Switching of a quantum dot spin valve by single molecule magnets

Abstract

We explore theoretically the spin transport in nanostructures consisting of a gold quantum dot bridging nonmagnetic electrodes and two Mn12-Ph single molecule magnets (SMMs) that are thiol-bonded to the dot but are not in direct contact with the electrodes. We find that reversal of the magnetic moment of either SMM by the application of a magnetic field leads to a large change in the resistance of the dot, i.e., a strong spin valve effect. We show that this phenomenon arises from a novel physical principle: The spin-dependent molecular orbitals that extend over the dot and both SMMs change drastically when the magnetic moment of either SMM is reversed, resulting in a large change in the conduction via those orbitals. The same physics may also be responsible for the spin valve phenomena discovered recently in carbon nanotube quantum dots with rare earth SMMs by Urdampilleta, Klyatskaya, Cleuziou, Ruben and Wernsdorfer [Nature Mater. 10, 502506 (2011)]. PACS numbers: 85.75.Bb,75.50.Xx, 85.35.Be Molecular spintronics 1 combines two active fields of study: molecular electronics and spintronics. The ultimate goal of this field is to control the electronic spin and charge on the molecular scale where quantum effects emerge. In the search for new molecular spintronic nano devices, single molecule magnets (SMMs) 2 appear to be natural candidates. 3 A SMM is a molecular-size nano-magnet that exhibits quantum behaviour such as quantum tunneling of the magnetization (QTM) 2 , Berry phase interference 4 , the Kondo effect 5 , and magnetoresistance phenomena. 3,6 Spin valve-like effects have also been predicted in SMMs with magnetic electrodes. 7,8 In a recent advance, experimental detection of spin valve-like behavior has been reported 6 in a system with non-magnetic electrodes and SMMs coupled to a (non-magnetic) carbon nanotube quantum dot. It was suggested 6 that this behavior was due to the magnetic moments of individual SMMs bound to the dot reversing at different values of the applied magnetic field as the field was swept. It was proposed 6 that this would result in changes in the resistance of the dot due to modulation of the spin transport through the dot, i.e., a spin valve effect. However, to our knowledge, no relevant quantitative theory has as yet been reported. Thus the physics responsible for the observed novel behavior 6 has not been definitively identified.Here we explore spin valve phenomena in devices with a pair of SMMs bound to a non-magnetic quantum dot bridging non-magnetic electrodes theoretically for the first time. Our results reveal that spin valve functionality in such devices can arise from current carrying electronic resonant states that extend over the whole device, including the quantum dot and both SMMs that are bound to it. When the magnetic moment of one of the SMMs is reversed, these states, being spin-dependent, are strongly modified. Consequently the resistance of the device changes. Thus the system exhibits a spin valve effect. Thus our findings demonstrate a novel principle for spin...

Quantum turnstile operation of single-molecule magnets

“…A complementary approach to transport properties of SMMs relies on density functional theory (DFT) [31,32]. In the DFT approach the molecular structure and the contact regions are carefully taken into account, while the many-body correlations within the SMM are accounted for within various approximations.…”