Takaya Mitsui
Magnetism Research Group, Synchrotron Radiation Research Center

A synchrotron Mössbauer source (SMS) is a powerful tool forthe local analysis of iron-based materials. The SMS filters thesingle-line 57Fe-Mössbauer radiation from synchrotron radiation(SR) using pure nuclear Bragg reflection (PNBR) of the57
FeBO3 crystal near the Néel point (TN), and it allows us toconduct conventional 57Fe Mössbauer spectroscopy, whereby
information on the local magnetic state, valence, and electron
density can be obtained via the electron nuclear hyperfine
couplings. Additionally, the high degree of polarization, lowdivergence, and small beam size of SR enables an advanced
Mössbauer experiment [1]. Moreover, SMS has great potential
to improve the radiation properties because the magnetic controlof 57FeBO3 crystal significantly affects the PNBR. As a firstattempt, we have developed a new method of varying thebandwidth of SMS by controlling the magneto-acousticvibrations in a 57FeBO3 crystal [2]. The scheme is as follows.
Below the TN, nuclear Zeeman splitting is observed in 57FeBO3.
In this case, the PNBR has a multiline resonance structure,corresponding to the four absorption lines of m= ±1 nuclear
transitions. At room temperature, the resonance energies spreadin the region of ~10 6eV, and the photon flux is more than 10times larger than that of single-line PNBR from the 57FeBO3
crystal near the TN. However, multiline PNBR is not suitable asa probe in spectroscopy. To solve this problem, a radio
frequency magnetic field (HRF) is applied to the57FeBO
temperature-controlled
crystaltoexcite
3
magneto-acoustic vibrations. In this case, the random vibrationsof 57Fe atoms are excited in the crystal via magneto-elastic
coupling [3], and single-line PNBR below the TN is obtained by
the collapse of nuclear Zeeman splitting in 57FeBO3. The
linewidth is controllable in the range of 10 8 to 10 6eV byadjusting the HRF and the crystal temperature. Figure 1(a) shows
the optical system of variable bandwidth 57Fe-SMS. The-polarized X-rays were monochromatized to a bandwidth of
2.5 meV at the 57Fe nuclear resonance energy (14.4keV) by ahigh-resolution monochromator consisting of Si(511) and (975)
crystals. It was incident on the RF vibrating 57FeBO3 (95%enriched in57Fe): the magneto-acoustic vibrations were excitedby the HRF of f= 8.0 MHz and of variable amplitude (0 3.6 Oe);the HRF was generated by a Helmholtz coil ( 20mm); the crystal
temperature was controlled in the range of 25 76 °C. 57Fe
Mössbauer radiation was filtered from the SR by the 57FeBO3(333) PNBR and the bandwidth (energy distribution) of thePNBR was evaluated by measuring the Mössbauer spectra of aDoppler-vibrating 90% 57Fe-enriched 2µm-thick stainless-steel(SS) foil.
The observed spectra are shown in Figure 1(b). Ata glance,
one can see that the results give clear evidence that single-line
absorption profiles with different linewidths are obtained byadjusting the HRF and the crystal temperature: the bandwidths of
SMS show continuous variation in the range of 10 8 to 10 6 eV,
depending on HRF and T; the photon flux markedly increases
with linewidth-broadening. Table I summarizes the linewidthand beam flux of the RF collapsed 57Fe-SMS.

possibilities of SR-Mössbauer experiments. As the first feasible-ray scattering of liquid glycerol wasmeasured. Figure 2(a) shows the experimental setup: the HRF(0.75Oe,8 MHz) was applied to the 57FeBO3 crystal at T= 25°C.
The glycerol was placed on the Cu-stage in a LN2-cryostat;the57Fe-SR-Mössbauerradiation, whose bandwidth was ~1 eV,
was incident on the sample and a Mössbauer reference absorber(MRA), whose absorption linewidth was 50neV, was used todetect the linewidth broadening of the scattered radiation.
Initially, to obtain an instrumental function, the absorption

The variable-bandwidth 57Fe-SMS provides new application

52spectrum of an MRA placed in the forward direction wasmeasured with no sample. As shown in the upper part ofFigure2(b), the spectrum was a single-line profile and was wellfitted by the Voigt function with a linewidth of ~1µeV.
-ray scattering of glycerol wasobserved at 123 and 25°C. The former was below theglass-transition temperature Tg~115°C and the latter was overthe melting point Tm~17°C. The quasielastic -ray photons
scattered by the sample were transmitted through the MRA
placed at the scattering angle of 12°. The results in Figure 3(b)show that the spectrum of glycerol at 123 °C is almost thesame as that of the instrumental function, meaning that the line
broadening of the quasielastic scattering of polycrystallized
glycerol at a low temperature (T < Tg) is too small to bemeasured with the resolution of this spectrometer. In contrast,
the spectrum at T= 25°C shows a marked linewidth broadening.
Considering the linewidth of the instrumental function, the line
broadening due to atomic vibration is estimated to be ~1µeV.

3. T. Mitsui et al., Nucl. Instr. and Meth. B.199 (2003) 75.

This result shows that the variable-bandwidth57Fe-SMS has ahigh potential to determine the energy transfer of soft condensed
matter (e.g., proteins, colloids, or polymers). The
bandwidth-controllable single-line 57Fe-SMS will open new
possibilities of SR-based Mössbauer studies with an ultrahighresolution of 10 8 to 10 6 eV.

This work was supported by the Grants-in-Aid for Scientific
Research (B,25287086) from the Japan Society for thePromotion of Science (JSPS).

1. T. Mitsui et al., J. Synchrotron Radiat. 16 (2009) 723.
2. T. Mitsui et al., J. Phys. Soc. Jpn. 87 (2018) 093001.

53