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The Spin Polarised Hot Neutron Beam Facility D3

D3 is a modular polarised neutron instrument viewing to the hot source which is used for the study of magnetisation densities and magnetic structures

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Applications

  • High field configuration
    • Magnetic form factors (reflecting the state of the magnetic ions)
    • Magnetisation distributions
  • Zero-field configuration
    • Non-collinear magnetic structures
    • Antiferromagnetic form factors
    • Spherical polarimetry
  • Non-polarized beam with high magnetic field
    • ..

Selected examples

Determination of the magnetization distribution in Cr2O3 using spherical neutron polarimetry

P. J. Brown, J. B. Forsyth, E. Lelièvre-Berna and F. Tasset
J. Phys.: Condens. Matter 14 (2002) 1957–1966

The magnetization distribution due to the Cr3+ ion in Cr2O3 has been determined using spherical neutron polarimetry. The magnetic structure factors of h 0 l reflections have been measured out to sin / = 0.75 Å-1. It has been shown that SNP can be used to determine precise values of the magnetic interaction vectors in the class of antiferromagnetic materials in which nuclear and magnetic scattering appear in the same reflections and are in phase quadrature. The method has enabled the magnetization distribution in Cr2O3 at 25 K to be determined with good precision.

The distribution can be fitted to a first approximation by a model in which the unpaired electrons are all in those trigonal, a1 and e, 3d orbitals of the Cr3+ ion derived from the cubic orbitals of t2g symmetry. However the total moment associated with each Cr3+ ion is only 2.48 µB rather than the 2.97 µB which would be predicted from the measured g-factor of 1.98. The loss of moment can be attributed to covalent mixing of the Cr 3d electrons with O 2p orbitals in pi-type antibonding orbitals. Positive and negative spin transferred to O from oppositely polarized Cr3+ ions is superposed and so does not contribute to the magnetization. The magnitude of the moment deficit corresponds to a covalent mixing factor of &Mac197; 0.18. There are some significant features in the magnetization distribution which are not accounted for by the ionic model; they occur in regions where the Cr and O radial wavefunctions overlap strongly and are hence probably due to covalent overlap.
The magnetization distribution has a gradient at the Cr3+ sites which is consistent with a parallel ME coefficient with the same sign as that implied by the combination of magnetic and electric fields needed to stabilize the domain in question.

 

Maximum-entropy reconstruction of the density corresponding to the difference between the observed magnetization distribution and that calculated from the multipole model. The section shown is perpendicular to [0 1 0] and passes through the origin. The contours are logarithmically spaced with a factor of two between successive levels. The highest contour is at 1.0 µB Å3; negative contours are dashed. The filled triangles mark the Cr3+ ion positions; the one farthest to the right in the diagram has positive spin.


The cerium magnetic form factor and diffuse polarization in CeRh3B2 as functions of temperature

F. Givord, J.-X. Boucherle, E Lelièvre-Berna and P. Lejay
J. Phys.: Condens. Matter 16 (2004) 1211–1230

In the compound CeRh3B2, a rather special polarization of the conduction electrons along the c-chains of ceriumatoms had been previously reported at low temperatures (Alonso et al, J. Magn. Magn. Mater. 177–181 (1998) 1048). The distribution of the CeRh3B2 magnetization has now been studied as a function of temperature up to 150 K - that is, above the Curie temperature of 115 K. The magnetization density maps have been obtained from polarized neutron diffraction experiments by using the Maximum Entropy method. The cerium form factor has also been analysed. Calculations of the form factor including several multiplets are developed and it is shown that it is necessary to take into account the influence of the higher multiplet of the Ce3+ ion. This result is coherent with the observation of a peak at high energy in the inelastic neutron spectra, indicating a very large crystal electric field splitting. Both analyses lead to the same conclusion that, on heating, the diffuse negativemagnetization observed at lowtemperature along the ceriumchains disappears at the magnetic ordering temperature. The influence of the second multiplet of the Ce3+ ion could be part of the explanation for the low value of the 4f moment and the large Curie temperature in CeRh3B2.

(a) Thermal variation of the measured macroscopic magnetization M (full circles), of the calculated 4f moment µ4f (open circles) and of the diffuse negative polarization (crosses). Lines are guides for the eye. Squares are the mean values deduced from the maps between two cerium atoms in the directions [001] (full dots) and [210] (open dots). The data at T = 150 K were obtained in a different magnetic field, H = 9 T instead of 5 T. (b) Diffuse negative polarization versus the spin part of the 4f moment.

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