We report a single-crystal study on the magnetism of the rare-earth compound ${\mathrm{PrTiNbO}}_6$ that experimentally realizes the zigzag pseudospin-1/2 quantum antiferromagnetic chain model. Random crystal electric field caused by the site mixing between nonmagnetic ${\mathrm{Ti}}^{4+}$ and ${\mathrm{Nb}}^{5+}$ results in the non-Kramers ground-state quasidoublet of ${\mathrm{Pr}}^{3+}$ with the effective pseudospin-1/2 Ising moment. Despite the antiferromagnetic intersite coupling of about 4 K, no magnetic freezing is detected down to 0.1 K, while the system approaches its ground state with almost zero residual spin entropy. At low temperatures, a sizable gap of about 1 K is observed in zero field. We ascribe this gap to off-diagonal anisotropy terms in the pseudospin Hamiltonian, and argue that rare-earth oxides open an interesting venue for studying magnetism of quantum spin chains.

• Received 22 November 2017
• Revised 2 April 2018

DOI:https://doi.org/10.1103/PhysRevB.97.184434