Theoretical study of the conductance of ferromagnetic atomic-sized contacts

摘要

Recently, different experiments on the transport through atomic-sizedcontacts made of ferromagnetic materials have produced contradictory results.In particular, several groups have reported the observation of half-integerconductance quantization, which requires having full spin polarization andperfectly conducting channels. Motivated by these surprising results, we havestudied theoretically the conductance of ideal atomic contact geometries of theferromagnetic 3d materials Fe, Co, and Ni using a realistic tight-bindingmodel. Our analysis shows that in the absence of magnetic domains, the d bandsof these transition metals play a key role in the electrical conduction. In thecontact regime this fact has the following important consequences for the threematerials: (i) there are partially open conduction channels and thereforeconductance quantization is not expected, (ii) the conductance of the lastplateau is typically above G_0=2e^2/h, (iii) both spin species contribute tothe transport and thus there is in general no full current polarization, and(iv) both the value of the conductance and the current polarization are verysensitive to the contact geometry and to disorder. In the tunneling regime wefind that a strong current polarization can be achieved.