Exploring the Single Domain Limit of High Magnetization Nanoparticles: A Combined Theoretical and Experimental Study

Abstract

Developments in the design and fabrication of the high saturation magnetization nanoparticles in the single-domain size regime are highly desirable due to their extensive applications in biomedicine, magnetic resonance imaging, microelectromechanical systems, radio frequency electronics, and soft robotics. In this work, we present for the first time an in-depth investigation of different-sized iron cobalt (Fe65Co35) nanoparticles covering magnetization configurations from flower to vortex to multi-domain states.A finite element micromagnetic model was used to predict magnetic properties of Fe65Co35 nanoparticles ranging in size from 10 to 100 nm. The simulation results reveal that the flower-to-vortex transition of the Fe65Co35 nanoparticles occurs at around 20 nm, corresponding to the particle size with the highest coercivity. A decrease in remanence is expected for larger particle sizes, where simulations showed the most dramatic drop for particles between 40 and 60 nm. Furthermore, the simulated behaviors were verified experimentally.By systematically varying the reaction parameters, Fe65Co35 nanocubes with tunable edge lengths were synthesized. Scanning electron microscope (SEM) images of as-synthesized samples indicate a narrow size distribution. The measured hysteresis loops confirmed the maximum coercive field for cubes of size 17 nm and the sudden decrease in remanence for samples within the range of 40–60 nm. By combining chemical synthesis and micromagnetic calculations, we have experimentally unveiled the size-dependent spin configurations in Fe65Co35 nanoparticles. These results open the door to a more accurate design and control of spin texture, remanence, and coercivity in high magnetization nanoparticles which are vital building blocks of new biomedical and electronic materials.  Fig. 1 SEM images of Fe65Co35 nanocubes with various edge lengths, (a) 17 nm, (b) 34 nm, (c) 40 nm, (d) 62 nm, (e) 77 nm, (f) 92 nm, and (g) 97 nm. The scale bars are 200 nm.  Fig. 2 (a) The simulated spin configuration for a cube of each respective size. (b) Simulated hysteresis loops of a 5×5×5 sphere array with different sizes. (c) Measured hysteresis loops of as-synthesized Fe65Co35 nanocubes at 300 K.