Time-resolved magnetic force microscopy of all-optical magnetization switching

Understanding spin dynamics at ever shorter time and smaller length scales is one of the major challenges in fundamental and applied magnetism, leading in particular to the discovery of giant magnetoresistance and All-Optical Switching (AOS) of magnetization and motivating the development of large-scale facilities such as x-ray free-electron lasers. Here, we propose a conceptually distinct approach to explore ultrafast laser-induced spin dynamics at the nanoscale by combining tabletop Magnetic Force Microscopy (MFM) with femtosecond laser excitation. By writing magnetic domains not with a single but with a pair of mutually delayed ultrashort laser pulses, we observe the spin dynamics by analyzing the final static nanotextured domain pattern as a function of pump-to-pump delay. We show that using MFM, we are able to deduce not only the average reversed magnetization but also the switched areas of nanoscale domains with picosecond temporal resolution. The capabilities of the technique are further demonstrated by applying it to study ultrafast helicity-dependent AOS of magnetization in ferromagnetic Pt/Co/Pt, where magnetization can be reversed with a pair of 100-fs and 3-ps laser pulses. Tracking the laser-induced area of the nanotextured domains as a function of the time separation between the pump pulses reveals critical slowing down of the spin dynamics near the Curie temperature of Co, thereby increasing the efficiency of dual-pulse AOS.