Films are originally synthesized to replace bulk monocrystals out of economy and effectiveness concern. Derived from film fabrication, an emerging technology known as interface control creates completely new structures by selecting every atomic or molecular layer. Previous research focuses mainly on molecular multilayer, while the mechanism of monolayers remains unclear.

In a study published in Science Advances, a group led by Prof. LU Yalin from University of Science and Technology of China (USTC) of the Chinese Academy of Sciences, Assoc. Prof. ZHAI Xiaofang from USTC (currently working for Shanghai Tech University), Assoc. Prof. CHEN Hanghui from New York University Shanghai, and the collaborators, discovered new magnetic anisotropy (MA) in a SrRuO₃ monolayer and the interplay between electron correlation and MA. 

The researchers fabricated SrRuO₃/(SrTiO₃)_N (N=1 to 5) superlattices by pulsed laser deposition (PLD) to explore magnetism of a molecular monolayer. The SrRuO₃ superlattice is the ideal model by virtue of its strong spin-orbit coupling (SOC), stable long-range magnetic ordering and relatively high Curie temperature. 

Serial experiments were then conducted to analyse typical properties of SrRuO₃/(SrTiO₃)N superlattices. 

Structure analyses indicated that the in-plane and out-of-plane spin of Oxygen octahedron. Magnetic tests including magneto-optic Kerr effect and polarized neutron reflectometry proved that ferromagnetism only exists in a SrRuO₃ monolayer. 

Magnetic field angle-dependent resistance measurements revealed that when N>3, magnetic easy axis of Ru spins is transformed from uniaxial〈001〉direction to eightfold〈111〉directions. This eightfold anisotropy enables unprecedented 71° and 109° spin switching in SrRuO₃ monolayers. 

First-principle calculations revealed that increasing the SrTiO₃ layer thickness (N from 1 to 5) weakens the out-of-plane coupling and induces an emergent correlation-driven orbital ordering. For N>3, the orbital reorientation followed by the opening of energy gap leads to a metal-to-semiconductor transition, which influences spin-orbit interactions within a SrRuO₃ monolayer. These two phenomena combined ultimately change MA. 

This study sheds new lights on the electron correlation system, the forefront of quantum materials. The asymmetry of space inversion and translation in correlated oxide interfaces enables malleability which is not exhibited by their bulk counterpart. As a result, the electron correlation system promises great potential in novel quantum materials. It is not only a crucial platform of fundamental science but only the basis of embryonic electron correlation physics. 

Besides, this study has profound implications on quantum effects. As Jochen Mannhart from Max Planck Institute puts it, exotic quantum effects come into being through the tight correlation of interfaces as well as adjustments of stress or electronic field when you stack quantum materials and put them in the quantum well. 

At past, the most common research object is molecular multilayer where spin-orbit exchange and coupling exist between layers. When it comes to a monolayer, however, will new quantum states generate from strongly correlated electrons in the magnetic interface without above-mentioned coupling between layers? Given that MA is the prerequisite of long-range magnetic ordering stability in 2D materials, does a magnetic oxide monolayer show different MA from normal 2D materials? Answering these two questions, the research pushes back the frontier of quantum effects in 2D materials.