Revealing a hidden magnetic order in the triangular lattice antiferromagnet CuNdO₂

We investigate the magnetic ground state of CuNdO₂, which is a delafossite with a triangular lattice of magnetic Nd³⁺ ions that are well separated by non-magnetic Cu spacer layers. From inelastic neutron scattering measurements of the crystal electric field, we determine the strong Ising character of the pseudo-spin 1/2 Nd³⁺ moments. Magnetic susceptibility and heat capacity measurements reveal the onset of long-range antiferromagnetic order at TN=0.78 K. While the magnetic transition is definitively observed with muon spin relaxation, accompanied by the formation of a weakly dispersing spin wave excitation, no dipole-ordered moment is detected with neutron diffraction. We show that the apparent absence of a dipolar ordered moment is a consequence of the dominant Ising character of the Nd³⁺ moments, which experience extreme frustration on the triangular lattice. Consequently, the frustration in CuNdO₂ is relieved through in-plane ordering of the substantially smaller perpendicular component of the Nd³⁺ moments into a 120\ structure, with a nearly vanishing ordered moment.