Two-dimensional Phyllosilicates as an Air-stable Platform for Layered Magnetic Materials

Since the discovery of graphene, intrinsic magnetic ordering in two-dimensional (2D) materials was not observed for over a decade. Several efforts such as proximity effect, defect engineering and doping were employed to induce magnetism in originally nonmagnetic 2D materials. The intrinsic 2D ferromagnetism was first observed in chromium-based compounds, and ever since many other van der Waals (vdW) materials with intrinsic magnetism has been reported. The significance of intrinsic magnetism in 2D materials rely on the fact that these layers can be integrated into vdW heterostructures, enabling functionalities in spintronics and data storage applications. To reach their full potential, 2D magnetic materials must have ambient stability and critical ordering temperature above the room temperature. While chromium-based compounds lack ambient stability and show intrinsic magnetic ordering at cryogenic temperatures, an alternate approach was employed to introduce magnetic dopants (Fe, V and Co) in diamagnetic vdW materials to form diluted magnetic layered systems. However, the efficiency of dilute magnetic system strongly relies on a precisely controlled dopant concentration. This work aims to propose a class of 2D magnetic materials which are ambient stable, can be thinned down, and integrated in the device applications. In contrast to the synthetic system, Phyllosilicates are naturally occurring layered materials, which integrate local magnetic baring ions via magnesium substitution in their functional octahedral sites. Despite of extensive research on magnetic properties of bulk phyllosilicates, their potential at 2D limit have been overlooked. Structural and magnetic characterization of iron-rich members of phyllosilicates (minnesotaite, annite and biotite) established that mixture of iron oxidation state (Fe2+ and Fe3+) strongly influence their critical ordering temperatures. The estimated near neighbour interaction obtained from total energy calculations suggest that presence of randomly distributed Fe3+ ions enhances the ferromagnetic interactions, when surrounded by Fe2+ ion. The correlation between mixture of Fe oxidation state and magnetic properties, suggest a strategy for design of materials with higher critical ordering temperature through oxidation sate tuning. This thesis also reports on the visualization of magnetic domains and layer dependent magnetic ordering in 2D flakes of annite. Using scanning superconducting quantum interference device (SQUID) microscopy - SSM, the stray magnetic fields above the step edges of single crystalline flakes were mapped. The SSM maps demonstrate that the layers of annite order intrinsically as A-type in-plane layer by layer antiferromagnet. Given the biocompatibility and biodegradability of phyllosilicates, the study of evolution of magnetic ordering and domain formation will open new pathways in in-vivo bio-sensing applications. In parallel, the work on this thesis aims to propose a scalable approach to induce magnetism in originally paramagnetic talc (Mg-rich member of phyllosilicates) using broad beam ion implantation. The technique effectively transformed the magnetic state, showing room temperature ferromagnetic ordering. The emergence of room temperature ferromagnetism in implanted talc crystal suggest potential applications in magnetic storage and sensing technologies. The results demonstrate that implantation at higher temperature is favourable, as it helps the structure to self-heal and enable the dopants to occupy energetically favourable positions in the lattice. Our findings contribute to the understanding of magnetic properties driven by defects and dopants in phyllosilicates and enable new pathways to fabricate ambiently stable 2D magnetic insulators with tuneable magnetic properties.