Magnetorelaxation (MRX) Measurements With DC-SQUID Gradiometers

Magnetic nanoparticles (MNP) show different relaxation mechanisms after switching off an external field. The Brownian relaxation of the free rotation of the whole particles and the Néel relaxation of the magnetization vector inside a fixed particle have different time scales. While the Brownian relaxation time is in the order of milliseconds the Néel relaxation time is in the second range. Thus a clear separation of free and fixed MNP is possible. To measure the very small magnetic relaxation fields we have developed MRX systems using low temperature superconductor thin film second order SQUID gradiometers working in unshielded environment. By standard MRX we are able to separate both relaxation processes to determine the amount of bounded MNP as results of binding reactions. By Néel measurements with a x-y scanning MRX system a spatial resolved imaging of the bounded particles is possible even for living objects. We try to improve the spatial resolution of our MRX system by a small ferromagnetic needle. Within an anti-cryostat we can measure the temperature dependence of small samples (TMRX) between 4 K and 300 K. From Néel signals using some theoretical modeling important parameters of the MNP such as energy barrier distribution, activation energy, anisotropy constant and diameter of the magnetic core were obtained.