Probing Magnetic Ordering in Air Stable Iron‐Rich Van der Waals Minerals

Magnetic monolayers show great promise for future applications in nanoelectronics, data storage, and sensing. The research in magnetic two-dimensional (2D) materials focuses on synthetic iodides and tellurides, which suffer from a lack of ambient stability. So far, naturally occurring layered magnetic materials have been overlooked. These minerals offer a unique opportunity to explore complex air-stable layered systems with high concentration of magnetic ions. Magnetic ordering in iron-rich phyllosilicates is demonstrated, focusing on minnesotaite, annite, and biotite. These naturally occurring layered materials integrate local moment baring ions of iron via magnesium/aluminum substitution in their octahedral sites. Self-inherent capping by silicate/aluminate tetrahedral groups enables air stability of ultra-thin layers. Their structure and iron oxidation states are determined via Raman and X-ray spectroscopies. Superconducting quantum interference device magnetometry measurements are performed to examine the magnetic ordering. Paramagnetic or superparamagnetic characteristics at room temperature are observed. Below 40 K ferrimagnetic or antiferromagnetic ordering occurs. In-field magnetic force microscopy on exfoliated flakes confirms that the paramagnetic response at room temperature persists down to monolayers. Further, a correlation between the mixture of the oxidation states of iron and the critical ordering temperature is established, indicating a path to design materials with higher critical temperatures via oxidation state engineering.