1 Tuning Molecule-mediated Spin Coupling in Bottom-up Fabricated Vanadium-TCNE Nanostructures Daniel Wegner Institute of Physics and Center for Nanotechnology (CeNTech) University of Münster Germany

2 (drosophila of SMMs) Why Bottom-Up ? Challenges Low temperatures Leads Molecule orientation Deposition Controlled model systems Single molecule Magnet: Mn 12 -acetate +− Bottom-up STM

3 molecule Our Goal Bottom-up design by STM manipulation Spin-coupling via molecules molecule 3d

4 Flexibility Engineer spin structures (1D, 2D) Tune spin coupling spin chain spin ladder triangular spin lattice square spin lattice Kagome spin lattice

5 Outline Motivation: molecular spin networks V + TCNE – promising candidates Building V x (TCNE) y molecules Magnetic properties of V x (TCNE) y Outlook

6 Vanadium + TCNE Bulk V x [TCNE] y Room-temperature magnet (T C ≈ 400 K) Crystal structure unclear  magnetic coupling unclear! Manriquez et al., Science (1991) tetracyanoethylene ? V 3d = molecule = 300 K magnetization magnetic field

7 Sample Preparation Molecules via leak valve at 300 K V deposition at low-temperature crystalline TCNE TCNE gas Ag(100) UHV leak valve LT-STM (T = 7 K) in-situ V e-beam evaporator

8 Outline Motivation: molecular spin networks V + TCNE– promising candidates Building V x (TCNE) y molecules Magnetic properties of V x (TCNE) y Outlook

9 V sample bias (mV) d 2 I/dV 2 TCNE V 20 Å Building V x (TCNE) y Complexes Synthesis of V-TCNE V-TCNE V TCNE V-TCNE V 2 TCNE -0.3 Å0 Å1.5 Å0.5 Å1.0 Å Reaction of V-TCNE with V  trans-V 2 TCNE LUMO Wegner et al., Nano Lett. 8, 131 (2008) Wegner et al., PRL 103, (2009) topodI/dV -0.6V

10 V x (TCNE) y Structure Analysis Model “rules” TCNE on bridge site V on hollow site V-N bond Å V-V Å > 10.4 Å -0.3 Å0 Å1.5 Å0.5 Å1.0 Å long V 2 TCNEshort V 2 TCNE Wegner et al., PRL 103, (2009)

11 Outline Motivation: molecular spin networks V + TCNE– promising candidates Building V x (TCNE) y molecules Magnetic properties of V x (TCNE) y Outlook

12 Kondo resonance conduction electrons screen impurity spin STS Probes the Spin  5Å  V-TCNE dI/dV sample V state E−E F (eV) Majority Minority V-LDOS dI/dV sample bias (mV) EFEF metal U V atom two local probes of V-spin DFT    5Å dI/dV sample bias (V) V(TCNE) 2 dI/dV sample bias (mV) Wegner et al., PRL 103, (2009)

13 Spin in V 2 TCNE Long V 2 TCNE -0.2 V state Kondo resonance Short V 2 TCNE -0.2 V state STS virtually identical No Kondo resonance! sample bias (V) 1 dI/dV 5Å    long V 2 TCNE 5Å    short V 2 TCNE sample bias (V) Magnetic coupling! sample bias (mV) dI/dV sample bias (mV) dI/dV ! Wegner et al., PRL 103, (2009)

14 Magnetic Coupling and Kondo Effect Single spin impurity Two spin impurities weakly coupled Two spin impurities strongly coupled weak coupling strong coupling long V 2 TCNE short V 2 TCNE Two-impurity Kondo problem Jayaprakash et al., PRL (1981) strong AFM coupling → no weak coupling → T K strong FM coupling → T K FM « T K T exp

15 Symmetry arguments Even number of electrons on TCNE (TCNE 0, TCNE 2− ) Odd number of electrons on TCNE (TCNE − ) FM vs. AFM Coupling 5Å short V 2 TCNE V-spins couple AFM V-spins couple FM Determine the charge state

16 DFT vs. Experiment V 2 TCNE dI/dV sample bias (V) V 2 TCNE exp. short long (V 2 TCNE) − DOS fits best ferromagnetic coupling Wegner et al., PRL 103, (2009)

17 Double Check: V-TCNE dI/dV sample V-TCNE exp. (V-TCNE) − DOS fits best isolated molecule: TCNE − Wegner et al., PRL 103, (2009)

18 Outline Motivation: molecular spin networks V + TCNE– promising candidates Building V x (TCNE) y molecules Magnetic properties of V x (TCNE) y Outlook

19 Outlook Weaken substrate coupling reduce screening increase spin coupling Build larger structures various symmetries, geometries engineered spin structures Substrate TCNE V Insulating monolayer

20 Summary Create V+TCNE complexes: - atomic-scale precision - strong chemical bond short V 2 TCNE V 2 TCNE molecules: Increase orbital overlap  Ferromagnetic coupling STS: two spin probes longV 2 TCNE sample bias (V) V majority d-state Kondo resonance sample bias (mV) Wegner et al., PRL 103, (2009)

21 Acknowledgements Michael F. Crommie Ryan Yamachika Xiaowei Zhang (UC Berkeley Physics) Jeffrey R. Long Bart M. Bartlett (UC Berkeley Chemistry) Mark Pederson Tunna Baruah (NRL Washington DC / UT El Paso) Alexander von Humboldt Foundation National Science Foundation Department of Energy