Revealing the Magnetic Structure and Properties of Mn(Co,Ge)2.

The atomic and magnetic structures of Mn(Co,Ge)2 are reported herein. The system crystallizes in the space group P63/mmc as a superstructure of the MgZn2-type structure. The system exhibits two magnetic transitions with associated magnetic structures, a ferromagnetic (FM) structure around room temperature, and an incommensurate structure at lower temperatures. The FM structure, occurring between 193 and 329 K, is found to be a member of the magnetic space group P63/mm'c'. The incommensurate structure found below 193 K is helical with propagation vector k = (0 0 0.0483). Crystallographic results are corroborated by magnetic measurements and ab initio calculations.

Localized versus itinerant character of 4f-states in cerium oxides.

The electronic structure of cerium oxide is investigated here using a combination of ab initio one-electron theory and elements from many-body physics, with emphasis on the nature of the 4f electron shell of cerium ions. We propose to use the hybridization function as a convenient measure for the degree of localization of the 4f shell of this material, and observe that changing the oxidation state is related to distinct changes in the hybridization between the 4f shell and ligand states. The theory reveals that CeO2 has essentially itinerant 4f states, and that in the least oxidized form of ceria, Ce2O3, the 4f states are almost (but not fully) localized. This conclusion is supported by additional calculations based on a combination of density functional theory and dynamical mean field theory. Most importantly, our model points to the fact that diffusion of oxygen vacancies in cerium oxide may be seen as polaron hopping, involving a correlated 4f electron cloud, which is located primarily on Ce ions of several atomic shells surrounding the vacancy.

The CeFe11Ti permanent magnet: a closer look at the microstructure of the compound.

High-performance permanent magnets (PM) are compounds with outstanding intrinsic magnetic properties. Most PMs are obtained from a favorable combination of rare earth metals (RE = Nd, Pr, Ce) with transition metals (TM = Fe, Co). Amongst them, CeFe11Ti claims considerable attention due to its large Curie temperature, saturation magnetization, and significant magnetocrystalline anisotropic energy. CeFe11Ti has several potential applications, in particular, in the development of electric motors for future automatic electrification. In this work, we shed some light on the mictrostructure of this compound by performing periodic hybrid-exchange density functional theory (DFT) calculations. We use a combined approach of atom-centered local orbitals, plane waves and full-potential linear muffin-tin orbital (LMTO) for our computations. The electronic configuration of the atoms involved in different steps of formation of the crystal structure of CeFe11Ti gives an explanation on the effect of Ce and Ti on its magnetic properties. While Ti stabilizes the structure, atomic orbitals of Ce hybridizes with Fe atomic orbitals to a significant extent and alters the electronic bands. Our calculations confirm a valence of 3+ for Ce, which has been deemed crucial to obtain a large magnetocrystalline anisotropy. In addition, we analyze several spin configurations, with the ferromagnetic configuration being most stable. We compare and contrast our data to those available and provide an insight for further development of optimized high-performance PMs. Moreover, we compute the Magnetocrystalline Anisotropy of this compound by means of two approaches: the Force Theorem and a full-potential LMTO method.

Ligand Effects on the Linear Response Hubbard U: The Case of Transition Metal Phthalocyanines.

It is established that density functional theory (DFT) + U is a better choice compared to DFT for describing the correlated electron metal center in organometallics. The value of the Hubbard U parameter may be determined from linear response, either by considering the response of the metal site alone or by additionally considering the response of other sites in the compound. We analyze here in detail the influence of ligand shells of increasing size on the U parameter calculated from the linear response for five transition metal phthalocyanines. We show that the calculated multiple-site U is larger than the single-site U by as much as 1 eV and the ligand atoms that are mainly responsible for this difference are the isoindole nitrogen atoms directly bonded to the central metal atom. This suggests that a different U value may be required for computations of chemisorbed molecules compared to physisorbed and gas-phase cases.

Influence of Electron Correlation on the Electronic Structure and Magnetism of Transition-Metal Phthalocyanines.

There exists an extensive literature on the electronic structure of transition-metal phthalocyanines (TMPcs), either as single molecules or adsorbed on surfaces, where explicit intra-atomic Coulomb interactions of the strongly correlated orbitals are included in the form of a Hubbard U term. The choice of U is, to a large extent, based solely on previous values reported in the literature for similar systems. Here, we provide a systematic analysis of the influence of electron correlation on the electronic structure and magnetism of several TMPcs (MnPc, FePc, CoPc, NiPc, and CuPc). By comparing calculated results to valence-band photoelectron spectroscopy measurements, and by determining the Hubbard term from linear response, we show that the choice of U is not as straightforward and can be different for each different TMPc. This, in turn, highlights the importance of individually estimating the value of U for each system before performing any further analysis and shows how this value can influence the final results.

Elucidating the 3d electronic configuration in manganese phthalocyanine.

To shed light on the metal 3d electronic structure of manganese phthalocyanine, so far controversial, we performed photoelectron measurements both in the gas phase and as thin film. With the purpose of explaining the experimental results,three different electronic configurations close in energy to one another were studied by means of density functional theory. The comparison between the calculated valence band density of states and the measured spectra revealed that in the gas phase the molecules exhibit a mixed electronic configuration, while in the thin film, manganese phthalocyanine finds itself in the theoretically computed ground state, namely, the b1(2g)e3(g)a1(1g)b0(1g) electronic configuration.

Electron correlation effects in the Fe dimer.

The potential energy surface of the Fe dimer is investigated on the basis of density functional theory in the generalized gradient approximation (GGA). Electron correlation effects are taken into account explicitly within the GGA+U approach. We find a value of 2.20 eV for the Coulomb repulsion parameter U to describe the Fe dimer best, yielding a 9 Sigma(g)- ground state with an interatomic separation of 2.143 A. Agreement of the associated vibrational frequency, binding energy, ionization potential, and electron affinity with experimental data as well as corresponding results calculated within a high-level ab initio approach is improved significantly compared to conventional GGA. The effect of U on calculated geometric and magnetic properties of larger Fe clusters is discussed.