Vanishing ordered moment in the frustrated triangular lattice antiferromagnet CuNdO${}_2$

Abstract

We investigate the magnetic ground state of CuNdO${}_2$, which is a delafossite with a triangular lattice of magnetic Nd${}^{3+}$ 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 Nd${}^{3+}$ moments. Magnetic susceptibility and heat capacity measurements reveal the onset of long-range antiferromagnetic order at $T_N=0.78$ 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 antiferromagnetically coupled Nd${}^{3+}$ moments, which experience extreme frustration on the triangular lattice. Consequently, the frustration in CuNdO${}_2$ is relieved through in-plane ordering of the substantially smaller perpendicular component of the Nd${}^{3+}$ moments into a 120\textdegree\ structure, with a nearly vanishing ordered moment.