A unified theory of ferromagnetic quantum phase transitions in heavy fermion metals

Motivated by the recent discovery of a continuous ferromagnetic quantum phase transition in CeRh6Ge4 and its distinction from other U-based heavy fermion metals such as UGe2, we develop a unified explanation of their different ground state properties based on an anisotropic ferromagnetic Kondo-Heisenberg model. We employ an improved large-N Schwinger boson approach and predict a full phase diagram containing both a continuous ferromagnetic quantum phase transition for large magnetic anisotropy and first-order transitions for relatively small anisotropy. Our calculations reveal three different ferromagnetic phases including a half-metallic spin selective Kondo insulator with a constant magnetization. The Fermi surface topologies are found to change abruptly between different phases, consistent with that observed in UGe2. At finite temperatures, we predict the development of Kondo hybridization well above the ferromagnetic long-range order and its relocalization near the phase transition, in good agreement with band measurements in CeRh6Ge4. Our results highlight the importance of magnetic anisotropy and provide a unified theory for understanding the ferromagnetic quantum phase transitions in heavy fermion metals.