Effect of Inter-Particle Interactions in Magnetic Nanoparticle Ensembles

Ensembles of single domained ferromagnetic (FM) nanoparticles, in which magnetic interparticle interactions are sufficiently weak, show superparamagnetic (SPM) behavior. However, at increased interactions the system eventually shows collective behavior, which overcomes the individual anisotropy properties of the particles. Different collective states of magnetism were first recognized on so-called discontinuous magnetic metal-insulator multilayers (DMIMs) consisting of focused ion-beam grown Co80Fe20 nanoparticles on glassy Al2O3. At sufficiently small interparticle distances as controlled by the nominal Co80Fe20 film thickness, 0.5 nm < t(n) < 1.1 nm, dipolar interaction enables superspin glass (SSG) properties. At increased concentration, but still below physical percolation, 1.1 nm <t(n) <1.3 nm, stronger interactions give rise to superferromagnetic (SFM) states with domain formation similar to that in conventional FM films. At t(n) > 1.3 nm the metal-insulator multilayers (MIMs) become continuous owing to physical interparticle percolation. Owing to inherent layer roughness competing ANNNI-type dipolar interactions give rise to layer-by-layer magnetization reversal as evidenced by domain imaging performed by magneto-optical Kerr effect microscopy.