  We present a new approach to femtosecond direct laser writing lithography to
pattern nanocavities in ferromagnetic thin films. To demonstrate the concept we
irradiated 300~nm thin nickel films by single intense femtosecond laser pulses
through the glass substrate and created complex surface landscapes at the
nickel-air interface. Using a fluence above the ablation threshold the process
is destructive and irradiation leads to the formation of 200~nm thin flakes of
nickel around the ablation crater as seen by electron microscopy. By
progressively lowering the peak laser fluence, slightly below the ablation
threshold the formation of closed spallation cavities is demonstrated by
interferometric microscopy. Systematic studies by electron and optical
interferometric microscopies enabled us to gain an understanding of the
thermo-mechanical mechanism leading to spallation at the solid-molten
interface, a conclusion supported by molecular dynamics simulations. We
achieved a control of the spallation process that enabled the fabrication of
closed spallation nanocavities and their periodic arrangements. Due to their
topology closed magnetic nanocavities can support unique couplings of multiple
excitations (magnetic, optical, acoustic, spintronic). Thereby, they offer a
unique physics playground, before unavailable, for magnetism, magneto-photonic
and magneto-acoustic applications.
