Cooling rate-dependent charge-transfer-induced spin transition in K_0.4Co_1.3[Fe(CN)₆]·nH₂O Prussian blue analog

The Prussian blue analog K_0.4Co_1.3[Fe(CN)₆]·nH₂O was synthesized and observed to exhibit charge-transfer-induced spin transition (CTIST) behavior that resulted in three different magnetic phases at low temperature. The highest χT value is observed at room temperature (RT), corresponding to the largest number of the magnetic spins. Upon slow cooling (0.5 K/min) to 100 K, the number of the magnetic spins is reduced due to the CTIST effect, producing the low-temperature (LT) phase. In this state, the sample exhibited a low value of χT and showed a hysteric transition back to the RT phase upon warming (0.5 K/min). When the compound was rapidly cooled (i.e. quenched faster than ∼100 K/min) to 100 K, a large fraction of the spins bypassed the CTIST effect and stayed in the high-temperature (HT) phase, which is assumed to be similar to the RT phase. However, upon warming (0.5 K/min), the trapped spins relax and the system reaches a new low-temperature (NLT) phase for which the χT data show the lowest values. The result is that in the temperature range below ∼150 K, three phases with different χT values are accessible. This unique NLT phase can only be achieved by the rapid cooling and slow warming, and gives an additional spin control mechanism that might be useful for modern magnetic applications.