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SINGLE MOLECULE MAGNETS: HISTORY AND MODERN TRENDS S.M. Aldoshin

SINGLE MOLECULE MAGNETS: HISTORY AND MODERN TRENDS S.M. Aldoshin Institute of Problems of Chemical Physics, RAS Chernogolovka , Moscow Region, Russia. Southern Federal University 14 October, 2016. First Single Molecule Magnet: Mn 12 -acetate.

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SINGLE MOLECULE MAGNETS: HISTORY AND MODERN TRENDS S.M. Aldoshin

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  1. SINGLE MOLECULE MAGNETS: HISTORY AND MODERN TRENDS S.M. Aldoshin Institute of Problems of Chemical Physics, RAS Chernogolovka, Moscow Region, Russia Southern Federal University 14 October, 2016

  2. First Single Molecule Magnet: Mn12-acetate [Mn12O12(CH3COO)16(H2O)4]·2CH3COOH·4H20 T. Lis, Acta Crystallogr. Sect. B 36, 2042 (1980) Sessoli, R., Gatteschi, D., Caneschi, A. & Novak, M. A., Nature 365, 141–143 (1993) In the ground state it behaves as giant spin S=10 S=10 S1= 2 Mn(III) S2= 3/2 Mn(IV) Oxygen Carbon S= 8S1- 4S2 = 10

  3. High-spin molecules (SMM, SIM) S=3 S=3 S=4 Strong ferromagnetic exchange interaction between the unpaired electrons forms a high-spin ground state

  4. Internal Magnetic Anisotropy ТЕНЗОР РАСЩЕПЛЕНИЯ В НУЛЕВОМ ПОЛЕ ZERO-FIELDSPLITTING (ZFS) TENSOR(FINE-STRUCTURE TENSOR) S = 6

  5. Single Molecule Magnets (SMM’s): bistable molecular magnetic units MS= 0 |DS| S2 +8 -8 +9 -9 MS= +10 MS= -10 •SMMs are magnetically bistablesystems that require an applied field to invert theirmagnetization direction below blocking temperature “Frozen” superparamagnetic states •Bistability (SMM) stems from large spin [S = 10] and negativemagnetic anisotropy b=|DS| S2 - spin-reversal barrier

  6. Magnetization Reversal Mechanisms Spin-phonon cascade relaxation Ground state quantum tunneling HZ=0 HZ=0 Phonon assisted quantum tunneling HZ=0

  7. Experimental Evidences of Single Molecule Magnetism The Arrhenius law  = 0 exp[b/(kBT)]  - relaxation time Temperature and frequency dependences of real (’) and imaginary (’’) parts of ac -susceptibility

  8. Experimental Evidences of Single Molecule Magnetism Magnetization hysteresis loops Mn12-ac L.Thomas et al. Nature, 383 (1996) p.145 It looks like hysterezis in bulk magnet but  It is not a bulk property (long-range magnetic order, domains, etc.)  It is essentially a single-molecule property !!!  It appears due to slow relaxation of magnetization

  9. Experimental evidences of Single Molecule Magnetism Steps in hysteresis loops arise from quantum tunneling of magnetization Tunneling Cannot appear in bulk magnets !!! Mn12-ac R. Sessoli

  10. Potential applications of SMMs  Magnetic memory of extremely high density – 104 times higher than existing memory densities  Quantum computers–SMMs as qubits • Molecular spintronics –SMMs as spin transistors L. Bogdani, W. Wernsdorfer, Nature Materials 2008, 7(3), 179-186 Qubits M. N. Leuenberger, D. Loss, Nature 2001, 410, 789.

  11. Another Well Known Single Molecule Magnet [Fe8O2(OH)12(tacn)6]Br8.9H20 tacn=1,4,7-Triazacyclononane Fe3+ S = 5/2 S =10 K. Wieghardt, K. Pohl, I. Jibril and G. Huttner, Angew. Chem. Int. Ed. Engl. 1984, 23, 77. As distinguished from Mn12-ac exhibiting only phonon-assisted QTM in Fe8 pure QTM does exist

  12. More Recent Examples - Giant Mn70 Single Molecule Magnets  4nm Diameter [Mn70O60(O2CMe)70(OEt)20(EtOH)16(H2O)22]  [Mn70O60(O2CMe)70(OC2H4Cl)20(ClC2H4OH)18(H2O)22]  A. Vinslava, A. J. Tasiopoulos, W. Wernsdorfer, K. A. Abboud, G. Christou Inorganic Chemistry, Publication Date (Web): February 9, 2016 

