Effect of Fe substitution on the magnetic ordering in Ca₃(Co_1-xFe_x)₂O₆

Abstract

Mössbauer studies over 4.2–550 K of iron doped quasi-one-dimensional spin chain compounds Ca₃(Co_1-xFe_x)₂O₆, crystallizing in rhombohedral structure (space group R3c) for x⩽0.2, reveal that the iron resides only in the Co2 (6a) site. The compounds are paramagnetic at 90K, and the iron exhibits slow spin relaxation phenomena even at room temperature. However, for the x = 0.5 compound, the crystal structure is triclinic (P1), and the iron resides almost equally in two different crystallographic sites and exhibits a very high magnetic ordering temperature T_N∼500 K. Both iron sites exhibit quadrupole interactions of almost equal size; however, with opposite sign (1/4e²qQ≈ ± 0.62 mm/s). At T_sr = 193 K, a spin reorientation of the iron magnetic moments is observed. Below 150 K the magnetic moments are tilted at 20° relative to the local electric field gradient axis and above 230 K they are aligned perpendicular to this axis. These results agree well with magnetic susceptibility measurements, in which a sharp drop in magnetization is observed above 200 K. The magnetic hyperfine fields in the two sites at 4.2 K are 53.3 and 45.7 T, respectively. The net magnetic interaction is antiferromagnetic, however, with a small ferromagnetic component displaying hysteresis along with a saturation magnetization of about 0.02μ_B∕f.u. at 1.5 K.