R. M. Nicklow

High-Pressure Cell for Neutron Scattering

We have developed a high-pressure cell for neutron scattering. The cell uses a barrel-shaped support cylinder made of sintered ceramics. Samples 6 mm in diameter and 10 mm long can be accommodated. The cell is suitable for use at low and high temperatures, after a pressure generated at room temperature is locked. The change of pressure on cooling to cryogenic temperatures is estimated by measuring the lattice parameter of NaCl. Besides the pressure locking technique, continuous pressurization can be achieved at room temperature by using a built-in mini-press while the cell is mounted on a goniometer of neutron spectrometer.

Neutron diffraction study on the hyperfine-enhanced nuclear spin order of HoVO4☆

Abstract A neutron diffraction experiment on the hyperfine enhanced nuclear spin system of HoVO4 has been carried out following adiabatic demagnetization cooling. Below 4.5 mK we observed magnetic diffraction peaks due to antiferromagnetic order of the hyperfine enhanced nuclear spin system. A spin-flop transition in an applied magnetic field of about 80 Oe was also observed. The neutron diffraction results are consistent with the magnetic structural model proposed by Bleaney on the basis of dipolar energy considerations.

Spin fluctuations in small γ-Fe precipitates

Abstract Spin dynamics of an incommensurate spin-density wave (SDW) in small γ-Fe precipitates in Cu was studied by neutron inelastic scattering. Strong spin fluctuations were observed around the broad SDW satellite peak positions even above the Neel temperature. This is not the magnon excitation because no such inelastic scattering is observed at the well defined SDW satellite peak positions for large γ-FeCo precipitates. Similarities to the SDW clusters in CuMn spin-glass alloys are discussed.

Spin‐Wave Dispersion Relation in Rare‐Earth Metals

Recent measurements of the spin‐wave dispersion relations for Gd, Dy, and Ho metal by triple‐axis neutron spectrometry will be reviewed. Measurements along the c axis have been carried out for each metal at several temperatures, and for Gd additional measurements along the a and b axes have been made at 78°K. Information about the Fourier‐transformed exchange interaction J(q) has been obtained from data analyses, which are based on a simple Hamiltonian containing only exchange interactions for Gd and a Hamiltonian containing exchange interactions plus appropriate anisotropy interactions (the usual frozen lattice model) for Dy and Ho. For both Dy and Ho a satisfactory analysis of the data requires the use of anisotropy interaction parameters which are not in good agreement with the results of macroscopic measurements. The neutron data suggest the existence of other anisotropy interactions, perhaps magnetoelastic, in addition to those usually considered in the frozen lattice Hamiltonian. The dispersion rela...

Separation of crystal-field levels and phonons in Pr dideuteride using polarized neutrons

Polarized neutrons with polarization analysis are used for the first time successfully for distinguishing crystal‐electric‐field (CEF) excitations from phonons by investigating both the spin‐flip and non‐spin‐flip cross sections. From this, it is shown that the 20 meV peak in Pr dideuteride, observed previously with unpolarized neutrons, arises from CEF excitations. Since this energy is nearly half the value of the CEF level at 41 meV known before, the 20 meV peak is composed of two contributions. At low temperatures it only arises from transitions from the ground state into the 20 meV level. At room temperature, on the other hand, the 20 meV level is thermally populated, so that transitions from the 20 meV into the 41 meV level are also obtained. This leads to the unexpected increase of the intensity of the 20 meV peak at increasing temperatures observed previously. The present result gives a strong contribution to the knowledge of the CEF parameters of the Pr3+ ions in PrD2.

Temperature dependence of the magnetic excitations in Gd

We have made inelastic neutron scattering studies of the magnetic excitations in gadolinium above and below the Curie temperature (Tc = 293 K). The constant Q measurements were made out to the Brillouin zone boundaries in the [001] and [100] directions in the temperature range from 9 to 593 K. The small q spin‐wave energies renormalize in approximate proportionality to the net magnetization and approach zero at Tc. There is very little broadening of these peaks except at temperatures very near Tc. At large q, however, there is appreciable broadening but a much slower decrease of spin‐wave energies with increasing temperature. The large q peaks do not vanish at Tc; instead, they persist as broad shoulders in the magnetic excitation spectra up to about 1.2 Tc.

Nuclear spin ordering observed by neutron diffraction (invited)

A review is given of recent neutron scattering experiments which have been carried out at milli‐Kelvin temperatures to study the hyperfine‐enhanced nuclear‐spin ordering in the singlet electronic ground state compounds PrSn3, PrCu2, and HoVO4. Bragg peaks were observed in PrSn3 which are due solely to the nuclear spin polarization of 141Pr caused by the hyperfine field from the antiferromagnetically ordered electronic moment. The temperature dependencies of the nuclear polarization and the additional electronic polarization induced by the nuclear polarization have been measured in detail. A magnetic phase transition in PrCu2 which is believed to correspond to a simultaneous ordering of both the nuclear and electronic spin systems has been observed at 58 mK. The positions of the magnetic diffraction peaks in reciprocal space are represented by G±τ, where G is a vector of the reciprocal lattice and τ=0.24a*±0.68c*, indicating that the magnetic periodicity is incommensurate with the chemical lattice. In HoVO...

The phonon dispersion relations in a disordered Ni1-xPtx system

Abstract The phonon dispersion relations of disordered alloys Ni 1- x Pt x were measured for five specimens covering the full range of atomic concentration by means of neutron inelastic scattering. At the resonant frequencies well separated double peak structures were observed in the phonon spectra. For the specimens with x ≧ 0.30 , the energy splitting of the phonon dispersion curve for the L -branch takes place at a higher frequency than that for the T -branch. The maximum phonon energy decreases with increasing Pt concentration but deviates from the theoretical prediction based on the mass defect CPA model. For x = 0.95 the localized mode cannot be observed as a well defined peak and does not seem to be split off from the main phonon band.

Spin‐Wave Dispersion Relation for Holmium in the Spiral Magnetic Phase

The spin‐wave dispersion relation for the c direction of holmium metal in its plane‐spiral magnetic phase has been studied by coherent inelastic scattering of thermal neutrons. Constant‐Q measurements of neutron energy loss processes, with the scattered neutron energy held fixed at 8.5 meV, were made at 78° and 48°K. Preliminary least‐squares analyses of the data indicate that the out‐of‐plane anisotropy constant is smaller than that for Dy and Tb, and that five interplanar exchange constants are required in order to fit the data satisfactorily. The resulting exchange constants are larger in magnitude than the anisotropy constant and are both positive and negative in sign as expected for the stability of the spiral structure.

Neutron scattering investigation of magnetic phase transitions in CoCl2-graphite intercalation compounds

The stage 2 CoCl2-graphite intercalation compound exhibits two magnetic phase transitions at TCl (=8.0 K) and TCu (=9.1 K). We have performed neutron scattering experiments on this compound which revealed at T