Emergent Magnetic Anisotropy in the '115' Cerium-based Heavy Fermions | Shull Wollan Center
ABSTRACT: Metals containing cerium exhibit a diverse range of fascinating phenomena including heavy fermion behavior, quantum criticality, and novel states of matter such as unconventional superconductivity. The antiferromagnetic system CeIn3, which becomes superconducting under pressure, is a prototypical case of such physics. We have measured the magnetic H-T phase diagram of CeIn3, and show that above 40 T a surprising anisotropy develops. By theoretical modeling we demonstrate that the origin of this anisotropy is a consequence of the changing orbital character of the 4f wavefunctions in an applied field, which results in the effective exchange constants and their anisotropy becoming field dependent. In a related work, by measuring the spin waves of CeRhIn5 in an applied magnetic field we directly show that the exchange interaction develops a uniaxial anisotropy in an applied magnetic field as predicted. Interestingly, because of the magnetic frustration present in CeRhIn5 this results in ANNNI physics emerging in a metallic system. Since crystal field splitting in 4f materials is typically of order 10 meV we expect that field dependent exchange interactions are a general phenomenon. What remains to be seen is how these anisotropic magnetic interactions are intertwined with the conduction electrons. In fields of 30 T, still within the antiferromagnetic state of CeRhIn5 a density wave emerges with electronic nematic character, with no noticeable change of the magnetic structure. We will discuss these results and the puzzles that lay ahead for the 115 family of heavy fermion superconductors.