Magnetization measurements on frozen ferrofluids: an attempt to separate interaction and anisotropy influences

In this contribution we focus on a novel way to perform and evaluate magnetization experiments on frozen ferrofluids or magnetic nanoparticles fixed in space. The basic idea is to project the behaviour of a real particle system onto a Stoner–Wohlfarth (SW) particle ensemble behaviour. Therefore first an ideal SW particle system acting as a reference system with unique particle sizes and orientation of easy axis was studied numerically by introducing a particle–particle interaction via a demagnetizing field. We have shown that for non-interacting SW particles a universal mean value rule holds that the magnetization measured after ZFC (zero field cooling) can be expressed as a mean value of the magnetization measured after PHFC and NHFC (positive and negative high field cooling) processes. Any deviation from this mean value rule is therefore due to non-SW behaviour (multidirectional anisotropy and interaction) only and can be derived from the magnetization measurements and be compared with simulations. By this new method it is possible to determine the mean value of the particle's anisotropy and the mean interaction which is expressed here as a magnetic field. We will report on experiments performed on frozen ferrofluids because these systems offer the possibility of complete thermal demagnetization after melting. In this contribution we describe experiments performed on two different frozen Co-based ferrofluids by SQUID magnetometry.