  Magnetic nanomaterials record information as fast as picoseconds in computer
memories but retain it for millions of years in ancient rocks. This exceedingly
broad range of times is covered by hopping over a potential energy barrier
through temperature, ultrafast optical excitation for demagnetization or
magnetization manipulation, mechanical stress, or microwaves. As switching
depends on nanoparticle size, shape, orientation, and material properties, only
single-nanoparticle studies can eliminate ensemble heterogeneity. Here, we push
the sensitivity of photothermal magnetic circular dichroism down to individual
20-nm magnetite nanoparticles. Single-particle magnetization curves display
superparamagnetic to ferromagnetic behaviors, depending on size, shape, and
orientation. Some nanoparticles undergo thermally activated switching on time
scales of milliseconds to minutes. Surprisingly, the switching barrier appears
to vary in time, leading to dynamical heterogeneity. Our observations will help
to identify and eventually control the nanoscale parameters influencing the
switching of magnetic nanoparticles, an important step for applications in many
fields.
