Modelling of Packed Co Nanorods for Hard Magnetic Applications

Abstract

Abstract. We present a numerical algorithm based on the bullet physics library to generate densely packed (39% -41%) structures of high-aspect-ratio nanorods for finite element micromagnetic simulations. The coercivities μ 0 H c of the corresponding Cobalt nanorod structures vary between 0.50T and 0.67T, depending on the overall orientation of nanorods, which is in good agreement with experimental results. The simulations make it possible to estimate the coercivity loss due to incoherent reversal processes (27%) as well as the gain due to shape anisotropy (59%). Our calculations show permanent magnets consisting of packed Co nanorods with an energy density product (BH) max of 83kJ/m³. We estimate that this value can be increased to 103kJ/m³ by increasing the packing density from 40% to 45%. Another way to optimize (BH) max is the usage of novel materials. By varying the anisotropy constant K 1 and the saturation polarisation J S we found lower limits for these parameters to reach a certain energy density product. To increase (BH) max to 160 kJ/m³, K 1 and J S have to be in the order of 450kJ/m³ and 2.25T, respectively. The thermal stability of this approach was verified by elastic band calculations. Cobalt nanorods with a diameter of 10nm and a height of 50nm are thermally stable at room temperature, but problematic at 900K. Doubling the nanorods' height to 100nm increases that limit considerably.

Magnetic hardening of Fe30Co70nanowires

“…Calculations showed that for these two different diameters the magnetic reversal mode usually changes from vortex (for d=40 nm) to transverse DWs (for d=20 nm) [20,21]. The DW nucleation and reversal types have the most significant influence on the coercive field of NWs [20,21,34,35]. In our samples the difference in H c for NWs with two diameters is small (about 10%, cf.…”

“…This large difference in the simulations is a consequence of the different mechanisms of magnetic reversal. Calculations showed that for these two different diameters the magnetic reversal mode usually changes from vortex (for d=40 nm) to transverse DWs (for d=20 nm) [20,21]. The DW nucleation and reversal types have the most significant influence on the coercive field of NWs [20,21,34,35].…”

“…Micromagnetic simulations revealed that this softening in magnetic NWs derives from the formation of a vortex and the nucleation of domain walls (DWs) at the tips of the NWs. Subsequent depinning and propagation of DWs along the length of the NW reduces H c [19][20][21][22]. The nucleation starts at the tips where the local demagnetizing field, which is almost parallel to the external field, has the largest value [19].…”