<i>(Invited)</i> Giant Anisotropy of the Magnetoresistance in Few-Layer α-RuCl₃ Tunnel Junctions

The magnetic insulator α-RuCl 3 is proximate to a quantum spin liquid (QSL) [1] described by the Kitaev model [2]. A promising route to realizing a true Kitaev QSL in α-RuCl 3 is to reduce its dimensionality via mechanical exfoliation. In addition to enhancing magnetic fluctuations, exfoliating α-RuCl 3 opens the door to manipulating its magnetic state by coupling it to other two-dimensional materials. However, measuring the magnetic properties of such small samples represents a great technical challenge. Moreover, to harness the technological potential of this predicted Kitaev QSL phase and its non-Abelian anyonic excitations, an electrical probing technique is highly desirable but limited by the insulating nature of the material. Here, we present angle-dependent tunneling magnetoresistance (TMR) measurements on ultrathin α-RuCl 3 crystals with various layer numbers (Fig. 1) to probe their magnetic, electronic and crystal structure [3]. We observe a giant change in resistance – as large as ∼ 2500 % – when the magnetic field rotates either within or out of the α-RuCl 3 plane. This is a manifestation of the anisotropic spin interactions arising from the strong spin-orbit coupling in this material. Using TMR as a probe, we track the magnetic phase diagram of α-RuCl 3 as a function of temperature, applied magnetic field and its angle relative to the crystallographic axes. Our results show that few-layer α-RuCl 3 hosts a zigzag antiferromagnetic order with a Néel temperature of ~ 14 K, higher than the ~ 7 K measured in bulk samples with a rhombohedral stacking. We explain this surprising result by showing that exfoliated flakes maintain a monoclinic structure at low temperature, while bulk α-RuCl 3 is believed to undergo a monoclinic-to-rhombohedral phase transition. This conclusion is supported by our scanning transmission microscopy study of isolated flakes. Our study provides a deeper understanding of how the magnetic properties of α-RuCl 3 depend on its stacking order and layer number, which helps lay the groundwork for the van der Waals engineering of exotic magnetic phases such as QSLs. References [1] Takagi, H., Takayama, T., Jackeli, G., Khaliullin, G. & Nagler, S. E. Nat. Rev. Phys . 1 , 264–280 (2019). [2] Kitaev, A. Ann. Phys. 321, 2-111 (2006). [3] Massicotte, M. et al . ACS Nano , 18 , 25118–25127 (2024). Figure 1. Schematic representation of a graphite/α-RuCl 3 /graphite magnetic tunnel junction Figure 1