Reversed (negative) magnetization for electrochemically deposited high-Tc thin films of chromium hexacyanide magnets

Thin films of chromium cyanide Prussian blue-like materials, KnCrx[Cr(CN)(6)] (ca. 1.0 +/- 0.5 mu m thick) are electrodeposited on glassy carbon electrodes in amorphous (a) and crystalline (alpha) forms. The crystalline film shows a X-ray diffraction pattern typical of a face-centered cubic (fcc) Prussian blue-type cubic lattice [a = 10.41(2) Angstrom]. The films exhibit quasi-reversible cyclic voltammetric waves, distinct morphologies in SEM images, and strong field-dependent magnetic behavior. The films undergo two one-electron processes, Cr-II[Cr-II(CN)(6)](2-) reversible arrow Cr-III[Cr-II(CN)(6)](-) + e(-), and Cr-III[CrII(CN)(6)](-) reversible arrow Cr-III[Cr-III(CN)(6)] + e(-), with some residual Cr-II being present. The composition is determined from an analysis of the upsilon(CN) IR data {[Cr-II(CN)(6)](4-): upsilon(CN) = 2072s cm(-1); [Cr-III(CN)(6)](3-): upsilon(CN) = 2185m cm(-1)} and X-ray photoelectron (XPS) spectral data. The composition of the oxidized and reduced films are Cr-III[Cr-III(CN)(6)](0.98)[Cr-II(CN)(6)](0.02) and K2.0CrII[Cr-II(CN)(6)], respectively, for both amorphous and crystalline forms. These materials exhibit broad intervalence charge-transfer bands in their oxidized [lambda(max) = 18 800 cm(-1) (epsilon similar to 1280 cm(-1) M-1); 35 500 cm(-1) (epsilon similar to 1180 cm(-1) M-1)] and reduced [lambda(max) = 20 200 cm(-1) (epsilon similar to E 870 cm(-1) M-1); 36 700 cm(-1) (epsilon similar to 770 cm(-1) M-1)] forms. The crystal field splitting Delta(o)'s for Cr-II(NC)(6) and Cr-III(NC)(6) are 19 500 and 26 600 cm(-1), respectively. The magnetic ordering temperatures, T-c, are a function of Cr oxidation state and range between 135 and 260 K for both film types. The T-c's are independent of crystallinity, suggesting that the magnetic domains are smaller than the short-range structural order for both a- and alpha-films, Hysteresis is observed for the films, with coercive fields as high as 830 Oe at 20 K, confirming the existence of bulk ferrimagnetic behavior below T-c. We determined that the coercivity of the amorphous films is larger than that of the crystalline films and suggest that this due to the larger number of defects in the a-films.