Electronic and magnetic properties of the charge ordered phase of LuFe 2 O 4 are investigated by means of x-ray spectroscopic and theoretical electronic structure approaches. LuFe 2 O 4 is a compound showing fascinating magnetoelectric coupling via charge ordering. Here, we identify the spin ground state of LuFe 2 O 4 in the charge ordered phase to be a 2:1 ferrimagnetic configuration, ruling out a frustrated magnetic state. An enhanced orbital moment may enhance the magnetoelectric coupling. Furthermore, we determine the densities of states and the corresponding correlation potentials by means of x-ray photoelectron and emission spectroscopies, as well as electronic structure calculations. DOI: 10.1103/PhysRevB.80.220409 PACS number͑s͒: 75.80.ϩq, 71.20.Ϫb, 78.70.Dm, 78.70.En Multiferroic transition metal oxides, i.e., compounds in which more than one ferroic phase coexist, have gained enormous attention during the last few years. 1-4 Besides a number of perovskites and related compounds, 2,5,6 the charge frustrated layered compound LuFe 2 O 4 has attracted intense interest due to its fascinating ferroelectric and magnetoelectric properties. 7,8 LuFe 2 O 4 has a rhombohedral crystal structure ͑space group R3m͒. The underlying layered structure consists of W-like hexagonal Fe 2 O 2.5 and U-like LuO 1.5 layers. 9 The W layers comprise two triangular nets of Fe ions; the resulting electric polarization is induced via a frustrated charge ordering of Fe 2+ and Fe 3+ ions on the resulting honeycomb lattice below 330 K. [10][11][12] Below 240 K a longrange ferrimagnetic order sets in. 7 The fact that the ferroelectricity is caused by correlated electrons from the Fe ions leads to unusual properties and unique capabilities of LuFe 2 O 4 . A large response of the dielectric constant by applying small magnetic fields has been found, opening a possible route for future devices. 8 Phase transitions from the charge ordered ͑CO͒ phase have been very recently associated with a nonlinear current-voltage behavior and an electric-field-induced phase transition, which might be of interest for potential electric-pulse-induced resistive switching applications. 13,14 The large magnetoelectric coupling has been attributed to an intricate interplay between charge and spin degrees of freedom with the crystal lattice and external electrical and magnetic fields to some extent on a short-range order. [15][16][17][18][19] However, there is still some confusion about the nature of spin-charge coupling in LuFe 2 O 4 . In particular a model finding a ͱ 3 ϫ ͱ 3 CO ground state 20,21 is challenged by simulations implying that the electrical polarization in LuFe 2 O 4 is due to spin-charge coupling and a spin frustrated magnetic ground state in a chain CO state. 22,23 On the other hand the first model finds a ferrimagnetic spin ground state where Fe 2+ and 1/3 of Fe 3+ make up the majority spin, and 2/3 of Fe 3+ make up the minority spin.X-ray magnetic circular dichroism ͑XMCD͒ is a very powerful tool to investigate the internal magnetic stru...

Molecular magnets incorporate transition-metal ions with organic groups providing a bridge to mediate magnetic exchange interactions between the ions. Among them are star-shaped molecules in which antiferromagnetic couplings between the central and peripheral atoms are predominantly present. Those configurations lead to an appreciable spin moment in the nonfrustrated ground state. In spite of its topologically simple magnetic structure, the [Cr(III)Mn(II)(3) (PyA)(6)Cl(3)] (CrMn(3)) molecule, in which PyA represents the monoanion of syn-pyridine-2-aldoxime, exhibits nontrivial magnetic properties, which emerge from the combined action of single-ion anisotropy and frustration. In the present work, we elucidate the underlying electronic and magnetic properties of the heteronuclear, spin-frustrated CrMn(3) molecule by applying X-ray magnetic circular dichroism (XMCD), as well as magnetization measurements in high magnetic fields, density functional theory, and ligand-field multiplet calculations. Quantum-model calculations based on a Heisenberg Hamiltonian augmented with local anisotropic terms enable us not only to improve the accuracy of the exchange interactions but also to determine the dominant local anisotropies. A discussion of the various spin Hamiltonian parameters not only leads to a validation of our element selective transition metal L edge XMCD spin moments at a magnetic field of 5 T and a temperature of 5 K but also allows us to monitor an interesting effect of anisotropy and frustration of the manganese and chromium ions.

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Several studies of the magnetic properties of Sr2FeMoO6, a half metallic double perovskite showing large magneto resistance effects at room temperature, by means of site specific x-ray absorption spectroscopy (XAS) have led to very different results concerning the Fe valence state. We present a detailed study of a Sr2FeMoO6 sample, which has been probed by means of XAS and x-ray magnetic circular dichroism (XMCD) over several years. We find a mixed valent Fe2+, Fe3+ state, which shifts toward Fe3+ with time. An understanding of such a chemical change is of importance for potential applications of Sr2FeMoO6 and related transition metal oxides.

Giant Keplerate-type molecules with a {Mo72Fe30} core show a number of very interesting properties, making them particularly promising for various applications. So far, only limited data on the electronic structure of these molecules from X-ray spectra and electronic structure calculations have been available. Here we present a combined electronic and magnetic structure study of three Keplerate-type nanospheres--two with a {Mo72Fe30} core and one with a {W72Fe30} core by means of X-ray absorption spectroscopy, X-ray magnetic circular dichroism (XMCD), SQUID magnetometry, and complementary theoretical approaches. Furthermore, we present detailed studies of the Fe(3+)-to-Fe(2+) photoreduction process, which is induced under soft X-ray radiation in these molecules. We observe that the photoreduction rate greatly depends on the ligand structure surrounding the Fe ions, with negatively charged ligands leading to a dramatically reduced photoreduction rate. This opens the possibility of tailoring such polyoxometalates by X-ray spectroscopic studies and also for potential applications in the field of X-ray induced photochemistry.