Magnetic Frustration and Quantum Disorder in J_eff = 1/2 Lanthanide-Based ALnX₂ Materials

Magnetic frustration is a generous source of phases of matter with unusual physical properties where magnetic order is dictated by the interdependence of local electronic states and their real-space interactions. These local states can compete to drive differing magnetic phases that range from typical collinear (anti)ferromagnets to more exotic highly-entangled ground states with emergent phenomena. Typically, these entangled phases are sought after in materials comprised of small spin 1/2 moments decorating lattice geometries inciting multiple equivalent interactions. These small moments can form from crystalline electric field interactions splitting a total angular momentum manifold of a Kramers’ magnetic ion producing an effectively small J_eff = 1/2 moment at low temperature.

In this work, magnetic frustration and J_eff = 1/2 moments are studied in select members of the ALnX₂ family of materials (A = alkai; Ln = lanthanide; X = chalcogenide) with bulk magnetic measurements and neutron scattering. Triangular lattice antiferomagnet NaYbO₂ shows no signs of conventional magnetic order to 50 mK and a continuum of magnetic excitations consistent with a Dirac spin liquid. Its magnetic ground state evolves under an external field into an equal moment up-up-down structure originating from the triangular lattice XXZ Hamiltonian. On the other hand, triangular lattice KCeO₂ shows signatures of magnetic ordering below 300 mK, yet contains an unexplained additional crystalline electric field mode. The elongated diamond lattice of LiYbO₂ is tied to the J₁-J₂ Heisenberg model, where an incommensure spiral magnetic ground state undergoes a lock-in transition to a commensurate spiral in an external field. Overall, our investigations establish the ALnX₂ materials as promising materials for researching exciting physics.