T Jarlborg

Nesting Properties and Anisotropy of the Fermi Surface of LuNi{sub 2}B{sub 2}C

The rare-earth nickel borocarbides, with the generic formula RNi{sub 2}B {sub 2}C , have recently been shown to display a rich variety of phenomena. Most striking has been the competition between, and even coexistence of, antiferromagnetism and superconductivity. We have measured the Fermi surface (FS) of LuNi{sub 2}B {sub 2}C , and shown that it possesses nesting features capable of explaining some of the phenomena experimentally observed. In particular, it had previously been conjectured that a particular sheet of FS is responsible for the modulated magnetic structures manifest in some of the series. We report the first direct experimental observation of this sheet. (c) 1999 The American Physical Society.

Calculation of the electronic structure and the magnetic properties of and

We study the two pseudo-cubic perovskite ruthenates and , by means of LSDA electronic structure calculations using the LMTO method for both the idealized cubic and the real orthorhombic structures. The LSDA calculations predict that both orthorhombic structures are ferromagnetic, with magnetic moments of 2.0 and respectively, while in the cubic structure, only is magnetic . The general features of the DoS are in reasonable agreement with the experimental photoemission and EELS spectra. However, the fine details of the DoS of these structures are sharply peaked near the position of and this contributes to the high sensitivity of many of the calculated results as well as to strong variations of the properties of the real material. Magnetic moments and transport properties are examples of quantities exhibiting strong variations, but the calculations clearly show that the orthorhombic distortion is favourable for large spin splitting and low conductivity. A gap structure in the majority band just above can be important for semi-metallic properties induced by distortions or charge transfers.

Spin density in ferromagnetic nickel : a magnetic Compton scattering study

The magnetic Compton profiles (MCPs) measured in the [100], [110], [111] and [112] directions in single-crystal nickel with an incident photon beam of energy 224 keV are presented and discussed. The momentum resolution achieved, of 0.43 atomic units, improves on previous studies by almost a factor of two, and facilitates the interpretation of the MCPs in terms of the underlying spin-dependent momentum densities. Calculations have been performed using the linear muffin-tin orbital method, within both the local spin-density approximation (LSDA) and the generalized gradient approximation (GGA). Comparison with experiment reveals the limitations of the LSDA at low momentum, where the GGA is better able to reproduce the contribution of the s- and p-like electrons. All of the calculations overestimate the moment associated with the d-like electrons, for momenta corresponding to the first Brillouin zone. We also confirm the existence of the so-called Umklapp shoulders, which derive from the Fermi surface topology.

Fermi surface of the colossal magnetoresistance perovskite La0.7Sr0.3MnO3

Materials that exhibit colossal magnetoresistance (CMR) are currently the focus of an intense research effort, driven by the technological applications that their properties suggest. Using the angular correlation of photons from electron-positron annihilation, we present a first view of the Fermi surface of a material that exhibits CMR, supported by `virtual crystal electronic structure calculations. The Fermi surface is shown to be sufficiently cubic in nature that it is likely to support nesting.

Spin density in Gd studied by magnetic Compton scattering

The spin-dependent momentum density of single-crystal ferromagnetic gadolinium was probed by the magnetic Compton profile technique. Comparison with electronic structure calculations indicate that the local spin-density approximation is adequate for describing the magnetic Compton profiles. Furthermore, the linear combination of muffin-tin orbitals prescription compared favourably with a full-potential method. We find that testing theory against the experimental density of states is problematic. Our calculations also indicate that the spin density is sensitive to the treatment of the 5p electrons.

Ferromagnetism below the stoner limit in La-doped SrB6

Spin-polarized band calculations for LaSr7B48 show a weak ferromagnetic state. This is despite a low density of states (DOS) and a low Stoner factor. The reason for the magnetic state is found to be associated with a gain in potential energy in addition to the exchange energy, as a spin splitting is imposed. A DOS with an impuritylike La band is essential for this effect. It makes a correction to the Stoner factor and provides an explanation of the recently observed weak ferromagnetism in doped hexaborides.

Fermiology of Cr and Mo

An interpretation of the two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) for paramagnetic chromium (Cr) and molybdenum (Mo) is presented. Rather than explaining the significant differences in the resulting k-space occupancies in terms of different Fermi surface (FS) topologies, the recovery of a FS topology for paramagnetic Cr in agreement with band theory (and similar to the Mo experiment) through the application of a recently introduced maximum-entropy-based filtering technique suggests an explanation related to positron wavefunction perturbations.