First-order transitions at the Neel temperatures of trigonal SrMn2P2 and CaMn2P2 single crystals containing corrugated-honeycomb Mn sublattices

TL;DR

This study investigates the magnetic phase transitions in SrMn2P2 and CaMn2P2 single crystals, revealing unique first-order antiferromagnetic transitions and structural details that challenge existing understanding of magnetic ordering in similar quasi-two-dimensional systems.

Contribution

It reports the discovery of first-order Neel transitions in SrMn2P2 and CaMn2P2, contrasting the typical second-order transitions in related compounds, and provides detailed structural and magnetic characterizations.

Findings

First-order AFM transition at 53 K in SrMn2P2

First-order AFM transition at 69.8 K in CaMn2P2

CaMn2P2 has a commensurate AFM structure, SrMn2P2 incommensurate

Abstract

Single crystals of SrMn2P2 and CaMn2P2 were grown using Sn flux and characterized by single-crystal x-ray diffraction, electrical resistivity rho, heat capacity Cp, and magnetic susceptibility chi = M/H measurements versus temperature T and magnetization M versus applied magnetic field H isotherm measurements. The x-ray diffraction results show that both compounds adopt the trigonal CaAl2Si2-type structure. The rho(T) measurements demonstrate insulating ground states for both compounds. The chi(T) and Cp(T) data reveal a weak first-order antiferromagnetic (AFM) transition at the Neel temperature TN = 53(1) K for SrMn2P2 and a strong first-order AFM transition at TN = 69.8(3) K for CaMn2P2. Both compounds show an isotropic and nearly T-independent chi(T < TN). {31}P NMR measurements confirm the strong first-order transition in CaMn2P2 but show critical slowing down near TN for SrMn2P2 thus evidencing second-order character. The NMR measurements also 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 trigonal (Ca, Sr, Ba)Mn2(P, As, Sb, Bi)2 compounds which otherwise exhibit second-order AFM transitions. This result presents a challenge to understand the systematics of magnetic ordering in this class of materials in which magnetically-frustrated antiferromagnetism is quasi-two-dimensional.