Glassy magnetic dynamics emerging in a robust diamagnetic state of Ba2Co3.5Cl1.5O7 single crystal: A new phase of the mixed-anion cobalt oxyhalide

Glassy magnetic states typically develop on top of a paramagnetic or weakly ferromagnetic background. However, their emergence in a diamagnetic state remains largely unexplored. In this study, we report a single-crystal investigation of a mixed-anion cobalt oxyhalide with an energy dispersive x-ray spectroscopy (EDS)-estimated composition close to Ba2Co3.5Cl1.5O7, in which glassy magnetic dynamics emerge within a predominantly negative magnetic response. The composition Ba2Co3.5Cl1.5O7 refers to the EDS-estimated stoichiometry, whereas the crystallographic refinement follows a Ba2Co4ClO7-type structural model. Direct current magnetic susceptibility measurements revealed a pronounced diamagnetic response over a broad temperature range, accompanied by a freezing-like anomaly near 50 K. Assessments of alternating current susceptibility further showed an unusual dynamical behavior: the real component exhibited weak frequency dependence, whereas the imaginary component displayed strongly frequency-dispersive dissipation. The extracted Mydosh parameter, δTf = 0.035, significantly exceeded values typical of canonical spin glasses, indicating a noncanonical glassy magnetic state. These results establish Ba2Co3.5Cl1.5O7 as an unconventional glassy magnetic system and highlight mixed-anion cobalt oxyhalides as a platform for realizing frustrated magnetic relaxation in a diamagnetic state.