Nanostructured Hard Magnets: A Micromagnetic Study

Hard magnetic nanostructures have several applications such as magnetic recording media, sensors, and microelectromechanical systems applications containing permanent magnets. In this paper, we performed micromagnetic simulations on Co nanorods on a regular grid and in an irregular packing. Both types of models describe structures which can be produced experimentally: regular grids of nanorods can be created by electro deposition in an alumina template and irregularly packed structures can be obtained by compacting nanorods synthesized in a polyol process. In addition, we also studied the increase of coercivity of capped Fe nanorods with an antiferromagnetic layer. The packed structures have been generated by a packing algorithm on the basis of gravity and collision forces and have a packing density of up to 58%. The simulations on the regular hexagonal grid of nanorods show that energy density products (BH)_max of up to 200 kJ/m³ can be reached with this packing density, but the calculations on irregularly packed structures exhibit only energy density products of up to 85 kJ/m³. The discrepancy can be explained by the misalignment of nanorods which reduces both coercive field and remanence of the structure. The antiferromagnetic caps on Fe nanorods suppress the formation of nucleation centers at the tips and increase the coercivity of the nanorod by 11% in the case of asymmetric hysteresis loops with hard caps (K_1,cap = 4.6 MJ/m³) and 25% in the case of symmetric hysteresis with softer caps (K_1,cap = 460 kJ/m³).