XMaS has in routine use a prototype adaptation of a closed-cycle refrigerator system developed by the cryogenics group at the ILL. A continuous flow of 4He gas, forced through a Joule-Thomson jet, provides the additional cooling at the displex tail that reduces the sample environment base temperature from the 10 K of the standard displex down to 1.7 K. The system can be used in both vertical and horizontal scattering geometries and can be combined with a number of other experimental techniques, for example, with azimuthal scans and also with the XMaS 1.0 Tesla electromagnet.

Recently, an experiment was performed with 3He replacing the 4He flowing through the Joule-Thomson jet. A base temperature of 1.0 K was achieved in a resonant X-ray magnetic scattering (RXMS) experiment to investigate the spin density wave (SDW) antiferromagnetism formed in TmNi2B2C. TmNi2B2C is a celebrated compound because of the co-existence of superconductivity (Tc = 11 K) and magnetism (TN = 1.5 K) at low temperatures. The (1± 10) spin density wave satellites, at T = 1 K, together with the (1 1 10) Bragg intensity are shown in Figure 171. Polarisation analysis was used to measure the s component of the scattered intensity (i.e. the component of the incident s photons scattered with a -phase shift).


Fig. 171: The (1± 10) SDW satellites and central (1 1 10) Bragg peak of TmNi2B2C at 1.0 K.


Dipole RXMS scatters photons uniquely with this polarisation, whereas the presence of the central (110) Bragg peak in the channel arises only through 'cross-talk' ( leakage through the analyser). Figure 172 shows the temperature dependence of the (1+ 1+ 10) SDW together with data from neutron scattering. Very good agreement is found between the thermal evolution of the magnetic sublattice and TN for both scattering techniques.


Fig. 172: The (1± 10) SDW satellites and central (1 1 10) Bragg peak of TmNi2B2C at 1.0 K.


It was found necessary, during the acquisition of the RXMS data, to attenuate the beam by two orders of magnitude to avoid excessive local heating of the sample by the intense X-ray beam. Future measurements are planned using an exchange gas to try to reduce this effect.

D. Mannix (a), P. Thompson (a), S. Pujol (b) and X. Tonon (b).
(a) XMaS UK CRG Beamline, ESRF
(b) ILL