Details of a Researcher - Einaga, Yasuaki

Papers - Einaga, Yasuaki

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Photoinduced long-range magnetic ordering of a cobalt-iron cyanide

Sato O., Einaga Y., Fujishima A., Hashimoto K.

Inorganic Chemistry 38 ( 20 ) 4405 - 4412 1999

ISSN 0020-1669

Two kinds of cobalt-iron cyanides (Rb_0.66Co_1.25[Fe(CN)₆]·4.3H₂O and Co_1.5[Fe(CN)₆]·6H₂O) with different electronic structures have been investigated to understand the photoinduced long-range magnetic ordering. Rb_0.66Co_1.25[Fe(CN)₆]·4.3H₂O produces a photomagnetic effect, whereas Co_1.5[Fe(CN)₆]·6H₂O does not respond to light. FT-IR and Mossbauer studies revealed that their oxidation states are expressed as Rb_0.66Co^III_0.84Co^II _0.41-[Fe_II(CN)₆] and Co^II_1.5[Fe^III(CN)₆], respectively. The difference in the oxidation states of the metal atoms in these two compounds has been explained by the Co coordination with H₂O or CN ligands. In the case of Rb_0.66Co_1.25[Fe(CN)₆]·4.3H₂O, more CN ligands are involved in coordination than expected in the case of Co_1.5[Fe(CN)₆]· 6H₂O. A charge-transfer (CT) band from Fe^II to Co^III is observed at around 550 nm for Rb^0.66Co^1.25[Fe(CN)₆]· 4.3H₂O. The magnetism of Rb_0.66Co_1.25[Fe(CN)₆]·4.3H₂O changed from paramagnetic to ferrimagnetic due to the CT from Fe^II to Co^III when illuminated at low temperature. The Curie temperature after illumination was 22 K. This metastable state was stable for more than several days at 5 K. The metastable state was restored back to its original one when the sample was heated to 120 K. It is considered that the interconversion proceeded via a pronounced domain formation. © 1999 American Chemical Society.

Local structure of a trapped photoexcited state of a Fe-Co cyanide studied by x-ray-absorption fine-structure spectroscopy

Yokoyama Toshihiko, Kiguchi Manabu, Ohta Toshiaki, Sato Osamu, Einaga Yasuaki, Hashimoto Kazuhito

Physical Review B - Condensed Matter and Materials Physics 60 ( 13 ) 9340 - 9346 1999

ISSN 0163-1829

The local structure of a trapped photoexcited state of a Fe-Co cyanide magnetic material, Na_0.4Co_1.3Fe(CN)₆·5H₂O, was investigated by Fe and Co K-edge x-ray-absorption fine-structure spectroscopy. The excited state was prepared by visible-light irradiation at 36 K. In order to obtain detailed structural information, temperature dependence of the local structure was also studied without light irradiation. The Fe-C distance shows almost no change by light irradiation (1.92 Å before and after irradiation), while the average Co-N/O distance is noticeably elongated (1.94 and 2.08 Å before and after irradiation, respectively). This phase transformation is caused by the charge transfer from the Fe^II(S=0)·CN-Co^III(S=0) configuration to Fe^III(S =1/2)-CN-Co^II(S=3/2). The local structure of the trapped photoexcited state was found to be identical with that of the high-temperature phase, although the high-temperature phase was created much more effectively by light irradiation than thermal activation. ©1999 The American Physical Society.

Mössbauer study on characterization of Co[Fe(CN)₅NH₃] · xH₂O

Einaga Y., Gu Z. Z., Kobayashi Y., Sato O., Iyoda T., Ambe F., Hashimoto K., Fujishima A.

Hyperfine Interactions 116 ( 1-4 ) 159 - 166 1998

ISSN 0304-3843

External magnetic field effect on the photoinduced magnetization in a cobalt iron cyanide

Einaga Yasuaki, Ohkoshi Shin Ichi, Sato Osamu, Fujishima Akira, Hashimoto Kazuhito

Chemistry Letters ( 7 ) 585 - 586 1998

ISSN 0366-7022

We studied the external magnetic field effects on the photoinduced magnetization of a cobalt iron cyanide, K_0.4Co_1.3[Fe(CN)₆]·5H₂O. Photoinduced magnetization value was increased ca. 20 % at maximum under the magnetic field of 50000 G at 5 K. In addition, in the ferrimagnetic region (T < T_c = 26 K), the external magnetic field accelerated the rate of photoinduced spin generation. Those data suggest that external magnetic field increases magnetic interaction between the excited state and spin clusters.

