The main interest of this thesis relies on the structural and magnetic investigation of magnetic particles with dimensions in the nano-and micrometer range. Three types of particles have been investigated, namely: aqueous-based Fe3O4 nanoparticles, with an average diameter of 12±3 nm, as well as, core-shell particles and composite particles, both with diameters ranging from 680 nm to 1000 nm. The Fe3O4 nanoparticles are negatively charged and tetrabutylammonium hydroxide-stabilized. The core-shell particles consist of an anionic polystyrene core, with a diameter of 640 nm coated with multilayers of Fe3O4 nanoparticles and polyelectrolytes. The composite particles have the same polystyrene core coated with Fe3O4 nanoparticles, polyelectrolytes and silica-encapsulated gold nanoparticles with a diameter of 15 nm. The core-shell and composite particles were prepared by using a recently developed method, called layer-by-layer technique. The uniformity and regularity of the coating has been confirmed by Transmission Electron Microscopy and Atomic Force Microscopy measurements. The structure of the Fe3O4 nanoparticles has been investigated by High Resolution Transmission Electron Microscopy, X-ray Diffraction, X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy. The pattern formation of the core-shell and composite particles in water solution under applied magnetic field has been studied. An in-plane magnetic field arranged the microspheres into a chain-like structure. Chains up to 2 mm in length have been observed. The motion of the magnetic colloids in an aqueous solution when a magnetic field is applied has been visualized in real-time by Optical Microscopy. Ferromagnetic Resonance, SQUID-magnetometry and Magnetic Force Microscopy have been used to investigate the magnetic properties of the nano-and micrometer-size particles. The polar angular dependence of ferromagnetic resonance for Fe3O4 nanoparticles reveals an effective anisotropy field of 0.12 T and a negative out-of-plane anisotropy constant at 300 K. The magnetic measurements on the core-shell particles aligned into a chain-like structure confirm a long-range dipolar order at room temperature, an in-plane easy axis of the magnetization along the chains and a reduction of the magnetic moment of Fe3O4.