Itinerant spin and valley ferromagnetism in rhombohedral trilayer graphene (Part II)

Under an applied electrical displacement field, rhombohedral (ABC-stacked) trilayer graphene is a semiconductor with an unusually flat band minimum. Here we show that the resulting high density of states near the band edges leads to an instability towards ferromagnetic states in which one or more of the spin and valley symmetries are broken. In this second of two presentations, we will focus on the phenomenology of the observed magnetism within the combined spin- and valley isospin. Using low magnet field quantum oscillations and tilted field magnetocapacitance, we classify the competing ferromagnetic states by their spin- and valley symmetry breaking. Remarkably, measurements of similarly fabricated devices in which the trilayer graphene is aligned to a hexagonal boron nitride substrate reveal a pattern of ferromagnetic phase transitions that is only weakly perturbed relative to the superlattice free case. Our measurements show that ABC trilayer graphene hosts rare high mobility, two dimensional “half” and “quarter” metal states, and provides insight into the origin of magnetism in moire systems.