Thickness Dependent Magnetic Ordering in Two-Dimensional Magnetic Insulator

Magnetic domain formation in two-dimensional magnetic insulators offers insight into the fundamentals of magnetism and serve as a catalyst for the advancement of spintronics device technology. One such example is magneto-resistive random-access memory (MRAM). In case of 2D magnetic insulators in MRAMs, ultra-thin antiferromagnetic barriers in combination with regular (non-magnetic) metals could lead to the development of future memory elements, computing in memory concepts, and self-reconfiguring circuits.^3,4 In order to propel these developments, its crucial to acquire understanding of the evolution of magnetic ordering at nanometer scale, as a function of number of atomic layers and applied magnetic field.<br/>From an implementation prospective, 2D van der Waal magnets present significant challenges related to stability, size, and critical temperature. One primary issue is the degradation of 2D magnets when exposed to ambient conditions. We propose to overcome this by exploiting naturally occurring iron-rich phyllosilicates. Phyllosilicates offer a unique opportunity to explore complex air-stable layered systems with high concentration of magnetic ions. These naturally occurring layered materials are inherently magnetic and are wide band gap insulators (i.e. 5-6 eV).<br/>In this study, we visualize few-layer annite (mica) (KFe₃²⁺AlSi₃O₁₀(OH)₂) by employing a hybrid approach involving a scanning superconducting quantum interference device (SQUID) and an atomic force microscopy (AFM) probe. Annite is a naturally occurring van der Waals (vdW) magnetic insulators which incorporate local moment baring ions of iron (Fe) via in their octahedral sites.^1,2 The simultaneous imaging capability of the SQUID-on-tip microscope, capturing both the surface features and its stray magnetic field, enabled us to establish correlations between layer thickness and magnetic ordering. External field dependent maps of annite’s stray magnetic field reveal coercivity of 70 mT for the 2D flakes, and capture the domain formation and flipping in the external field.<br/>We demonstrate correlations between the iron concentration, layer structure, iron oxidation states, and their magnetic response, indicating a path to design materials with higher critical temperatures via tuning the concenteration of magnetic species and oxidation state engineering.<br/><br/><b>References: </b><br/>1-Matković , et al. <i>npj 2D Materials and Applications</i> 5.1 (2021): 94.<br/>2-Khan, et al. <i>Advanced Physics Research</i> 2.12 (2023): 2300070.<br/>3-Mak, K. F., et.al., Nature Reviews Physics 1, 646, 2019.<br/>4-Chu, J., et.al.,Advanced Materials 33, 2004469, 2021.