Fluctuating magnetism and Pomeranchuk effect in multilayer graphene

Magnetism typically arises from the effect of exchange interactions on highly localized fermionic wavefunctions in f- and d-atomic orbitals. By contrast, in rhombohedral multilayer graphene (RMG), magnetism—manifesting as spontaneous polarization into one or more spin and valley flavours^{1, 2, 3, 4, 5, 6–7}—originates from itinerant electrons near a Van Hove singularity. Here we show experimentally that the electronic entropy in this system indicates signatures typically associated with disordered local magnetic moments, unexpected for electrons in a fully itinerant metal. Specifically, we find a contribution ΔS ≈ 1 k_B per charge carrier that begins at the Curie temperature and survives more than one order of magnitude in temperature. First-order phase transitions show an isospin ‘Pomeranchuk effect’ in which the fluctuating moment phase is entropically favoured over the nearby symmetric Fermi liquid^8,9. Our results imply that, despite the itinerant nature of the electron wavefunctions, the spin and valley polarization of individual electrons is decoupled, a phenomenon typically associated with localized moments, as happens, for example, in solid ³He (ref. ¹⁰). Transport measurements, surprisingly, show a finite-temperature resistance minimum in the fluctuating moment regime, which we attribute to the interplay of fluctuating magnetic moments and electron–phonon scattering. Our results highlight the universality of soft isospin modes to two-dimensional flat-band systems.