Polarised neutron diffraction: D7, ILL

D7 is a cold neutron diffuse scattering diffractometer that is equipped withXYZpolarisation analysis at the Institut Laue-Langevin in Grenoble, France [73, 74, 89, 94]. D7 can be used in two separate configurations:XYZpolarisation analysis and uniaxial polarisation analysis (used for looking at scattering from non-magnetic samples and non-collinear magnets). Be- low, only the polarisation analysis configuration will be described. On D7,Qresolution is

relaxed in order to gain flux to look at diffuse scattering which is not expected to have any sharp features. The background is kept as low as possible because the diffuse features are also, in general, expected to be much weaker than Bragg scattering. Measurements of the XYZpolarisation cross-sections described in Section 3.3.3 allow for the unambiguous sep- aration of nuclear, magnetic, nuclear-spin-incoherent and isotope incoherent contributions to the scattering function simultaneously over a large range of scattering vectorQ.

The wavelength of the incident neutrons on D7 is selected by moving the entire instrument to access three different angles of the vertically and horizontally focusing py- rolytic graphite monochromators, which translate into incident wavelengths of 3.1, 4.7 or 5.8 ˚A. A Beryllium filter is used to suppress higher order contamination. Polarisation is achieved on D7 using a polariser, flipper and aXYZ alignment coil assembly. First, the incident neutron beam is polarised with high efficiency in theZdirection using a focusing Sch¨arpf supermirror bender, and a small guide field of∼20 G ensures that this polarisation is effectively transmitted through the instrument. A Mezei-type flipper (not used for the non-spin-flip measurements) is a rectangular coil that produces a well defined horizontal magnetic field, and when calibrated for the correct neutronλit rotates neutron spin polar- isation byπ radians with respect to the guide field. A set of three orthogonal field coils around the sample position rotate the beam polarisation in theX, Y orZ directions, and thus define the incident neutron spin polarisation. It should be noted that theX,Y andZ

polarisation directions are fixed with respect to the instrument, and only theZpolarisation direction is parallel and perpendicular to the wavevectorQ. The Sch¨arpf angleα, discussed in Section 3.3.3, is then the angle betweenQand the X polarisation axis. The neutrons scattered by the sample are analysed by more supermirror benders that are situated in front of an array of 1323He detectors arranged in three banks, which can be moved around, and these allows D7 to cover a significant angular range of 4◦ <2θ <145◦. The sample space to detector/supermirror analyser banks is encased in a vacuum box to reduce air scattering, and a schematic of the instrument layout is shown in Fig. 4.10. By measuring the amount of neutrons that have been spin-flipped (SF) and non-spin-flipped (NSF) by the sample for the three separate coordinate axes (i.e. six separate measurements) it is possible to carry out a fullXYZpolarisation analysis to isolate the coherent nuclear, incoherent nuclear, spin

incoherent and magnetic contributions to the scattering.

Before each experiment, several calibration measurements have to be performed. A vanadium standard is used for normalising the detector efficiency (and analyser trans- mission) of the instrument because it is a purely incoherent (isotropic) scatterer. Vanadium calibration also allows the intensity to be converted to absolute units. A quartz (amorphous silica) standard was used for normalising the polarisation efficiency because its scattering is perfectly coherent, and hence any scattering in the SF channel will arise from non perfect polarisation. An empty sample holder must also be run in order to determine the back- ground. Scans with the sample are performed by rotating the crystal about the vertical axis, typically in steps of 1◦. The reciprocal space maps are then constructed in the usual manner using standard D7 data reduction functions [95].

For the single crystal SrHo2O4 experiments carried out on D7, 0.9 g samples of

SrHo2O4 (one for the (hk0) and one for the (h0l) scattering planes) were aligned using

the backscattering x-ray Laue technique and cut perpendicular to the principal crystal axes, within an estimated accuracy of2◦. The samples were fixed to oxygen free copper sample holders using Kwikfill resin, and equivalent sample mountings used for background mea- surements. Cold neutrons monochromated to a wavelength of 3.1 ˚A were used in order to look at the magnetic reflections at the lowest scattering angles, resulting inQ-space cov- erage of 0.14 to 3.91 ˚A−1. The polarisation ratio was found to be 0.95 for the zero-field diffraction experiments, and 0.85 for the experiment with a cryomagnet. During the exper- iments, the samples were rotated in steps of 1◦, and different detector positions, separated by 4.5◦ were used (to cover the “dead” angles between the detector banks). The SrHo2O4

