Rdsum6.-On expose une thdorie gdndrale des fluctuations de spins et les propriiStds thermodynamiques du magndtisme dlectronique itingrant, interpolation entre les limites faible et forte du ferromagndtisme. On donne une expression unifide de la tempdrature de Curie et on discute la signification physique de la susceptibilit6 magn6tique statique de Curie-Weiss. En tant que phdnomSnes nouveaux ddcoulant de cette thdorie, on discute les moments magndtiques locaux ddpendant de la tempdrature c o m e ceux observds dans CoS2, CoSez, etc. et les propridtds magnstiques et thermiques des semiconducteurs presque magndtiques connus de Si.Abstract.-A general theory of spin fluctuations and thermodynamical properties of itinerant electron magnets is developed, interpolating between the weakly and strongly ferromagnetic limits. A unified expression is given for the Curie temperature and the physical meaning of the Curie-Weiss magnetic susceptibility is discussed. As new phenomena derived from this theory the temperature-induced local magnetic moments as observed in CoSz, CoSe2, etc. and peculiar magnetic and thermal properties of nearly ferromagnetic semiconductors such as FeSi are discussed.

We investigate the local electronic structure and magnetic properties of the group-IV-based ferromagnetic semiconductor, Ge1−xFex (GeFe), using soft X-ray magnetic circular dichroism. Our results show that the doped Fe 3d electrons are strongly hybridized with the Ge 4p states, and have a large orbital magnetic moment relative to the spin magnetic moment; i.e., morb/mspin ≈ 0.1. We find that nanoscale local ferromagnetic regions, which are formed through ferromagnetic exchange interactions in the high-Fe-content regions of the GeFe films, exist even at room temperature, well above the Curie temperature of 20–100 K. We observe the intriguing nanoscale expansion of the local ferromagnetic regions with decreasing temperature, followed by a transition of the entire film into a ferromagnetic state at the Curie temperature.

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Growth, electronic and magnetic properties of γ ′ -Fe4N atomic layers on Cu(001) are studied by scanning tunneling microscopy/spectroscopy and x-ray absorption spectroscopy/magnetic circular dichroism. A continuous film of ordered trilayer γ ′ -Fe4N is obtained by Fe deposition under N2 atmosphere onto monolayer Fe2N/Cu(001), while the repetition of a bombardment with 0.5 keV N + ions during growth cycles results in imperfect bilayer γ ′ -Fe4N. The increase in the sample thickness causes the change of the surface electronic structure, as well as the enhancement in the spin magnetic moment of Fe atoms reaching ∼ 1.4 µB/atom in the trilayer sample. The observed thickness-dependent properties of the system are well interpreted by layer-resolved density of states calculated using first principles, which demonstrates the strongly layer-dependent electronic states within each surface, subsurface, and interfacial plane of the γ ′ -Fe4N atomic layers on Cu(001).

Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy magnetization axis depending on the epitaxial strain and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism (XMCD) at the Ru M2,3 edge. Results have shown that, with decreasing film thickness, the film changes from ferromagnetic to non-magnetic around 3 monolayer thickness, consistent with previous magnetization and magneto-optical Kerr effect measurements. The orbital magnetic moment perpendicular to the film was found to be ∼ 0.1 µB/Ru atom, and remained nearly unchanged with decreasing film thickness while the spin magnetic moment decreases. Mechanism for the formation of the orbital magnetic moment is discussed based on the electronic structure of the compressively strained SRO film.

The coupled electronic-structural modulations of the ligand states in IrTe2 have been studied by x-ray absorption spectroscopy (XAS) and resonant elastic x-ray scattering (REXS). Distinctive preedge structures are observed at the Te-M4,5 (3d → 5p) absorption edge, indicating the presence of a Te 5p -Ir 5d covalent state near the Fermi level. An enhancement of the REXS signal near the Te 3d → 5p resonance at the Q = (1/5, 0, −1/5) superlattice reflection is observed below the structural transition temperature Ts ∼ 280 K. The analysis of the energy-dependent REXS lineshape reveals the key role played by the spatial modulation of the covalent Te 5p -Ir 5d bond-density in driving the stripe-like order in IrTe2, and uncovers its coupling with the charge and/or orbital order at the Ir sites. The similarity between these findings and the charge-ordering phenomenology observed in the high-Tc superconducting cuprates suggests that the iridates may harbor similar exotic phases.PACS numbers: 71.45. Lr, 78.70.Ck, 78.70Dm, 71.20.Be Transition-metal compounds exhibit surprisingly rich electronic and magnetic properties due to the partially filled d orbitals. The fundamental properties of the electronic structure of transition-metal compounds can be described within the Zaanen-Sawatzky-Allen (ZSA) scheme. This differentiates between the Mott-Hubbard regime (U < ∆) and the charge-transfer regime (∆ < U ), depending on the relative balance of the on-site Coulomb interaction U between the d electrons and the chargetransfer energy ∆ between the ligand states and the transition-metal d states [1]. When ∆ approaches zero, the ligand states are almost degenerate in energy with the transition-metal d levels. As a result, the ligand states may participate in those spin, charge, and/or orbital ordering phenomena that are peculiar to the correlated nature of the d-orbitals. As an example of such phenomenology, ordering of the oxygen 2p holes is realized in the stripe-ordered phase of layered cuprates [2-6], or in the ladder-type Cu oxides [7].Very recently, a first-order structural transition was discovered in the 5d transition-metal chalcogenide IrTe 2 at T s ∼ 280 K. This attracted great interest due to the concomitant discovery of superconductivity in the Ptand Pd-substituted or intercalated compounds [8, 9]. Clarifying the origin of the structural phase transition might be a critical step towards the understanding of superconductivity itself; however, to date several mechanisms have been debated, with a universal consensus still lacking. The phase transition is accompanied by the emergence of a superstructure lattice modulation in electron diffraction [9] -with wavevector Q = (1/5, 0, −1/5) as expressed in reciprocal lattice units in tetragonal notation -which is here illustrated in Fig. 1. The main elements are the Ir-Ir dimerization along the a axis with period 5a, and the consequent distortion of the triangular Ir sublattice in the a − b plane, conflating to an overall trigonal-to-triclinic symmetry reduction. The IrIr dimeriza...

In scanning tunneling microscopy, orbital selectivity of the tunneling process can make the topographic image dependent on a tip-surface distance. We have found reproducible dependence of the images on the distance for a monatomic layer of iron nitride formed on a Cu(001) surface. Observed atomic images systematically change between a regular dot array and a dimerized structure depending on the tip-surface distance, which turns out to be the only relevant parameter in the image variation. An accompanied change in the weight of Fe-3d local density of states to a tunneling background was detected in dI/dV spectra. These have been attributed to a shift in surface orbitals detected by the tip from the d states to the s/p states with increasing the tip-surface distance, consistent with an orbital assignment from first-principles calculations.

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