C. Stassis

Polarized neutron study of CeSn3 (invited)

Over the past several years there has been considerable interest in materials exhibiting a nonintegral valence. As a result the properties of cerium metal and its compounds have been the subject of many theoretical and experimental investigations. In the present experiment polarized neutron scattering techniques have been used to study the spatial distribution and temperature dependence of the magnetization induced in a single crystal of CeSn3 by a magnetic field of 42.5 kG. In the 40–300 K temperature range the measured magnetic form factor is in good agreement with the 4f magnetic form factor of Ce3+. Below 40 K, on the other hand, the magnetic form factor measurements suggest that the induced magnetization contains a large component of 5d electronic character of eg symmetry, which can account, at least partly, for the increase in the magnetic susceptibility of CeSn3 at low temperatures. The results will be discussed in terms of a model in which the 4f level is positioned slightly above the Fermi energy...

A comparative study of the post-quench behaviour of Cu–Al–Be and Cu–Zn–Al shape memory alloys

Abstract This paper reports a comparative investigation of the effect of quenching on the Cu–Al–Be and Cu–Zn–Al shape memory alloys by the use of several experimental techniques. In a first stage, the order–disorder transitions in these alloys have been characterized by means of modulated calorimetry. Results have proved that the A2⇋DO3 transition in Cu–Al–Be is first order with a latent heat of 1160 J/mol; the B2⇋L21 transition in Cu–Zn–Al is second order, and a peak in the specific-heat vs temperature curve has been observed. Secondly, the post-quench behaviour of these alloys, when subjected to some of the typical heat treatments used to stabilize the β phase, has also been studied by means of neutron diffraction, positron annihilation and highly sensitive calorimetry. A different post-quench time evolution of the martensitic transition temperatures has been found for the two alloys. For Cu–Al–Be, this evolution has been shown to be correlated with positron annihilation data, while, for Cu–Zn–Al, a correlation with neutron diffraction data has been established. These results show that the measured shifts in the transition temperatures induced by a quench are mostly due to an excess of vacancies in the case of Cu–Al–Be, and to an incomplete degree of L21 atomic order in Cu–Zn–Al.

Probing spin frustration in high-symmetry magnetic nanomolecules by inelastic neutron scattering

We report cold-neutron inelastic neutron scattering measurements on deuterated samples of the giant polyoxomolybdate magnetic molecule {Mo72Fe30}. The 30 s = 5/2 Fe+3 ions occupy the vertices of an icosidodecahedron, and interact via antiferromagnetic nearest neighbor coupling. The measurements reveal a band of magnetic excitations near E ~ 0.6 meV. The spectrum broadens and shifts to lower energy as the temperature is increased, and also is strongly affected by magnetic fields. The results can be interpreted within the context of an effective three-sublattice spin Hamiltonian.

Electrical resistivity and magnetization of the intermediate valence compound CePd3

The electrical resistivity and magnetization have been measured for a high quality single crystal of the intermediate valence compound CePd3. The magnetization was measured in the temperature range 4–175 K. After correcting for a small Curie‐like impurity contribution, the susceptibility χ decreased with increasing temperature from the 4 K value of 1.85×10−3 emu/mole of CePd3. The electrical resistivity ρ was measured from 1.2–300 K. A resistance ratio ρ(300 K)/ρ(4.2 K)=11.5 was obtained. At low temperature (T<8 K), ρ varied quadratically with temperature having an intercept of 9.3×10−8 Ω m at 0 K.

Temperature dependence of the field induced magnetic form factor of CeSn3

Polarized neutron scattering techniques have been used to study the spatial distribution and temperature dependence of the magnetization induced in a single crystal of CeSn3 by a magnetic field of 42.5 kG. We find that in the 300–40 K temperature range the measured form factor is in good agreement with the free ion 4f magnetic form factor of Ce3+. Below 40 K large deviations from a 4f magnetic form factor were observed. A satisfactory fit to the data below 40 K can be obtained by assuming that the induced magnetization contains a large component of 5d electronic character of eg symmetry.

Temperature dependence of the field induced magnetic form factor of the intermediate valence compound CePd3

Polarized neutron scattering techniques have been used to obtain information regarding the ground state electronic properties of the intermediate valence compound CePd3. The spatial distribution of the magnetization induced by a magnetic field of 42.5 kG in a single crystal of CePd3 has been studied at 100 and 4.2 K. We find that at 100 K the measured form factor is in good agreement with the free‐ion 4f magnetic form factor of Ce3+. At 4.2 K on the other hand, deviations from a 4f magnetic factor were observed. An adequate fit to the experimental data taken at 4.2 K can be obtained by assuming that the induced magnetization contains a significant component (∼17% of the total) of Ce–5d electronic character.

