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SINGLE MOLECULE MAGNETS QUANTUM EFFECTS AND QUANTUM CONTROL

SINGLE MOLECULE MAGNETS QUANTUM EFFECTS AND QUANTUM CONTROL. The first single molecule magnet - Mn 12 -acetate. Ferro- or ferrimagnetically coupled magnetic ions Giant spin - up to 26 m B Organic ligands isolate the molecules. Well defined giant spin ( S = 10) at low temperatures (T < 10 K).

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SINGLE MOLECULE MAGNETS QUANTUM EFFECTS AND QUANTUM CONTROL

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  1. SINGLE MOLECULE MAGNETS QUANTUM EFFECTS AND QUANTUM CONTROL

  2. The first single molecule magnet - Mn12-acetate • Ferro- or ferrimagnetically coupled magnetic ions • Giant spin - up to 26mB • Organic ligands isolate the molecules • Well defined giant spin (S = 10) at low temperatures (T < 10 K)

  3. Other single molecule magnet examples Structure of [Ni(hmp)(t-BuEtOH)Cl]4 • The 4 Os in the hmp ligands bridge 4 S = 1 Ni ions to form a cube • The Ni ions couple ferromagnetically to give total spin S = 4

  4. Other single molecule magnet examples Mn4O3(OSiMe3)(O2CMe)3(dbm)3 Distorted cubane • MnIII (S = 2) and MnIV (S = 3/2) ions couple ferrimagnetically to give a total ground state spin of S = 9/2 • Christou/Hendrickson have discovered over 75% of all known SMMs

  5. energy magnetization orientation Magnetic Properties of Mn4 Complexes [Mn4O3X]R • A significant S value. S = 9/2 • Easy axis (Ising) type anisotropy D = -0.65 to -0.75 K • ΔE = (S2 -1/4)|D| = 20 D E • Advantages • Variation in core X group, with constant ligand groups • Variation in ligand R groups, for a constant Mn4O3X core • Soluble in organic solvents • Sub-nanoscale dimensions (core volume ~ 0.01 nm3)

  6. Site-selective reactivity o N c N N N Mn Mn Mn Mn Mn Mn O O O O O O Mn R Mn O O N Br O Mn Mn Mn Mn Mn Mn C O O O O O O O O Mn Mn Mn Mn Mn O O O H H Mn H C Cl Mn Mn O Mn O O F Mn O Mn Mn Mn Mn Mn O O Mn O O O O Mn Mn By varying the symmetry of the core, one can control tunneling rates

  7. Axial crystal field Thermally activated relaxation S2D • Barrier against magnetic relaxation • Magnetic bistability Ms is good quantum number

  8. Magnetic quantum tunneling Pure tunneling • Transverse anisotropy, i.e. terms that do not commute with Ŝz • Mixes states in the two wells  quantum tunneling

  9. Magnetic quantum tunneling Thermally assisted tunneling • Transverse anisotropy, i.e. terms that do not commute with Ŝz • Mixes states in the two wells  quantum tunneling

  10. Other sources of transverse anisotrpy Thermally assisted tunneling Pure tunneling • Higher order crystal field terms in Ô(4); disorder, strain, inter-molecular exchange and dipolar interactions, hyperfine fields in Ĥ'

  11. ` • Slow relaxation • In some systems t may exceed the time of the measurement, i.e. days, weeks or years • Temperature-independent magnetization relaxation at low temperature. • Highly axially symmetric systems show the slowest relaxation, e.g. Mn12-Ac.

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