Ferromagnetism is a collective order phenomenon that is intimately related to the solid state. In general, magnetic forces are comparatively small and thus ferromagnetic order is destroyed for elevated temperatures. Another consequence of the competition with thermal energy is that ferromagnetic order disappears when reducing the size of a solid below the so-called superparamagnetic limit. This property is one of the main problems hindering the further downscaling of magnetic storage devices . On the other hand it has been predicted, that upon reduction of the dimension, some elements, which in bulk solids are not ferromagnetically ordered, may reveal ferromagnetic order. Examples include atomic chains of strong paramagnetic metals, such as platinum or iridium . The existence of magnetic order in these chains is still under debate, because a direct measurement of the magnetic moment of only a few spins is below the resolution limit of basically all known magnetometer concepts. Thus an indirect measurement scheme is needed. Here we report an experimental study of the electrical conductance in magnetic fields of atomic-size contacts and mono-atomic chains of platinum, palladium, and iridium. We find strong magnetoresistance effects which vary when stretching the contact by subatomic distances [3,4]. These findings can be interpreted as a signature of local magnetic order in the contact, which may be of particular importance for the application of atomic-sized contacts in spintronic devices of the smallest possible size.
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