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The Eu{+2} spins S = 7/2 in the metallic compound EuCo{2-y}As2 order into anantiferromagnetic helical structure below a Neel temperature TN = 40 to 45 K.The effective magnetic moment mu_eff of the Eu spins in the paramagnetic statefrom 100 to 300 K is found from static magnetic susceptibility measurements tobe enhanced by about 7% compared to the value expected for spectroscopicsplitting factor g = 2, and the saturation moment at high applied fields H andlow temperatures T is also sometimes enhanced. Hereelectron-paramagnetic-resonance (CW EPR) measurements versus applied magneticfield H at fixed X-band rf (microwave) angular frequency omega were carried outusing a linearly-polarized rf magnetic field oriented perpendicular to H tostudy the microscopic magnetic properties of the Eu spins. In order to analyzethe data, the complex magnetic susceptibility chi(omega) at fixed H was usedthat was derived for linearly-polarized rf fields from the modified Blochequations [M. A. Garstens and J. I. Kaplan, Phys. Rev. 99, 459 (1955)] (GK). Itis shown that their formulation when applied to calculate the Dysonianabsorptive susceptibility chi_D''(H) of local magnetic moments in metals yieldsa prediction that can be very different from the traditionally-used form ofchi_D''(H). By fitting the derivative of the field-swept CW EPR data forEuCo{2-y}As2 by chi_D''(H) at fixed omega derived from the GK chi_D''(omega) atfixed H, the Eu spin spectroscopic splitting factor (g-factor) is found to beapproximately 2.00 from 300 to ~125 K, and then to continuously increase toapproximately 2.16 on further cooling to 50 K. We speculate that theenhancement of the Eu g-factor on cooling from 125 to 50 K arises fromcontinuously-increasing local short-range ferromagnetic correlations betweenthe Co 3d-band electrons and the Eu spins.

EPR measurements of Eu{+2} spins in metallic EuCo{2-y}As2 single crystals