  In condensed matter systems, higher temperatures typically disfavors ordered
phases leading to an upper critical temperature for magnetism,
superconductivity, and other phenomena. A notable exception is the Pomeranchuk
effect in 3He, in which the liquid ground state freezes upon increasing the
temperature due to the large entropy of the paramagnetic solid phase. Here we
show that a similar mechanism describes the finite temperature dynamics of spin
and valley-isospins in magic-angle twisted bilayer graphene. Most strikingly a
resistivity peak appears at high temperatures near superlattice filling factor
nu = -1, despite no signs of a commensurate correlated phase appearing in the
low-temperature limit. Tilted field magnetotransport and thermodynamic
measurements of the inplane magnetic moment show that the resistivity peak is
adiabatically connected to a finite-field magnetic phase transition at which
the system develops finite isospin polarization. These data are suggestive of a
Pomeranchuk-type mechanism, in which the entropy of disordered isospin moments
in the ferromagnetic phase stabilizes it relative to an isospin unpolarized
Fermi liquid phase at elevated temperatures. Measurements of the entropy, S/kB
indeed find it to be of order unity per unit cell area, with a measurable
fraction that is suppressed by an in-plane magnetic field consistent with a
contribution from disordered physical spins. In contrast to 3He, however, no
discontinuities are observed in the thermodynamic quantities across this
transition. Our findings imply a small isospin stiffness, with implications for
the nature of finite temperature transport as well as the mechanisms underlying
isospin ordering and superconductivity in twisted bilayer graphene and related
systems.
