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 chi and heat capacity versus temperature T data reveal a weak first-order antiferromagnetic (AFM) transition at the Neel temperature T-N = 53(1) K for SrMn2P2 and a strong first-order AFM transition at T-N = 69.8(3) K for CaMn2P2. Both compounds exhibit isotropic and nearly T-independent chi(T <= T-N), suggesting magnetic structures in which nearest-neighbor moments are aligned at approximate to 120 degrees to each other. The P-31 NMR measurements confirm the strong first-order transition in CaMn2P2 but show critical slowing down above T-N for SrMn2P2, thus also evidencing second-order character. The P-31 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)Mn-2 (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.

Automated summary

SrMn2P2 and CaMn2P2 are insulators with corrugated Mn honeycomb layers; they exhibit weak and strong first-order antiferromagnetic transitions at 53K and 69.8K respectively. The magnetic structures of these compounds show a mixture of first-order and second-order characteristics.