First-order antiferromagnetic transitions of SrMn2P2 and CaMn2P2 single crystals containing corrugated-honeycomb Mn sublattices.

SrMn₂P₂ and CaMn₂P₂ are insulators that adopt the trigonal CaAl2Si2-type structure containing corrugated Mn honeycomb layers. Magnetic susceptibility χ and heat capacity versus temperature T data reveal a weak first-order antiferromagnetic (AFM) transition at the Néel temperature TN = 53(1) K for SrMn₂P₂ and a strong first-order AFM transition at TN = 69.8(3) K for CaMn₂P₂. Both compounds exhibit isotropic and nearly T-independent χ(T ≤ TN), suggesting magnetic structures in which nearestneighbor moments are aligned at ≈120◦ to each other. The ³¹P NMR measurements confirm the strong first-order transition in CaMn₂P₂ but show critical slowing down above TN for SrMn₂P₂, thus also evidencing second-order character. The ³¹P NMR measurements indicate that the AFM structure of CaMn₂P₂ is commensurate with the lattice whereas that of SrMn₂P₂ is incommensurate. These first-order AFM transitions are unique among the class of (Ca, Sr, Ba)Mn₂ (P, As, Sb, Bi)₂ compounds that otherwise exhibit second-order AFM transitions. This result challenges our understanding of the circumstances under which first-order AFM transitions occur. [ABSTRACT FROM AUTHOR]