First-order antiferromagnetic transitions of SrMn2P2 and CaMn2P2 single crystals containing corrugated-honeycomb Mn sublattices期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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

Authors:	N S Sangeetha Santanu Pakhira Qing-Ping Ding Lennard Krause Hyung-Cheol Lee Volodymyr Smetana Anja-Verena Mudring Bo Brummerstedt Iversen Yuji Furukawa David C Johnston

Institution:	^aAmes Laboratory, Iowa State University, Ames, IA 50011;^bDepartment of Physics and Astronomy, Iowa State University, Ames, IA 50011;^cCenter for Materials Crystallography, Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark;^diNANO, Aarhus University, DK-8000 Aarhus C, Denmark;^eDepartment of Materials and Environmental Chemistry, Stockholm University, 106 91 Stockholm, Sweden

Abstract:	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 $T_{N} = 53 (1)$ 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 $χ (T \leq T_{N})$ , suggesting magnetic structures in which nearest-neighbor moments are aligned at $\approx 120^{°}$ to each other. The ³¹P 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 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. The Mn-based 122-type pnictides $A {Mn}_{2} P n_{2}$ ( $A =$ Ca, Sr, Ba; Pn = P, As, Sb, Bi) have received attention owing to their close stoichiometric 122-type relationship to high- $T_{c}$ iron pnictides. The undoped Mn pnictides are local-moment antiferromagnetic (AFM) insulators like the high- $T_{c}$ cuprate parent compounds (1 –3). The ${BaMn}_{2} P n_{2}$ compounds crystallize in the body-centered tetragonal ${ThCr}_{2} {Si}_{2}$ structure as in $A {Fe}_{2} {As}_{2}$ (A = Ca, Sr, Ba, Eu), whereas the ${(Ca,Sr) Mn}_{2} P n_{2}$ compounds crystallize in the trigonal ${CaAl}_{2} {Si}_{2}$ -type structure (4). Recently, density-functional theory (DFT) calculations for the 122 pnictide family have suggested that the trigonal 122 transition-metal pnictides that have the ${CaAl}_{2} {Si}_{2}$ structure might compose a new family of magnetically frustrated materials in which to study the potential superconducting mechanism (5, 6). It had previously been suggested on theoretical grounds that CaMn₂Sb₂ is a fully frustrated classical magnetic system arising from proximity to a tricritical point (7 –9).The electrical resistivity ρ and heat capacity $C_{p}$ versus temperature T of single-crystal CaMn₂P₂ were reported in ref. 10. The compound is an insulator at T = 0 and undergoes a first-order transition of some type at 69.5 K. The Raman spectrum of CaMn₂P₂ at T = 10 K showed new peaks compared to the spectrum at 300 K, whereas the authors’ single-crystal X-ray diffraction measurements showed no difference in the crystal structure at 293 and 40 K. They suggested that the results of the two types of measurements could be reconciled if a superstructure formed below 69.5 K (10). The authors’ magnetic susceptibility $χ (T)$ measurements below 400 K revealed no evidence for a magnetic transition.Here we report the detailed properties of trigonal CaMn₂P₂ and SrMn₂P₂ (11) single crystals. We present the results of single-crystal X-ray diffraction (XRD), electrical resistivity ρ in the ab plane (hexagonal unit cell) versus temperature T, isothermal magnetization versus applied magnetic field M(H), magnetic susceptibility $χ (T)$ , heat capacity $C_{p} (H, T)$ , and ³¹P NMR measurements. We find from $C_{p} (T), χ (T)$ , and NMR that CaMn₂P₂ exhibits a strong first-order AFM transition at $T_{N} = 69.8 (3)$ K whereas SrMn₂P₂ shows a weak first-order transition at $T_{N} = 53 (1)$ K but with critical slowing down on approaching $T_{N}$ from above as revealed from NMR, a characteristic feature of second-order transitions. Thus, remarkably, the AFM transition in SrMn₂P₂ has characteristics of both first- and second-order transitions. The $χ (T)$ data also reveal the presence of strong isotropic AFM spin fluctuations in the paramagnetic (PM) state above $T_{N}$ up to our maximum measurement temperatures of 900 and 350 K for SrMn₂P₂ and CaMn₂P₂, respectively. This behavior likely arises from spin fluctuations associated with the quasi–two-dimensional nature of the Mn spin layers (12) together with possible contributions from magnetic frustration. Our single-crystal XRD data at room temperature and high-resolution synchrotron XRD data at T = 20 K for SrMn₂P₂ and CaMn₂P₂ demonstrate conclusively that there is no structure change of either compound on cooling below their respective $T_{N}$ .Our studies of SrMn₂P₂ and CaMn₂P₂ thus identify the only known members of the class of materials with general formula AMn $_{2} P n_{2}$ containing Mn $^{2 +}$ spins S = 5/2 that exhibit first-order AFM transitions, where A = Ca, Sr, or Ba and the pnictogen $P n =$ P, As, Sb, or Bi. In particular, only second-order AFM transitions are found in ${CaMn}_{2} {As}_{2}$ (13), ${SrMn}_{2} {As}_{2}$ (13 –15), ${CaMn}_{2} {Sb}_{2}$ (8, 9, 16 –19), ${SrMn}_{2} {Sb}_{2}$ (16, 19), and ${CaMn}_{2} {Bi}_{2}$ (20).

Keywords:	first-order antiferromagnetic transitions SrMn₂P₂ CaMn₂P₂ trigonal CaAl₂Si₂ structure incommensurate and commensurate antiferromagnetic structures

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