Space-Charge Modulation of Antiferromagnetic-Ferromagnetic Core-Shell Magnetism in Ruthenium Nanoparticles.

Here we report antiferromagnetic-ferromagnetic core-shell magnetism in electrochemically synthesized elemental ruthenium nanoparticles. The core exhibits strong antiferromagnetic interactions, while uncompensated surface spins drive emergent ferromagnetism. This is evidenced by a large negative Weiss constant (-72.5 K), nonsaturating magnetization at high fields, and temperature-dependent coercivity reaching 430 Oe at 2 K, a value 3 times larger than that reported for ferromagnetic ruthenium films. By employing space-charge engineering to modulate the surface electron density, we achieve reversible magnetization changes up to 46% at room temperature. Furthermore, significant voltage-dependent coercivity changes at low temperatures provide strong support for the direct modulation of surface ferromagnetism and hint at a tight coupling to the antiferromagnetic core. These results uncover distinctive magnetism in ruthenium nanoparticles and establish space-charge engineering as a general route to voltage-controlled magnetism in metallic nanomaterials, advancing antiferromagnetic spintronics.