Particle motion artifacts in equilibrium magnetization measurements of large iron oxide nanoparticles

Iron oxide nanoparticles find many applications due to their response when subjected to externally applied magnetic fields. Equilibrium magnetization measurements (commonly known as magnetization curves) are an essential characterization tool to evaluate if particles display hysteresis, and to obtain magnetic properties such as the saturation magnetization. For superparamagnetic particles, one can obtain a magnetic size distribution by fitting the data to a theoretical model, such as the Langevin function, in what is called magnetogranulometric analysis. If one wishes to use the resulting size estimates as a predictor of particle performance in applications, magnetization data must be obtained under conditions that capture the response of the particles with minimal artifacts. In this paper, we used selected iron oxide nanoparticle batches with physical size ranging from 20 to 45 nm to demonstrate the influence of sample preparation methods on the magnetization data obtained. We show that measurements in powder form and in liquid solvents display varying degrees of particle interaction artifacts at low fields, depending strongly on particle size and on the thickness of the surface coating. In addition, measurements in 'solid' waxy hydrocarbon matrices are shown to be susceptible to particle rotation artifacts for large particle sizes. Hard crosslinked polymer matrices are shown to restrict particle motion completely, resulting in magnetization data that follows the Langevin function if the measurement is performed above the blocking temperature of the particles. We end with a discussion of how the presence of matrix-dependent measurement artifacts influence the magnetic diameter fits obtained using magnetogranulometry, and how measuring above and below the blocking temperature can affect fit results.