Phase diagram of magnetic nanodisks measured by scanning electron microscopy with polarization analysis

We use scanning electron microscopy with polarization analysis (SEMPA) to image the magnetic domain structures of individual ferromagnetic nanodisks with different diameters and thicknesses, and thereby determine the phase diagram of the magnetic ground states in these technologically important magnetic structures. Depending on the nanodisk dimensions, we observe magnetic structures based on one of three configurations: a single-domain in-plane, a single-domain out-of-plane, or a vortex state. By imaging the in-plane and out-of-plane magnetization components of identically prepared Ni₈₀ Fe₁₅ Mo₅ nanodisks with diameters that range from 35 to 190 nm and with thicknesses that range from 10 to 65 nm, we are able to locate phase boundaries between the three different phases and the triple point. The phase boundaries are not sharply defined, however. Near the boundaries and especially near the triple point, we observe disks in a mixture of the different metastable ground phases, and we observe variations of the basic states, such as a tilted vortex configuration. A magnetic phase diagram generated by a micromagnetic simulation is found to be in good qualitative agreement with the phase diagram determined by the SEMPA measurements. The ability to determine the magnetic phases in sub-100 nm nanodisks enables tailoring material properties and geometry of nanodisks for various potential applications.