  13. ΔE/k~ 29.4 K 0~ 4.26∙10-8 с Calix[4]arene-based SMM * Karotsis G., Teat Simon J., Wernsdorfer W., Piligkos S., Dalgarno Scott J. and Brechin Euan K. (2009), Angewandte Chemie, 121: 8435–8438. * *S.M. Aldoshin, I.S. Antipin, V.I. Ovcharenko, et al, Russ.Chem.Bull., Int.Ed., 62 (2) (2013) 536-542. Temperature dependence of χT (exp. 300K 14.18; Calc. 300K 14.75) Dependence of normed magnetization (M/NµB) on H/T at various intensity of external magnetic field. M→ 16, S=8 (Smax=9: 4 Mn(III), 5 Mn(II)) In calixarene-containing [MnIII2MnII2] clusters, the replacement of a peripheral ligand by the pyridine molecule results in the increase of the ferromagnetic transition temperature (from 1.1*to 5 K * *).

  14. Calix[4]arene manganese complex of [MnII2MnIII2] type with the dipyridyl ligand S.M. Aldoshin, I.S. Antipin, V.I. Ovcharenko, etc, Russ.Chem.Bull., Int.Ed., 62 (2) (2013) 536-542. I II (a) I II Diagrams of the energetic levels of I (a) and II (b) obtained by diagonalization of matrix presentation of the spin-Hamiltonian. I II Complex II does not behave as SMMs S.M. Aldoshin, I.S. Antipin, V.I. Ovcharenko, etc, Russ.Chem.Bull., Int.Ed., 62 (2) (2013) 536-542 (for I); S. M. Aldoshin, et al. J. of Mol. Structure (2015), 1081, 227 (for II). I rhomb II parallelogram

  15. Might a molecular spin cluster serve as a memory storage unit? For a distance 5nm between neighboring spins, a disc with the area 100 cm2 will hold: 4 ·1014 spins The state MS=S of each cluster (spin) would be used to store a classical bit (it remembers magnetization !!!): 1spin↔1bit Thedisc holds a staggering amount of memory: 50 000 gigabytes !!! At T=1.5K the relaxation time for Mn12-ac is 108s (3 years). This is not enough for computers elements (even if the refrigeration problem would be solved). An acceptable relaxation time: at least 15 years at room temperature!!!

  16. Main aim –to increase the barrier b=|DS| S 2in order to increaseTb How could we reach this aim ? By creating larger systems in order to increase the spin? No progress in increasing Tb in this way. Why?

  17. There is no progress in increasing Tb by increasing S. Why? Does it mean that is increased with the increase of as ? No, because O. Waldmann, Inorg. Chem., 2007, 46, 10035–10037. is almost independent of The only way to increase (and hence Tb) is to try to increase Since is mainly due to single-ion ZFS that is small (1-10 cm-1) for “spin clusters” (clusters without orbital angular momentum) so we must go beyond spin clusters!!!

  18. New trend: to use highly anisotropic ions with unquenched (or not fully quenched orbital angular momenta) Diversity of new classes of SMMs based on clusters and mononuclear complexes of highly anisotropic 3d, 4d, 5d, 4f and 5f ions • Mononuclear 4f – complexes -Single Ion Magnets (SIMs) and clusters composed of 4f – ions-SMMs Mixed SMMs of 4f-nd – type • SMMs based on clusters containing orbitally-degenerate nd-ions • SIMs based on momonuclear complexes of highly anisotropic nd-ions

  19. New trend: to use highly anisotropic ions with unquenched (or not fully quenched orbital angular momenta) Diversity of new classes of SMMs based on clusters and mononuclear complexes of highly anisotropic 3d, 4d, 5d, 4f and 5f ions • Mononuclear 4f – complexes -Single Ion Magnets (SIMs) and clusters composed of 4f – ions-SMMs Mixed SMMs of 4f-nd – type • SMMs based on clusters containing orbitally-degenerate nd-ions • SIMs based on mononuclear complexes of highly anisotropic nd-ions

  20. First Ln-based SIMs –lanthanide double-decker complexes N. Ishikawa et al, JACS, 2003 For Ln =Tb For Ln =Dy

  21. Origin of strong magnetic anisotropy in lanthanide double-decker complexes Some 4f- ions with orbital angular momentum Crystal field Hamiltonian of approximately D4d -symmetry , ,  -Stevens coefficients, - operator equivalents • plays the same role as spin-Hamiltonian of zero field splitting • in spin clusters giving rise to the magnetic anisotropy that is • however much strongerthan in pure spin systems • Another asvantage of 4f – ions – weak electron-phonon interaction • and hence slow relaxation SMM behavior can be obtained even for mononuclear complexes !!!

