Yasuhito Isozumi

Preparation and characterization of (111)-oriented Fe3O4 films deposited on sapphire

Films of various iron oxides including FeO, Fe3 O4 , and α‐Fe2 O3 were formed on the α‐Al2 O3 (0001) surface by a reactive vapor deposition method and characterized by x‐ray diffraction and conversion electron Mossbauer spectroscopy (CEMS). The formation range for each phase was determined as a function of the substrate temperature (Ts ) and oxygen partial pressure (PO2 ). Typically, the deposition of (111)‐oriented epitaxial magnetite films could be performed at low temperatures of Ts =523∼623 K and PO2 =1.0–5.0×10−4 Torr. Good stoichiometry of the as‐grown films were confirmed by CEMS, and the Verwey transition was clearly detected by measurements of resistivity and CEMS down to 77 K. However, the large lattice mismatch between the substrate and Fe3 O4 resulted in a columnar particle growth. The initially grown phase on such a mismatched substrate was specifically characterized by depositing the Mossbauer active isotope, 57 Fe, only at the deepest layers. On the other hand, by depositing 57 Fe only in t...

Effects of 2.45-GHz Electromagnetic Fields with a Wide Range of SARs on Micronucleus Formation in CHO-K1 Cells

There has been considerable discussion about the influence of high-frequency electromagnetic fields (HFEMF) on the human body. In particular, HFEMF used for mobile phones may be of great concern for human health. In order to investigate the properties of HFEMF, we have examined the effects of 2.45-GHz EMF on micronucleus (MN) formation in Chinese hamster ovary (CHO)-K1 cells. MN formation is induced by chromosomal breakage or inhibition of spindles during cell division and leads to cell damage. We also examined the influence of heat on MN formation, since HFEMF exposure causes a rise in temperature. CHO-K1 cells were exposed to HFEMF for 2 h at average specific absorption rates (SARs) of 5, 10, 20, 50, 100, and 200 W/kg, and the effects on these cells were compared with those in sham-exposed control cells. The cells were also treated with bleomycin alone as a positive control or with combined treatment of HFEMF exposure and bleomycin. Heat treatment was performed at temperatures of 37, 38, 39, 40, 41, and 42°C.The MN frequency in cells exposed to HFEMF at a SAR of lower than 50 W/kg did not differ from the sham-exposed controls, while those at SARs of 100 and 200 W/kg were significantly higher when compared with the sham-exposed controls. There was no apparent combined effect of HFEMF exposure and bleomycin treatment. On heat treatment at temperatures from 38–42°C, the MN frequency increased in a temperature-dependent manner. We also showed that an increase in SAR causes a rise in temperature and this may be connected to the increase in MN formation generated by exposure to HFEMF.

Surface and interface properties of epitaxial Fe3O4 films studied by Mössbauer spectroscopy

Abstract Stoichiometric Fe 3 O 4 films have formed epitaxially on α-Al 2 O 3 and MgO single-crystal substrates by a reactive vapor deposition method. In order to apply conversion electron Mossbauer spectroscopy depth-selectively, a 5–7 Athick probe layer containing 57 Fe was formed at various depths in inactive 56 Fe 3 O 4 matrix films. At the topmost surfaces and also at the interfaces, the essential electronic features of bulk Fe 3 O 4 are retained, including a rapid electron hopping between the Fe 2+ and Fe 3+ ions at B sites. Minor depth-dependent changes are confined to a few outermost atomic layers, and the changes depend on the orientation and the lattice mismatch with the substrate. For (111) growth on α-Al 2 O 3 , the surface layer seems to be strongly relaxed to reduced the electric polarization, while a high density of defects seems to be concentrated at the interface with α-Al 2 O 3 . For (001) growth on MgO, the surface retains the spinel lattice though slightly oxidized, while the interface with MgO has good crystallinity and stoichiometry. An enhanced thermal fluctuation of the Fe 3+ -spins in contact with the MgO substrate and in the topmost surface layer can be seen in their reduced magnetic hyperfine field at 300 K.

CEMS study of the growth and properties of Fe3O4 films

Abstract Stoichiometric Fe3O4 films have been epitaxially formed on both α-Al2O3 and MgO single-crystalline substrates by a reactive vapor deposition method. Their crystalline qualities were examined by various methods including conversion electron Mossbauer spectroscopy (CEMS). A large lattice mismatch of Fe3O4 with the α-Al2O3 substrate resulted in a granular particle growth. But with Fe3O4/MgO, where the mismatch was only 0.3%, the structural and magnetic properties were found to be very sensitive to an excellent interfacial lattice matching.

