Determination of the anisotropy constant and saturation magnetization of magnetic nanoparticles from magnetization relaxation curves

We have developed a new method for the determination of the anisotropy constant and saturation magnetization of magnetic nanoparticles. This method deals with the approximation of magnetization relaxation curves measured upon application and further fast switching off the dc magnetizing field. The relaxation process is registered in the time interval from 6 μs to several minutes by using a scanning high-T _C SQUID-microscope equipped with a specially designed electronic circuit composed of a fast solid-state switch and a low-inductance magnetizing coil. The algorithm for calculating the approximation data is based on the activation Néel–Arrhenius law and takes into account the size distribution of the nanoparticles and the angular distribution of their easy axes. The performance of the method is demonstrated on dilute (∼0.2 vol%) ensembles of near-spherical Fe₃O₄ nanoparticles with a mean size of 7.7 nm and a standard deviation of 45% as determined from transmission electron microscopy data.