Visible-light-induced reversible photomagnetism in rubidium manganese hexacyanoferrate

The photoreversibility of a photoinduced phase transition was investigated in a rubidium manganese hexacyanoferrate, Rb _0.88 Mn[Fe(CN) ₆ ] _0.96 .0.5H ₂ 0. The present material shows a charge-transfer phase transition from the Mn ^II -Fe ^III [high-temperature (HT)] phase to the Mn ^III -Fe ^II [low-temperature (LT)] phase, and the LT phase shows ferromagnetism. Spectroscopic ellipsometry measurements of the dielectric constant suggest that the optical transitions in the LT and HT phases are a metal-to-metal charge transfer (Fe ^II -Mn ^III ) band at 420-540 nm and a ligand-to-metal charge transfer (CN ^- → Fe ^III ) band of [Fe ^III (CN) ₆ ] at 410 nm, respectively. By irradiation with 532 nm light, the LT phase is transmitted to the photoinduced (PI) phase, which has a valence state similar to that of the HT phase, and photodemagnetization is observed. In contrast, irradiating the PI phase with 410 ±30 nm light causes the reverse phase transition. Neutron powder diffraction measurement of an analogue compound, Rb _0.58 Mn[Fe(CN) ₆ ] _0.86 ·2.3H ₂ O, which does not show a charge-transfer phase transition and maintains the Mn ^II -Fe ^III phase at a very low temperature, confirms that the PI phase is an antiferromagnet. Hence, the present visible-light-induced reversible photomagnetic effect is due to optical switching between the ferromagnetic LT phase and the antiferromagnetic PI phase.