3d-Metal Induced Magnetic Ordering on U(IV) Atoms as a Route toward U(IV) Magnetic Materials

Uranium(IV) 5f² magnetism is dominated by a transition from a triplet to a singlet ground state at low temperatures. For the first time, we achieved magnetic ordering of U(IV) atoms in a complex fluoride through the incorporation of 3 d transition metal cations. This new route allowed us to obtain an unprecedented series of U(IV) ferrimagnetic materials of the new composition Cs₂MU₃F₁₆ (M = Mn²⁺, Co²⁺, and Ni²⁺), which were comprehensively characterized with respect to their structural and magnetic properties. Magnetic susceptibility measurements revealed ferromagnetic-like phase transitions at temperatures of ∼14.0, 3.5, and 4.8 K for M = Mn²⁺, Co²⁺, and Ni²⁺, respectively. The transition is not observed when the magnetic M cations are replaced by a diamagnetic cation, Zn²⁺. Neutron diffraction measurements revealed the magnetic moments of 0.91(6)-1.97(3) μ_B on the U atoms, which are only partially compensated by antiparallel moments of 1.53(14)-3.26(5) μ_B on the 3 d cations. This arrangement promotes suppression of the transition to a diamagnetic ground state characteristic of U(IV), and in doing so, induces magnetic ordering on uranium via 3 d-5 f exchange coupling.