Contrary to the antiferromagnetic and insulating character of bulk NiO, one-dimensional chains of this material can become half metallic due to the lower coordination of their atoms. Here we present ab initio electronic structure and quantum transport calculations of ideal infinitely long NiO chains and of more realistic short ones suspended between Ni electrodes. While infinite chains are insulating, short suspended chains are half-metallic minority-spin conductors that displays very large magnetoresistance and a spin-valve behavior controlled by a single atom. DOI: 10.1103/PhysRevB.74.081402 PACS number͑s͒: 73.63.Rt, 75.47.Jn, 75.75.ϩa In going from bulk to lower dimensions material properties often change drastically. A recent example is that of interfaces between different insulators which can actually be metallic.1 Even more recently, it has been predicted theoretically that certain oxygen surfaces of some insulating ceramic oxides can exhibit magnetism and half-metallicity.2 The ultimate limit in this respect can be found in atomic chains formed in metallic nanocontacts that allow one to study the transport properties of one-dimensional systems of atomic size.3 Due to the lower coordination of the atoms the properties of metallic atomic chains formed in nanocontacts can be remarkably different from those in the bulk. For example, Pt nanocontacts can exhibit magnetic order when atomic chains are formed. 4 More complex one-dimensional systems like carbon-cobalt atomic chains 5 or organometallic benzenevanadium wires 6 have even been predicted to be halfmetallic conductors.However, not all metals form atomic chains in nanocontacts, although recently it has been found that the presence of oxygen favors their formation.7 For example, experiments with Ni nanocontacts 8,9 have never shown evidence of chain formation. Nevertheless, the presence of oxygen in the contact region could possibly lead to the formation of NiO chains. In this context it has also been proposed that the rather moderate magnetoresistive properties of pure Ni nanocontacts 10,11 could be enhanced considerably by the presence of oxygen adsorbates on the surface of the Ni electrodes.12 On the other hand, bulk NiO is a common example of a correlated insulator with antiferromagnetic ͑AF͒ order ͑see, e.g., Refs. 13 and 14͒, which remains insulating even above the Néel temperature when the AF order is lost. Thus it is not at all obvious whether or not oxidized Ni nanocontacts or NiO chains should be conductors.In this Rapid Communication we investigate the electronic and magnetic structure and the transport properties of one-dimensional NiO chains, both idealized infinite ones and more realistic short ones suspended between Ni nanocontacts. Anticipating our most important results, our ab initio quantum transport calculations show that short NiO chains suspended between Ni nanocontacts can become halfmetallic conductors, i.e., carry an almost 100% spinpolarized current. This result holds true even for a single O atom in between Ni electrodes. Con...