Magnetic techniques for molecular and nanometric materials

1. Magnetic techniques for molecular and nanometric materials Dante Gatteschi & Roberta Sessoli February 2008

2. Diapositive disponibili: ftp://lamm21.chim.unifi.it/pub/Corso_Gatteschi_Sessoli Per ogni problema scrivere a: federico.totti@unifi.it

3. Molecular Magnetic Materials (nano) Magnetic Techniques (Sessoli) EPR (Gatteschi)

4. Molecular magnetic materials • simple paramagnets: step 1 • Interacting paramagnets: step 2 • Size effects: step 3

5. Bulk 3D magnets

6. The first molecular ferromagnet Miller, Epstein et al. MolCrystLiqCryst 1985

7. The first room temperature molecular magnet Miller, Epstein et al. Science, 1991

8. Nitroxides Tc= 0.6 K TC= 1.5 K

9. Fullerene Alemand et al. Science 1991 TC= 16 K

10. p-NC-C6F4-CNSSN•: a monomeric S-based radical

11. Single molecule magnets, SMM

12. MS=-10 Easy axis of magnetization MS= 10 The first single molecule magnet:Mn12-acetate top view T. Lis Acta Cryst.1980, B36, 2042. S4||z Mn(AcO)2•4H2O + KMnO4 in 60% v/v AcOH/H2O [Mn12O12(OAc)16(H2O)4]·2AcOH·4H2O lateral view z Ground state S = 8*2 - 4*3/2 = 10 Msaturation = 2.S = 20B Barrier 60 K

13. The library of molecular magnets: single chain magnets

14. More complex structures

15. Three different organizations • 2: embedded in amorphous silica • 3: LB film • 4: SAM • Bogani et al. Adv Mater in press

16. Reducing the size super paramagnet paramagnet magnet Quantum mechanics Classical physics ????????????

17. Inorganic radicals O2, NO.. Organic radicals Tyrosyl, nitroxides Paramagnet RE coordination compounds TM coordination compounds

18. Outline of the EPR section EPR in a nutshell: • The principle of the experiment • Basic EPR: the spin Hamiltonian HF experiments: • Radicals and Biological systems • Clusters

19. Outline of the EPR section 2 Spin interactions: • The spin hamiltonian of pairs • SH parameters of pairs The Mn12 testing ground: • Epr • Nmr

20. EPR Spectroscopy in a Nutshell • It is like NMR but is limited to paramagnetic systems • Invented by Zavoiski in Kazan in 1944 • It needs a magnetic field and electromagnetic radiation • Unlike NMR the field is scanned and the frequency is fixed

21. General design of an EPR spectrometer . Transmission line rectangular waveguides up to 150 GHz) corrugated waveguides. via space with refocusing devices oversized waveguides Source klystron (conventional) FIR lasers ( > 240 GHz) Gunn diodes (95-400 GHz) Carcinotron (very High power) Magnet electromagnets (up to 1.5 Tesla) superconductive magnets (up to 17 Tesla) resistive magnets (30 Tesla) hybrid magnets (45 Tesla) pulsed magnets (hundreds of Tesla) Detector crystal diodes bolometers Schottky diodes Sample environment resonating structure temperature control multiple irradiation

22. The microvave techniques are used in conventional EPR. The propagation of the radiation is made by using mono- modal metallic rectangular waveguides, metallic cavities and the other devices present in a typical microwave bridge. . • Most of the efforts for the development of EPR at high frequeny are aimed at the extention at millimeter and sub-millimeter waves of the general design of the conventional microwave bridge. • The main problem along this path is the availability and/or the design and realization of devices (magic Tees, circulator, phase shifter etc.) able to carry on the function of the low-frequency analogoue. The microwave techniques can be successfully extended up to 150 GHz ca. Above this frequency waveguides become eccessively lossy (typical figure of merit 12 dB/m at 250 GHz) and the rectangular or cylindric cavities eccessively small.

23. EPR Spectroscopy in a Nutshell: Zeeman Term In a system with S= 1/2, when the static magnetic field is parallel to z, E(M)= MgμBH a transition is observed when gzmBH= hν= gemBH0 Similar expressions hold for x, and y. The g values and their anisotropy depend on the chemical environment, therefore they provide structural information

24. Zeeman Splitting

25. 1 GHz= 3.3561x10-2 cm-1 Some Useful Relations Res. Freq. Band Res. Field (GHz) g=2.00 9 X 0.3234 35 Q 1.2578 95 W 3.1441 200 7.1876 300 10.7814 500 17.9690

26. Polycrystalline Powder EPR Spectra The EPR spectra of polycrystalline powders or frozen solutions provide the gx, gy, and gz values directly provided that the linewidth is smaller than the anisotropy

27. Polycrystalline Powder Spectra rhombic axial isotropic

28. The Spin Hamiltonian H = BB.g.S+S.D.S+ kIk.Ak.S Hyperfine Zeeman Fine

29. Interazione iperfine e superiperfine Cu2+ S=1/2 63Cu I=3/2 69% 65Cu I=3/2 31% 2nI+1 n=2, I=1 Informazioni sull’intorno di coordinazione 1- Termine di contatto: Axx=Ayy=Azz=8/3(gegnBn)|n(0)|2 2- Termine dipolare: anisotropo, traccia nulla (Axx+Ayy+Azz=0) 3- Pseudo- contatto : Interazione spin nucleare-momento orbitalico: è funzione dell’anisotropia di g Traccia non nulla, anisotropo

