Tuning the sign of photoinduced changes in magnetization: Spin transitions in the ternary metal prussian blue analogue Na_αNi _1-xCo_x[Fe(CN)₆]_β·nH ₂O

Tuning the composition of the ternary transition-metal Prussian blue analogue Na_αNi_1-xCo_x[Fe(CN) ₆]_β·nH₂O allows the sign of the photoinduced change in magnetization to be controlled. The parent cobalt hexacyanoferrate material is well-known to display photoinduced and thermal charge-transfer-induced spin transitions (CTISTs). Upon partial replacement of Co ion sites with Ni^II, irradiation with halogen light can cause either an increase or a decrease in magnetization, depending upon the extent of Ni^II substitution, the applied field, and the temperature. For all compositions with x > 0, photoexcitation generates new moments according to the same mechanism observed for the parent x = 1 compound. However, the presence of Ni^II introduces a superexchange of opposite sign, providing a mechanism for controlling the sign of the change in magnetization with applied light. Additionally, dilution of the spin-crossover material reduces the magnitude and hysteresis of the thermal CTIST effect. These effects can be qualitatively explained by simple mean-field models.