Synthesis, Structures, and Magnetic Properties of Rare-Earth Cobalt Arsenides, RCo₂As₂ (R = La, Ce, Pr, Nd)

Abstract

Four rare-earth cobalt arsenides, RCo 2 As 2 (R = La, Ce, Pr, Nd), were obtained by reactions of constituent elements in molten Bi. The use of Bi flux also allowed the growth of representative single crystals. All compounds are isostructural and belong to the ThCr 2 Si 2 type (space group I4/mmm). The formation of Co vacancies is observed in all structures, while the structures of La-and Ce-containing compounds also show incorporation of minor Bi defects next to the R crystallographic site. Correspondingly, the general formula of these materials can be written as R 1−x Bi x Co 2−δ As 2 , with x/δ = 0.03/0.1, 0.05/0.15, 0/0.2, and 0/0.3 for R = La, Ce, Pr, and Nd, respectively. All compounds exhibit high-temperature ferromagnetic ordering of Co magnetic moments in the range 60−200 K. Electronic band structure calculations revealed a high peak in the density of states at the Fermi level, thus supporting the itinerant nature of magnetism in the Co sublattice. The magnetic ordering in the lanthanide sublattice takes place at lower temperatures, with the R moments aligning antiparallel to the Co moments to give a ferrimagnetic ground state. The measurements on oriented single crystals demonstrated significant magnetic anisotropy in the ferrimagnetic state, with the preferred moment alignment along the c axis of the tetragonal lattice. Neutron powder diffraction failed to reveal the structure of magnetically ordered states but confirmed the presence of Co vacancies. X-ray absorption near-edge structure spectroscopy on Ce 1.95 Bi 0.05 Co 1.85 As 2 showed the average oxidation state of Ce to be +3.06. Solid state NMR spectroscopy revealed a substantially reduced hyperfine field on the Co atoms in the vicinity of Bi defects.

Synthesis, crystal structure, and magnetism of A2Co12As7 (A=Ca, Y, Ce–Yb)

“…This assumption is further supported by properties of the later members of R2Co12As7 series. It is also in agreement with our findings of AFM 4f-3d coupling in RCo2P2 and RCo2As2 structures with early lanthanides [25,26,33].…”

“…Such behavior is characteristic of AFM coupling between the R 4f and Co 3d magnetic moments, suggesting the ferrimagnetic (FiM) ground state for these materials. This finding is also in accord with the FiM behavior reported for Pr2Co12P7 and Nd2Co12P7 [13], as well as for PrCo2As2 and NdCo2As2 [33]. The AFM 4f-3d coupling is well illustrated by the magnetization isotherms recorded for Nd2Co12As7 at different temperatures (Fig.…”

“…In an attempt to find alternative pathways to single crystals of R-Co arsenides, we have recently reported the use of molten Bi as an effective reaction medium for the crystal growth of RCo2As2 (R = La, Ce, Pr, Nd) [32,33]. Furthermore, this method afforded phase-pure bulk materials, thus allowing physical characterization to be carried out on both powder and single-crystal samples.…”

Nd8Co4−Al Ge2C3: A case study in flux growth of lanthanide-rich intermetallics

“…This is further evidence that the magnetic moments prefer to align along a-axis. Similar lowered remanent magnetization and larger coercivity when the field is applied perpendicular to the easy axis of magnetization was also observed for Ce 0.95 Bi 0.05 Co 1.85 As 2 [33]. …”

Flux Growth of Phosphide and Arsenide Crystals

“…When switching to isostructural arsenides, tin flux failed to produce any RCo 2 As 2 phases. Instead Bi flux was successfully applied (Thompson et al, 2011(Thompson et al, , 2014bTan et al, 2016aTan et al, , 2018. Detailed structural characterizations revealed that Bi is capable of partially replacing R 3+ cations, probably due to its trivalent nature (Thompson et al, 2011).…”