Characterization of the S 9 excited state in Mn12bromoacetate by electron paramagnetic resonance

1
Characterization of the S 9 excited state in
Mn12-bromoacetate by electron paramagnetic
resonance

• Konstantin Petukhov (UF, Physics)
• Stephen Hill (UF, Physics)
• Nicole Chakov (UF, Chemistry)
• George Christou (UF, Chemistry)

2
Motivation
Mn12O12(O2CCH3)16(H2O)4 2CH3CO2H 4H2O
Mn12-Ac
S4 symmetry only allows tunneling every fourth
step, for strictly axial case
But selection rules for QTM imposed by S4 is not
strictly obeyed by Mn12-Ac
S 10 ground state
S4 symmetry group
Source of transverse anisotropy to facilitate
tunneling?
3
Solvent discorder
Mn12O12(O2CCH3)16(H2O)4 2CH3CO2H 4H2O
A. Cornia et al., Phys. Rev. Lett. 89, 257201
(2002) A. Cornia et al., Acta Cryst. C 58, m371
(2002) S. Hill et al., Phys. Rev. Lett. 90,
217204 (2003) E. del Barco et al., Phys. Rev.
Lett. 91, 047203 (2003)
average molecular symmetry of Mn12-Ac clusters is
lower than axial
4
Mn12- bromoacetate
Mn12O12(O2CCH2Br)16(H2O)4 4CH2Cl2 Mn12-BrAc
Ideal candidate!

• No BrAcH / H2O molecules of crystallization
  only CH2Cl2
• Only one type of molecule in crystal
• should not show disorder in the hard axis spectra
• no variation in E ? no fine structure in the EPR

S 10 ground state S4 symmetry group
5
Mn12-BrAc vs. Mn12-Ac
Mn12-BrAc Mn12-Ac
6
Experimental techniques

• High-frequency EPR (cavities) Sample rotation
  capability
• M. Mola et al., Rev. Sci. Inst. 71, 186
  (2000) S.Takahashi et al., Rev. Sci. Inst.
• (to be published)
• 51.5, 65.4, and 76.9 GHz
• Magnetic field up to 9 T

700 GHz up to 45 T (NHMFL)
B
B
7
Experimental techniques
8
Angle-dependent EPR
simulation
9
Angle-dependent EPR
51.5 GHz, T 15 K
Appliyng different transverse fields one can
switch on and off transitions There is a
magic transverse field of 0.49 T
10
Angle-dependent EPR
11
Angle-dependent EPR
T 20 K, hard plane
51.5 GHz, T 15 K
51.5 GHz, T 15 K
12
Closer look to a9
S10
S9
51.5 GHz, hard plane
S9 excited state D - 0.430 cm-1 (-0.62
K) Anisotropy barrier DS2 50 K
Conclusion a9 S9 excited state
13
Closer look to a9
Energy level diagram for B ? z-axis
14
Closer look to a9
15
Conclusions
arXiv cond-mat/0403435
? 40 2 K

• S9 state is located at 40 2 K above the S10,
  MS 10
• ground state
• In perfect agreement with latest calculations
• K. Park et al., Phys. Rev. B 69, 014416
  (2004)
• Mn12-BrAc is intrinsically cleaner system than
  Mn12-Ac ?
• Interesting parity effect was observed