Magnetic Force Microscopy of Micropatterned Clusters of Superparamagnetic Iron Oxide Nanoparticles

Magnetic force microscopy (MFM) was used to characterize micropatterned clusters of superparamagnetic iron oxide nanoparticles (SPIONs). Top-down lithography was used to create SPION aggregates with well-defined geometries. The micrometer-scale aggregates exhibited different properties from individual particles and from smaller clusters containing just a few particles. The MFM phase shift from magnetic interactions between the sample and probe tip could be detected at lift heights of several hundred nanometers. The experimental data was compared to a magnetic dipole-dipole interaction model to understand the relationship between MFM phase shift and lift height. Magnetic interactions between the probe tip and the sample also led to an apparent "ballooning" of the feature size, where the aggregates appeared larger with MFM than their physical size obtained from scanning electron microscopy. These results can guide emerging applications of MFM, such as the detection of SPIONs within biological environments.