High spin-low spin transition in insulating CaMn$_2$Sb$_2$

Layered manganese pnictides are often interesting compounds to compare with the iron pnictide superconductors. To this end, we have synthesized high quality flux-grown single crystals of CaMn$_2$Sb$_2$, which forms in a trigonal CaAl$_2$Si$_2$-type structure characterized by corrugated triangular Mn layers. Previously reported as a bad metal, we observe instead that this compound exhibits a distinct insulating trend in temperature-dependent resistivity measurements, including an enhancement of up to two orders of magnitude between 200 K and T$_N$ = 85 K. Measurements of ac susceptibility exhibit an orientation- and highly field-dependent plateau across the same temperature range, while heat capacity measurements reveal a sharp feature at 85 K as well as a broad anomaly centered near 195 K. Curie-Weiss behavior above 300 K indicates the presence fluctuating moments with prevailing ferromagnetic interactions, corresponding to less than half the static moment reported for the antiferromagnetic ordered state. These results imply a temperature-induced high spin-low spin insulator-insulator transition.