Magnetic fields in monoclinic $\alpha$-RuCl$_3$ reveal rhombohedral inclusions underlying apparent oscillations

Abstract

The majority of research on $\alpha$-RuCl$_3$ has focused on applying in-plane magnetic fields to suppress antiferromagnetic order and induce a quantum spin liquid (QSL). However, this effort has been complicated by the materials temperature-dependent crystal structure and sensitivity to strain-induced stacking disorder, making interpretation of field-induced phenomena contentious. The crystal structure of $\alpha$-RuCl$_3$ has recently been clarified as a function of temperature and sample size, motivating a reassessment of its magnetic properties and connection to proposed spin-liquid signatures. Here, we show that the monoclinic structure can be isolated in nanogram-scale crystals, enabling the study of Kitaev physics in a new regime. We focus on a structurally well-defined monoclinic crystal at low temperature and perform high-resolution magnetotropic susceptibility measurements in several crystal planes. Mapping the AFM phase boundary versus temperature, field, and orientation, we find the monoclinic phase diagram closely resembles rhombohedral crystals but is systematically shifted to higher transition temperatures and critical fields. For $B \parallel a$, we observe a two-step suppression of AFM order, indicating an intermediate ordered phase analogous to the ZZ2 phase reported in rhombohedral samples. Our results show that transitions previously observed beyond the AFM regime under in-plane fields arise from multiple shifted AFM phase boundaries associated with monoclinic inclusions, rather than non-magnetic phases. These findings indicate that features attributed to a QSL are instead due to an incomplete transition from the high-temperature monoclinic to the low-temperature rhombohedral structure. They also highlight the role of structural symmetry and sample homogeneity in interpreting field-induced phenomena in $\alpha$-RuCl$_3$ and related two-dimensional quantum magnets.