Magnetic Ground State and Tunable Néel Temperature in the Spin ½ Linear Chain Antiferromagnet Co(OH)(2−x)Fx

Abstract

The ground state of Co2+ in the octahedral CoO6 crystal field has often been assumed to be in high spin state with S = 3/2. As a result, the effective magnetic moments of Co2+ are overestimated in Co(OH)(2−x)F x . Herein, it is argued that the ground state of Co2+ in Co(OH)(2−x)F x has effective spin 1/2 but not S = 3/2, due to spin–orbit coupling in distorted octahedral crystal field. Analysis of the published magnetic susceptibility (χ) data on three samples of Co(OH)(2−x)F x is reported here with the new result that the Néel temperature T N decreases with an increase in χ as T N = 36.5, 30.5, and 26.5 K are determined for x = 1.09, 1.14, and 1.26, respectively. The employment of modified Curie–Weiss (CW) law along with S = 1/2 ground state eliminates the reported disagreement between the magnetic moment μ(Co2+) obtained from CW law above and neutron powder diffraction (NPD) below The fits of the data to the predictions of the linear chain model yield the exchange constant J/k B = −30.5, −28.1, and −24.7 K for x = 1.09, 1.14, and 1.26, respectively, supporting the chain‐like antiferromagnetic ordering observed using NPD.