All-optical magnetic switching promises ultrafast magnetization
control without a magnetic field. Existing schemes typically require
power-hungry femtosecond-pulsed lasers and complex magnetic
materials. Here, we demonstrate deterministic, all-optical magnetic
switching in simple ferromagnetic nanomagnets (Ni81Fe19, Ni50Fe50)
with sub-diffraction limit dimensions using a focused low-power,
linearly polarized continuous-wave laser. Isolated nanomagnets
are switched across a range of dimensions, laser wavelengths, and
powers. All square-geometry artificial spin ice vertex configurations
are written at low powers (2.74 mW). Usually, switching with linearly
polarized light is symmetry forbidden; here, the laser spot has a
similar size to the nanomagnets, producing an absorption distribution that depends on the nanoisland-spot displacement. We attribute the deterministic switching to the transient dynamics of this
asymmetric absorption. No switching is observed in Co or Ni nanostructures, suggesting the multi-species nature of NiFe plays a role.
These results usher in inexpensive, low-power, optically controlled
devices with impact across data storage, neuromorphic computation, and reconfigurable magnonics.