Isospin Pomeranchuk effect and finite temperature resistance minimum in rhombohedral graphene

Magnetism typically arises from the effect of exchange interactions on highly localized fermionic wave functions in f- and d-atomic orbitals. In rhombohedral graphene multilayers, in contrast, magnetism---manifesting as spontaneous polarization into one or more spin and valley flavors originates from fully itinerant electrons near a Van Hove singularity. Here, we study the electronic compressibility, proximal exciton sensing, and electronic transport measurements of Bernal Bilayer Graphene (BBG) and Rhombohedral Trilayer Graphene (RTG) devices as a prototypical example for itinerant magnetism. While magnetic ordering - observed most clearly via the onset of the anomalous Hall effect in valley polarized phases—occurs only below 2-3K, we find a fluctuation regime persisting to temperature more than one order of magnitude higher with an associated large excess entropy of ~0.8k_B/charge carrier. As a consequence, increasing temperature can favor the formation of a fluctuating ordered phase, analogous to the Pomeranchuk effect in 3He. In temperature dependent transport measurements, we observe a decreasing resistance with increasing temperature throughout the fluctuation regime, a highly unusual behavior for metals. The negative temperature coefficients lead to a minimum of R(T) within the fluctuation phases of around 10-20K. We connect the negative dR/dT to the fluctuation regime and discuss possible mechanisms.