Single crystal neutron diffraction techniques have been employed to study the evolution of magnetic structures in RNi2B2C compounds in an attempt to understand the relationship between magnetic ordering and superconductivity in several members of this series. For HoNi2B2C, below the superconducting transition (Tc = 8 K), an incommensurate magnetic structure characterized by two wave vectors (0.585 a* and 0.915 c*) is found in a narrow temperature range between 4.7 K and 6 K. This is the same temperature range where bulk measurements find a deep minimum in the upper critical field, Hc2. Below 4.7 K, HoNi2B2C is a simple collinear antiferromagnet. ErNi2B2C (Tc = 11 K) orders in an incommensurate modulated antiferromagnetic state characterized by an ordering wave vector 0.553 ā * below 7 K, which coexists with superconductivity.

Low temperature inelastic neutron scattering studies have been performed to characterize the low energy magnetic excitation spectrum of the magnetic nanomolecule ${{\mathrm{Mo}}_{72}{\mathrm{Fe}}_{30}}$. This unique highly symmetric cluster features spin frustration and is one of the largest discrete magnetic molecules studied to date by inelastic neutron scattering. The $30\phantom{\rule{0.3em}{0ex}}\mathrm{s}=5∕2$ ${\mathrm{Fe}}^{\mathrm{III}}$ ions, embedded in a spherical polyoxomolybdate molecule, occupy the vertices of an icosidodecahedron and are coupled via nearest-neighbor antiferromagnetic interactions. The overall energy scale of the excitation and the gross features of the temperature dependence of the observed neutron scattering are explained by a quantum model of the frustrated spin cluster. However, no satisfactory theoretical explanation is yet available for the observed magnetic field dependence.

Inelastic neutron scattering techniques have been used to measure the phonon dispersion curves of bcc ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}$ ($x=10.8$, 13.3, 16.0, 22.5) alloys as a function of Ga concentration. The phonon frequencies of every branch were found to decrease significantly with increasing Ga concentration. The softening was most pronounced for the ${T}_{2}[\ensuremath{\xi}\ensuremath{\xi}0]$ branch and, to a lesser extent, the $L[\ensuremath{\xi}\ensuremath{\xi}\ensuremath{\xi}]$ branch in the vicinity of $\ensuremath{\xi}=\frac{2}{3}$. The concentration dependence of the shear elastic constant ${C}^{\ensuremath{'}}=1∕2({C}_{11}\ensuremath{-}{C}_{12})$, calculated from the slope of the ${T}_{2}[\ensuremath{\xi}\ensuremath{\xi}0]$ branch, was found to agree with the results of sound velocity measurements. For the higher concentration sample measured, 22.5 at. % Ga, new branches appeared, an effect associated with the increase in the number of atoms per unit cell.

V. O. Garlea, ∗ S. E. Nagler, J. L. Zarestky, C. Stassis, D. Vaknin, P. Kögerler, D. F. McMorrow, C. Niedermayer, D. A. Tennant, B. Lake, 1 Y. Qiu, M. Exler, J. Schnack, and M. Luban Ames Laboratory, Department of Physics and Astronomy, Iowa State University, Ames, IA, 50011, USA Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA Risø National Laboratory, DK-4000, Roskilde, Denmark Department of Physics and Astronomy, University College London, UK Laboratory for Neutron Scattering ETHZ & PSI, CH-5232 Villigen PSI, Switzerland School of Physics and Astronomy, University of St Andrews, St Andrews, FIFE KY16 9SS, Scotland, UK Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK NIST Center for Neutron Research, Gaithersburg, MD & U. Maryland, College Park, MD, USA Universität Osnabrück, Fachbereich Physik D-49069 Osnabrück, Germany (Dated: May 4, 2005)

We report the results of a neutron diffraction study, carried out on both single crystalline and polycrystalline samples of Tb5Si2.2Ge1.8 and polycrystalline Tb5Si2.5Ge1.5. On cooling, at approximately 120 K, the Tb5Si2.2Ge1.8 system undergoes a magnetoelastic transition from a high-temperature monoclinic-paramagnetic to a low-temperature orthorhombic-ferromagnetic structure. Between 120 K and 75 K, the magnetic structure has a net ferromagnetic component along the a axis direction. The moments are slightly canted with respect to the a axis, while the components along the b and c axes are ordered antiferromagnetically. A second magnetic transition occurs at approximately 75 K. Below this temperature, the magnetic structure consists of ferromagnetically aligned mu(x) and mu(z) projections of the magnetic moments and an antiferromagnetic arrangement of the mu(y) moment components. Magnetic structures of Tb5Si2.5Ge1.5 are nearly identical to those of Tb5Si2.2Ge1.8.

