Toward Quantitative Magnetic Information from Transition Metal K-Edge XMCD of Prussian Blue Analogs

Abstract

Two series of Prussian blue analogs (PBA) were used as model compounds in order to disentangle the information contained in X-ray Magnetic Circular Dichroism (XMCD) at the K-edges of transition metals. The number of 3d electrons on one site of the bimetallic cyanide polymer has been varied by associating to the [Fe(CN) 6 ] 3or the [Cr(CN) 6 ] 3precursors various divalent metal ions A 2+ (Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ and Cu 2+ ). The compounds were studied by X-ray diffraction and SQUID magnetometry, as well as by X-ray absorption spectroscopy and XMCD at the K-edges of the A 2+ transition metal ion. The study shows that the 1s → 4p contribution to the A K-edge XMCD signal can be related to the electronic structure and the magnetic behavior of the probed A 2+ ion: the shape of the signal to the filling of the 3d orbitals, the sign of the signal to the direction of the magnetic moment with respect to the applied magnetic field, the intensity of the signal to the total spin number S A and the area under curve to the Curie constant C A . The whole study hence demonstrates that PBAs are particularly well adapted for understanding the information contained in the transition metals K-edge XMCD signals. It also offers new perspectives towards the full disentangling of the information contained in these signals and access to new insights into materials magnetic properties.

Interplay between Transition-Metal K-edge XMCD and Magnetism in Prussian Blue Analogs

“…The sign of the high-energy lobe of the main δ contribution to the XMCD signal of molecular compounds is assigned in the literature to the direction of the magnetic moment carried by the absorber atom with regard to that of the applied magnetic field. 28,54 This can be verified here too. The magnetic moments borne by the A 2+ ions point in the same direction as the applied magnetic field for all compounds (Figure 1b).…”

“…The shape of the contribution with one dominant lobe is in line with the presence of one electron pair in the t 2g orbitals of the Fe 3+ low spin ion. 28 Surprisingly, the low-energy contribution to the XMCD signal (α peak) is located over the pre-edge energy range of the XANES spectra (below 7115 eV). At the Fe K-edge, the XANES spectra of the AFe series (S2) exhibit two weak preedge peaks located at 7112 and 7115 eV (peaks α 1 and α 2 on S2), which can be assigned to 1s → 3d transitions.…”

“…The XANES spectra of all compounds exhibit one single pre-edge peak with variable broadness and intensity, assigned to the 1s → 3d transitions. 28 At all A Kedges, XMCD gains intensity at the energy of the XANES preedge peak, whatever the population of the 3d orbitals (from d 5 to d 8 ). From a molecular point of view, the coordination sphere of the A 2+ ion is mainly made of four −NC and two −OH 2 ligands.…”

“…Our work belongs within this kind of less developed approach and invites us to take a fresh look at this long-standing issue. Our strategy, presented in a previous work, 28 A large variety of A and B TM ions can thus be combined in isostructural compounds, allowing for the independent variation of the magnetic orbitals on both A and B sites and consequently for the analysis of their effect on the XMCD signal at both TM K-edges. Nevertheless, it has to be noticed that PBAs are usually obtained in powder form and crystals having a size big enough to be analyzed by singlecrystal diffraction are scarce.…”

“…The hexacyanometallate entities are thus stable, rigid, and usually assumed to have a perfect O h symmetry. 40,41 In our previous work mentioned above, 28 we focused on the XMCD signal at the A K-edges of two series of PBAs of the chemical formula A 4…”