Crystal and magnetic structure of CaCo1.86As2studied by x-ray and neutron diffraction

Abstract

Neutron and x-ray diffraction measurements are presented for powders and single crystals of CaCo 2 As 2 . The crystal structure is a collapsed-tetragonal ThCr 2 Si 2 -type structure as previously reported, but with 7(1)% vacancies on the Co sites corresponding to the composition CaCo 1.86(2) As 2 . The thermal expansion coefficients for both the a and c axes are positive from 10 to 300 K. Neutron diffraction measurements on single crystals demonstrate the onset of A-type collinear antiferromagnetic order below the Néel temperature T N = 52(1) K with the ordered moments directed along the tetragonal c axis, aligned ferromagnetically in the ab plane and antiferromagnetically stacked along the c axis.

Antiferromagnetic order and spin dynamics in iron-based superconductors

“…(b) The AF structure of BaMn2As2, where the ordered moments on Mn are along the c-axis direction (the G-type) . (c) The crystal structure of CaCo2As2, where the ordered moments on Co form the Atype AF structure (Quirinale et al, 2013). (d) The collinear AF order in NaFeAs doubles the crystalline unit cell along the c-axis (Li et al, 2009c).…”

Effective One-Dimensional Coupling in the Highly Frustrated Square-Lattice Itinerant Magnet CaCo2−yAs2

“…INS experiments on SrCo 2 As 2 observe stripe-type AFM spin fluctuations peaked at Q stripe similar to AFe 2 As 2 [19]. The anisotropy of the spin fluctuations gives η ≈ −0.6 [19], and indicates a moderate frustration.Here, we report inelastic neutron scattering measurements of magnetic excitations in CaCo 2−y As 2 at T = 8 K, below the AFM ordering temperature T N = 52 K [15,20], that reveal planes of magnetic scattering in reciprocal space perpendicular to the Co square-lattice planes. This is in sharp contrast to rods of scattering expected perpendicular to quasi-two-dimensional planes of spins but similar to the expectation for quasi-one-dimensional spin chains.…”

“…The vacancies on the Co sites are randomly distributed, as x-ray and neutron diffraction measurements find no evidence for vacancy ordering [15,20]. INS experiments were performed on the ARCS Spectrometer [21] at the Spallation Neutron Source, at Oak Ridge National Laboratory.…”

“…Much of the recent motivation for the study of these materials is due to the proximity of antiferromagnetic (AFM) order and hightemperature superconductivity in the doped variants of TM = Fe compounds [10][11][12][13]. The ATM 2 As 2 materials adopt several different magnetic structures, including Néel-(or checkerboard)type AFM (e.g., BaMn 2 As 2 [14]), stripe-type AFM (e.g., AFe 2 As 2 [13]), and A-type (e.g., CaCo 2−y As 2 [15]). The AFM order in the TM = Fe and Co variants is itinerant in nature, possessing an ordered moment of µ 1µ B /TM.…”

“…Single crystals of CaCo 2−y As 2 (y 0.14), were grown using Sn flux [15,20]. The vacancies on the Co sites are randomly distributed, as x-ray and neutron diffraction measurements find no evidence for vacancy ordering [15,20].…”