V. O. Garlea,1,* J. L. Zarestky,1 C. Y. Jones,2 L.-L. Lin,1 D. L. Schlagel,3 T. A. Lograsso,3 A. O. Tsokol,3 V. K. Pecharsky,3,4 K. A. Gschneidner, Jr.,3,4 and C. Stassis1 1Ames Laboratory and Department of Physics, Iowa State University, Ames, Iowa 50011, USA 2National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA 3Materials and Engineering Physics Program, Ames Laboratory, Ames, Iowa 50011, USA 4Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA Received 5 March 2005; published 23 September 2005

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.

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 [12̄0] direction. Measurements at 4.2 K, with a magnetic field applied along the [12̄0] direction, revealed metamagnetic transitions at approximately 20 kG and 40 kG.

We have measured the adiabatic second order elastic constants of two Ni-Mn-Ga magnetic shape memory crystals with different martensitic transition temperatures, using ultrasonic methods. The temperature dependence of the elastic constants has been followed across the ferromagnetic transition and down to the martensitic transition temperature. Within experimental errors no noticeable change in any of the elastic constants has been observed at the Curie point. The temperature dependence of the shear elastic constant C' has been found to be very different for the two alloys. Such a different behavior is in agreement with recent theoretical predictions for systems undergoing multi-stage structural transitions.

The temperature dependence of the Δ4 acoustic and optic mode, measured at q=(0.5,0,0) in LuNi2B2C, YNi2B2C were extended to 1000 K and new measurements on the mixed compound, Y0.5Lu0.5Ni2B2C, were undertaken to explore the mode coupling in these materials. The temperature behavior of each is different and cannot be explained by a simple mode coupling scheme of the acoustic and optic modes.

The TA(2) phonon dispersion curves of Ni-Mn-Ga alloys with different compositions which transform to different martensitic structures have been measured over a broad temperature range covering both paramagnetic and ferromagnetic phases. The branches show an anomaly (dip) at a wave number that depends on the particular martensitic structure, and there is softening of these anomalous phonons with decreasing temperature. This softening is enhanced below the Curie point, as a consequence of spin-phonon coupling. This effect is stronger for systems with higher electronic concentration.

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.

The phonons that exhibit anomalous temperature dependence in these compounds have wave vectors close to ξ→m=(ξ,0,0), with ξ≅0.5, and belong to two Δ4[ξ00] branches (one acoustic and the other optic) which cannot cross by symmetry. The behavior of these phonon modes was studied systematically between 2 and 620 K. We find that above Tc the frequencies of the acoustic modes decrease with decreasing temperature, and there is intensity transfer from the upper (optic) to the lower (acoustic) mode in both compounds. As the temperature decreases towards Tc, the two branches approach each other in the vicinity of ξ→m for the Lu compound, whereas they separate in the Y compound. This behavior, above Tc, can be satisfactorily explained by a couple-mode model. Below Tc the phonon spectrum changes dramatically: in both compounds, it consists of a sharp peak at approximately 4 meV with a broad weak shoulder on the high-energy side. The width of the sharp peak remains practically constant but its intensity decreases with increasing temperature and vanishes in the vicinity of Tc. The results below Tc are in qualitative agreement with recent theoretical calculations.

Polarized neutron-diffraction experiments have been performed in search of the orbital magnetic moments recently predicted in a theory for the cuprate metals. Both La2-xSrxCuO4 [LSCO(x)] and YBa2Cu3O6+x [YBCO(x)] compounds were investigated. No definitive evidence for the existence of such a three-dimensionally ordered moment was found in any of these samples within experimental uncertainty, which was as low as 0.01 mu(B). We have also investigated the possibility that a magnetic rod exists parallel to the c axis in the case of two- or quasi-two-dimensional ordering in the (a,b) plane and have set an upper limit for the total moment to be 0.1 mu(B) in LSCO (x = 0.1). [S0163-1829(99)02938-0].

The low-lying phonon dispersion curves of a DO3 ordered Cu3Al(+Be) shape-memory alloy were measured, at room temperature, along the [xi 00], [xi xi 0], [xi xi xi], [xi xi 1], and [1/2, 1/2, xi] symmetry directions. As in other bcc metals and alloys, we find that the frequencies of the TA(2)[xi xi 0] modes are relatively low and that the L[xi xi xi] branch exhibits a pronounced dip at xi=2/3. The measured low-lying phonon dispersion curves are quite similar to those of a bcc structure. Actually, a fifth-nearest-neighbor force constant bcc model provides a satisfactory fit to the low-lying phonon dispersion curves of this alloy. This model was used to evaluate the elastic constants, the phonon density of states, and the lattice specific heat. The results of these calculations are in good agreement with macroscopic measurements of the elastic constants and specific heat of this alloy. [S0163-1829(99)02114-1].