Periodic atom chains of carbon-cobalt compounds, (CnCo) ∞, comprise both conducting and insulating electronic properties simultaneously depending on the spin type of electrons, and hence are half-metals. Their band gap and the net magnetic moment oscillate with the number of carbon atoms in a unit cell. Finite segments of these chains also show interesting magnetic and transport properties. When connected to appropriate metallic electrodes the antiferromagnetic CoCnCo segments behave like molecular spin-valves, which can be conveniently manipulated. © EDP Sciences.