Synthetic Antiferromagnet Disk Particles for Hyperthermia Applications

Abstract

Magnetic hyperthermia is a technique for the destruction of cancerous cells by heat arising from magnetization losses from magnetic nanoparticles (MNPs) in an oscillatory magnetic field. For the typically employed chemically synthesized superparamagnetic nanoparticles the heat generation remains limited by the low magnetization of the oxidic materials, the wide particle size distribution, and the narrow shape of their magnetic hysteresis loop. To overcome these limits, synthetic antiferromagnet magnetic disk particles (SAF MDP) consisting of two ferromagnetic (F) layers separated by a non-magnetic layer were designed. The geometry and magnetic system parameters were optimized via micromagnetic modeling to obtain an antiferromagnetically coupled (zero moment) ground state and an abrupt switching into a ferromagnetically aligned state in an applied field H_AF->F to maximize the hysteretic loss. The magnetic multilayer was sputter-deposited onto a 50nm-thick Ge sacrificial layer on a silicon wafer. Self—assembled polysterene spheres served as an etch mask for the successive nanopatterning of disk-shaped islands. These were then detached from the supporting wafer by dissolution of the Ge layer and subsequently encapsulated in thin silica shells using tetraethyl orthosilicate (TEOS). The magnetic properties of the SAF MDP analyzed by vibrating sample and Kerr magnetometry, high-resolution in-field magnetic force microscopy closely matched the design goals and micromagnetic simulation results. A turn-on/turn-off magnetism of the SAF MDP and a hysteretic loss close to the theoretical limit given by the magnetic material with the highest saturation magnetization and a rectangular SAF hysteresis loop could be demonstrated. Experiments mapping the hysteretic loss of SAF MDP suspensions for different operation conditions demonstrated a superior performance compared to classical superparamagnetic nanoparticles.