Enhanced moments of Eu in single crystals of the metallic helical antiferromagnet EuCo2 yAs2

The compound EuCo{2-y}As2 with the tetragonal ThCr2Si2 structure is known to contain Eu{+2} ions with spin S = 7/2 that order below a temperature TN = 47 K into an antiferromagnetic (AFM) proper helical structure with the ordered moments aligned in the tetragonal ab plane, perpendicular to the helix axis along the c axis, with no contribution from the Co atoms. Here we carry out a detailed investigation of the properties of single crystals. Enhanced ordered and effective moments of the Eu spins are found in most of our crystals. Electronic structure calculations indicate that the enhanced moments arise from polarization of the d bands, as occurs in ferromagnetic Gd metal. Electrical resistivity measurements indicate metallic behavior. The low-field in-plane magnetic susceptibilities chi{ab}(T < TN) for several crystals are reported that are fitted well by unified molecular field theory (MFT), and the Eu-Eu exchange interactions Jij are extracted from the fits. High-field magnetization M data for magnetic fields H||ab reveal what appears to be a first-order spin-flop transition followed at higher field by a second-order metamagnetic transition of unknown origin, and then by another second-order transition to the paramagnetic (PM) state. For H||c, the magnetization shows only a second-order transition from the canted AFM to the PM state, as expected. The critical fields for the AFM to PM transition are in approximate agreement with the predictions of MFT. Heat capacity Cp measurements in zero and high H are reported. Phase diagrams for H||c and H||ab versus T are constructed from the high-field M(H,T) and Cp(H,T) measurements. The magnetic part Cmag(T, H = 0) of Cp(T, H = 0) is extracted and is fitted rather well below TN by MFT, although dynamic short-range AFM order is apparent in Cmag(T) up to about 70 K, where the molar entropy attains its high-T limit of R ln8.Comment: 29 pages, 30 figures including 62 subfigures, 8 tables, 84 reference

Incommensurate and commensurate antiferromagnetic states in CaMn2As2 and SrMn2As2 revealed by 75As NMR

We carried out As-75 nuclear magnetic resonance (NMR) measurements on the trigonal CaMn2As2 and SrMn2As2 insulators exhibiting antiferromagnetic (AFM) ordered states below Neel temperatures T-N = 62 and 120 K, respectively. In the paramagnetic state above T-N, typical quadrupolar-split As-75 NMR spectra were observed for both systems. The As-75 quadrupolar frequency nu(Q) for CaMn2As2 decreases with decreasing temperature, while nu(Q) for SrMn2As2 increases, showing an opposite temperature dependence. In the AFM state, the relatively sharp and distinct As-75 NMR lines were observed in SrMn2As2 and the NMR spectra were shifted to lower fields for both magnetic fields H parallel to c axis and H parallel to ab plane, suggesting that the internal fields B-int at the As site produced by the Mn ordered moments are nearly perpendicular to the external magnetic field direction. No obvious distribution of B-int was observed in SrMn2As2, which clearly indicates a commensurate AFM state. In sharp contrast to SrMn2As2, broad and complex NMR spectra were observed in CaMn2As2 in the AFM state, which clearly shows a distribution of B-int at the As site, indicating an incommensurate state. From the analysis of the characteristic shape of the observed spectra, the AFM state of CaMn2As2 was determined to be a two-dimensional incommensurate state where Mn ordered moments are aligned in the ab plane. A possible origin for the different AFM states in the systems was discussed. Both CaMn2As2 and SrMn2As2 show very large anisotropy in the nuclear spin-lattice relaxation rate 1/T-i in the paramagnetic state. 1/T-i for H parallel to ab is much larger than that for H parallel to c, indicating strong anisotropic AFM spin fluctuations in both compounds.This article is published as Ding, Q-P., N. S. Sangeetha, Abhishek Pandey, D. C. Johnston, and Y. Furukawa. "Incommensurate and commensurate antiferromagnetic states in Ca Mn 2 As 2 and Sr Mn 2 As 2 revealed by As 75 NMR." Physical Review B 104, no. 22 (2021): 224413. DOI: 10.1103/PhysRevB.104.224413. Copyright 2021 American Physical Society. DOE Contract Number(s): AC02-07CH11358. Posted with permission

Carrier Tuning of Stoner Ferromagnetism in ThCr2Si2-Structure Cobalt Arsenides

CaCo2-yAs2 is an unusual itinerant magnet with signatures of extreme magnetic frustration. The conditions for establishing magnetic order in such itinerant frustrated magnets, either by reducing frustration or increasing electronic correlations, is an open question. Here, we use results from inelastic neutron scattering and magnetic susceptibility measurements and density functional theory calculations to show that hole doping in Ca(Co1-xFex)(2-y)As-2 suppresses magnetic order by quenching the magnetic moment while maintaining the same level of magnetic frustration. The suppression is due to tuning the Fermi energy away from a peak in the electronic density of states originating from a flat conduction band. This results in the complete elimination of the magnetic moment by x approximate to 0.25, providing a clear example of a Stoner-type transition.This article is published as Ueland, B. G., Santanu Pakhira, Bing Li, A. Sapkota, N. S. Sangeetha, T. G. Perring, Y. Lee, Liqin Ke, D. C. Johnston, and R. J. McQueeney. "Carrier tuning of Stoner ferromagnetism in ThCr 2 Si 2-structure cobalt arsenides." Physical Review B 104, no. 22 (2021): L220410. DOI: 10.1103/PhysRevB.104.L220410. Copyright 2021 American Physical Society. DOE Contract Number(s): AC02-07CH11358. Posted with permission

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.This is a manuscript of an article published as Sangeetha, N. S., Santanu Pakhira, Qing-Ping Ding, Lennard Krause, Hyung-Cheol Lee, Volodymyr Smetana, Anja-Verena Mudring, Bo Brummerstedt Iversen, Yuji Furukawa, and David C. Johnston. "First-order antiferromagnetic transitions of SrMn2P2 and CaMn2P2 single crystals containing corrugated-honeycomb Mn sublattices." Proceedings of the National Academy of Sciences 118, no. 44 (2021): e2108724118. DOI: 10.1073/pnas.2108724118. Copyright 2021 The Authors. DOE Contract Number(s): AC02-07CH11358. Posted with permission