Collapse of a region of the magnetic phase diagram of elemental terbium under a strain-induced Lifshitz transition

AC susceptibility and neutron scattering measurements are used to study the magnetic field-temperature magnetic phase diagram of a Tb single crystal under uniaxial tension along its hexagonal $c$ axis. An external magnetic field is applied in the basal plane. We focus on the region in the phase diagram that corresponds to the helical antiferromagnetic phase and find that this region collapses when the uniaxial tension is increased beyond a critical value as low as 600 bar. There are strong reasons to associate this collapse with an underlying Lifshitz transition in the Fermi surface of Tb's valence electrons. We use a finite-temperature ab initio theory to analyze our measurements, obtaining a pressure-temperature magnetic phase diagram in very good agreement with experiment. Our calculations indicate that short- and long-range magnetic order has a crucial effect on the Fermi surface nesting and consequent magnetism of Tb.