Magnetic nanoparticles with a high specific absorption rate (SAR) have been developed and used in mouse models of cancer. The magnetic nanoparticles are comprised of predominantly iron oxide magnetic cores surrounded by a dextran layer for colloidal stability. The average diameter of a single particle (core plus dextran) is 92±14 nm as measured by photon correlation spectroscopy. Small angle neutron scattering measurements under several H2O/D2O contrast conditions and at varying nanoparticle concentrations have revealed three length scales: ≫10 μm, several hundred nanometers, and tens of nanometers. The latter corresponds to the particle diameter; the several hundred nanometers corresponds to a hard sphere interaction radius of the core/shell nanoparticles; ≫10 μm corresponds to the formation of long-range, many-particle structures held together by magnetic interactions and dextran. The long-range collective magnetic behavior appears to play a major role in enhancing the SAR. For samples having nominally equal concentrations and similar saturation magnetizations, the measured SAR is 1075 W/(g of Fe) for tightly associated nanoparticles and 150 W/(g of Fe) for very loosely associated nanoparticles at an applied field of 86 kA/m (1080 Oe) and 150 kHz.