Micromagnetic Study of Magnetization Reversal and Dipolar Interactions in NiFe Nano Disks

To achieve the high areal densities in storing the information in the hard disk magnetic material, one need to look for the small bit size with minimum separations possible without erasing the information by the dipolar fields of neighbor bits. Bit patterned media is one of the proposed magnetic media wherein each bit is stored in an isolated magnetic dot in contrast to conventional granular media. Micromagnetic simulations are very helpful in finding the optimum bit size and their separations for this purpose. We used Nmag in understanding the magnetization relaxation and reversal process in the NiFe nano disks. We considered 5nm thick disks with diameters ranging from 80 to 300nm for the reversal study. All the disks show single step hysteresis loops which indicates that the reversal is governed by coherent rotations where the entire disk is behaving like a giant spin. Dipolar interactions between the disks of diameters ranging from 50 to 80 nm, with 5, 10 and 15 nm thicknesses are considered with different initial magnetizations and distances. This study shows that the closest packing is limited by the distance between 90 degrees oriented disks, which generally occurs during the reversal of neighbor bits.