  Light-matter strong coupling in the vacuum limit has been shown to enhance
material properties over the past decade. Oxide nanoparticles are known to
exhibit weak ferromagnetism due to vacancies in the lattice. Here we report the
700-fold enhancement of the ferromagnetism of YBa$_2$Cu$_3$O$_{7-x}$
nanoparticles under cooperative strong coupling at room temperature. The
magnetic moment reaches 0.90 $\mu_{\rm B}$/mol, and with such a high value, it
competes with YBa$_2$Cu$_3$O$_{7-x}$ superconductivity at low temperature. This
strong ferromagnetism at room temperature suggest that strong coupling is a new
tool for the development of next generations of magnetic and spintronic
nanodevices.
