Maskless etching of single crystalline Mn-Zn ferrite in an H3PO4 aqueous solution by Ar+ laser irradiation has been investigated to obtain high etching rates and aspect-ratios of etched grooves. The etching processes have been found to be photochemical in a low laser power region and thermochemical in a high laser power region. High etching rates of up to 340 pm/s and an aspect-ratio of 50 for slab structures have been achieved. Periodic ripple structures have been observed under specific etching conditions due to the local surface melting.