Some New Geometric Phase Effects in Mn12 Variants

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Some New Geometric Phase Effects in Mn12 Variants
Jonathan FriedmanEduardo H. da Silva
Neto Michael Foss-FeigAmherst College
Christos LampropoulosGeorge Christou UFL -
Chemistry
Nurit AvrahamYuri MyaesoedovHadas ShtrikmanEli
ZeldovWeizmann Institute of Science
Funding NSF, Research Corporation and Amherst
College Dean of Faculty
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Mn12 Ac
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The Effect of a Transverse Field

• The tunneling rate for a particular pair of
  resonant levels depends on the transverse field
  (H-).
• H- increases the tunneling rate and reduces the
  barrier.

Thermal Activation
New Barrier
-9
9
-10
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D. A. Garanin and E. M. Chudnovsky, Phys. Rev. B
56, 11 102 (1997) J. R. Friedman, Phys. Rev. B,
57, 10291 (1998)
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Interference between Tunneling Paths in Fe8
Theoretical Prediction A. Garg., EPL, 1993.
W. Wernsdorfer and R. Sessoli, Science, 1999.
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Predicted Interference Effect for Mn12
Park and Garg, PRB, 2002
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Solvent Disorder in Mn12Ac
Acetic acid of solvation
Angle-selected relaxation rate. (del Barco, et
al., PRL, 2003)
Two positions of C deduced from X-ray diffraction
? induced 2nd order anisotropy
ESR spectrum as a function of azimuthal angle.
(Edwards, et al., PRL, 2003)
Cornia, et al., PRL, 2002
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A new SMM - Mn12 tBuAc
Mn12-Ac

Mn12-tBuAc

• Mn12-tBuAc has less solvent disorder and weaker
  dipole interactions.
• S. Hill et al., J. Appl. Phys. 97 (2005).
• W. Wernsdorfer et al. Phys. Rev. Lett. 96, 057208
  (2006).
• A.-L. Barra et al., JACS, 129, 10754 (2007).

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Controlling the Relaxation Rate of Mn12-tBuAc
with a Transverse Field
Applied Magnetic Field
position 2
?
Sample is rotated to position 1
T 2.98(3) K HT 4200 Oe HL-500 Oe
Large component of magnetic field along easy axis
Lower well populated
Sample is rotated to position 2
Small component of magnetic field along easy axis
Large component of magnetic field transverse to
easy axis
Quasi-exponential decay in magnetization.
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Longitudinal Field Rate Distribution
T 3.21 (3) K HT 4800 Oe
ON Resonance Relaxation Rate
OFF Resonance Relaxation Rate
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T 3.21 (3) K
Thermal Activation
-9
9
-10
10
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Rates on and off resonance
Off-resonance
On-resonance
Plateaus indicate dominant tunneling resonance

• Plateaus (much flatter!) in the off-resonance
  relaxation.
• Steps and plateaus occur at different fields on
  and off resonance

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Predicted Dependence of Relaxation Rate on
Transverse Field
On resonance
Off resonance
D. Garanin, arXiv0805.0391
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Now Add Transverse Anisotropy (Fourth Order!)
HT along hard axis.
S. Hill et al., J. Appl. Phys. 97, 10M510
(2005) A.-L. Barra et al., JACS, 129, 10754 (2007)
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Calculated Tunnel Splittings
Park and Garg, PRB, 2002
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Full Data vs Simulations
T 3.21(3) K
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Predicted Pressure-Induced Interference Effect
M. S. Foss-Feig and JRF, ArXiv 0809.2289
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Conclusions

• Measured relaxation rate as a function of
  transverse field in highly symmetric Mn12-tBuAc.
• Steps and plateaus in the relaxation rate as a
  function of transverse field both on and off
  resonance.
• Off-resonance results appear to be a remnant of a
  geometric-phase interference effect.
• Predicted new interference effect induced by
  uniaxial pressure.