R. Caudron

Electronic structure of transition metal diborides

Abstract Electronic specific heat, magnetic susceptibility and V 51 Knight shift measurements on the transition metal diborides of the first series (ScMn) show that the values of the density of states at the Fermi level are high and typical for a dominantly d -character band. Very high values of the density of states for CrB 2 , (VCr)B 2 and (MnCr)B 2 are related to strong electron-electron correlation effects.

Electronic structure of face centred cubic titanium and vanadium carbide alloys

Abstract Specific heat, magnetic susceptibility and N.M.R. measurements have been carried out on TixV1-xC1-y compounds to check some details of the various band models for f.c.c. transition metal carbides.

Electronic structure, cohesive properties and phase stability in Ni3V, Pd3V, and Pt3V compounds

Abstract The electronic structure and the cohesive properties of Ni 3 V, Pd 3 V and Pt 3 V compounds are studied by using first-principles calculations based on the linear-muffin-tin-orbital method. The relative structural stability of the three compounds among the three different (L1 2 -, D0 22 -, D0 23 -type) structures is studied, bringing to light the effects of the shape approximation to the potential or of the self-consistency. Our calculations are also used to study the stability of the L1 2 - and D0 22 - type structures as a function of the number of valence electrons per atom. We finally discuss our results in connection with data deduced from in situ neutron scattering experiments.

Low temperature specific heat and magnetization of Sc-Gd and Y-Gd spin glasses

Abstract For Sc -Gd and Y -Gd spin glasses, in the dilute limit, the RKKY interaction parameter V0, determined by the approach to saturation, has the same value of about 470 KA3; but the scaling laws are not well obeyed by the Y -Gd system. The 3% Sc -Gd specific heat behaves as T1.6 at very low temperatures.

Experimental Study of the Short Range Order in the PT-V System: Effective Pair Interactions as a Function of the Concentration

We present a detailed study of interactions in two alloys, Pt 3 V and Pt 8 V, and preliminary results on Pt 4 V. In situ neutron scattering experiments were performed in order to measure the short range order parameters in the disordered phase for the two concentrations. We will show the drastic effect of the concentration on the Short Range Order (SRO), particulary in Pt 8 V, where the diffuse intensity maxima do not peak at special points of the fcc lattice. Using inverse Monte Carlo (IMC) simulations, two sets of Effective Pair Interactions (EPI) were deduced from the experimental SRO parameters within the Ising Model. Surprisingly, despite quite different SRO patterns, the EPI seem nearly concentration independant. We will show, using MC simulation, that the two very different diffuse intensity maps are well reproduced with the same interaction set. The concentration independence of the EPI allowed us to describe also properties of the ordered states and more generally the whole phase diagram. In particular, we predict the existence, at very low temperature, of a new phase of stoechiometry A 5 B which has never been observed until now.

Diffuse Scattering of Neutrons in Ni3V and Pt3 V: Test of the Gamma Expansion Method Approximation in a Degenerate Case

Our results of in situ diffuse scattering of neutrons in Ni3V and Pt3V are presented. In the Ising model framework, effective pair interactions (EPI) up to the 9th neighbor were extracted from the data. These EPI were used to explain successfully the transition temperature of both compounds, the core structure of the dislocations in Ni3V, and the occurrence of long periods in Pt3V. The need of approximations yielding long-range EPI is stressed. We give a description of the Gamma Expansion Method (GEM). Because of its mean field nature, we expected the GEM to fail in the highly degenerate case of Pt3V. We tested it in that case, and found it completely successful. This gives more confidence in the mean field approximations.