experiments required dilution fridge temperatures, and 360◦planes were measured at 0.055, 0.75 and 4.5 K in zero field. For the zero-field experiment, six measurements required to carry out fullXYZpolarisation analysis were performed at each temperature for both sample orientations, and 36 hours were required to measure each plane at each temperature with good statistics. For the in field measurements, 360◦ planes were also measured in fields of 0.1, 0.6, 0.8, 1.2 and 2.0 T at the lowest reached temperature of 0.15 K. With the cryo- magnet, only theZpolarisation SF andZ NSF cross-sections can be collected, because the large magnet severely depolarises the neutron beam.

Figure 4.11: Schematic of the D10 instrument at a thermal neutron guide at ILL, France. This figure made with reference to the D10 page in [89].

4.3.5 4-circle diffraction: D10, ILL

D10 [89] is a high-flux single-crystal diffractometer located on a thermal neutron guide at the Institute Laue-Langevin in Grenoble, France. Exceptionally goodQresolution and low background make this instrument very useful for careful studies of both nuclear and mag- netic scattring from small single crystal samples. Additionally, the optional energy analysis mode allows to restrict the incident neutron energy range and an even greater reduction in the background is achieved, which is useful when investigating diffuse scattering. Four different configurations of the instrument are possible, and these are standard four-circle, standard two-axis, four-circle with energy analysis and two-axis with energy analysis. For the experiment performed on SrHo2O4, D10 was used in the four-circle dilution refrigera-

tor mode and in four-circle with energy analysis mode to collect precise neutron scattering data.

Many incident energies may be selected on D10, and for the SrHo2O4experiment

a vertically focusing pyrolytic graphite (002) monochromator that gave an incident wave- length of 2.36 ˚A was chosen. The half-wavelength contamination was suppressed by a PG filter. For the SrHo2O4 measurements, single crystal samples of SrHo2O4 (weighing

0.3 g and 0.1 g) were fixed to oxygen free copper pins using Stycast resin. The sample pins are attached to the Eulerian cradle (which allows for three separate axes of rotation for the crystal) inside a helium flow cryostat with the dilution option to allow scanning along all reciprocal lattice directions at low temperature. Measurements on SrHo2O4were

made in the temperature range 0.15 to 10 K. Air cushions underneath the sample and the analyser tables allow them to move indepentently. In the diffraction configuration of D10 the detector rotates around the sample position, and two different detectors can be used - a 80×80 mm microstrip detector and a single3He detector. The out of plane coverage on D10 can be increased by inclining the detector by up to 30◦. In order to create an orienta- tion (UB) matrix using the positions of the nuclear reflections from the sample, the detector is placed at a value of2θwhich satisfies the Bragg condition for a reflection from (hkl), and then rotating the crystal while measuring the scattered intensity. Once several strong reflections are measured, the UB matrix may be found and hence the sample can be easily moved to any required position inQ. For the SrHo2O4experiment an analyser was used to

give improved resolution and suppressed background for some of the measurements. The two-dimensional area detector was used for all the measurements except for those made with the energy analyser, where the single3He detector was used instead. The basic layout and the main features of the D10 instrument are shown in Fig. 4.11.

D10 is controlled via a command lines from a computer. The data analysis includes the usual routines for integrating Bragg reflections, fitting peak profiles and plotting the scattering profiles. From the data collected during the SrHo2O4experiment, the integrated

intensity of the magnetic diffuse scattering around the nuclear Bragg peaks was extracted in the usual manner [96]. However, the plane like diffuse scattering features often had a broad intensity profile which covered a large proportion of the two-dimensional D10 detector. This meant that the selection of the integration box for these features required a compromise between getting adequate resolution and the correct representation of the diffuse scattering

Figure 4.12: Schematic of the IN4 instrument at a thermal neutron guide at ILL, France. This figure made with reference to the IN4 page in [89].

profile.