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

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 [Formula: see text] K for SrMn₂P₂ and a strong first-order AFM transition at [Formula: see text] K for CaMn₂P₂ Both compounds exhibit isotropic and nearly T-independent [Formula: see text], suggesting magnetic structures in which nearest-neighbor moments are aligned at [Formula: see text] to each other. The ³¹P NMR measurements confirm the strong first-order transition in CaMn₂P₂ but show critical slowing down above [Formula: see text] 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.