Tuning the sign of photoinduced changes in magnetization: spin transitions in the ternary metal Prussian blue analogue Na(alpha)Ni(1-x)Co(x)[Fe(CN)6](beta) x nH2O.

ABSTRACT

Tuning the composition of the ternary transition-metal Prussian blue analogue Na(alpha)Ni(1-x)Co(x)[Fe(CN)(6)](beta) x nH(2)O 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.