resolved photoemission spectroscopy

 

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Seeing the magnetic superexchange interaction at work (October 2017)

 

In transition metal compounds the metal ions are separated by large distances which rule out direct magnetic exchange interactions between the metal sites. The hybridization of the metal states with valence orbitals of the ligands opens the possibility to exchange electrons over longer distances, between the metal sites via the ligand. For instance, in FeCr2S4, the 3d3 ions of the Cr3+ sublattice, situated on octahedral (Oh) sites, and the 3d6 ions of the Fe2+ sublattice, located at the tetrahedral (Td) sites, can exchange electrons through S 3p states (see figure below). We demonstrated that the spectrum of orbital excitations measured at the Fe sites at the L3 resonance, displays dd excitations excited on the Cr3+ sites. This clear signature of superexchange indicate that orbital excitations can be excited on the chromium sites even when the energy is added on the iron sites in the inelastic scattering process.

SEI in FeCr2S4.

Related publication: Sorin G. Chiuzbăian, Stefania Brignolo, Coryn F. Hague, Renaud Delaunay, Marco Guarise, Alessandro Nicolaou,, Zhaorong Yang, Haidong Zhou and Jean-Michel Mariot, Spectroscopic Evidence for Superexchange in the Ferrimagnetic Spinel FeCr2S4, Journal of Physical Chemistry C 121, 22369–22376 (2017) (view online)

 

Focus on multiferroics (November 2017)

 

Multiferroics are materials that display simultaneously ferroelectricity and magnetic order. Due to potential technological applications and the discovery of compositions with strong coupling between charge and spin degrees of freedom, multiferroics have recently sparked considerable interest.
In structures with spiral or helical magnetic ordering (multiferroics of type II), the charge disproportionation is supposed to arise solely from multiferroic coupling, independently from the electronic structure. Using the resonant inelastic x-ray scattering (RIXS), we studied the orbital excitations for compounds of the the RMnO3 family and were able to describe the crystal field acting on the Mn sites with unprecedented precision.

Related publication: Jiatai Feng, Amélie Juhin, Renaud Delaunay, Romain Jarrier, Nicolas Jaouen, Alessandro Nicolaou, Ryan Sinclair, Haidong Zhou, Jean-Michel Mariot and Sorin G. Chiuzbăian, Crystal-field excitations in multiferroic TbMnO3 by Mn L3 and O K resonant inelastic ray scattering, Journal of Applied Physics 122, 194101 (2017) (view online)

 
Spin flip excitations without spin-orbit coupling in the core-hole state
 

Resonant inelastic x-ray scattering (RIXS) at the L-edge of transition metal elements is now commonly used to probe single magnon excitations. Here we show that single magnon excitations can also be measured with RIXS at the K-edge of the surrounding ligand atoms when the center heavy metal elements have strong spin–orbit coupling. This is demonstrated with oxygen K-edge RIXS experiments on the perovskite Sr2IrO4, where low energy peaks from single magnon excitations were observed. This new application of RIXS has excellent potential to be applied to a wide range of magnetic systems based on heavy elements, for which the L-edge RIXS energy resolution in the hard x-ray region is usually poor.

Fig. Momentum dependence of the RIXS spectra with p incident polarization.

Related publication: X. Liu, M.P.M. Dean, J Liu, S.G. Chiuzbăian, N. Jaouen, A. Nicolaou, W.G. Yin, C. Rayan Serrao, R. Ramesh, H. Ding, and J.P. Hill, Probing single magnon excitations in Sr2IrO4 using O K-edge resonant inelastic x-ray scattering, Journal of Physics: Condensed Matter 27, 202202 (2015) (view online)

 

 Joint resonant inelastic x-ray scattering studies of 3d transition metal compounds: the CoO case

 

We performed resonant inelastic x-ray scattering (RIXS) measurements jointly across the M2,3 and L3 absorption thresholds in order to study CoO local electronic structure. Our high-resolution experimental data are supported by state-of-the-art model calculations: the same core-hole independent set of parameters is used within the single impurity Anderson model is used to describe both the M (3p-3d) and L (2p-3d). Measured M2,3-RIXS spectra are observed to be free of charge-transfer or normal fluorescence contributions. Moreover the cross section of M-RIXS final states with change in spin multiplicity is low. Combining this information with L-edge studies establishes an appealing means of making a better separation between on-site and intersite ligand to metal electronic excitations. Experimental and theoretical features specific to RIXS studies performed at M and L edges are summarized as a basis for future studies on complex transition metal compounds.

 

Fig. CoO RIXS measurements displayed after substraction of the elastic peak.

Related publication: S.G. Chiuzbăian, T. Schmitt, M. Matsubara, A. Kotani, G. Ghiringhelli, C. Dallera, A. Tagliaferri, L. Braicovich, V. Scagnoli, N. B. Brookes, U. Staub and L. Patthey, Combining M- and L-edge resonant inelastic x-ray scattering for studies of 3d transition metal compounds,  Physical Review B 78, 245102 (2008)

 
First evidence of spin flip excitations with resonant inelastic x-ray scattering
 

We studied the neutral electronic excitations of NiO localized at the Ni sites by measuring the resonant inelastic x-ray scattering (RIXS) spectra at the Ni M2,3 edges. The good energy resolution allows an unambiguous identification of several spectral features due to dd excitations. The dependence of the RIXS spectra on the excitation energy gives evidence of local spin flip and yields a value of 125±15 meV for the antiferromagnetic exchange interaction. Accurate crystal field parameters are also obtained.

Fig. X-ray scattering results for NiO with incoming photon energies encompassing Ni M absorption edges. The spectra deliver information on the crystal-field and the magnetic exchange energy between Ni ions.

Related publication: S.G. Chiuzbăian, G. Ghiringhelli, C. Dallera, M. Grioni, P. Amann, X. Wang, L. Braicovich and L. Patthey, Localized Electronic Excitations in NiO Studied with Resonant Inelastic X-Ray Scattering at the Ni M Threshold: Evidence of Spin Flip, Physical Review Letters 95, 197402 (2005) (view online)