In the search for new spintronic materials with high spin-polarization at room-temperature, we have synthesized an osmium based double perovskite with a Curie-temperature of 725 K. Our combined experimental results confirm the existence of a sizable induced magnetic moment at the Os site, supported by band-structure calculations in agreement with a proposed kinetic energy driven mechanism of ferrimagnetism in these compounds. The intriguing property of Sr2CrOsO6 is that it is at the endpoint of a metal-insulator transition due to 5d band filling, and at the same time ferrimagnetism and high-spin polarization is preserved. PACS numbers: 61.12.Ld 75.50.Gg 75.50.Pp 75.50.Vv 81.05.ZxA so-called half-metal is a highly desired material for spintronics, as only charge carriers having one of the two possible polarization states contribute to conduction. In the class of the ferrimagnetic double perovskites such half-metals are well known, e. g. Sr 2 FeMoO 6 [1]. The here described compound Sr 2 CrOsO 6 is special, as it has a completely filled 5d t 2g minority spin orbital, while the majority spin channel is still gapped. It is thus at the endpoint of an ideally fully spin-polarized metal-insulator transition. At the metallic side of this transition we have the half-metallic materials Sr 2 CrWO 6 [2] and Sr 2 CrReO 6 [3,4]. Within the unique materials class of double perovskites, therefore, one can find high Curie-temperature ferrimagnets with spin-polarized conductivity ranging over several orders of magnitude from ferrimagnetic metallic to ferrimagnetic insulating tunable by electron doping. Note that Sr 2 CrOsO 6 , where a regular spin polarized 5d band is shifted below the Fermi level, is fundamentally different from a diluted magnetic semiconductor, where spin-polarized charge carriers derive from impurity states.While for simple perovskites as the half-metallic ferromagnetic manganites the Curie-temperature, T C , is in the highest case still close to room-temperature, halfmetallic ferrimagnetic double perovskites can have a considerably higher T C [5]. It has been suggested that ferrimagnetism in the double perovskites is kinetic energy driven [6,7,8]. In short, due to the hybridization of the exchange split 3d-orbitals of Fe 3+ (3d 5 , spin majority orbitals fully occupied) or Cr 3+ (3d 3 , only t 2g are fully occupied), and the non-magnetic 4d/5d-orbitals of * Electronic address: alff@oxide.tu-darmstadt.de Mo, W, Re or Os (N -sites), a kinetic energy gain is only possible for the minority spin carriers. This will lead to a corresponding shift of the bare energy levels at the non-magnetic site, and a strong tendency to half-metallic behavior. This mechanism is operative for the Fe 3+ and Cr 3+ (M sites) compounds [2], where all 3d majority spin states resp. all t 2g majority spin sates are fully occupied and represent localized spins. In agreement with band-structure calculations [1, 2,6,9,10,11] this mechanism is naturally associated with half-metallic behavior, as the spin-polarized conduction electrons mediate ...

A general reformulation of the exchange energy of 5f -shell is applied in the analysis of the magnetic structure of various actinides compounds in the framework of LDA+U method. The calculations are performed in a convenient scheme with essentially only one free parameter, the screening length. The results are analyzed in terms of different polarization channels, due to different multipoles. Generally it is found that the spin-orbital polarization is dominating. This can be viewed as a strong enhancement of the spin-orbit coupling in these systems. This leads to a drastic decrease in spin polarization, in accordance with experiments. The calculations are able to correctly differentiate magnetic and non-magnetic Pu system. Finally, in all magnetic systems a new multipolar order is observed, whose polarization energy is often larger in magnitude than one of spin polarization. INTRODUCTIONThe magnetism of actinide systems shows a very rich variety of magnetic properties [1]. There are variations from itinerant magnetic systems to systems showing characteristics of localized magnetism. In the border between these extremes one have the so-called heavy fermions, which show many peculiar and anomalous properties, one of which is the co-existence of superconductivity and magnetism [2]. One aspect that makes the magnetism of the actinides unique is the presence of strong spin-orbit coupling (SOC) together with strong exchange interactions for the 5f electrons, which are the ones responsible for the magnetism.From a theoretical point of view, a standard density functional approach, either in the local density approximation (LDA) or generalized gradient approximation (GGA), describes quite well the equilibrium properties of at least metallic systems. However, these functionals are known to underestimate the orbital moments which are induced by the relatively strong SOC [3,4,5]. This can be remedied by allowing for the so-called orbital polarization [5], responsible for Hund's second rule in atomic physics, either through adding an appropriate orbital depending term to the Hamiltonian or by adopting the so-called LDA+U approach [6,7,8]. In the latter method a screened Hartree-Fock (HF) interaction is included among the 5f states only.There is a drastic difference between the itinerant magnetism of a 3d shell and that of the 5f shell. In the former the orbital degrees of freedom are quenched due to the process of hopping between different atoms, while in the latter case the stronger SOC un-quenches them again. Magnetic ordering is relatively abundant among actinide systems due to the strong exchange interactions, but generally the spin moments are strongly reduced compared to a fully spin polarized value, which sometimes is ascribed to crystal field effects and other times to hybridization. This paper will focus on the role of the local screened exchange interactions and it will aim to convincingly argue that these interactions, together with an appreciable SOC interaction, are responsible for the reduced spin polarizations ...

A broken symmetry ground state without any magnetic moments has been calculated by means of the local-density approximation to density functional theory plus a local exchange term, the so-called LDA+U approach, for URu(2)Si(2). The solution is analyzed in terms of a multipole tensor expansion of the itinerant density matrix and is found to be a nontrivial magnetic multipole. Analysis and further calculations show that this type of multipole enters naturally in time reversal breaking in the presence of large effective spin-orbit coupling and coexists with magnetic moments for most magnetic actinides.

The electronic structure of the anomalous δ-phase of Pu is analyzed by a general and exact reformulation of the exchange energy of the f -shell. It is found that the dominating contribution to the exchange energy is a polarization of orbital spin-currents that preserves the time reversal symmetry, hence a non-magnetic solution in accordance with experiments. The analysis brings a unifying picture of the role of exchange in the 5f -shell with its relatively strong spin-orbit coupling. The results are in good accordance with recent measurements of the branching ratio for the d to f transition in the actinides.

PACS 71.20.Rv -Electron density of states and band structure of crystalline solids -Polymers and organic compounds PACS 79.20.Hx -Electron and ion emission by liquids and solids; impact phenomena -Electron impact: secondary emissionAbstract. -Radiation damage is an unavoidable process when performing structural investigations of biological macromolecules with X-rays. In crystallography this process can be limited through damage distribution in a crystal, while for single molecular imaging it can be outrun by employing short intense pulses. Secondary electron generation is crucial during damage formation and we present a study of urea, as model for biomaterial. From first principles we calculate the band structure and energy loss function, and subsequently the inelastic electron cross section in urea. Using Molecular Dynamics simulations, we quantify the damage and study the magnitude and spatial extent of the electron cloud coming from an incident electron, as well as the dependence with initial energy.