Fe/V and Fe/Co (001) superlattices: growth, anisotropy, magnetisation and magnetoresistance

Some physical properties of bcc Fe/V and Fe/Co (001) superlattices are reviewed. The dependence of the magnetic anisotropy on the in-plane strain introduced by the lattice mismatch between Fe and V is measured and compared to a theoretical derivation. The dependence of the magnetic anisotropy (and saturation magnetisation) on the layer thickness ratio Fe/Co is measured and a value for the anisotropy of bcc Co is derived from extrapolation. The interlayer exchange coupling of Fe/V superlattices is studied as a function of the layer thickness V (constant Fe thickness) and layer thickness of Fe (constant V thickness). A region of antiferromagnetic coupling and GMR is found for V thicknesses 12-14 monolayers. However, surprisingly, a 'cutoff' of the antiferromagnetic coupling and GMR is found when the iron layer thickness exceeds about 10 monolayers.Comment: Proceedings of the International Symposium on Advanced Magnetic Materials (ISAMM'02), October 2-4, 2002, Halong Bay, Vietnam. REVTeX style; 4 pages, 5 figure

First-order transition between a small-gap semiconductor and a ferromagnetic metal in the isoelectronic alloys FeSi1−x_{1-x}Gex_x

The contrasting groundstates of isoelectronic and isostructural FeSi and FeGe can be explained within an extended local density approximation scheme (LDA+U) by an appropriate choice of the onsite Coulomb repulsion, $U$ on the Fe-sites. A minimal two-band model with interband interactions allows us to obtain a phase diagram for the alloys FeSi$_{1-x}$Ge$_{x}$. Treating the model in a mean field approximation, gives a first order transition between a small-gap semiconductor and a ferromagnetic metal as a function of magnetic field, temperature, and concentration, $x$. Unusually the transition from metal to insulator is driven by broadening, not narrowing, the bands and it is the metallic state that shows magnetic order.Comment: 4 pages, 5 figure

Optical evidence for heavy charge carriers in FeGe

The optical spectrum of the cubic helimagnetic metal FeGe has been investigated in the frequency range from 0.01 - 3.1 eV for different temperatures from 30 K to 296 K. The optical conductivity shows the evolution of a low energy (0.22 eV) interband transition and the development of a narrow free carrier response with a strong energy and temperature dependence. The frequency dependent effective mass and scattering rate derived from the optical data indicate the formation of dressed quasi-particles with a mass renormalization factor of 12. Similar to FeSi the spectral weight in FeGe is not recovered over a broad frequency range, an effect usually attributed to the influence of the on-site Coulomb interaction.Comment: 5 pages, 5 figure

Magnetism of PdNi alloys near the critical concentration for ferromagnetism

We report results of a muon spin rotation and relaxation ($\mu$SR) study of dilute Pd$_{1-x}$Ni$_x$ alloys, with emphasis on Ni concentrations $x =$ 0.0243 and 0.025. These are close to the critical value $x_\mathrm{cr}$ for the onset of ferromagnetic long-range order (LRO), which is a candidate for a quantum critical point. The 2.43 and 2.5 at.% Ni alloys exhibit similar $\mu$SR properties. Both samples are fully magnetic, with average muon local fields $\langle B^\mathrm{loc}\rangle =$ 2.0 and 3.8 mT and Curie temperatures $T_C =$ 1.0 and 2.03 K for 2.43 and 2.5 at.% Ni, respectively, at $T = 0$. The temperature dependence of $\langle B^\mathrm{loc}\rangle$ suggests ordering of Ni spin clusters rather than isolated spins. Just above $T_C$ a two-phase region is found with separate volume fractions of quasistatic short-range order (SRO) and paramagnetism. The SRO fraction decreases to zero with increasing temperature a few kelvin above $T_C$. This mixture of SRO and paramagnetism is consistent with the notion of an inhomogeneous alloy with Ni clustering. The measured values of $T_C$ extrapolate to $x_\mathrm{cr}$ = 0.0236 $\pm$ 0.0027. The dynamic muon spin relaxation in the vicinity of $T_C$ differs for the two samples: a relaxation-rate maximum at $T_C$ is observed for $x$ = 0.0243, reminiscent of critical slowing down, whereas for $x =$ 0.025 no dynamic relaxation is observed within the $\mu$SR time window. The data suggest a mean-field-like transition in this alloy.Comment: 15 pages, 15 figures, to be published in Phys. Rev.

TEXTURE EFFECTS IN 3/2-1/2 MÖSSBAUER SPECTRA

Une méthode d'éviter les effets de texture dans les spectres Mössbauer en conservant les propriétes autosupportantes et la bonne conductivité thermale des absorbants est introduite. La méthode doit être d'une importance particulière dans la minéralogie et pour l'étude des effets Goldanskii-Karyagin sur les spécimens polycristallins.A way of avoiding texture effects in Mössbauer spectra while retaining self-supporting, good thermal conductivity absorbers is introduced. The method should be of particular importance in mineralogy and in the study of Goldanskii-Karyagin effects on polycrystalline samples

Clay mineral fractions studied by Mössbauer spectroscopy and conventional methods

A X-ray, wet-chemical and thermal study of different fractions of a clay sample has been combined with a Mössbauer spectroscopic investigation. It is shown that conventional wet-chemical analysis can give ferric-ferrous ratios that are very much higher than those obtained from a Mössbauer analysis. The need for a systematic Mössbauer study of well characterized standard clay minerals and the possibilities to use Mössbauer spectroscopy to study the weathering of minerals is stressed

Interface quality and short-range order of

The dependence of the interface quality on growth temperature in \chem{{}^{57}Fe/V} (001) single-crystal multilayers was investigated using conversion electron Mössbauer spectroscopy (CEMS) and X-ray diffraction (xrd). The samples, with 7 atomic layers (\chem{AL}) of isotope-enriched \chem{Fe} and 5 \chem{AL} of \chem{V}, were grown by dc magnetron sputtering onto \chem{MgO} (001) substrates at temperatures T_{\ab{g}} in the range 230–430\un{{}^{\circ}C}. The xrd results confirmed the previous findings on optimum temperature for sharp interfaces (T_{\ab{g}}=300–330\un{{}^{\circ}C}), whereas the CEMS data suggested that the short-range order was similar in all samples with T_{\ab{g}}\leq 330 \un{{}^{\circ}C}. The film grown at 430\un{{}^{\circ}C} was significantly different, with clear signs of alloying throughout the \chem{Fe} layers. Additional magneto-optical measurements showed the expected ferromagnetic behaviour