Fluctuating magnetism and Pomeranchuk effect in multilayer graphene.

Abstract

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^1-7-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.