30. Il Cu2+ nei prioni 1 eq. 2 eq. 3 eq. 4 eq. 5 eq. 6 eq. 5.3 eq. Cu2+ Bassa conc. Alta conc. 7 linee  min. 3 N leganti Cu2+ legato pH=7.40 Affinità per il Cu2+ a pH>6 pH=4.00 Cu2+ libero Determinazione dei diversi siti leganti e della stechiometria Determinazione del numero di azoti leganti per uno dei siti coordinanti Biochemistry 2003 42, 6794

32. Q-band of 6

33. 6 in solution. RT, X-band

34. The spin hamiltonian and the parameters

35. High Frequency EPR: Why? • increased resolution • simpler spectra • orientation effects • spectra from integer spin systems with large zero field splitting • sign of the zero field splitting • different time scale

36. Enhanced Resolution • The g tensor anisotropy of tyrosyl radicals present for instance in Photosystem II is completely resolved at high frequency. This provides important structural information, like their main orientation in the membranes.

37. Tyrosyl Radicals • They are present in RNR and in Photosystem II • RNR: ribonucleotide reductase catalyzes the reduction of ribonucleotide to deoxyribonucleotides

38. EPR of Tyrosyl rad. of S. typhymurium gx=2.0090 gz=2.0022 gy=2.0044 250 GHz 9.45 GHz

39. Tyrosyl Radical The g values are sensitive to the environment x y gx is the most sensitive, because of the interaction of the non-bonding oxygen orbitals Un et al. JACS 1999, 121, 5743

40. Resolution effect P700+ radical cation of PSI

41. Tyrosyl Radical in Different Environments N-ac-L-tyr L-tyr-HCl RNREC PSII YD PSII YZ gx 2.0094 2.0067 2.00868 2.00740 2.00750 gy na 2.0045 2.00430 2.00425 2.00422 gz na 2.0023 2.00203 2.00205 2.00225 giso 2.0055 2.0045 2.00500 2.00466 2.00466 Brustolon et al. J Phys Chem A 1999, 103, 9636

42. Tyrosyl Radical in Different Species Tyrosyl radical of RNR of different species E.coli 2.0091 mouse 2.0076 herpes 2.0076 typhimurium 2.0089 JACS 120, 5080, 1998

43. Orientation Effects in membranes

44. Il tensore g nei metalli di transizione

45. z2 6 6 z 2 x xz yz y 2 2 Es: Cu2+ elongato 2 2 x2-y2 xy dx2-y2 dz2 dxy dxz,dyz 8 L’anisotropia del fattore g gi=ge+ g<ge dn n=1-4 g>ge dn n=6-9 Per un elettrone spaiato si ha: g// = ge + 8 /(Edxy-Edx2-y2) g = ge + 2 /(Edyz- Edx2-y2)

46. eg dx2-y2, dz2 Es: Fe3+ alto spin t2g dxy,dxz, dyz d5, 6A1g d1, 2T2g Es: Ti3+ Es: Fe3+ basso spin Es: V3+ d5, 2T2g d2, 3T1g Es: Fe2+ alto spin Es: Cr3+ d3, 4A1g d6, 5T2g Stati fondamentali in campo ottaedrico -1

47. Es: Co2+ Es: Ni2+ d7, 4T2g d8, 3A2g dx2-y2 dz2 dxy dxz,dyz d9, 2Eg Cu2+ d4 , 5Eg Mn3+ dx2-y2 dz2 dxy dxz,dyz elong. elong. dz2 dx2-y2 dxz,dyz dxy dz2 dx2-y2 dxz,dyz dxy comp. comp. Stati fondamentali in campo ottaedrico -2 Stati fondamentali Eg sono instabili rispetto alla distorsione Jahn-Teller e danno luogo a stati fondamentali orbitalmente non-degeneri

48. Perturbative Approach = ±/2S g= 

49. Valori di g per coordinazione pseudo-ottaedrica Conf. elett. S Stato fond. gx gy gz d1 1/2 2T2g ge-2/1 ge-2/2 ge-8/3 d2 1 3T1g ge-9/ ge-9/ ge d3 3/2 4A2g ge-8/1 ge-8/2 ge-8/3 d4 2 5Eg comp. ge-6/1 ge-6/2 ge elong.ge-2/1 ge-2/2 ge-8/3 d5 HS 5/2 6A1g ge ge ge d6 2 5T2g ge+2/1 ge+2/2 ge+2/3 d7 3/2 4T2g Oh 2(5-)/3 2(5-)/3 2(5-)/3 elong. 0 0 2(3-)/3 comp. 4 4 2 d8 1 3A2g ge+8/1 ge+8/2 ge+8/3 d9 1/2 2Eg elong. ge+2/1 ge+2/2 ge+8/3 comp. ge+6/1 ge+6/2 ge

50. g values for some ions