  With the goal to elucidate the nature of spin-dependent electronic transport
in ferromagnetic atomic contacts, we present here a combined experimental and
theoretical study of the conductance and shot noise of metallic atomic contacts
made of the 3d ferromagnetic materials Fe, Co, and Ni. For comparison, we also
present the corresponding results for the noble metal Cu. Conductance and shot
noise measurements, performed using a low-temperature break junction setup,
show that in these ferromagnetic nanowires: (i) there is no conductance
quantization of any kind, (ii) transport is dominated by several partially-open
conduction channels, even in the case of single-atom contacts, and (iii) the
Fano factor of large contacts saturates to values that clearly differs from
those of monovalent (nonmagnetic) metals. We rationalize these observations
with the help of a theoretical approach that combines molecular dynamics
simulations to describe the junction formation with nonequilibrium Green's
function techniques to compute the transport properties within the
Landauer-Buettiker framework. Our theoretical approach successfully reproduces
all the basic experimental results and it shows that all the observations can
be traced back to the fact that the d bands of the minority-spin electrons play
a fundamental role in the transport through ferromagnetic atomic-size contacts.
These d bands give rise to partially open conduction channels for any contact
size, which in turn lead naturally to the different observations described
above. Thus, the transport picture for these nanoscale ferromagnetic wires that
emerges from the ensemble of our results is clearly at variance with the well
established conduction mechanism that governs the transport in macroscopic
ferromagnetic wires, where the d bands are responsible for the magnetism but do
not take part in the charge flow.
