Ferromagnetism in rhombohedral graphene, Part I

Rhombohedral graphene multilayers host ferromagnetism and superconductivity when the fermi level is tuned to saddle-point van Hove singularities near the band edge. Recently, new superconducting pockets were observed in rhombohedral bi-, tri-, and tetralayer samples with enhanced spin-orbit coupling induced by a transition metal dichalcogenide substrate, raising the question of the role of the spin degree of freedom in superconductivity. We use high resolution nanoSQUID-on-tip (nSOT) magnetometry to explore the nature of spin ferromagnetism in rhombohedral trilayer graphene supported by WSe2 for both strong and weak proximity induced spin orbit coupling. Focusing on the spin polarized half metal phase hosting enhanced superconductivity, we use the spatial pattern of the fringe magnetic fields as a signature of in- and out-of-plane spin polarization. We find the spin moments to be isotropic for weak spin orbit coupling; however, the out-of-plane spin polarization is strongly suppressed when Ising spin orbit coupling is strong. Instead, the half metal phase shows a remnant in-plane magnetic polarization, as predicted for a 'canted' ferromagnetic phase arising from the interplay of spin orbit coupling and the ferromagnetic intervalley Hund's coupling. We discuss the implications for the nature and origin of the superconducting state observed in this regime. (Part 1 or 2)