  The current challenge to realizing continuously tunable magnetism lies in our
inability to systematically change properties such as valence, spin, and
orbital degrees of freedom as well as crystallographic geometry. Here, we
demonstrate that ferromagnetism can be externally turned on with the
application of low-energy helium implantation and subsequently erased and
returned to the pristine state via annealing. This high level of continuous
control is made possible by targeting magnetic metastability in the ultra-high
conductivity, non-magnetic layered oxide PdCoO2 where local lattice distortions
generated by helium implantation induce emergence of a net moment on the
surrounding transition metal octahedral sites. These highly-localized moments
communicate through the itinerant metal states which triggers the onset of
percolated long-range ferromagnetism. The ability to continuously tune
competing interactions enables tailoring precise magnetic and magnetotransport
responses in an ultra-high conductivity film and will be critical to
applications across spintronics.