  22. Another example of mononuclear 4f –based SIM , Ln

  23. Example of SMMs based on clusters of 4f –ions b=227 cm-1

  24. New trend: to use highly anisotropic ions with unquenched (or not fully quenched orbital angular momenta) Diversity of new classes of SMMs based on clusters and mononuclear complexes of highly anisotropic 3d, 4d, 5d, 4f and 5f ions • Mononuclear 4f – complexes -Single Ion Magnets (SIMs) and clusters composed of 4f – ions-SMMs  Mixed SMMs of 4f-nd – type • SMMs based on clusters containing orbitally-degenerate nd-ions • SIMs based on mononuclear complexes of highly anisotropic nd-ions

  25. Example of 3d-4f cluster showing SMM behavior b=127 K (temperature 7.5 -9.5 K) b=51 K (temperature 4.5 -7.5 K) •  Advantage of 4f- ion – high magnetic anisotropy • Advantage of 3d – ion – increases magnetic moment due to ferromagnetic 3d-4f - exchange interaction

  26. In Search of New 3d-4f –SMMs: Series of Thiacalix[4]areneTetranuclear MII2LnIII2 Clusters (M=Mn, Co, Ln = Gd, Eu, Pr,Tb, Dy) Already studied - do not behave as SMMs We expect them to be SMMs Fragment of the structure S.M. Aldoshin, N.A. Sanina, et al, Russ.Chem.Bull, 2014, 63, 1465-1474. S. M. Aldoshin,  N. A. Sanina,  A. V. Palii,   B. S. Tsukerblat Inorg. Chem.,  2016, 55, 3566−3575.

  27. Specific structural features and magnetic properties of thiacalix[4]arene-containing M2Ln2-complexes (Mn2Pr2 – I; Mn2Eu2 – II; Co2Eu2 -III) Independent part of the structure. The hydrogen atoms are not shown II I The central fragment of the metal complex III The temperature dependences of the effective magnetic moment µeff The temperature dependence of the effective magnetic moment µeff The temperature dependences of the effective magnetic moment µeff Dependence of the magnetic moment (M) on the intensity of the magnetic field (T=2K). The solid line shows the calculated plot of Brillouin function for the total magnetic moment J-1/2 Dependence of the magnetic moment (M) on the intensity of the magnetic field Dependence of the magnetic moment (M) on the intensityof the magnetic field

  28. Exchange Coupling and Magnetic Anisotropy in theCoII2EuIII2 cluster Temperature dependence of the effective magnetic moment of the powder sample of thiacalix[4]areneCoII2EuIII2 complex. Experimental dataare shown by red circles, and the theoretical curve) calculated with the best-fit parameters) is shown by the solid black line. Temperature dependence of the effective magnetic moment of the EuIII ion. • The seven-coordinated CoII (3/2) ions: • the first order orbital angular momentum is quenched; • The anisotropy can be described by the ZFS spin Hamiltonian: • µeff can be calculated as The model for the CoII2EuIII2 complex includes uniaxial anisotropy of the seven-coordinate CoII ions and an isotropic exchange interaction in the CoII2 pair, while the EuIII ions are diamagnetic in their ground states. Best-fit analysis of χT(T) –dependence showed that the anisotropic contribution (arising from a large zero-field splitting in CoIIions) dominates (weak exchange limit) in the CoII2EuIII2 complex. Best fit parameters: D = 20.5 cm−1, J = −0.4 cm−1, gCo = 2.22 (a strong orbital contributaion).

