The Ferromagnetic Pair
# The Ferromagnetic Pair
Superconductivity and ferromagnetism are conventionally antagonistic. Cooper pairs in standard superconductors form between electrons with opposite spins — singlet pairing. A ferromagnetic exchange field aligns spins in the same direction, breaking the singlet and destroying superconductivity. Materials are usually either superconducting or ferromagnetic, not both.
Zhang and colleagues (arXiv:2603.25807, March 2026) directly image the Meissner effect in rhombohedral graphene and find superconductivity emerging during a continuous transition to a canted spin ferromagnetic state. The material is simultaneously ferromagnetic and superconducting. The superconductivity does not fight the ferromagnetism — it arises from it.
The imaging reveals that the superconductivity screens only approximately 100 parts per million of the applied magnetic field — extraordinarily weak diamagnetism, far below what conventional superconductors produce. The superfluid stiffness — the energy cost of a phase twist in the superconducting order parameter — is linearly proportional to Tc, not following the BCS expectation where stiffness scales with the gap squared divided by the Fermi energy. The linear scaling indicates that the pairing mechanism is intrinsically different from the phonon-mediated pairing of conventional superconductors.
The zero-temperature superfluid stiffness being proportional to Tc (rather than much larger) means the superconductor is in an extreme strong-coupling or low-density regime where all the available spectral weight is used for pairing. There is no "overhead" — the superconducting condensate is as weak as the critical temperature allows, and strengthening one strengthens the other in lockstep.
The structural observation: a superconductor born from a ferromagnetic state violates the conventional antagonism between the two orders because the pairing symmetry accommodates the magnetism rather than competing with it. The canted spin state provides a compromise — the spins are partially aligned (ferromagnetic) but canted enough to permit triplet pairing (superconducting). The weakness of the Meissner screening and the linear stiffness-Tc scaling are signatures of a pairing mechanism that is intimately tied to the magnetic order rather than independent of it.
