Sample thickness dependence of structural and magnetic properties in $α$-RuCl$_3$

TL;DR

This study investigates how exfoliation-induced thickness changes affect the structural and magnetic transitions in α-RuCl$_3$, a candidate Kitaev QSL material. Using non-destructive cleaving, the authors track magnetic susceptibility across 3 crystals as they are thinned to ~30 µm, linking structural hysteresis width to sample quality and identifying emergent 10–12 K magnetic features associated with surface-induced persistence of the high-temperature $C2/m$ stacking. A simple Cl-layer electrostatics model provides a mechanistic explanation for the $C2/m$ vs $R\bar{3}$ competition and its sensitivity to lattice constants and strain, suggesting a critical thickness around 10–30 µm where the low-temperature structure fully transitions. The work delivers a practical exfoliation protocol and reveals thickness- and damage-dependent pathways to access or suppress specific magnetic orders, informing efforts to realize Kitaev physics in thin α-RuCl$_3$ samples.

Abstract

The layered transition metal trihalide $α$-RuCl$_3$ has been studied extensively in recent years as a promising candidate for a proximate Kitaev quantum spin liquid state. In high quality samples, a complete structural transition from room-temperature C2/m to low-temperature R$\bar{3}$ is consistently observed, with a single magnetic transition to antiferromagnetic ordering at $\sim$7K. However, magnetic and physical properties have been shown to depend heavily on both sample size and sample quality, with small and damaged samples exhibiting incomplete structural transitions and multiple magnetic anomalies. Although large high quality samples have been well studied, an understanding of the features attributed to low quality or small sample size is limited. Here, we probe the structural and magnetic transitions of $α$-RuCl$_3$ single crystal samples via magnetic susceptibility through a range of thickness, manipulated through careful mechanical exfoliation. We present a non-destructive protocol for exfoliating crystals and show success to 30 $μ$m, where sample quality is observed to improve with successive cleaving. Higher temperature magnetic features at 10 K/12 K are found to emerge through cleaving, both with and without induced sample damage. In both cases, we link these additional magnetic features to a persistence of C2/m structure to the low-temperature regime.

Figures (4)

• Figure 1: Crystal structures of $\alpha$-RuCl$_3$ including (c) high-temperature monoclinic (C2/m) structure and (d) low-temperature hexagonal (R$\bar{3}$) structure. The black arrows in (a) and (b) indicate the stacking sequence of neighbouring layers with a-directional honeycomb stacking [-1/3,0,1] in the C2/m phase and b-directional stacking [0,1/3,1] in the R$\bar{3}$ phase.
• Figure 2: Schematic of the cleaving process between magnetic susceptibility measurements generated by Gemini (2.5 Flash). (b) $\alpha$-RuCl$_3$ samples (black) are mounted on a plastic straw (grey) using a GE varnish (yellow). (c) Samples are mechanically exfoliated using scotch tape, discarding the removed portion and continuing with what remains of the bulk. (a) After cleaving, samples are secured vertically, and thickness is estimated using a calibrated optical microscope.
• Figure 3: (a) Normalized magnetic susceptibility as a function of temperature for $\alpha$-RuCl$_3$ (a) S1, (e) S2, and (i) S3 through a series of mechanical exfoliations. Trials reported with the same thickness were cleaved, but the measurement change was below the uncertainty. Magnetic transition close-up shown for (b) S1, (f) S2, and (j) S3, marked with grey dashed lines at observed magnetic transitions (T = 6.75 K, 9.5 K, and 12 K). First temperature derivative of normalized magnetic susceptibility of (c) S1, (g) S2, and (k) S3 in the low-temperature range is plotted to show magnetic features more clearly, with dashed grey lines again marking observed transitions. The difference between field heating and cooling magnetic susceptibility data is shown for (d) S1, (h) S2, and (l) S3 over the temperature range capturing the structural transition. Microscope views of sample faces are shown in figure insets, with corresponding thickness labeled. Insets in (g) are shown from top to bottom in order of cleaving, corresponding to the yellow, orange, and purple data sets, respectively. See text for further details.
• Figure 4: (a) Pseudocolor contour plot of Lennard-Jones electrostatic potential 3Å above the triangular Cl layer, with yellow 'x's marking Cl$^-$ positions and the underlying lattice shown with dashed black lines. Possible positions of Cl ions in the next layer are marked as positions A/B/C, with the A/C positions occupied in the C2/m structure and the B positions occupied in the R$\bar{3}$ structure. Solid red line traces the line cut shown in (b), where A/B/C positions are marked with red dashed lines. Triangular Cl lattice with labeled bond lengths is shown in (c), made more transparent in (d)/(e) where the overlaid Cl lattice corresponds to C2/m structure (d) or R$\bar{3}$ structure (e).