  29. Exchange Coupling and Magnetic Anisotropy in MnII2(5/2)GdIII2(7/2) Classical representation of the spin excitations in the MnII2GdIII2 tetramer, which is modeled by a linear chain according to the topology of the exchange network: (a) ground-state configuration, in which the spins of the MnII ions are antiparallel while the spins of The GdIII ions are parallel to the spins of the neighboring MnII ions; (b) spin excitation in the (MnIIGdIII)−(MnIIGdIII) dimer, in which spins of the pairs MnIIGdIIIare tilted with respect to one another; (c) representation of the excitation in terms of the effective spins of the dimers. The MnII,GdIII ions have half-filled 3d and 4f sub-shells. The ions do not carry orbital angular momentum. The non-monotonic temperature dependence of the χT product observed for the MnII2GdIII2 complex is attributed to the competitive influence of the ferromagnetic Mn−Gd and antiferromagnetic Mn−Mn exchange interactions, the latter being stronger. Best fit parameters:J(Mn, Mn) = −1.6 cm-1 J(Mn, Gd) = 0.8 cm-1, g = 1.97. Classical representation of the spin structure of the tetramer in the ground state. Scheme of the exchange pathways in the tetranuclear MnII2GdIII2 cluster. Spin Hamiltonian including HDVV exchange interactions and the isotropic Zeeman interaction.

  30. New trend: to use highly anisotropic ions with unquenched (or not fully quenched orbital angular momenta) Diversity of new classes of SMMs based on clusters and mononuclear complexes of highly anisotropic 3d, 4d, 5d, 4f and 5f ions • Mononuclear 4f – complexes -Single Ion Magnets (SIMs) and clusters composed of 4f – ions-SMMs  Mixed SMMs of 4f-nd – type • SMMs based on clusters containing orbitally-degenerate nd-ions • SIMs based on mononuclear complexes of highly anisotropic nd-ions

  31. Mn(III) Mn(II) N C Example of SMM containing ions with unquenched orbital angular momenta - {[MnII(tmphen)2]3[MnIII(CN)6]2} -cyanide based cluster tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline Mn5-cyanide

  32. Mn(III) e Mn(II) t2 N C Mn5 -cyanide SMM – origin of localmagnetic anisotropy Two apical Mn(III) ions in strong cubic crystal field of six carbon atoms S=1 d4 Carbon surroundings of Mn(III) ions are slightly trigonally distorted Unquenched orbital angular momentum L=1 Splitting of L=1 -term in trigonal crystal field gives rise to negative magnetic anisotropy and thus to the barrier for magnetization reversal A. V. Palii, S. M. Ostrovsky, S. I. Klokishner, B. S. Tsukerblat, C. P. Berlinguette, K. R. Dunbar, J. R. Galán-Mascarós, J. Am. Chem. Soc., 2004, Vol. 126, p. 16860-16867.

  33. Example of linear trimeric 3d-5d SMM with strong exchange anisotropy 5-Brsalen=N,N’-ethylenebis(5-bromosalicylideneiminato) MnIII OsIII MnIII 5d-orbitals give rise to strong exchange coupling K. S. Pedersen, M. Schau-Magnussen, J. Bendix, H. Weihe, A. V. Palii, S. I. Klokishner, S. Ostrovsky, O. S. Reu, H. Mutka, P. L. W. Tregenna-Piggott, Chem. Eur. J. 2010, 16, 13458–13464. Strong Ising-type superexchange is responsible for SMM properties

  34. New trend: to use highly anisotropic ions with unquenched (or not fully quenched orbital angular momenta) Diversity of new classes of SMMs based on clusters and mononuclear complexes of highly anisotropic 3d, 4d, 5d, 4f and 5f ions • Mononuclear 4f – complexes -Single Ion Magnets (SIMs) and clusters composed of 4f – ions-SMMs  Mixed SMMs of 4f-nd – type • SMMs based on clusters containing orbitally-degenerate nd-ions • SIMs based on mononuclear complexes of highly anisotropic nd-ions

  35. The role of internal magnetic anisotropy in the formation of magnetization reversal barrier MS=±S Ueff = |D|S2 Ueff = |D|(S2-1/4) Eeff S–an integer S–a half of an integer MS= 0 D<0 D>0 D ~ 1/S2 To increase the total system spin To increase the internal magnetic anisotropy ? • Spin-Spin (dipole-dipole) interaction (SS) • Spin-Orbit interaction (SOC)

  36. First example of SIM based on transition metal ion N b= 42 cm−1 FeII N N N field induced SIM Acts as SIM in applied dc field 1500 Oe Freedman, D. E.; Harman, W. H.; Harris, T. D.; Long, G. J.; Chang, C. J.; Long, J. R. J. Am. Chem. Soc. 2010, 132, 1224.

