152 High Quality Iron Borate Crystal for Nuclear Resonant Scattering Experiment 57 Fe-enriched FeBO 3 single crystal is one of the most advantageous optical elements of the nuclear resonant scattering of synchrotron radiation (SR). Since pure nuclear Bragg scattering (NBS) in the vicinity of Néel temperature T N can emit an extremely narrow band single-line X-rays on the order of 10 -8 eV [1], this allows one to perform energy domain Mössbauer measurement using excellent SR properties (i.e., polarization, small beam size, etc.). However, high reflectivity and small divergence of NBS are achieved only when the nuclear monochromator crystal has crystallinity on few seconds order. Recently, we have successfully obtained a centimeter-sized very high- quality 57 FeBO 3 single crystal by flux crystal growth [2]. Crystal perfection and magnetic domain structure were investigated by double-crystal topography using a non-dispersive (+, - ) setting, i.e., asym-Si(331) × sym-FeBO 3 (444). As shown in Fig. 1, the measured rocking curve of 57 FeBO 3 (444) reflection revealed that the value of full width at half maximum (FWHM) was only 4.48 arcsec for λ = 1.24 Å. This assures that this 57 FeBO 3 crystal has an ideal crystal perfection suited for an optical element using NBS. Magnetic domain structure was observed using precise X-ray topograph. In the topograph recorded at the peak position, homogeneous X-ray diffraction contrast is obtained from almost the whole of the crystal (see Fig.1 (a)). This indicates that the crystal surface is free from serious lattice defects such as dislocations and inclusions. On the contrary, topographs recorded at low and high-angle positions of FWHM show many straight-line contrasts crossing over the crystal surface (see Figs. 1(b) and 1(c)). The line contrasts are caused by magnetostriction across 90 º magnetic domain walls. They are composed many regularly arranged black and white diamond shaped strain sectors with a typical sector size of a few millimeters. Because the black and white contrasts of each sector have been reversed in Figs. 1(b) and 1(c) respectively, it is found that adjacent domains have a slight inclination to the opposite direction of each other with angular misorientation below a few arcsec. These results are the first observation of a unique case showing that regularly arranged multimagnetic domains play a vital role in the high-crystal perfection of 57 FeBO 3 . The optics for the generation of ultrafine monochromatic X-rays at a SR facility are shown in Fig. 2. The experiment was performed at beamline BL11XU . A high-resolution monochromator (HRM) was used for the monochromatization of SR X-ray. Then, the σ - polarized 14.4 keV X-ray of 2.5 meV bandwidth was incident on a 57 FeBO 3 crystal that was mounted in a heater. An external magnetic field of 150 Oe was applied along the 57 FeBO 3 (111) plane to magnetize it Fig. 1. Rocking curve and X-ray double crystal topograph of 57 FeBO 3 (444) plane with X-ray illumination of whole sample in H ex = 0 Oe. The dashed line is a Gaussian fitting curve. Topographs (a) , (b) and (c) are recorded at the following angles: (a) ∆θ = 0.0 s, (b) ∆θ = -2.24 s and (c) ∆θ = +2.24 s. Intensity (arb. units) (a) 0 2000 Angle (sec) (b) (c) (a) (b) (c) FWHM = 4.48” –10 0 10 –20 –30 –40 20 30 40 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 153 perpendicular to the scattering plane. In this optical system, a π -polarized ultrafine monochromatic X-ray beam could be emitted by the electronically forbidden pure nuclear Bragg reflection of 57 FeBO 3 (333). In Fig. 3, the Mössbauer spectra of a 57 Fe- enriched stainless steel foil are displayed at different temperatures in the range from room temperature up to the Néel temperature. This shows that narrow- band single-line X-rays are obtained at 75.8 deg. From the theoretical fitting on the absorption spectra of thin iron foils, it was estimated that the energy resolution is 15 neV. Intensity and angular divergence were evaluated by measuring a rocking curve of 57 FeBO 3 (333). As the result, their values were 12,000 cps and 3.8 arcsec, respectively (see Fig. 4). These results indicate that our obtained ultrafine monochromatized X-ray beam has a very high quality. Therefore, by utilizing an excellent neV bandwidth probe beam, Mössbauer microanalysis, precise γ -ray optics, and other new techniques of SR neV spectroscopy will be greatly advanced. Takaya Mitui SPring-8 / JAEA E-mail: taka@spring8.or.jp References [1] G.V. Smirnov et al. : Phys. Rev. B 55 (1997) 5811. [2] T. Mitsui, H. Takei, S. Kitao, M. Seto, T. Harami, X. Zhang, Y. Yoda and S. Kikuta: Trans. Mater. Res. Soc. Jpn. 30 (1) (2005) 7. Fig. 4. Rocking curve of NBS from 57 FeBO 3 (333) at temperature of T = 75.8 deg. Intensity (arb. units) Angle (sec) 0 –20 –10 0 10 20 2500 5000 7500 10000 12500 Fig. 3. Mössbauer absorption spectra of 2.5- μ m-thick stainless steel foil (90% 57 Fe) measured with reflected Bragg radiation from 57 FeBO 3 (333) at different temperatures. Intensity (arb. units) ch (1ch = 0.0457 mm/s) 0 75.8 ºC 100 200 300 400 500 75.7 ºC 74.5 ºC 28.0 ºC Fig. 2. Optics for ultrafine monochromatization of synchrotron radiation. BL11XU Si (111) HRM @ 14.4 keV H ex = 1500 Oe NaI Detector Absorber 57 FeBO 3 (333) Si (111) Si (511) Si (975) V 1 mm 4mm Slit
home
- JP
- EN