We here present a versatile process for the preparation of maghemite/polyaniline (γ-Fe2O3/PAn) nanocomposite films with macroscopic processibility, electrical conductivity, and magnetic susceptibility. The γ-Fe2O3 nanoparticles are coated and the PAn chains are doped by anionic surfactants of ω-methoxypoly(ethylene glycol) phosphate (PEOPA), 4-dodecylbenzenesulfonic acid (DBSA), and 10-camphorsulfonic acid (CSA). Both the coated γ-Fe2O3 and the doped PAn are soluble in common organic solvents, and casting of the homogeneous solutions gives free-standing nanocomposite films with γ-Fe2O3 contents up to ∼50 wt %. The morphology of the γ-Fe2O3 nanoparticles are characterized by transmission electron microscopy, UV−vis spectroscopy, and X-ray diffractometry. The γ-Fe2O3/PAn films prepared from chloroform/m-cresol solutions of DBSA-coated γ-Fe2O3 and CSA-doped PAn are conductive (σ = 82−237 S/cm) and superparamagnetic, exhibiting no hysteresis at room temperature. The zero-field-cooled magnetization experiment reveals that the nanocomposite containing 20.8 wt % γ-Fe2O3 has a blocking temperature (T b) in the temperature region of 63−83 K.

A systematic study of Co(SiO 2 ) granular films by means of transmission electron microscopy ͑TEM͒, dc and ac initial magnetic susceptibility, and thermoremanent magnetization ͑TRM͒ is presented. The experimental results are compared with simulations of zero-field-cooled ͑ZFC͒ and field-cooled ͑FC͒ magnetization and TRM curves obtained using a simple model of noninteracting nanoparticles. The simulated ZFC/FC curves, using the actual parameters obtained from the TEM images, show a different behavior than the experimental magnetic data. The effect of the dipolar interaction among particles introduces a self-averaging effect over a correlation length ⌳, which results in a larger average ''magnetic'' size of the apparent particles together with a narrower size distribution. The analysis of the ZFC/FC curves in the framework of independent ''particle clusters'' of volume ⌳ 3 , involving about 25 real particles, explains very well the observed difference between the experimental data for the median blocking temperature ͗T B ͘ and their distribution width with respect to the ones expected from the structural observations by TEM. The experimental TRM curves also differ from those obtained from the theoretical model, starting to decrease at a lower temperature than expected from the model, also indicating the strong influence of dipole-dipole interactions.

Using complementary experiments we show that the room temperature ferromagnetism observed in anatase Co:TiO(2) films is not carrier mediated, but coexists with the dielectric state. TEM and x-ray absorption spectroscopy reveal a solid solution of Co in anatase, where Co is not metallic but in the +2 state substituting for Ti. Measurements at 300 K yield a M(S) of 1.1 mu(B)/Co atom, while all films are highly insulating. The evidence of intrinsic ferromagnetism in the dielectric ground state of Co:TiO(2) leads to new considerations for the origin of ferromagnetism in transition metal doped oxides.

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