Neutron diffraction studies of water

Neutron diffraction measurements have been made for heavy water at five temperatures in the range 11–75°C using the diffractometers D2 and D4 at ILL. A first-order difference method has been used to determine the change in the intermolecular contributions to the observed cross-section. Fourier transformation techniques are used to give the changes in the real space distribution function. The results confirm the preliminary observations obtained in an earlier experiment and give a more precise indication of the structural variation. A detailed analysis shows that the intermolecular OD separation which corresponds to an effective hydrogen bond distance increases as the temperature rises. At larger distances there is a systematic but more complex variation which may be attributed to a change in the relative orientations of neighbouring molecules.     

Neutron diffraction investigation of the magnetic ordering of TiBe2

A neutron diffraction measurement on a polycrystalline sample of TiBe₂ did not reveal any onset of antiferromagnetic ordering at temperatures as low as 1.4 K. The maximum value of the ordered magnetic moment on titanium which might have gone undetected in the present experiment is shown to be 0.2 Bohr magnetons for a transverse spin density wave of arbitrary periodicity.     

Neutron diffraction study of magnetic order in TbRh2Ge2 and TbRu2Ge2

TbRh₂Ge₂ orders antiferromagnetically in the AFI type structure with a magnetic moment of 9.4(3)μ_B at 4.2 K localized on the Tb³⁺ ion. TbRu₂Ge₂ exhibits a square modulated magnetic structure with moments aligned along the c-axis of the crystallographic unit cell. The magnitude of the magnetic moment localized on the Tb³⁺ ion is 9.1(1)μ_B at 4.2 K.     

Neutron diffraction studies of simple molecular liquids

Summary A brief review is presented of the structural characteristics of diatomic and tetrahedral molecular liquids. Paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May, 1989.     

Neutron diffraction studies of liquid carbon suboxide

The structure factor S _m(Q) for liquid carbon suboxide has been determined for a Q-value range of 0·4 to 60 Å^-1 by neutron diffraction measurements using a steady-state (reactor) and a pulsed (linac) neutron source. The bond lengths of the molecule have been determined from the data and give good agreement with the results of electron diffraction measurements on the vapour phase after application of a molecular recoil correction term. The quasi-linear nature of the molecule is confirmed but the shape of the form factor indicates that large amplitude bending motion probably occurs in the liquid phase.

Oscillations in the intermolecular pair correlation function are observed to have a regular periodicity extending to 12 Å but details of orientational effects cannot be established from a single diffraction measurement.     

Neutron diffraction studies of transition metal nitrides

The magnetic structures of Fe₄N and Mn₄N have been redetermined using neutron diffraction. The magnetic form factors, obtained from polarised neutron data have been shown to be different for the face-centred and corner atoms. A qualitative explanation of the structures of Fe₄N and Mn₄N has been provided from the shapes of the magnetic form factors. Since deceased.     

Neutron diffraction study of UAs in high magnetic fields

Magnetization experiments have shown that at low temperature and in an applied magnetic field of ～90 kOe antiferromagnetic (AF) uranium arsenide transforms to a new ferrimagnetic state. The high-field transition produces a state with magnetic components $\text{parallel}$ to the applied field. A small ferromagnetic component (0.4 μ_B) is seen in magnetization experiments but with neutron diffraction we have observed directly the large AF component (～ 1.8 μ_B) which has a $\text{q}$ value of 0.58 ± 0.01 $\text{c}$¹ in contrast to the zero field type IA structure with $\text{q}\text{= (0,0,0.50)}$. This transition is quite different from that occuring in the conventional spin-flop transition.     

Neutron diffraction study of the magnetic structure of Na2 RuO 4

We present a novel model to simulate real social networks of complex interactions, based in a system of colliding particles (agents). The network is build by keeping track of the collisions and evolves in time with correlations which emerge due to the mobility of the agents. Therefore, statistical features are a consequence only of local collisions among its individual agents. Agent dynamics is realized by an event-driven algorithm of collisions where energy is gained as opposed to physical systems which have dissipation. The model reproduces empirical data from networks of sexual interactions, not previously obtained with other approaches.     

Neutron diffraction study of magnetic properties of TmCo2

The present neutron diffraction studies have confirmed that TmCo₂ represents an exception within the RCo₂ series. It was found that, in contrast with other heavy RCo₂ compounds, the Co sublattice in TmCo₂ does not order magnetically below the Curie temperature (T_c = 3.9 K). This is assumed to be due the fact that in TmCo₂ the intersublattice (f-d) molecular field is smaller than the critical field necessary to induce magnetic order within the Co sublattice, as is the case in other RCo₂ compounds. Furthermore, we show that the magnetic structure and the onset of long-range order in TmCo₂ depend sensitively on the sample preparation, which partly explains the differing results published earlier.     

Bragg diffraction of surface magnetostatic waves from magnetic lattices

We present a theoretical and experimental investigation of Bragg scattering of surface magnetostatic waves (SMSW) by a time-independent, spatially periodic magnetic field when the wave orientation is arbitrary with respect to the magnetization field. In the theoretical section the theory of single-mode Bragg diffraction is generalized to the case of waves with arbitrary dispersion propagating through an anisotropic medium. The calculated results are, on the whole, supported by experimental measurements on SMSW. We demonstrate that a geometry which in isotropic media leads to a sinusoidal distribution of diffraction order amplitudes as a function of penetration into the differing lattice, can lead to a nearly exponential distribution of such amplitudes in anisotropic media. The anisotropy of the interaction between SMSW and the magnetic diffracting lattice is manifested by anomalously high scattering efficiencies for certain cases of relative orientation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 76–85, November, 1988.     

Neutron diffraction and electrical transport studies on the incommensurate magnetic phase transition in holmium at high pressures

Neutron diffraction and electrical transport measurements have been made on the heavy rare earth metal holmium at high pressures and low temperatures in order to elucidate its transition from a paramagnetic (PM) to a helical antiferromagnetic (AFM) ordered phase as a function of pressure. The electrical resistance measurements show a change in the resistance slope as the temperature is lowered through the antiferromagnetic Néel temperature. The temperature of this antiferromagnetic transition decreases from approximately 122 K at ambient pressure at a rate of -4.9 K GPa(-1) up to a pressure of 9 GPa, whereupon the PM-to-AFM transition vanishes for higher pressures. Neutron diffraction measurements as a function of pressure at 89 and 110 K confirm the incommensurate nature of the phase transition associated with the antiferromagnetic ordering of the magnetic moments in a helical arrangement and that the ordering occurs at similar pressures as determined from the resistance results for these temperatures.     

Neutron diffraction evidence for magnetic phase transition in Europium selenide

The results of a single crystal neutron diffraction study of europium selenide EuSe in zero external magnetic field are reported. At 4.2°K europium selenide is antiferromagnetically ordered according to an oscillating type of spin configuration. Partial ferromagnetism is observed at 1.9°K, in agreement with previous measurements [1, 2]. Concerning this low temperature phase a model with two coexistent magnetic phases is proposed.