Prima-Garcia et al. Nanoscale Research Letters 2012, 7:232 http://www.nanoscalereslett.com/content/7/1/232 mode operation was used in the morphological study of the electrodeposited films. Root mean square (RMS) roughness and average particle size were determined using WSx4.0 Develop 13.0 software [9], developed by Nanotec Electronics S.L. (Tres Cantos, Madrid, Spain). MOKE measurements The magneto-optical characterization was performed with a self-made Kerr magnetometer. A He-Ne laser with a wave- length of 633 nm and an output power of 12 mW was used as the light source producing a nearly linearly-polarized light beam. This beam was passed through a Glan-Laser calcite polarizer (Karl Lambrecht Corporation, Chicago, IL, USA) with an extinction coefficient of 10−5, which allows working with both s-polarized (electric field perpendicular to the plane of incidence) and p-polarized (electric field par- allel to the plane of incidence) configurations. The design of the electromagnets and the cryostat allows the use of longi- tudinal, polar, and transverse geometries. The external mag- netic field range in the polar configuration is ±150 mT. Results and discussion By reducing [Cr(H2O)6]3+ at a fixed potential of E = −0.88 V in an aqueous solution containing [Cr(CN)6]3− anions, it was possible to obtain thin films of Crx(CN)6 zH2O [10] (1). Thus, an electroactive transparent film is formed in situ by a reaction of the [Cr(H2O)6]2+ species with the hex- acyanometalate anion on the surface electrode resulting in the formation of the insoluble PBA. By changing the elec- trodeposition time in coulometry, it was possible to tune the thickness of the electrodeposited films. The relation- ship between the time of deposition and the film thickness was found to be nearly linear (Table 1). The use of this electrochemical method allowed us to fabricate films with thicknesses ranging from 1,500 nm for 100 s deposition time to 40 nm for films with a deposition time of 1 s. Optical absorption spectra of the electrodeposited films of 1 [11] and attenuated total reflectance infrared spectra (ATR-IR) have been studied somewhere else. The ATR-IR spectra showed for all the different thicknesses a principal band located at 2,186 cm−1 corresponding to the cyanide Table 1 Morphological parameters of different thin films of 1 Time of deposition (s) 100 50 25 10 2 1 a At 190 K. Thickness (nm) 1,500 ± 100 450 ± 30 250 ± 10 80 ± 10 50 ± 10 40 ± 10 Average particle size (nm) 600 ± 100 320 ± 80 180 ± 30 170 ± 40 120 ± 20 90 ± 20 RMS roughness (nm) 58 37 18 10 10 8 Coercive fielda (mT) 32 ± 5 49 ± 5 76 ± 5 85 ± 5 33 ± 5 10 ± 5 Page 2 of 4 stretching vibration of the [CrIII(CN)6]3− anion in con- cordance with the previous results [10]. The IR frequency associated with this specie is independent of the film thick- ness [11], indicating that the composition of the film is un- altered as the thickness is decreased. In order to characterize the morphology of the films of 1, an atomic force microscopy (AFM) study was per- formed (Figure 1). In this case, only the images corre- sponding to the thinner films obtained so far are showed. The images were acquired in tapping mode, with a 5 × 5-μm scan sizes for films with different depos- ition times. For the thicker films, it is possible to distin- guish a polycrystalline structure composed of pyramidal particles of different sizes [11]. This type of morphology has been observed in the epitaxial electrodeposition of Prussian blue on Au(110) surface [12]. For the thinner films, the average particle size (Table 1) increases from a value of 90 nm (lateral size) for the film prepared after 1-s electrodeposition (40-nm thickness) to a value of 170 nm for the film of 80 nm. During the initial (nucleation) stages of the electrodeposition process, the reaction takes place at the naked surface of the electrode, and its kinet- ics is only limited by diffusion of the species, resulting in very small particles that cover homogenously the elec- trode surface. At longer deposition times, the intensity of the electrochemical current decreases, indicating a slower kinetics that results in the growth of the particles as pyramidal microcrystals as has been studied at thicker films of sample (1) [11]. The roughness of the film sur- face shows a slight increase with the deposition time, passing from a value of 8 nm for the 40-nm thickness film to a value of 10 nm for the 80-nm thickness film. The magnetic properties of the films at different thick- ness have been studied somewhere else [11] exhibiting negligible changes on the magnetic properties when changing the film thickness. Magneto-optical Kerr effect (MOKE) measurements were performed in order to probe directly the magnetic properties of the different thin films described in Table 1. A source of linearly polarized light of high wavelength (λ = 633 nm) and low power (12 mW) was used in order to prevent any photomagnetic effect. The Kerr rotation (θk) of the films, directly proportional in a first approximation to the magnetization of the samples, was recorded as a func- tion of the applied magnetic field for different tempera- tures. The first striking result from the MOKE study is that a relatively large hysteresis loops appeared just below Tc for all the films measured in polar configuration (magnetic field perpendicular to the surface sample). The difference between MOKE and other magnetometer like supercon- ducting quantum interference device (SQUID) is probably due to the local surface character of the MOKE technique, which is more sensitive to subtle variations in the morph- ology of the sample. Also, it has to be considered that the