Synthesis of monodisperse iron-platinum (FePt) nanoparticles by reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine stabilizers is reported. The FePt particle composition is readily controlled, and the size is tunable from 3- to 10-nanometer diameter with a standard deviation of less than 5%. These nanoparticles self-assemble into three-dimensional superlattices. Thermal annealing converts the internal particle structure from a chemically disordered face-centered cubic phase to the chemically ordered face-centered tetragonal phase and transforms the nanoparticle superlattices into ferromagnetic nanocrystal assemblies. These assemblies are chemically and mechanically robust and can support high-density magnetization reversal transitions.

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We have used oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) to grow CoxTi1−xO2 anatase on SrTiO3(001) for x=∼0.01–0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or polycrystalline, these CoxTi1−xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1−xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous pulsed laser deposition-grown material.

The magnetic domain structure and magnetization curves of chemically ordered epitaxial FePt (001) films with perpendicular magnetic anisotropy are discussed. Films were dc magnetron sputtered from a Fe50Pt50 alloy target onto Pt seeded MgO (001) at substrate temperatures of 550 °C. The thickness of the FePt layers was varied between 18 and 170 nm. Specular and grazing incidence x-ray diffraction measurements confirm the presence of the anisotropic, face centered tetragonal (L10) crystal structure. Long range chemical order parameters of up to 0.95 and small mosaic spread, similar to results reported for FePt (001) films grown by molecular beam epitaxy. For film thicknesses ⩾50 nm in-plane and out-of-plane hysteresis measurements indicate large perpendicular magnetic anisotropies and at the same time low (about 10%) perpendicular remanence. Magnetic force microscopy reveals highly interconnected perpendicular stripe domain patterns. From their characteristic widths, which are strongly dependent on the film thickness, a value of the dipolar length D0∼50±5 nm is derived. Assuming an exchange constant of 10−6 erg/cm, this value is consistent with an anisotropy constant K1∼1×108 erg/cc.