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Internal Magnetic Structure of Nanoparticles Dominates Time-Dependent Relaxation Processes in a Magnetic Field



Cindi L. Dennis, Kathryn L. Krycka, Julie Borchers, Ryan D. Desautels, Johan van Lierop, Natalie F. Huls, Andrew J. Jackson, Cordula Gruettner, Robert Ivkov


Time-dependent relaxation of nanomagnets, such as magnetic nanoparticles, have applications in many disciplines of science and technology because these magnets can exhibit hysteresis and loss power when exposed to AC magnetic fields. In particular, the responsiveness of magnetic nanoparticles to static and AC magnetic fields has found use for cancer therapy and imaging. For therapy, magnetic nanoparticles introduced to cancerous tissue (and the tissue region) are subjected to an alternating magnetic field (AMF) thereby depositing energy as heat through loss power into the tumor. The total amount of energy deposited depends upon the local nanoparticle concentration, specific loss power (SLP) of the particles, duration of treatment, and AMF frequency and amplitude. Pre-clinical and clinical testing suggest that clinically realistic AMF parameters include fixed frequency and variable amplitude within specific limits. Therefore, it is critical to understand the amplitude-dependent response of a magnetic nanoparticle system at a fixed frequency. Here, we considered three nanoparticle systems, all approximately 100nm in hydrodynamic diameter, and composed of magnetic iron oxide and dextran, but with varying iron oxide grain sizes and magnetic and crystalline structures. The 15% variation in the saturation magnetization, as determined by magnetometry, is insufficient to explain the observed variation of AMF-amplitude scattering (PASANS) to examine the internal magnetic domain structure of the nanoparticles. PASANS demonstrates that when multiple domains are present within a single nanoparticle, these domains must first be aligned with a large magnetic field before significant SLP can be generated. In addition, the stronger the coupling between the domains, the larger the field required before the SLP plateaus. Despite the similarities of size and composition amoung the nanoparticle systems, the dramatic differences in their heat generation originate primarily from the contrasting internal structures of these magnetic nanoparticles.
Advanced Functional Materials


hyperthermia, magnetic nanoparticles, polarization analyzer small angle neutron scattering, magnetic domains


Dennis, C. , Krycka, K. , Borchers, J. , Desautels, R. , van Lierop, J. , Huls, N. , Jackson, A. , Gruettner, C. and Ivkov, R. (2015), Internal Magnetic Structure of Nanoparticles Dominates Time-Dependent Relaxation Processes in a Magnetic Field, Advanced Functional Materials, [online], (Accessed May 24, 2024)


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Created June 1, 2015, Updated October 12, 2021