Magnetic force microscopy study of electron-beam-patterned soft permalloy particles: Technique and magnetization behavior

Abstract

Electron-beam-patterned submicron permalloy elements with different aspect ratios were studied by magnetic force microscopy (MFM). The MFM tip stray field can be used to control a particle's magnetic state. By suitably choosing the operating mode and tip coatings, the tip induced distortion of the magnetic structure of soft permalloy elements can be largely reduced. The particle switching field can be precisely obtained by operating MFM at remanence. Through studying the remanent magnetization behavior, it was revealed that for large aspect ratio elements $(>4:1)$ magnetization reversal occurs directly from one single domain state to the reversed single domain state, while for medium aspect ratio elements $(<~4:1)$ the magnetization reversal occurs in a two-step process with two characteristic switching fields. Initially single domain particles switch into a low moment state (vortex state) at the first field ${H}_{\mathrm{s}},$ while at the higher field ${H}_{\mathrm{a}},$ the magnetic moments form a reversed single domain state. The forming of the low moment state is due to the fact that the vortex states can be trapped in the elements during magnetization reversal. This is consistent with micromagnetic simulations and can be directly demonstrated by controlled local MFM tip induced switching. The variations in the measured distribution of the switching fields for both large and small aspect ratio particle arrays are attributed to different reversal mechanisms as well as individual difference in size, thickness, and edge roughness.