Magnetic x-ray scattering studies of holmium using synchro- tron radiation

Abstract

We present the results of magnetic x-ray scattering experiments on the rare-earth metal holmium using synchrotron radiation. Direct high-resolution measurements of the nominally incommensurate magnetic satellite reflections reveal new lock-in behavior which we explain within a simple spin-discommensuration model. As a result of magnetoelastic coupling, the spin-discommensuration array produces additional x-ray diffraction satellites. Their observation further substantiates the model and demonstrates additional advantages of synchrotron radiation for magnetic-structure studies.PACS numbers: 61.10. Fr, An x-ray incident on an electron is scattered by both the electron's charge and its magnetic moment. Charge scattering is the dominant mechanism and is the basis for structural investigations of condensed matter by x-ray diffraction. In the first application of magnetic x-ray scattering, de Bergevin and Brunei 1,2 performed experiments on the antiferromagnet NiO using a fixed-target x-ray tube. Subsequently, there have been experimental 3 and theoretical 4 developments, but progress to date has been limited by the fact that the magnetic x-ray scattering cross section is substantially smaller than the Thomson cross section for charge scattering. Even in the most favorable cases, 3,4 the ratio of magnetic to charge scattering is less than = 10" 5 . As we will show, however, easily measured signals are obtained with the intense synchrotron radiation beams available from wiggler sources.In this paper we present high-resolution magnetic x-ray scattering studies of the incommensurate magnetic spiral in the rare-earth metal holmium. During the last decade, investigations of various incommensurate systems, such as charge-density wave materials 5 and graphite intercalates, 6 have revealed microscopically inhomogeneous structures composed of commensurate regions separated by localized regions of large incommensurability called discommensurations. 7 The high resolution naturally provided with synchrotron x-ray scattering methods 8 permits direct study of structural correlations on the long spatial length scales inherent in these systems. High resolution is also crucial in the present experiments, which have led to a new model 9 of the magnetic structure of rare earths based on the concept of spin discommensurations. Results from both x-ray and complementary neutron scattering experiments on the same sample are reported.For the synchrotron experiments carried out at the Stanford Synchrotron Radiation Laboratory, a Ho single crystal (7x3x1 mm 3 ) grown by R. J. Gambino at IBM was mounted in a variable-temperature cryostat and studied in reflection. The initial experiments were performed on the Exxon/LBL/SSRL 54-pole wiggler beam line VI-2 with X= 1.7 A; further measurements involving polarization analysis were performed on the 8-pole wiggler beam line VII-2 with \= 1.5 A. In each experiment a Si or Ge double-crystal monochromator was used with a Ge analyzer in the standard vertical diffraction geometry. The illuminated sample are...

Exploring the magnetic phase diagram of dysprosium with neutron diffraction

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Threefold Symmetric Magnetic Two-Ion Coupling in hcp Rare-Earth Metals

“…Although the magnetic scattering intensity is weak, the narrow resolution in the X-ray experiments made it possible to detect two other commensurable structures in Ho below 25 K, at r = 2/11τ c and 5/27τc besides the one at r = ś τ c. These commensurable structures were interpreted in terms of the spin-slip model [13]. In the limit of very strong hexagonal anisotropy the moments would be confined to be aligned along one of the six (in the present system) b-directions.…”

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