Finite-Element Simulations and Experimental Study of Single Domain Behavior in Fe₆₅Co₃₅ Nanocubes: Implications for Bioimaging and MEMS Actuators

Developments in the design and fabrication of the high saturation magnetization magnetic nanoparticles in the single-domain size regime are highly desirable due to their significant potential for various near-future applications. In this work, we present an in-depth investigation of iron cobalt (Fe65Co35) nanocubes with different sizes covering magnetization configurations from flower to vortex to multidomain states. We synthesized Fe65Co35 nanocubes with tunable edge lengths by adjusting reaction parameters in a liquid-phase reduction reaction. Scanning electron microscope (SEM) images confirm a narrow size distribution. Measured hysteresis loops indicate the size dependence of coercivity and remanence, showing a peak at a size of 17 nm. To explain this phenomenon, a finite element micromagnetic model was used to predict the magnetic properties of Fe65Co35 nanocubes, ranging in size from 10 to 100 nm. Simulation results reveal that the flower-to-vortex transition occurs at around 20 nm, coinciding with the highest coercivity. Larger nanoparticle sizes exhibit a decrease in remanence, with the most significant drop observed between 40 and 60 nm. By combining chemical synthesis and micromagnetic calculations, we have experimentally elucidated the size-dependent spin configurations in Fe65Co35 nanocubes. These findings offer insights for the accurate design and control of spin texture, magnetic remanence, and coercivity in high magnetization nanoparticles. Such advancements can greatly enhance contrast agents for bioimaging, actuators for microelectromechanical (MEMS) systems, and magnetic composites for various applications.