  The Eu{+2} spins S = 7/2 in the metallic compound EuCo{2-y}As2 order into an
antiferromagnetic helical structure below a Neel temperature TN = 40 to 45 K.
The effective magnetic moment mu_eff of the Eu spins in the paramagnetic state
from 100 to 300 K is found from static magnetic susceptibility measurements to
be enhanced by about 7% compared to the value expected for spectroscopic
splitting factor g = 2, and the saturation moment at high applied fields H and
low temperatures T is also sometimes enhanced. Here
electron-paramagnetic-resonance (CW EPR) measurements versus applied magnetic
field H at fixed X-band rf (microwave) angular frequency omega were carried out
using a linearly-polarized rf magnetic field oriented perpendicular to H to
study the microscopic magnetic properties of the Eu spins. In order to analyze
the data, the complex magnetic susceptibility chi(omega) at fixed H was used
that was derived for linearly-polarized rf fields from the modified Bloch
equations [M. A. Garstens and J. I. Kaplan, Phys. Rev. 99, 459 (1955)] (GK). It
is shown that their formulation when applied to calculate the Dysonian
absorptive susceptibility chi_D''(H) of local magnetic moments in metals yields
a prediction that can be very different from the traditionally-used form of
chi_D''(H). By fitting the derivative of the field-swept CW EPR data for
EuCo{2-y}As2 by chi_D''(H) at fixed omega derived from the GK chi_D''(omega) at
fixed H, the Eu spin spectroscopic splitting factor (g-factor) is found to be
approximately 2.00 from 300 to ~125 K, and then to continuously increase to
approximately 2.16 on further cooling to 50 K. We speculate that the
enhancement of the Eu g-factor on cooling from 125 to 50 K arises from
continuously-increasing local short-range ferromagnetic correlations between
the Co 3d-band electrons and the Eu spins.