Single crystal neutron diffraction study of the magnetic structure of TmNi2B2C

Neutron diffraction techniques have been used to study the magnetic structure of single crystals of the magnetic superconductor (T{sub c} {congruent} 11K) TmNi{sub 2}B{sub 2}C. We find that below approximately 1.5K the magnetic moments order in an incommensurate spin wave with propagation vector q{sub m} = q{sub m} (a* +b*) (or q{sub m} = q{sub m} (a* + b*)) with q{sub m} = 0.094 {+-} 0.001. The spin wave is transverse with the moments aligned along the c-axis, and the observation of relatively intense higher order harmonics shows that the modulation is not purely sinusoidal but considerably squared. This incommensurate magnetic structure, which coexists with superconductivity below T{sub N} {congruent} 1.5K, is quite different from those observed in the magnetic superconductors HoNi{sub 2}B{sub 2}C and ErNi{sub 2}B{sub 2}C. The origin of diffraction peaks observed in scans parallel to a* is briefly discussed.

Neutron and resonant X-ray scattering studies of RNi2B2C (R=rare earth) single crystals

Abstract This family of intermetallic compounds is ideal for the study of the interplay between superconductivity and magnetism since, in several of these compounds (Ho, Er, Tm, Dy), superconductivity coexists with magnetic ordering. The most important findings of our scattering studies are (a) in the Ho-compound, a complex magnetic structure characterized by two incommensurate wave vectors, k → a =0.585 a → * and k → c =0.915 c → * , exists in the vicinity of 5 K, where the almost re-entrant behavior of this compound occurs (below 4.7 K, it is a commensurate antiferromagnet); (b) an incommensurate magnetic structure with wave vector along a → * , close to the zone boundary, is observed in several of these compounds (Ho, Er, Tb, Gd) and (c) pronounced soft-photon behavior was observed for both the acoustic and first optical Δ 4 [ ξ 00] branches in the superconducting Lu and Ho compounds, a behavior characteristic of strongly coupled conventional superconductors. Furthermore, these phonon anomalies occur at wave vectors close to those of the incommensurate magnetically ordered structures observed in the magnetic compounds of this family (see (b)). This observation suggests that both the magnetic ordering and phonon softening originate from common nesting features of the Fermi surfaces of these compounds. Band theoretical calculations are in qualitative agreement with these results.

Neutron scattering study of TbPtIn intermetallic compound

Neutron diffraction techniques have been used to study the magnetic properties of a TbPtIn single-crystal as a function of temperature and magnetic field. In the absence of an externally applied magnetic field, the compound orders, below approximately 47 K, in an antiferromagnetic structure with propagation vector k=(12,0,12); the magnetic moments were found to be parallel to the [120] direction. Measurements at 4.2 K, with a magnetic field applied along the [120] direction, revealed metamagnetic transitions at approximately 20 kG and 40 kG.

Magnetic properties of icosahedral R–Mg–Zn quasicrystals (R=Y, Tb, Dy, Ho and Er)

Abstract Large (up to 0.5 cm3) single grains of icosahedral R–Mg–Zn quasicrystals (R=Y, Tb, Dy, Ho and Er) have been grown via a self-flux technique. The samples have a composition of approximately R9Mg34Zn57, and are extremely well-ordered, with little or no evidence of phason strain. These samples have allowed a detailed investigation of the magnetic behaviour of local 4f moments in a quasiperiodic environment. Neutron scattering experiments using powdered single-grains show no evidence for long range magnetic order in the R–Mg–Zn quasicrystals. However, magnetization and magnetic susceptibility measurements indicate a spin-glass state, with freezing temperatures Tf=5.8, 3.6, 2.0 and 1.3 K for R=Tb, Dy, Ho and Er respectively. Deviations from a Curie-Weiss temperature dependence of the susceptibility are small, and only occur for temperatures up to 5 K above Tf, indicating the absence of any significant cluster glass behaviour. In addition, the rare earth dilution series (Y1−xGdx)–Mg–Zn have lower freezing temperatures than (Y1−xTbx)–Mg–Zn, despite the larger de Gennes factor of Gd3+ over Tb3+, indicating that crystal electric fields play a significant role in the freezing phenomenon. This difference between Heisenberg and non-Heisenberg moments is further explored via several (Gd1−xRx)–Mg–Zn pseudo-ternary quasicrystal series.