Photoinduced Long-Range Magnetic Ordering of a Cobalt-Iron Cyanide.

Two kinds of cobalt-iron cyanides (Rb(0.66)Co(1.25)[Fe(CN)(6)].4.3H(2)O and Co(1.5)[Fe(CN)(6)].6H(2)O) with different electronic structures have been investigated to understand the photoinduced long-range magnetic ordering. Rb(0.66)Co(1.25)[Fe(CN)(6)].4.3H(2)O produces a photomagnetic effect, whereas Co(1.5)[Fe(CN)(6)].6H(2)O does not respond to light. FT-IR and Mössbauer studies revealed that their oxidation states are expressed as Rb(0.66)Co(III)(0.84)Co(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 H(2)O or CN ligands. In the case of Rb(0.66)Co(1.25)[Fe(CN)(6)].4.3H(2)O, more CN ligands are involved in coordination than expected in the case of Co(1.5)[Fe(CN)(6)].6H(2)O. A charge-transfer (CT) band from Fe(II) to Co(III) is observed at around 550 nm for Rb(0.66)Co(1.25)[Fe(CN)(6)].4.3H(2)O. The magnetism of Rb(0.66)Co(1.25)[Fe(CN)(6)].4.3H(2)O 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.