Research Group for Nanoscale Structure and Function of Advanced Materials
Group Leader : FUKUTANI Katsuyuki
Members : HIGEMOTO Wataru, FUKAYA Yuki, ITO U.Takashi , YASUDA Satoshi, YANO
Masahiro, YAMAKAWA Koichiro, TERASAWA Tomoo, UETA Hirokazu, SHAMOTO Shinichi, MACHIDA Shinichi, MIYAZAKI Itsuki, SUZUKI Taiga

Macroscopic matter is assembly of microscopic matter andfunctional materials, whose characteristics are exposed byinvestigation of both local and whole state. An experimental
technique that can be used to selectively investigate a local state
can extend the field of materials research. Existing technicaldevelopments enable us to efficiently produce high-intensity
beams of muons, positrons, neutrons, and ions, which are usefulfor studying various atomic structures and dynamics offunctional materials at the length scale of advanced beams. In
addition, experiments that can obey local states at surface,interface, or around impurity have been developed as well, andthey can be used to obtain new scientific insights. Our
objectives are to study the nanoscale structures and functions ofmaterials, and develop advanced measurement technologies.
Hydrogenation and hydrogen diffusion at the anatase
TiO2(101) surface [1]Titanium dioxide (TiO2) has fascinating properties that enablehydrogen sensors and photocatalytic water splitting. Since bothphenomena are accompanied by proton motion and electrontransfer, it is important to understand the behaviour of hydrogenand its associated effects on the electronic structure of TiO2
surfaces in order to elucidate the mechanism behind thehydrogen-induced phenomena.
We have studied the hydrogenation of anatase TiO2 surfaces byirradiation with low-energy hydrogen ions. After 500-eVhydrogen ion irradiation, an in-gap state was formed at about 1
eV below the Fermi level with a downward band bending as
shown in the inset of Fig. 1, suggesting formation of a small
polaron state. This is in contrast to the H adsorption on theanatase surface, where no small polaronic state was observed,
probably because of oxygen vacancy formation under H ion
irradiation.

Fig. 1 Depth profile of H measured for anatase TiO2(101) after H ion
irradiation at 200 K. Inset: Ultraviolet photoemission spectra of cleanTiO2(101), after H ion irradiation at 200 K and subsequent annealing at200 K.

Nuclear reaction analysis with 1H(15N,αγ)12C shows that the H
depth profile reveals a maximum at a depth of about 1 nm withan enhanced intensity corresponding to 11.8 ML. In addition to

the surface peak, the H depth profile exhibits a broad
distribution extending to 50 nm with an average concentrationof 0.8 at. % indicating substantial diffusion of H at 200 K. The
diffusion coefficient is analysed to be 2.7x10-13 m2s-1 from the H
depth profile. This value indicates that H diffusion is much
faster in anatase TiO2 than in rutile TiO2.
Magnetic structure and phase transition at the surface
region of Fe3O4(100) [2]
Magnetite (Fe3O4) is a ferrimagnet, where spin 5/2 at thetetrahedral (A) site is antiferromagnetically coupled with spin
5/2 and 3/2 at the octahedral (B) site with a curie temperature of858 K. Lowering of symmetry at surfaces exerts significant
effects on the magnetic properties at surfaces such as spinorientation and phase transition, because the exchangeinteraction and magnetic anisotropy at surfaces are modified ascompared to the bulk.
We have investigated the magnetic structure and phasetransition of the near-surface region of Fe3O4 with Conversion
Electron Mössbauer spectroscopy (CEMS) (inset of Fig. 2) andtheoretical calculations. It is revealed that at 300K themagnetization is in-plane in the surface region and cants fromthe in-plane to the 〈111〉 direction in a deeper region suggesting
the presence of a noncollinear magnetic structure in the nearsurface region. From the temperature dependence of the internal
magnetic field obtained from the CEMS (Fig. 2), the criticalexponents for the tetrahedral and octahedral sites are estimatedto be 0.24±0.01 and 0.28±0.01, respectively, suggesting twodimensional features for the magnetic phase transition at thesurface.

Fig. 2 Temperature dependence of the internal magnetic fields at the A(□) and B (〇) sites. Solid curves are fits, and the dash-dotted curves
are the data taken from the bulk data. Inset: CEMS spectrum taken for
the Fe3O4 surface with fits shown by solid and dashed curves below.

References[1] N. Nagatsuka et al., J. Chem. Phys. 152, 074708 (2020).
[2] T. Kawauchi et al., J. Phys. Commun. 4, 115001 (2020).