  37. Linear two-coordinate Fe(I) SIM Fe(I) J. M. Zadrozny et al, Nat.Chem. 2013, 5(1), 577-581. - record for 3d-based SMMs!!! Low coordination number Strong anisotropy

  38. Influence of a heavy atom on the magnetic anisotropy of 3d complexes S. Yeand F. Neese, J. Chem. Theory Comput. 2012, 8, 2344−2351 Ni scorpionate

  39. Influence of a heavy atom on the increase of magnetic anisotropy in pseudotetrahedral complexes of Co(II) I [Co(EPh)4]2– (E = O, S, Se) (M. Zadrozny, J. Telser, J. R. Long, Polyhedron, 64 (2013) 209–217. J. M. Barony, J. R. Long, J. Am. Chem.Soc., 133 (2011) 20732−20734) Significant increase of the D parameter in the row of O,S, Se II [Co(L)(MeCN)X2] (L = O- or S-donor ligand, X = Cl– or Br–) further supported the observation of increased D in the complexes with sulfur atom analogues. (S. Vaidya, A. Upadhyay, et al. Chem. Commun., 51 (2015) 3739–3742). III Co(L)2I2 (L = quinoline, Ph3P, Ph3As); D increases in a series of N-, P-, As-donor ligand IV [Co(Ph3P)2X2] (X = Cl–, Br–, I–) (the heavier halogen, the higher is D parameter) (M.Saber, K. Dunbar, Chem. Commun., 50 (2014) 12266–12269). V [Co(qu)2X2] (qu = quinoline, and X =NCS, Cl–, Br–(D.V. Korchagin; G. V. Shilov; S. M. Aldoshin; et al, Polyhedron, 2015,102 , pp.147–151),I– . Small Effect!!!

  40. Examples of 3d complexes with large magnetic anisotropy D ~ −5.7 cm-1 Magnetization relaxation time, lnτvs. T–1 plot. The relaxation follows an Arrhenius plot with activation energy (Ueff) 25.6 cm–1 (17.8 K) and a pre-exponential factor (τ0) 1.4·10–8 s. No SMM D ~ + 6.1 cm-1 S. M. Aldoshin etal, in press

  41. UV photolysis of the precursor 1 (2,4,6-triazido-3,5-dibromopyridine ) in solid argon matrix at temperature 15 K Expected paramagnetic products: 1. Two triplet mononitrenes T-1 and T-2. 2. Two quintet dinitrenes Q-1 andQ-2. 3. Septet trinitreneS-1.

  42. MononitrenesS = 1 Curt Wentrup, at al., J. Org. Chem. 2006, 71, 4049-4058

  43. DinitrenesS = 2

  44. TrinitrenesS = 3 J. Phys.Chem. A, 119, 2413 (2015)

  45. Conclusions The addition of heavy atoms has been found to stimulate a strong spin-orbital component in the magnetic anisotropy. The studied polynitrenes are the first high-spin organic molecules with the contribution of spin-orbital interaction in the magnetic parameters being dominating over the spin-spin component. The spin-orbital interaction determines: • the value of D, • the sign of D, • the ratio of E/Dand • direction of the “light” magnetic axis This is a new type of organic high-spin molecules with the magnetic anisotropy forming preferably under the action of spin-orbital interactions.

  46. Two Co(II)-complexes exhibiting field induced SIM behavior Co(hfac)2(H2O)2 (2) Et4N+[Co(hfac)3] _ (1) hfac = CF3C(O)CH2C(O)CF3 hexafluoroacetylacetone

  47. Griffith Hamiltonian (GH) - fitting parameters - to be determined from quantum-chemical calculations Key question - the sign of which determines the sign of the anisotropy 1) - the ground state of the axial crystal field Hamiltonan is orbital singlet (ML = 0) 2) - the ground state is orbital doublet (ML = +1, -1) ZFS spin Hamiltonian is applicable ZFS spin Hamiltonian cannot be used 1) - pisitive magnetic anisotropy (ZZ< XX, YY) – easy plane 2) - negative magnetic anisotropy (ZZ > XX, YY) – easy axis

  48. Griffith diagram Spin Hamiltonian is applicable to describe this splitting Spin Hamiltonian cannot be used for the description of this splitting

  49. Crystal field splitting (case of ) + Octahedral Ground Excited Axially distorted One electron (orbital) energy scheme Many-electron energy scheme (terms)

  50. Crystal field splitting (case of ) + Ground Octahedral Excited Axially distorted One electron (orbital) energy scheme Many-electron energy scheme (terms)

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