Emergence of a zero-bias peak on the MgO/Fe(001) surface induced by the adsorption of a spin-1/2 molecule.

Engineering electronically decoupled spin states is essential for achieving robust spin by suppressing inelastic spin-flip scattering induced by conduction electrons. Accordingly, the fabrication of spins on insulating ultrathin films such as MgO or NaCl deposited on metallic substrates has been intensively investigated over the past decades to mitigate electronic hybridization. However, these studies have predominantly focused on non-magnetic noble metal substrates. In this work, we experimentally demonstrate that ultrathin MgO films grown on a ferromagnetic Fe(001) substrate, commonly employed in tunnel magnetoresistance sensors, can serve as an advanced platform for realizing electronically isolated spin states. As a prototypical system, we utilize a copper (Cu) ion (S = 1/2) embedded within a copper-phthalocyanine (CuPc) molecule. An atomically flat and clean insulating surface is obtained by optimizing the epitaxial growth conditions of ∼1 nm-thick MgO films on an Fe(001) whisker substrate precoated with a p(1 × 1) oxygen layer. Scanning tunneling microscopy (STM) conducted at 4.6 K under ultrahigh vacuum conditions shows individual CuPc molecules adsorbed on the MgO surface. Simultaneous scanning tunneling spectroscopy (STS) reveals a well-defined molecular energy gap. Remarkably, a pronounced zero-bias peak (ZBP) emerges within this gap, signifying the presence of an electronically isolated spin on the MgO/Fe(001) heterostructure. Moreover, STS measurements reveal the lateral extension of the ZBP across the insulating film. These findings pave the way for engineering isolated molecular spin states on ferromagnetic substrates, offering new possibilities for manipulating spin states through substrate-mediated magnetic interactions.