Cation-driven electron transfer involving a spin transition at room temperature in a cobalt iron cyanide thin film

Sato O., Einaga Y., Iyoda T., Fujishima A., Hashimoto K.

Journal of Physical Chemistry B 101 ( 20 ) 1997.05

ISSN 1089-5647

The electronic structure of a cobalt iron cyanide thin film, Na_0.4Co_1.3[Fe(CN)₆]5H₂O, was switched from Fe^III(low spin)-CN-Co^II(high spin) to Fe^II(low spin)-CN-Co^III(low spin) by exchanging cations in interstitial sites from Na⁺ to K⁺. The reverse process was also induced by replacing Na⁺ with K⁺. That is, the alkali cations in the interstitial sites dominate the electronic and spin states of the host compound. This type of switching phenomenon can be achieved by choosing a compound with both electronic degeneracy and zeolitic properties.

Reversible photoinduced magnetization

Sato O., Einaga Y., Iyoda T., Fujishima A., Hashimoto K.

Journal of the Electrochemical Society 144 ( 1 ) 1997.01

ISSN 0013-4651

Reversible photoinduced magnetization was observed in a cobalt iron cyanide thin film. Magnetic properties of the prussian blue analog, K_0.4Co_1.3[Fe(CN)₆] ·5H₂O, can be switched between ferrimagnetic and paramagnetic by visible and near-IR light illumination.The critical temperature after illumination was about 26 K. This material, obtained via an electrochemical route in the form of thin films on an electrode, provides a new system whose magnetic properties can be controlled by both photochemical and electrochemical redox reactions.

Mossbauer study on photochemistry of ferrocene isolated in low- temperature matrix

Einaga Y., Yamada Y., Tominaga T.

Journal of Radioanalytical and Nuclear Chemistry 218 ( 1 ) 97 - 100 1997

ISSN 0236-5731

Unstable novel species produced by photolysis of ferrocene (Cp₂Fe) isolated in low-temperature matrix were studied by means of the ⁵⁷Fe Mossbauer spectroscopy. On the UV-irradiation of ⁵⁷Fe-enriched ferrocene co-condensed with CCl₂ in low-temperature N₂ matrix (20 K), unknown high spin ferrous species (I.S. = 0.90 ± 0.02 mm/s), Q.S. = 1.94 ± 0.02 mm/s) were produced as well as ferricinium ion [Cp₂Fe]⁺ (I.S. = 0.52 ± 0.01 mm/s, Q.S. = 0.0 mm/s). On the annealing, these photoproducts disappeared to reform ferrocene by the reverse reaction in the matrix. The mechanisms of the photochemical reaction were discussed.

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Electronic states of cobalt iron cyanides studied by ⁵⁷Fe Mössbauer spectroscopy

Einaga Yasuaki, Sato Osamu, Iyoda Tomokazu, Kobayashi Yoshio, Ambe Fumitoshi, Hashimoto Kazuhito, Fujishima Akira

Chemistry Letters ( 4 ) 289 - 290 1997

ISSN 0366-7022

The electronic states of cobalt iron cyanides were observed using ⁵⁷Fe Mössbauer. It was revealed that Prussian blue analogs Co_1.5[Fe(CN)₆]6H₂O and K_0.5Co_1.25[Fe(CN)₆]3.6H₂0 have Fe^III(low spin) and Fe^II(low spin), respectively, at both room temperature and 4 K. In Co_1.5[Fe(CN)₆]6H₂O, a magnetic hyperfine interaction was observed at low temperature. Moreover, the Mössbauer spectra provided clear evidence for an internal electron transfer from Fe^II to Co^III in K_0.5Co_1.25[Fe (CN)₆]3.6H₂O by red light illumination at low temperature.

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