BaMn$_2$P$_2$: Highest magnetic ordering temperature 122-pnictide compound

Abstract

We report the growth of high-quality single crystals of ThCr$_2$Si$_2$-type tetragonal BaMn$_2$P$_2$ and investigation of its structural, electrical transport, thermal and magnetic properties. Our results of basal plane electrical resistivity and heat capacity measurements show that the compound has an insulating ground state with a small band gap. Anisotropic susceptibility $\chi_{ab,c}(T)$ data infer a collinear local-moment N\'eel-type antiferromagnetic (AFM) ground state below the ordering temperature $T_{\rm N} = 795(15)$~K, which is highest among all the ThCr$_2$Si$_2$- and CaAl$_2$Si$_2$-type 122-pnictide compounds reported so far suggesting that the strength of magnetic exchange interactions is strongest in this material. The magnetic transition temperatures of BaMn$_2$$Pn_{2}$ ($Pn$ = P, As, Sb, Bi) compounds exhibit a monotonic decrease with the increase of tetragonal unit cell parameters $a$ and $c$, suggesting a strong dependence of the strength of the decisive magnetic exchange interactions on the separation between the localized spins residing on the Mn-ions. The observed monotonic increase of both $\chi_{ab}$ and $\chi_{c}$ for $T > T_{\rm N}$ suggests that short-range dynamic quasi-two dimensional AFM correlations persist above the $T_{\rm N}$ up to the highest temperature of the measurements. The large $T_{\rm N}$ of BaMn$_2$P$_2$ demands for systematic hole-doping studies on this material as similar investigations on related BaMn$_2$As$_{2}$ with $T_{\rm N} = 618$~K have led to the discovery of an outstanding ground state where AFM of localized Mn-spins and itinerant half-metallic ferromagnetism with $T_{\rm c} \approx 100$~K originating from the doped holes coexist together.