Engineering Magnetic Phases in Two-Dimensional Non-van der Waals Transition-Metal Oxides.

The family of 2D magnetic materials is continuously expanding because of the rapid discovery of exfoliable van der Waals magnetic systems. Recently, the synthesis of non-van der Waals magnetic "hematene" from common iron ore has opened an unconventional route to 2D material discovery. These non-van der Waals 2D systems are chemically stable and easily available and may have different or enhanced properties compared to their van der Waals counterparts. In this work, we have investigated and explained the nature of magnetic ordering in non-van der Waals 2D metal oxides. Two-dimensional hematene is found to be fully oxygen-passivated and stable under ambient conditions. It exhibits a striped ferrimagnetic ground state with a small net magnetic moment. Superexchange interactions are predicted to control the magnetic ground state of hematene, where pressure-induced spin crossover results in an observable net magnetic moment. Modulating the superexchange by alloying hematenes alters the magnetic ordering, tuning the system to a ferromagnetic ground state. Extending this strategy to the design of a new 2D material, we propose 2D chromia (α-Cr2O3) or "chromene", which, because of larger inter-transition metal distances and suppressed AFM superexchange, has a ferromagnetic ground state. We also show that tuning the magnetic ordering in these materials controls the transport properties by modulating the band gap, which may be of use in spintronic or catalytic applications.