[en] We have performed full-potential linearized augmented plane wave calculations of the Gd(0001) surface using the local density approximation (LDA) together with the Hubbard U (LDA+U) total energy functional. The use of LDA+U instead of LDA total energy calculations leads to a ferromagnetic ground state for both bulk Gd and the Gd surface, in agreement with experimental observation. The calculated downward shift of 4f eigenvalues for the Gd surface is in agreement with experimentally observed binding energies. Surface strain relaxation leads to a 90% enhancement of the interlayer surface-to-bulk effective exchange coupling. Application of a Landau-Ginzburg-type theory yields a 30% enhancement of the Curie temperature at the surface, in very good agreement with the experiment. (c) 2000 American Institute of Physics

[en] Effects of electron correlation on the electronic structure and magnetic properties of the Gd(0001) surface are investigated using the full-potential linearized augmented plane-wave implementation of correlated band theory (''LDA+U''). The use of LDA+U instead of LDA (local-density approximation) total-energy calculations produces the correct ferromagnetic ground state for both bulk Gd and the Gd surface. Surface strain relaxation leads to a 90% enhancement of the interlayer surface-to-bulk effective exchange coupling. Application of a Landau-Ginzburg-type theory yields a 30% enhancement of the Curie temperature at the surface, in very good agreement with the experiment. (c) 2000 The American Physical Society

[en] Magnetic properties of the intermetallic compound U₂Fe₃Ge were studied on a single crystal. The compound crystallizes in the hexagonal Mg₂Cu₃Si structure, an ordered variant of the MgZn₂ Laves structure (C14). U₂Fe₃Ge displays ferromagnetic order below the Curie temperature T_C = 55 K and presents an exception to the Hill rule, as the nearest inter-uranium distances do not exceed 3.2 Å. Magnetic moments lie in the basal plane of the hexagonal lattice, with the spontaneous magnetic moment M_s = 1.0 μ_B/f.u. at T = 2 K. No anisotropy within the basal plane is detected. In contrast to typical U-based intermetallics, U₂Fe₃Ge exhibits very low magnetic anisotropy, whose field does not exceed 10 T. The dominance of U in the magnetism of U₂Fe₃Ge is suggested by the ⁵⁷Fe Mössbauer spectroscopy study, which indicates very low or even zero Fe moments. Electronic structure calculations are in agreement with the observed easy-plane anisotropy but fail to explain the lack of an Fe contribution to the magnetism of U₂Fe₃Ge.

[en] Rare-earth-based compounds R_2T_1_7 (R=Rare earth; T=Transition metal) have been extensively studied and developed for applications as permanent magnets. The actinide-based analogues, however, are much less documented and we report here about the magnetic properties of Np_2Co_1_7 and Np_2Ni_1_7, as inferred from "2"3"7Np Mössbauer spectroscopy, the best resonance in actinides, and specific heat.

[en] Valence-band photoelectron spectra of Pu₂C₃ and PuC_0.85 reveal the same type of 5f features below the Fermi level as observed for the majority of other Pu systems. They appear at the same energies (0-0.1, 0.5, and 0.85 eV) and their intensity is higher for Pu₂C₃ than for PuC_0.85. We deduce that they can be attributed to the 5f⁵ final-state multiplet originating in the 5f⁶ component of the ground state. The higher 5f occupancy (5.48), calculated in Pu₂C₃, explains its non-magnetic character

[en] ReT5 (Re = rare earth, T = Transition metal) compounds have been studied in the past in the frame of its magnetic properties (small Re to T stoichiometric ratio, large spontaneous magnetization and high Curie temperature) [1-2]. Extending these investigations to actinide analogues is interesting, as the wider radial extension of the 5f electronic shell compared to the 4f one should result in a larger inter-sublattice exchange interaction, and in non-negligible An-An exchange coupling. The spin-orbit interaction in the 5f configuration for the actinide atoms is also larger than the 4f spin-orbit interaction. For these reasons, a magnetic behaviour even richer than the one observed in the rare earth compounds is expected

[en] Two new transuranium metal boron carbides, NpBC and PuBC, have been synthesized. Rietveld refinements of powder XRD patterns of (Np,Pu)BC confirmed in both cases isotypism with the structure type of UBC. Temperature dependent magnetic susceptibility data reveal antiferromagnetic ordering for PuBC below T_N = 44 K, whereas ferromagnetic ordering was found for NpBC below T_C = 61 K. Heat capacity measurements prove the bulk character of the observed magnetic transition for both compounds. The total energy electronic band structure calculations support formation of the ferromagnetic ground state for NpBC and the antiferromagnetic ground state for PuBC

[en] A previously unknown neptunium-transition-metal binary compound, Np2Co17, has been synthesized and characterized by means of powder x-ray diffraction, 237NpMossbauer spectroscopy, SQUID magnetometry, and x-ray Magnetic Circular Dichroism (XMCD). The compound crystallizes in the Th2Ni17-type hexagonal structure, with room temperature lattice parameters a = 8.3107(1) A and c = 8.1058(1) A. Magnetization curves indicate the occurrence of ferromagnetic order below a TC > 350 K. Mossbauer spectra suggest a Np3+ oxidation state and give an ordered moment of fÊNp = 1.57(4) B and fÊNp = 1.63(4) fÊB for the Np atoms located, respectively, at the 2b and 2d crystallographic positions of the P63/mmc space group. Combining these values with a sum rule analysis of the XMCD spectra measured at the neptunium M4,5 absorption edges, one obtains the spin and orbital contributions to the site-averaged Np moment (fÊS = -1.88(9) fÊB, fÊL = 3.48(9) fÊB). The ratio between the expectation value of the magnetic dipole operator and the spin magnetic moment (fÊmd/fÊS = +1.36) is positive as predicted for localized 5f electrons, and lies between the values calculated in intermediate coupling (IC) and in jj approximations. The expectation value of the angular part of the spin-orbit interaction operator is in excellent agreement with the IC estimate. The ordered moment averaged over the four inequivalent Co sites, as obtained from the saturation value of the magnetization, is fÊCo∼1.6 fÊB. The experimental results are discussed against the predictions of first-principle electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction.

[en] Our ab initio theory calculations predict a semiconducting band structure of I-Mn-V compounds. We demonstrate on LiMnAs that high-quality materials with group-I alkali metals in the crystal structure can be grown by molecular beam epitaxy. Optical measurements on the LiMnAs epilayers are consistent with the theoretical electronic structure. Our calculations also reproduce earlier reports of high antiferromagnetic ordering temperature and predict large, spin-orbit-coupling-induced magnetic anisotropy effects. We propose a strategy for employing antiferromagnetic semiconductors in high-temperature semiconductor spintronics.