Dynamical Negative Differential Resistance in Antiferromagnetically Coupled Few-Atom Spin-Chains

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We present the appearance of negative differential resistance (NDR) in spin-dependent electron transport through a few-atom spin-chain. A chain of three antiferromagnetically coupled Fe atoms(Fe trimer) was positioned on a Cu2N/Cu(100) surface and contacted with the spin-polarized tip of a scanning tunneling microscope, thus coupling the Fe trimer to one non-magnetic and one magnetic lead. Pronounced NDR appears at the low bias of 7 mV where inelastic electron tunneling dynamically locks the atomic spin in a long-lived excited state. This causes a rapid increase of the magnetoresistance between spin-polarized tip and Fe trimer and quenches elastic tunneling. By varying the coupling strength between tip and Fe trimer we find that in this transport regime the dynamic locking of the Fe trimer competes with magnetic exchange interaction, which statically forces the Fe trimer into the high-magnetoresistance state and removes the NDR.