Effective Pairwise Interactions in Ni3V

We present a detailed study of the effective interactions in Ni3V. In situ neutron scattering experiments are performed to measure the short range order parameters in the disordered phase. An inverse Monte Carlo algorithm has been developed to calculate the effective pair interactions, using the measured short range order parameters. The relevance of the long ranged interactions is discussed. The effective interactions are then used to calculate the disordering temperature, the relative stability of the DO22 and L12 compounds, and the temperature dependance of the excess free energies of two different antiphases in the DO22 phase. These results are in good agreement with experimental data, particularly for the antiphase free energies. We finally discuss our results in connection with ab initio electronic structure calculations.

Paramagnetic critical scattering by a dilute incommensurate antiferromagnet YEr

Abstract We have studied the critical scattering of neutrons by an Y Er single crystal containing 3 at% Er ( T N =3.25 K). The experiment was performed between 3.309 and 4.39 K. The isointensity contours in the ( a, c ) plane are ellipses elongated along the a direction, with an anisotropy ratio of about 2.3, independent of the temperature. This ratio, probably specific to yttrium, is much stronger than for pure Ho and Dy. The thermal variation of the diffraction width is compatible with v =0.8±0.2, and the intensity variation with γ=2±0.5.

Low temperature specific heat of dilute iron-carbon martensite

Abstract In order to detect density-of-states variations due to carbon dilution in BCC iron, specific-heat experiments were undertaken between 1.2 to 10 K on water-quenched slabs of Fe -C martensites. The data was corrected for the residual-austenite content of the samples, as estimated from magnetization measurements. The results show a very strong increase (about 0.3 mJ/K 2 mol%) of the electronic specific heat coefficient γ.

Low temperature specific heat of the Y-Gd and Sc-Gd spin glasses

There is now much interest about the transition from paramagnetic to spin-glass state [l]. However, the ground-state is not yet perfectly known, so that the low temperature specific heat behaviour is not well understood. The controversy about the local field distribution theory Ising-Heisenberg [2] is typical of this situation. Recently, Walker and Walstedt [3] succeeded in reproducing the Cu-Mn behaviour by simulation. The obtained specific heat curve is bended at low temperature. Is that calculation only approximate a t low temperatures because of memory-core limitations (the lowerlying excited states being of larger extension), or is it true, so that experimenters confused the necessary inflection with a linear behaviour ? Some experiments indicate such a bending of the specific heat curves [4]. However, they were performed on systems showing Kondo effect in the dilute limit. Much more recently, other experiments [5] were performed in the Cu-Mn system. They also showed bending at low temperature, but substraction of the hyperfine specific heat is a delicate problem, though the procedure looks convincing. Carefully selected rare-earth systems are much better candidates for such studies : (i) There is no Kondo effect. (ii) Direct f-f effects do not exist. (iii) The hyperiine specific heat for Gd is very low [6]. We have undertaken specific heat experiments between 0.3 and 10 K for Y-Gd and ScGd systems, which have been reported to show spin glass behaviour [7]. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979599 LOW TEMPERATURE SPECIFIC HEAT OF THE Y-Gd AND SC-Gd SPIN GLASSES C5-267 For the Sc-Gd system, the scaling laws are approximately obeyed and the deviations are of the same type as in the more standard spin-glass system : when the concentration increases, both the slope at low temperature and the maximum of the specific heat per impurity increase. The Y-Gd behaviour is very different : the departure from the scaling law is much stronger and, when the concentration increases, the maximum specific heat increases, but the slope decreases. This result can be tentatively explained by the different behaviour of the generalized susceptibility ~ ( q ) of the matrixes (more peaked for Y). C / T vs. T plots show that C ( T ) is rather quadratic than linear at low temperature. The slope of the quasi-linear part of the specific heat curve is about 90 mJ/K2 mole for Sc-Gd and 45 mJ/KZ mole for Y-Gd. This is due to different strengths of the RKKY interaction. These differences are also reflected in the paramagnetic Curie and NBel temperatures of the more concentrated alloys. Let us further notice that, for Sc-Gd, the susceptibility maximum occurs for 0.6 K/% whereas the broad specific heat maximum occurs for 1.2 K/%.