Molecular spintronics with spin crossover compounds

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1 FunMol 2012, Bonn 2012 innovating nanoscience October Molecular spintronics with spin crossover compounds Stefano Sanvito School of Physics and CRANN, Trinity College Dublin, IRELAND

2 Spin in organic materials τ S =l S /v F l S Hyperfine weak Spin-orbit weak! S (sec) How to manipulate spins? S. Sanvito, Chem. Soc. Rev. 40, 3336 (2011) l S (nm) G. Szulczewski, S. Sanvito and J.M.D. Coey, Nature Materials 8, 693 (2009) 10 0 Alq 3 h Rubrene i MW-CNT f Co a T 6 m Graphene g Cu a CNT e Si d GaAs b Si c

3 Our strategy Use molecules that do something!! 1) Look at spins close to the interface: Spinterface S. Sanvito, Nature Physics 6, 562 (2010) S. Sanvito, Nature 467, 664 (2010) 2) Use magnetism: magnetic molecules 3) Use the exchange interaction not the anisotropy 4) Find a convenient handle to address spin (currents or fields)

4 Outline Manipulating Spins by electric means Exchange: electrostatic spin crossover effect Spin state: spin crossover, a tricky theory problem Valence: Driving Valence Tautomerism Reading Spins Quantitative Transport Theory: Smeagol Transport in spin crossover compounds

5 Nadjib Baadji Exchange: Electrostatic Spin Crossover Manuel Piacenza, Fabio Della Sala, Tugba Tugsuz, Giuseppe Maruccio (NNL, Lecce)

6 Exchange: ESCE Use Stark effect! N. Baadji et al., Nature Materials 8, 189 (2009); Phys. Rev. B 80, (2009)

7 ESCE N. Baadji et al., Nature Materials 8, 189 (2009)

8 ESCE s 1 p V s 2 N. Baadji et al., Nature Materials 8, 189 (2009)

9 ESCE J 0 ST!E GS!E GS Singlet Triplet J 0 ST V V cross V V bias V cross

10 ESCE CoCp 2 S=1/2 Then accurate DFT (B3LYP)

11 ESCE Effect can be tuned

12 Mechanisms for manipulating electrically the spin of a molecule Electrostatic spin crossover effect q Need to generate fields in the V/nm range q Problem: the critical field scale with the exchange à either large field or weak magnetic coupling

13 Andrea Droghetti Spin state: Spin crossover, a tricky problem for theory Dario Alfé (UCL)

14 Spin Crossover ) & ' % $ & '!!" $!" # $!!" "! #" $!" % # %&% $(%&' %&# $(%&' The environment is important % % M. Kepenekian et al., PRB 79, (2009) M. Kepenekian et al., JACS 131, (2009)

15 Spin Crossover!! (' "%# Which Theory?! &' "%# +,%-!! (' +,%-!! &' )*!! (' )*!! &'!! "#$" % (' % &' %

16 Spin Crossover Which Theory? 1) Which atomic coordinates? Do I need to relax? How? 2) Is DFT good enough? Do I have alternative? Which functional? 3) How to benchmark with experiments? Should I benchmark against experiments? 4) Is the environment important?

17 Spin Crossover Which functional? [Fe(H 2 O) 6 ] 2+ [Fe(NH 3 ) 6 ] 2+ [Fe(NCH) [Fe(CO) 6 ] 2+ 6 ] 2+ Ligand field S=2 S=2 S=2 S=0 A. Droghetti, D. Alfè and S. Sanvito, J. Phys. Chem. 137, (2012)

18 Spin Crossover [Fe(H 2 O) 6 ] 2+ [Fe(NH 3 ) 6 ] 2+ [Fe(NCH) 6 ] 2+ [Fe(CO) 6 ] 2+ DFT ΔE adia (ev) DFT ΔE adia (ev) DFT ΔE adia (ev) DFT ΔE adia (ev) LDA LDA 0.96 LDA 2.37 LDA 5.18 GGA GGA 0.08 GGA 1.04 GGA 3.42 B3LYP B3LYP B3LYP B3LYP 1.27 PB PB PB PB HH HH HH HH 0.24 DMC -2.54(1) DMC -1.59(1) DMC -1.37(3) DMC 0.37(3)

19 Spin Crossover [FeL 2 ](BF 4 ) 2 (L=2,6-dypirazol-1-yl-4-hydroxymethylpyridine) ΔE adia (ev) DMC -1.21(4) Experimental Structure DMC -0.36(4) Experimental estimate ev LS è 271K Madelung Corrections DMC 0.14(0)

20 Spin Crossover A. Droghetti, D. Alfè and S. Sanvito, arxiv: (2012) Experimental ev 20% 25% 50% DFT ΔE adia (ev) LDA 0.96 GGA 0.08 B3LYP PB HH DMC -1.21(4) Use DFT sensibly!!!

21 Andrea Droghetti Valence: driving valence tautomerism Giordano Poneti, Andrea Dei and Roberta Sessoli (Firenze)

22 Driving valence tautomerism Co(Me 2 tpa)(dbcat)(pf 6 ) Co-dioxolene π * semiquinonate catecholate

23 Driving valence tautomerism T C =597 K in solid state, T C ~300 K in solution G. Ponenti et al., Angw. Chem. Int. Ed. 49, 1954 (2009)

24 Driving valence tautomerism A. Droghetti and SS, Phys. Rev. Lett. 107, (2011) DFT-B3LYP or similar needed!

25 Driving valence tautomerism Now apply E e g e g π * π * π * e g t 2g t 2g t 2g 0.2 V/Å 0.6 V/Å 0.8 V/Å ε

26 Driving valence tautomerism Is that all? ε ε ε = 0.01 V/Å for ΔT C = 20 K

27 Conclusion Mechanisms for manipulating electrically the spin of a molecule Valence Tautomerism q Need to generate fields in the V/nm range q Will the molecules survive on a surface? q Can one address the single molecule? Transport properties?

28 Nadjib Baadji Transport in spin crossover molecules

29 NEGF+DFT NEGF + DFT (Siesta) G G [H EM ] Right lead Extended molecule Left lead ρ H EM [ρ] Σ L + H EM [ρ] + Σ R T(E; V)

30 Smeagol Also include: Spin-orbit, Spin-Torque, Forces, SIC, LDA+U, HF, Hybrids, order-n Phys. Rev. B 73, (2006); Nature Materials 4, 335 (2005); Phys. Rev. B 78, (2008)

31 Transport Transport in in spin spin crossover crossover molecules molecules E (ev) L 2 2,2';6',2"-terpyridine (c) X B3LYP HS LS !E g (ev)

32 Transport in spin crossover molecules T(E,V=0) 10 0 HS LS 10-2 T E-E F (ev)

33 Transport in spin crossover molecules I-V 10-5 I (Amp) HS LS V (Volt) R SCMR R = I HS I LS I LS V (Volt)

34 Transport in spin crossover molecules 2 V g LS V V g 1 V 2

35 Transport in spin crossover molecules F. Prins et al., Adv. Mat. 23, 1545 (2011)

36 Transport in spin crossover molecules T. Miyamachi et al., Nature Comm. 3, 938 (2012)

37 Spin crossover molecules ü Massive changes in I-V originating from spin crossover ü In general one expects larger currents for HS ü Will the molecules survive on a surface? q What kind of electronic coupling with the electrodes are possible? q Is charging good or bad? Need care in using electronic structure methods

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