C. J. Voyer

USING NEUTRON DIFFRACTION AND MÖSSBAUER SPECTROSCOPY TO STUDY MAGNETIC ORDERING IN THE R3T4Sn4 FAMILY OF COMPOUNDS

We review the complementary roles that 119Sn Mossbauer spectroscopy and neutron diffraction are playing in developing a complete description of magnetic ordering in the R3 Cu4Sn4 and R3Ag4Sn4 intermetallic compound series. We show that the two techniques yield consistent pictures of the order, and in many cases both are essential to obtaining a complete description. The recent neutron diffraction work on Sm3Cu4Sn4, Sm3Ag4Sn4 and Gd3Ag4Sn4 is highlighted.

Valence change and magnetic order in YbMn6Ge6 ? xSnx

The YbMn(6)Ge(6-x)Sn(x) compounds (0 < x < 6) have been investigated using x-ray diffraction, magnetic measurements, neutron diffraction and (170)Yb Mössbauer spectroscopy. The YbMn(6)Ge(6-x)Sn(x) system comprises three solid solutions: (i) 0 < x ≤ 1.1, (ii) 3.2 ≤ x ≤ 4.6 and (iii) 5.3 ≤ x < 6, all of which crystallize in the hexagonal (P6/mmm) HfFe(6)Ge(6)-type structure. The substitution of Sn for Ge yields changes in the type of magnetic order (antiferromagnetic, helimagnetic, ferromagnetic, conical and ferrimagnetic), in the easy magnetization direction (from easy axis to easy plane) as well as in the valence state of Yb (from trivalent to divalent). The Mn moments order at or above room temperature, while magnetic ordering of the Yb sublattice is observed at temperatures up to 110 K. While Yb is trivalent for x ≤ 1.1 and divalent for x ≥ 5.3, both magnetic and (170)Yb Mössbauer spectroscopy data suggest that there is a gradual reduction in the average ytterbium valence through the intermediate solid solution (3.2 ≤ x ≤ 4.6), and that intermediate valence Yb orders magnetically, a very unusual phenomenon. Analysis of the (170)Yb Mössbauer spectroscopy data suggests that the departure from trivalency starts as early as x = 3.2 and the loss of ytterbium moment is estimated to occur at an average valence of ∼2.5+.

Magnetic ordering in Gd3Cu4Sn4 and Gd3Ag4Sn4 studied using 119Sn Mössbauer spectroscopy

The magnetic properties of Gd3Cu4Sn4 and Gd3Ag4Sn4 have been investigated using 119 Sn Mossbauer spectroscopy. We find that the Neel temperature (TN) for Gd3Ag4Sn4 is 28.8(2) K, much higher than previously reported and fully consistent with de Gennes scaling for the R3T4Sn4 (T = Cu, Ag) compound series. The 8 K event previously identified as TN is most likely a spin reorientation transition. By contrast, TN for Gd3Cu4Sn4 is confirmed to be anomalously low at only 13.6(1) K, consistent with earlier specific heat data. The sub-splitting of the 119 Sn Mossbauer spectrum for Gd3Cu4Sn4 at 1.55 K points to a complex magnetic structure, but we find no evidence for the lower- temperature events that are apparent in susceptibility data.

Magnetic order of the rare earth sublattice in h-YbMnO3

Y170b-Mossbauer spectra have been recorded for h-YbMnO3 over the temperature range of 1.5–40 K. A rapid increase in the Yb 2a-site magnetic hyperfine field and electronic ground state fluctuation time provides microscopic evidence for long-range magnetic order of the Yb 2a-site sublattice below TN2≈5 K.

Magnetic ground state at the ytterbium site in YbNiAl4

Magnetic, specific heat, and Y170b-Mossbauer spectroscopy measurements are presented for polycrystalline YbNiAl4. Although the low temperature specific heat data are consistent with the early stages of a magnetic transition, there is no clear evidence for magnetic order down to the lowest experimental temperature of 1.5 K. It is concluded that this is due to an even stronger quenching effect than is predicted by the current crystal field theory.

Temperature dependence of induced Ni2+ moment fluctuations in the Eu2BaNiO5 Haldane system

Eu151 and Eu153 Mossbauer spectroscopies have been used to study the magnetism of the Ni2+ moments in Eu2BaNiO5 by way of the exchange-induced Eu3+ moments. These independent and complementary probes indicate that at high temperatures all of the moments in the Haldane quantum spin chains are paramagnetic. On cooling below 25 K, fluctuations of ∼90% of the Ni2+ moments slow down, while the remaining 10% remain paramagnetic (indicating the presence of chain defects). Both Eu151 and Eu153 Mossbauer measurements yield the same Ni2+ moment. Furthermore, below 5 K, the 90% chain component becomes static and ordered, in excellent agreement with the ordering temperature expected from the rare-earth exchange energy.

Magnetic structure of NdScGe

We have determined the magnetic structure of NdScGe (tetragonal I4∕mmm) by high-resolution neutron powder diffraction, ac-susceptibility and magnetometry. The magnetic ordering temperature is 194(2)K and the magnetic structure is characterized by ferromagnetic order of the Nd sublattice, lying in or close to the tetragonal basal plane. The Nd moment at 4K is 3.2(2)μB and, in contrast to the claims of a recent Perturbed Angular Correlation study, we find no evidence for a lattice softening effect suggested to occur around the Curie temperature.

Site disorder and spin-glass ordering in PrAu2Si2

We have used Au197 Mossbauer spectroscopy along with both neutron and x-ray diffraction to search for site disorder and noncollinear magnetic ordering in PrAu2Si2. There is no evidence of a transferred hyperfine field at the Au site down to 1.7 K allowing us to rule out both collinear antiferromagnetic order and spin-glass ordering of the Pr moments. Full-pattern Rietveld refinement of both neutron and x-ray diffraction data taken over a wide angle range allows us to exclude Au∕Si site disorder at the 1% level. We conclude that the Pr moments do not order above 1.7 K in PrAu2Si2, and that the susceptibility peak is likely a dynamic effect.

S119n Mössbauer spectroscopy investigation of Nd3Cu4Sn4, Nd3Ag4Sn4, and Ho3Cu4Sn4

Nd3Cu4Sn4, Nd3Ag4Sn4, and Ho3Cu4Sn4 have been studied using S119n Mossbauer spectroscopy. Contrary to recent neutron diffraction data, Nd3Cu4Sn4 shows a clear magnetic signal at 1.6 K and has an ordering temperature of 2.0(1) K. Nd3Ag4Sn4 orders at 4.8(1) K, in agreement with neutron diffraction measurements. We observe the ordering of the Ho 2d sublattice in Ho3Cu4Sn4 at 8.2(1) K but we also see some residual magnetic splitting up to 10.5(2) K, which was not previously detected by neutron scattering.

Cellulose-bound magnesium diboride superconductivity

Two-phase superconductor tapes were produced by blending high purity magnesium diboride powder with a liquid ethylcellulose-based polymeric binder. This procedure produced a material which is easily formable with a high superconducting transition temperature (38K). We show that the bulk superconducting properties are not affected by the presence of the binder, nor is there any evidence of a chemical reaction between the superconducting particles and the binder. However, the transport properties of the material are strongly affected by the presence of the binder, which leads to a seven order of magnitude increase of the normal state resistance along with a seven order of magnitude decrease of the transport critical current density. This new material is shown to be equivalent to a system of coupled Josephson junctions.