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Thermal neutron three-axis spectrometer with polarization analysis IN20

IN20 is a thermal beam three-axis spectrometer for inelastic scattering experiments in various sample environments (eg. magnets up to 15 T). It can be equipped with single-analyser-detector or Flatcone, with standard Helmholtz and CRYOPAD (zero-field) polarisation analysis and for high-resolution linewidth studies of dispersionless excitations using the spin-echo technique (TASSE).

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Applications

The typical applications of IN20 consist in the use of polarised neutrons to separate magnetic scattering from the nuclear one and, eventually, to obtain a more quantitative picture of its magnitude and anisotropy.
Two basic setups can be used:

  • combination of polarised incident beam with a non-polarising analyser (PG, Si), which avoids intensity losses on the analyser side, to study systems whose magnetic cross-section magnitude depends on the incident neutron spin orientation: magnons in ferromagnets (cf. Fig.4), excitations in chiral spin systems
  • full polarisation analysis (Heusler monochromator and analyser) to investigate scattering from antiferromagnetic and paramagnetic systems on the basis of spin-flip selection rules (cf. Fig. 5)

Despite the recent upgrade, the unique power of the polarisation analysis mode is paid for by a severe loss of luminosity (an order of magnitude and more) as compared to an equivalent setup with unpolarised beams. Therefore it is a good idea to start studies of a new system by a quick survey of the response in unpolarised mode (IN8C) to identify the ranges of interest so that unnecessary time losses in the IN20 experiments get avoided.

References: R.M. Moon et al. 181 (1969) 920; J. Kulda et al. Appl. Phys. A 74 (2002) S246.

Selected examples

Phonon-magnon coupling in CMR manganates

Despite large differences in their Curie temperatures, different ferromagnetic manganese perovskites exhibit very similar dispersions with marked zone-boundary magnon softening and broadening due to a strong coupling to optical phonons. Polarisation analysis provides an ultimate tool to separate unequivocally the magnetic part from the rest of the neutron response. The data displayed in the figure were collected with a 2 cm3 single crystal of La0.7Ca0.3MnO3 using the 4T horizontal cryomagnet and a "half-polarised" setup (Heusler 111 monochromator and PG 002 analyzer) (see figure).

Anisotropy of the magnetic incommensurate fluctuations

The unconventional superconductor Sr2RuO4 is isostructural with the high-Tc superconductor La2-xSrxCuO4 (LSCO) but exhibits an unusual triplet (p-wave) spin-pairing. Polarisation analysis on IN20 provides a tool to probe theoretical models requiring the presence of (quasi-)ferromagnetic or highly anisotropic antiferromagnetic fluctuations in the neutron response. The present IN20 results confirm the latter scenario qualitatively, with incommensurate fluctuations (similar to those observed in LSCO), whose anisotropy between the in-plane
(χ"ab) and out-of-plane (χ"c) components and energy dependence are weaker than predicted for p-wave pairing (see figure).

Phonon anharmonicity in Ge

The transverse acoustic phonons at the X point in Si, Ge and some III-V semiconductors (GaAs, InSb ...) exhibit a negative Gruneisen parameter and are responsible for the anomalous negative thermal expansion coefficients at low temperatures.
The IN20 spin-echo (TASSE) measurements on an isotopic crystal of 74Ge have revealed their metastable character (line-width close to zero) below 100 K, confirmed by subsequent ab initio calculations: the phonons required by the only possible difference process of decay are not thermally populated at such low temperatures. This behaviour is qualitatively different from that of optical phonons at the centre of the Brillouin zone, the only ones accessible to line-width measurements by Raman spectroscopy (see figure).

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