Investigating the Super Paramagnetic Limits of Ferromagnetic Nanowires

Highly sensitive focused magneto optical Kerr effect magnetometer (MOKE) has been used to explore the nanoscale limit of state of the art electron beam lithography (EBL) patterned permalloy (NiFe) nanowires. The nanowires are 20 μm long arranged in 20 μm × 20 μm arrays. Permalloy of different thicknesses, which includes 10, 8, 6, 4 and 3.5 nm were deposited on 5 different samples containing these arrays. Results from 10 nm thick permalloy nanowires shows superparamagnetic transition at 85 ± 5 nm wire width. For 6 nm thick array, superparamagnetic transition occurs at 125 ± 25 nm. We therefore show that with 10 nm thickness, instead of 6 nm thickness, more miniaturisation of nanowire width can be achieved before the onset of superparamagnetism. This may be seen as very efficient in achieving increased storage density in memory devices