Huge magnetically coupled orbital moments of Co porphyrin molecules and their control by CO adsorption

Abstract

Using x-ray magnetic circular dichroism, we show that Co porphyrin molecules adsorbed on graphenecovered Ni surfaces possess a huge in-plane (IP) orbital magnetic moment of the Co ions, despite their fourfold coordination, even comparable in size to the spin moment. Both are antiferromagnetically coupled to the Ni spins, leading to IP magnetization at zero external magnetic field. Adsorption of carbon monoxide on top of the molecules reduces the Co orbital moment by (77 ± 6)%. This is attributed to the change of the crystal field by the chemical stimulus which determines, together with the spin-orbit coupling (SOC), the orbital moment. Thermal desorption of the CO fully restores the initial electronic and magnetic properties.