Magnetic diffraction in solid 3He

2014 Magnetic diffraction in a bcc single crystal of 3He directly proves the occurrence of an antiferromagnetic ordering with a propagation vector ( 1 /2, 0, 0). J. Physique Lett. 46 (1985) L-923 L-927 ler OCTOBRE 1985, Classification Physics Abstracts 67. 80 75 . 25 75. 30E The originality of the magnetic ordering of solid 3 He is that the large zero point motion leads to atom-atom exchange processes. This mechanism involves the whole motion of the nucleus with its electronic shell. Thus solid 3 He represents one of the more attractive example of a magnetic coupling correlated with the direct displacement of the atoms. Evidence of the atom exchange is given by an ordering temperature TN ~ I mK [1] three orders of magnitude greater than that predicted for dipolar magnetic nuclear interactions. The tunnelling origin of the exchange leads also to a drastic diminution of TN with the decrease of the volume (Y) : TN ~ P~ [2]. Interest in the magnetic studies is reinforced by the fact that an ordinary nearest neighbour Heisenberg exchange model fails to explain the magnetic behaviour at low temperature ( T ~ TN) although it seems to describe the high temperature experiments. Due to the hard core potential, triple and four spin exchanges are important [3, 4]. Recently, it has been proposed [5] that a new magnetic ordering (a spin nematic state) may occur with the particularity that the atoms may carry no sublattice magnetization below TN at the opposite of the case of antiferromagnetic structures. Only neutron scattering experiments can answer whether or not a magnetization exist on each site below TN. Neutron diffraction is an unambiguous way to determine the existence of a sublattice magnetization and a magnetic structure. In the present case of nuclear moment ordering, magnetic diffraction would occur not through the usual dipole-dipole interaction which is too weak, but through the nuclear interaction which is spin dependent. The difference between the values b + and b _ of 3 He recently measured [6] is enough for a superstructure magnetic Bragg reflection to be detected. However, performing this experiment on solid 3He below 1 mK is a challenge. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:019850046019092300 L-924 JOURNAL DE PHYSIQUE LETTRES The magnetic ordering occurs at very low temperature. The huge neutron-3He absorption cross section implies a detection of a weak diffraction signal and a drastic sample heating by the neutron beam. Consequently the magnetic phase can be observed only for a short time [7]. We report the first observation of magnetic diffraction on a bcc single crystal of solid 3He. 1. The experimental background. 1.1 THE NEUTRON SPECTROMETER. The experiment has been performed on the polarized neutron spectrometer DN2 installed at the reactor Melusine of the « Centre d’Etudes Nucleaires de Grenoble ». The beam is made monochromatic and polarized by an Heussler crystal. Its wavelength, 1.74 A has been chosen large enough to produce larger intensities on the magnetic Bragg reflections of 3He, and to reduce the background around these low angle reflections. A set of filters (A1203 single crystal, Sm203 powder) permits use of either the A = 1.74 A beam (polarized) or the ~/2 = 0.87 A beam (practically unpolarized) to search for magnetic or nuclear reflections. Two counters are operated simultaneously (Fig. 1) : one (CT) for transmission measurements and the other (CD), which can be tilted above the basal plane, for Bragg reflection intensities. A concrete block with a large hole in its centre is mounted above the spectrometer. It lies on three pillars with rubber suspension. It eliminates vibrations and supports the rotating mechanism for the sample cryostat. 1.2 THE ULTRA LOW TEMPERATURE CRYOSTAT. Ultra low temperatures are obtained in two stages. The first stage consists of a dilution refrigerator which allows a pre-cooling down to 10 mK. The second stage is a nuclear demagnetization of copper which allows the sample container to cool down to 0.5 mK. It is made of a bundle of 10 moles of copper wires magnetized in a field of 8 T. The 2 stages are connected by a superconducting thermal switch made of pure lead wires, which conducts heat when the field is on and does not conduct when the field is off. Fig. 1. View of the experimental set up. No is the incoming neutron beam, M an Heussler crystal monochromator, RF a resonant flipper, F filters for producing unpolarized (~, = 0.87 A) or polarized (A = 1.74 A) beam, MG the magnetic guide, Cp and Cy the counters used respectively for the measurement of a (h, k, l) reflection and of the flipping ratio. A field H of 80 mT polarizes the 3He target. L-925 MAGNETIC DIFFRACTION IN SOLID 3He 2. Controlling the experimental parameters. Observing a magnetic Bragg reflection on a 3He single crystal, cooled down below 1 mK, requires that many difficulties be brought under control. 2.1 CONTROL OF THE 3He CRYSTAL GROWTH AND ORIENTATION. The 3He single crystal has to be grown in situ in a flat copper cell, under pressure, at very low temperature. Furthermore, at A = 1.74 A, the desirable thickness is less than 0.1 mm. To grow such a crystal, we have used a copper cell and filled it with liquid 3He through a very thin capillary. The geometry of the cell does not allow solidification at constant pressure because it implies a continuous flow of liquid in the cell. Crystallization occurs then at constant volume with a continuous decrease of pressure from 44 bar at T = 1.15 K to 34 bar at 0.8 K [8]. The orientation of such a crystal is not known. It is determined by a systematic search for the nuclear reflections of type (110), by rotating the cryostat around the vertical axis, and this for the different positions of the counter CD above the horizontal plane, along the Debye-Scherrer