  The magnetic properties of ThCr_2Si_2-type single crystals ACo_2Se_2 (A = K,
Rb and Cs) have been investigated by magnetic susceptibility and isothermal
magnetization measurements at various temperatures. The ferromagnetic phase
transition temperatures are estimated as $\approx$ 74 K and 76 K and 62 K for A
= K, Rb and Cs (in case of high magnetic field), respectively. The
susceptibility data in the paramagnetic state obey the modified Curie-Weiss law
quite well and the derived effective magnetic moments of the Co atom are about
2.21, 2.04 and 2.04 \mu_B/Co and the corresponding spontaneous moments derived
at the ground state are 0.72, 0.59 and 0.52 \mu_B/Co as well as the generalized
Rhodes-Wohlfarth ratios as 3.07, 3.42 and 3.96 for A= K, Rb and Cs,
respectively. The magnetic moment aligns within ab-plane and a
metamagnetism-like behavior occurs at a field of 3.5 T in CsCo_2Se_2 with
H//ab-plane. The magnetic properties in this system were discussed within the
frameworks of the self-consistent renormalization (SCR) and Takahashi's theory
of spin fluctuations.
