Enhancement of Curie temperature in electrochemically prepared crystalline thin films of Prussian blue analogs K_jFe_k^II[Cr^III(CN)₆]_l·mH₂O

Abstract

Structural and magnetic properties of electrochemically prepared crystalline films of Prussian blue analogs (PBAs) K_jFe_k^II[Cr^III(CN)₆]_l·mH₂O, with varying deposition time and electrode voltage, which result into change in film thickness and stoichiometry, respectively, have been investigated by using x-ray diffraction (XRD), infrared (IR) spectroscopy, and dc magnetization measurement techniques. An atomic force microscopy (AFM) and XRD study reveal uniform and crystalline nature of all films. As the film thickness increases from 1 μm to 5 μm, the Curie temperature (T_C), coercive field, and maximum magnetization increase from 11 K to 21 K, 20 Oe to 160 Oe, and 5.7 μ_B to 6.5 μ_B, respectively. For the films prepared with variation in electrode voltage, it has been found that the alkali metal ions are introduced into the films just by using suitable electrode voltage, contrary to usual method where alkali metal ions are intentionally introduced into the lattice by using additional compounds of alkali metals as starting materials. In addition, an enhancement in T_C with an increasing electrode voltage has been observed. The film deposited with a lower electrode voltage of −0.6 V shows a TC of ∼21 K, close to the previously reported value of T_C. Whereas, for films prepared with an electrode voltage of −0.9 V, an increase in T_C(∼65 K) is observed. The rise in T_C is attributed to the decrease in Fe^II/Cr^III ratio with an increasing electrode voltage. The ability of tuning T_C just by changing the electrode voltage could be useful in designing thin films of new molecule based magnets.