Size-dependent alternation of magnetoresistive properties in atomic chains*

Spin-polarized electronic and transport properties of carbon atomic chains are investigated when they are capped with transition-metal (TM) atoms like Cr or Co, using density functional theory. The magnetic ground state of the TM-C$_n$-TM chains alternates between the ferromagnetic (F) and antiferromagnetic (AF) spin configurations as a function of $n$. The desirable AF state is obtained for only even-$n$ chains with Cr; conversely only odd-$n$ chains with Co have AF ground states. We present a simple model that can successfully simulate these variations, and the induced magnetic moments on the carbon atoms. Depending on the relative strengths of the spin-dependent couplings and the on-site energies of the TM atoms there induces long-range spin polarizations on the carbon atoms which mediate the exchange interaction. When connected to appropriate electrodes these atomic chains display a strong spin-valve effect. Analysis of electronic and magnetic properties of these atomic chains, and the indirect exchange coupling of the TM atoms through carbon chain will be presented. $^1$E. Durgun \textit{et al.}, J. Chem. Phys. \textbf{125}, 121102 (2006). $^2$E. Durgun \textit{et al.}, Phys. Rev. B \textbf{74} (\textit{in press}).