Molecular Spintronics with Scanning Tunneling Microscopy
Prof. Dr. W. Wulfhekel (Physikalisches Institut + Center for Functional Nanostructures, Karlsruhe Institute of Technology)

Molecular spintronics is an emerging field that combines molecular electronics, i.e. electronics using individual molecules as functional building blocks, and spintronics, i.e. electronics utilizing the spin degree of freedom of electrons in transport.
Firstly, we demonstrate that with the help of spin-polarized Scanning Tunneling Microscopy (Sp-STM) the spin transport across single organic molecules can be investigated and a molecular GMR junction can be realized. For this, single hydrogen phthalocyanine molecules were contacted by two ferromagnetic electrodes, i.e. a magnetic substrate and a magnetic STM tip. As substrate, ferromagnetic Co nano-islands grown Cu(111) were used, onto which the molecules were deposited. The magnetic state of the islands was determined by Sp-STM with Co tips. Then, the tip of the STM was approached in a controlled way towards a single molecule to contact the molecule. Below 0.4 nm distance, an attractive interaction between the tip and the molecule leads to a jump to contact of one of the side groups of the molecule and to a well defined molecular junction. Through the contacted molecule a GMR of 60% was observed which is one order of magnitude larger than the magnetoresistance without the molecule. This is explained on basis of ab initio calculations showing a selective hybridization of the molecular states with minority states of the electrodes in combination with off resonance transport [1].
Secondly, one of the electrodes has been replaced by an antiferromagnet (bct Mn films on Fe(001)) forming an ideally hard magnetic layer. Due to the local character of the hybridization with the molecular states, a significant but negative magnetoresistance of -50% can be observed [2]. The negative magnetoresistance is due to resonance transport across the molecule.
Finally, we show that combined spin and conduction switching functionality can be realized on the level of individual molecules. Spin crossover molecules are known to be switchable between a high- and a low-spin state by external stimuli. We demonstrate that the spin and conductance of Fe based spin crossover molecules can reversibly and reproducibly be switched electrically, if the individual molecule is decoupled from a metallic substrate by a thin insulating layer. Thus, a digital spin memory can be realized with one bit per molecule [3].

[1] S. Schmaus, A. Bagrets, Y. Nahas, T.K. Yamada, A. Bork, M. Bowen, E. Beaurepaire, F. Evers, W. Wulfhekel, Nature Nanotechnology 6, 185 (2011).
[2] A. Bagrets, S. Schmaus, A. Jaafar, D. Kramczynski, T.K. Yamada, M. Alouani, W. Wulfhekel, F. Evers, submitted to Nanoletters.
[3] T. Miyamachi, M. Gruber, V. Davesne, M. Bowen, S. Boukari, L. Joly, F. Scheurer, G. Rogez, T.K. Yamada, P. Ohresser, E. Baeurepaire, W. Wulfhekel, Nature Communications accepted..