Theoretical study of the conductance of ferromagnetic atomic-sized contacts

TL;DR

This theoretical study investigates the conductance of ferromagnetic atomic contacts, revealing that conductance quantization is unlikely and polarization depends on geometry and disorder, contrasting with some experimental claims.

Contribution

The paper provides a detailed theoretical analysis of conductance in ferromagnetic atomic contacts, emphasizing the role of d bands and challenging previous experimental interpretations.

Findings

Conductance quantization is not expected in ideal ferromagnetic atomic contacts.

The conductance of the last plateau is typically above 2e^2/h.

Strong current polarization can occur in the tunneling regime.

Abstract

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