Field-induced transition of the magnetic ground state from A-type   antiferromagnetic to ferromagnetic order in CsCo2Se2
  We report on the magnetic properties of CsCo$_2$Se$_2$ with ThCr$_2$Si$_2$ structure, which we have characterized through a series of magnetization and neutron diffraction measurements. We find that CsCo$_2$Se2$_2$ undergoes a phase transition to an antiferromagnetically ordered state with a N\'eel temperature of $T_{\rm N} \approx$ 66 K. The nearest neighbour interactions are ferromagnetic as observed by the positive Curie-Weiss temperature of $\Theta \approx$ 51.0 K. We find that the magnetic structure of CsCo$_2$Se$_2$ consists of ferromagnetic sheets, which are stacked antiferromagnetically along the tetragonal \textit{c}-axis, generally referred to as A-type antiferromagnetic order. The observed magnitude of the ordered magnetic moment at $T$ = 1.5 K is found to be only 0.20(1)$\mu_{\rm Bohr}$/Co. Already in comparably small magnetic fields of $\mu_0 H_{MM}$(5K) $\approx$ 0.3 T, we observe a metamagnetic transition that can be attributed to spin-rearrangements of CsCo$_2$Se$_2$, with the moments fully ferromagnetically saturated in a magnetic field of $\mu_0 H_{\rm FM}$(5K) $\approx$ 6.4 T. We discuss the entire experimentally deduced magnetic phase diagram for CsCo$_2$Se$_2$ with respect to its unconventionally weak magnetic coupling. Our study characterizes CsCo$_2$Se$_2$, which is chemically and electronically posed closely to the $A_xFe_{2-y}Se_2$ superconductors, as a host of versatile magnetic interactions. 
F. von Rohr, A. Krzton-Maziopa, V. Pomjakushin, H. Grundmann, Z.   Guguchia, W. Schnick, and A. Schilling
cond-mat.str-el