Shot noise of spin current in ferromagnet-normal-metal systems

– We propose a three-terminal spin-valve setup, to determine experimentally the spin-dependent shot noise, which carries information on the spin-relaxation processes. Based on a spin-dependent Boltzmann-Langevin approach, we show that the spin Fano factor, defined as the spin shot noise to the mean charge current, strongly depends on the spin-flip scattering rate in the normal wire. While in the parallel configuration the spin Fano factor always decreases below its unpolarized value with increasing spin injection, for the antiparallel case it varies nonmonotonically. We also show that in contrast to the charge current Fano factor, which varies appreciably only in the antiparallel case, the spin Fano factor allows for a more sensitive determination of the spin-flip scattering rate. The importance of shot noise in mesoscopic systems has been recognized in the past years as a result of extensive experimental and theoretical studies of currents fluctuations in a wide variety of hybrid structures involving normal metals, semiconductors and superconductors [1,2]. Correlations of current fluctuations at low temperatures provide unique information about the charge, the statistics and the scattering of the current carriers. In spintronic structures [3–10], in which the transport involves both charge and spin degrees of freedom, the current fluctuations are expected to contain spin-resolved information on the conductance process. Consequently, spin-polarized current correlations can be used to extract information about spindependent scattering and spin accumulation in ferromagnet(F)-normal-metal(N) structures. Until very recently, spin-polarized shot noise has received little attention. Results of the earlier studies of shot noise in FNF [11] and FIF [12] systems have been explained in terms of the well-known results of the corresponding unpolarized systems for two spin directions [1]. Tserkovnyak and Brataas have found that shot noise in double-barrier FNF structures, in which the F-terminals have noncollinear magnetizations, depends on the relative orientation of the magnetizations of the terminals [13]. Results of more recent studies [14–19] have revealed that the spin-flip scattering in FNF structures can change the current correlations c © EDP Sciences Article published by EDP Sciences and available at http://www.edpsciences.org/epl or http://dx.doi.org/10.1209/epl/i2005-10036-0 818 EUROPHYSICS LETTERS