Ferromagnetic Doniach diagram for the underscreened Kondo lattice

Abstract

A coexistence between the Kondo effect and the ferromagnetic order has been observed in some Uranium compounds such as UTe or UCu 0.9Sb 2, which order ferromagnetically at roughly 100 K and present a Kondo resistivity decrease above it. Recently we have developed the underscreened Kondo lattice (UKL) model, where the competition between the ferromagnetic order and the underscreened Kondo effect is studied by considering a Kondo coupling J K of localized S f = 1 spins with a conduction band and an inter-site f–f ferromagnetic exchange interaction J H . Here we present a diagram for the UKL model, which appears to be significantly different from the Doniach diagram for the Kondo lattice model with localized spins S = 1 2 . We obtain that the ferromagnetic order exists for all values of the ratio J K / J H , while the Kondo-ferromagnetism coexistence takes place only for sufficiently large values of the ratio J K / J H . This new ferromagnetic diagram for the UKL model can, therefore, account for the strong Kondo-ferromagnetism coexistence observed in some Uranium compounds.