Superconductive radiation detector with large sensitive area (series‐connected STJ detector)

One of the serious problems in the development of particle detectors with superconducting tunnel junctions (STJ) is the difficulty in devising a large area that is sensitive to nuclear radiations. The problem has been solved by using an assembly of micro STJs connected in series (series‐connected STJ detector), which is designed for detecting nonthermal phonons created by irradiating the rear substrate (sapphire) with nuclear radiations. The present detector is sensitive not only to ionizing events but also to nonionizing events, providing us with a new method to detect nuclear radiations. Some features of the series‐connected STJ detector are described in this article.

Spin-flip anomalies in epitaxial α-Fe2O3 films by Mössbauer spectroscopy

The spin-flip transition of 1000 A-thick α-Fe2O3 films was investigated by conversion electron Mossbauer spectroscopy from 2.5 to 673 K. The films were epitaxially formed on α-Al2O3 single-crystalline substrates by a reactive vapor deposition method. Not only the transition temperature but also the spin axis were found to strongly depend upon the film orientation, probably as a result of anisotropic lattice strains caused by interactions with the substrate. For a (102)-oriented film the transition takes place at about 400 K, much higher the Morin transition temperature (260 K) of the free crystal, and the spin axis lying within the (102) plane above 400 K changes almost normal to the plane at room temperature and below: the normal makes 57.6° with the spin axis of the free crystal. A (001)-oriented film, on the other hand, does not show any transition at least down to 2.5 K.

Thermal phase transition of RbMnFe(CN)6 observed by X-ray emission and absorption spectroscopy

The thermal phase transition of RbMnFe(CN)6 has been observed by Mn and Fe 3p–1s X-ray emission spectroscopy (XES) and 1s X-ray absorption spectroscopy (XAS). The thermal variations of the spin states and the valences of Mn and Fe were determined to be Mn2+(S=5/2)–NC–Fe3+(S=1/2) for the high-temperature (HT) phase and Mn3+(S=2)–NC–Fe2+(S=0) for the low-temperature (LT) phase. These transitions are thus caused by charge transfer between Mn and Fe. The temperature dependences of Mn and Fe 3p–1s XES and 1s XAS were observed as the composition of the spectra of the HT and LT phases. The ratios of the HT component in each spectrum show good agreement with the thermal transition curves observed with magnetic susceptibility measurements.

Helium‐filled proportional counter for low‐temperature operation (1.75–4.2 K) and its application to cryogenic resonance‐electron Mössbauer spectroscopy II

The operation of the helium‐filled proportional counter (HFPC) has been investigated at low temperatures below liquid‐helium temperature, 4.2 K. It has been found that the gas gain as a function of the anode voltage is not appreciably changed by the temperature of the filling gas, but continuous electric discharges are induced at lower anode voltages as the gas temperature is decreased. The present counter technique has been applied to resonance‐electron Mossbauer spectroscopy in the temperature region below 4.2 K.

High‐temperature proportional counter and its application to resonance–electron Mössbauer spectroscopy

The operation of a proportional counter designed for resonance–electron Mossbauer spectroscopy measurement at high temperatures up to 900 °C has been investigated. In order to avoid insulation difficulties due to electric leakage on the surfaces of heated insulators, the counter is carefully constructed so that the sensitive volume of the counter can be warmed up while keeping the insulators at room temperature; all insulating materials between the anode and cathodes are located outside the electric furnace in the counter system. The counter operation is influenced by thermal electrons emitted from the cathode material; the maximum working temperature is about 800 °C when an a‐Fe foil is mounted as a cathode while operation at 900 °C is possible without the sample. 57Fe Mossbauer spectra for surface layers (∼1000 A) have been sucessfully obtained for samples of Fe3C, Fe2O3, and a Fe. Chemical changes of the sample surfaces have been observed at temperatures greater than 600 °C by the measurements. The hig...

Depth selective Mössbauer spectroscopic study of Fe3O4 epitaxial films

Several (111) oriented 56Fe3O4 films containing a 5‐A‐thick 57Fe3O4 probe layer at or below the surface were grown epitaxially on the α‐Al2O3 (0001) surface by a reactive vapor deposition method. Conversion electron Mossbauer spectroscopy was applied at 6, 78, and 300 K by using a recently developed helium‐filled proportional counter. The well‐crystallized surface has been found to be surprisingly stable even in air as characterized by the Mossbauer parameters that are almost the same as for the bulk. Moreover, the Verwey transition was detected clearly even in the 5‐A‐thick surface layer. However, the ferrous components seem to have changed their Mossbauer parameters probably because of their sensitivity to any crystalline field modifications in the surface. Generally speaking, the quality of epitaxial Fe3O4 films is very high: Any unusual surface state, if present, is confined in a shallow depth of ≤ 5 A.