Spin Blockade in Ballistic Nanocontacts
M.Ye. Zhuravlev, E. Y. Tsymbal, A. V. Vedyayev 1, S. S. Jaswal, and B. Dieny 2
1 Department of Physics, Moscow State University, Moscow, Russia
2 CEA, Département de Recherche Fondamentale sur la Matière Condensée, Grenoble, France
Using a free-electron model and a linear response theory the spin-dependent electronic transport across magnetic ballistic nanocontacts is studied in the regime of conductance quantization. Funded by NSF, NRI, and Keck Foundation.
The electrical conductance through a narrow constriction is quantized when the constriction width is comparable to the electron Fermi wavelength. When the constriction width changes the number of conducting channels and consequently the conductance vary in discrete steps. For diamagnetic nanowires the conductance is quantized in units of 2e2/h, where the factor 2 stands for spin degeneracy. If the constriction is made of a ferromagnetic metal, such as Ni, the exchange energy lifts the spin degeneracy, and the conductance is quantized in units of e2/h. Such a phenomenon was observed in Ni break junctions and in electrodeposited Ni nanocontacts.
An interesting observation that follows from these studies is the possibility to achieve the conductance value of e2/h, which implies that one spin channel is open, whereas the other spin channel is closed, resulting in fully spin-polarized electric current. This leads to a new phenomenon that might occur in atomic-size contacts. If there is a nonmagnetic region within the constriction that separates two ferromagnetic electrodes, electronic conduction can be blocked by the spin conservation rule. Indeed, if magnetizations of the two ferromagnets are antiparallel the spin channel that is open in one ferromagnet is closed in the other ferromagnet and vice versa. This spin blockade effect makes the conductance between the antiparallel-aligned electrodes equal to zero. This is opposite to the case of the parallel-aligned electrodes for which a conduction channel is open for up-spin electrons and the conductance is not equal to zero. The magnetoresistance of such an atomic-size constriction can be infinitely large.
Geometry of a nanocontact and spin-resolved conductance.
Paper:
M. Ye. Zhuravlev, E. Y. Tsymbal, A. V. Vedyayev, S. S. Jaswal, and B. Dieny, "Spin blockade in ferromagnetic nanocontacts", Applied Physics Letters 83, 3534-3536 (2003).