Enhanced Magnetic Resonance Contrast of Fe3O4 Nanoparticles Trapped in a Porous Silicon Nanoparticle Host A composite material consisting of Fe3O4 nanoparticles embedded in a 200-nm-diameter porous Si (pSi) nanoparticle "superstructure" is prepared as a potential magnetic resonance imaging contrast agent. Dipolar magnetic coupling between Fe3O4 nanoparticles is enhanced due to their proximity in the pSi host matrix, resulting in an increase in the saturation magnetization and coercivity of the composite. [GRAPHICS]