In this paper we study the possibility to control the size of iron oxide (Fe 3 O 4 ) nanoparticles by the microemulsion technique. We used a water-in-oil reverse microemulsion system with n-hexadecil trimethylammonium bromide (CTAB) as a cationic surfactant, n-butanol as a co-surfactant, n-hexanol as a continuous oil phase, and aqueous phase. The magnetite nanopowders were synthesized by a single microemulsion technique in which the aqueous phase contains only metal ions (Fe 2+ and Fe 3+). The particle size of the powders varied in the range of 14-36 nm depending on the preparation conditions. We studied the influence of changing the water/surfactant ratio (W 0 = 5, 10, 15, 20) and the metallic ion (Fe 2+ and Fe 3+ ) concentration on the particle size distribution and crystallinity of Fe 3 O 4 .

We report results from a study on the influence of the substitution of Zn 2+ cations in the Y-type Ba0.5Sr1.5Zn2Fe12O22 hexaferrite, known for strong magnetoelectric coupling, with magnetic cations, such as Ni 2+ , on its structural and magnetic properties. Polycrystalline samples of Ba0.5Sr1.5ZnNiFe12O22 were synthesized by citric acid solgel auto-combustion. The saturation magnetization value of 54.7 emu/g at 4.2 K was reduced to 37.2 emu/g at 300 K. The temperature dependence of the magnetization at magnetic fields of 50 Oe, 100 Oe and 500 Oe were used to determine the magnetic phase transition temperature. We demonstrate that the helical spin state, believed to cause the magnetoelectric effect, can be achieved by varying the magnetic field strength within a given temperature range.

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