AC Magnetization of Immobilized Magnetic Nanoparticles With Different Degrees of Parallel Alignment of Easy Axes

Abstract

Magnetic nanoparticles (MNPs) are physically immobilized in some biomedical applications. In this case, dynamic ac magnetization that determines MNPs' performance has strong dependence on the relative direction between the magnetic easy axes of MNPs and the applied excitation field. To study this dependence, we prepared immobilized MNP samples with different degrees of easy axes' alignment. The degree of alignment was changed by setting the strength of a dc field that is applied during the immobilization process. The magnetic properties of these samples, such as ac magnetization, coercive field, amplitude of third-harmonic signal, and hysteresis loss, are compared with those of a randomly oriented easy axes' sample. It was shown that these values are significantly affected by the degree of easy axes' alignment when the strength of the ac excitation field, H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , is relatively small. The alignment effect gradually becomes smaller when H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is increased. These findings are important for immobilized MNP applications in hyperthermia and magnetic particle imaging.