Single-Molecule Magnets (SMMs):A Molecular (Bottom-up) Approach to Nanoscale
Magnetic Materials
George ChristouDepartment of Chemistry, University of Florida
Gainesville, FL 32611-7200, USA
Lecture 1:The Mn12 Family of Single-Molecule Magnets
Single-Molecule Magnets (SMMs)Molecular Nanomagnets
magnetism intrinsic to the molecule;each molecule is a separate, nanoscale
magnetic particle
Types of Magnetic Materials
MagnetismTraditional Magnets
3D arrays of metal atoms or ions(metals, metal alloys, metal oxides
e.g. magnetite - Fe3O4)
Single-Chain Magnets (SCMs)
magnetism due to 1-D chains
Molecular Magnets
3D arrays of linked, interacting molecules
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Anisotropy barrier (U) = S2|D| Integer spinor (S2-1/4)|D| Half- integer spin
ms = -7
ms = -1ms = -2ms = -3
ms = -4
ms = -5
ms = -6
ms = -8
ms = -9
ms = -10
ms = 7
ms = 1ms = 2ms = 3
ms = 4
ms = 5
ms = 6
ms = 8
ms = 9
ms = 10
U
E
ms = 0
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The barrier to magnetization relaxation in SMMs is not due to intermolecularinteractions (as in traditional magnets) but to zero-field splitting (ZFS).
( ) 44
44
22 ˆBˆˆ OSSE yx +−04
04
2z
ˆBSD O+ HSg B ⋅�
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HSSSESD BYXZ ⋅+−+= ˆg)ˆˆ(ˆ 222 µH
rm Zeeman te termrhombic termaxial ++=H
Advantages of SMMs over Traditional Nanoscale Magnetic Particles
� truly monodisperse particles of nanoscale dimensions
� crystalline, therefore contain highly ordered assemblies
� well-defined ground state spin, S
� truly quantum spin systems
� synthesized by room temperature, solution methods
� enveloped in a protective shell of organic groups (ligands)
� truly soluble (rather than colloidal suspensions) in organic solvents
� the organic shell (ligands) around the magnetic core can be easily modified, providing control of separations between molecules, coupling with the environment, etc.
Synthesis
Properties
Major Potential Applications of SMMs
Quantum Computing- the use of quantum bits (qubits) rather than classical bits
as in present computation methods.- requires SMMs capable of existing in quantum
superpositions of two (or more) statesi.e. 1 and 0 instead of classical 1 or 0.
- requires SMM to show appropriate quantum properties.
Digital Information Storage- the storage of information at the molecular level as the
orientation direction of the magnetization vector. - each molecule stores one bit of information.- estimated 104 increase in storage density over present
devices.- requires ordered arrays of SMMs, either 2-D surfaces (present technology) or 3-D crystals (future technology).
Frequency-dependent out-of-phaseAC susceptibility signals (�M˝)
Hysteresis loops (with steps due to quantum tunneling of the
magnetization)
Experimental Identification of SMMs
They display the slow magnetization relaxation (reorientation) ratesof single-domain superparamagnets, even though they are much smaller
and the properties arise from a different origin (not long-range co-operativity)
Quantum Tunneling of the Magnetization (QTM)SMM’s are true mesoscale particles. They exhibit the macroscale
property of hysteresis, and the microscale property of QTM through the anisotropy barrier
Barrier (U) = S2|D| for integer S
= (S2-1/4)|D| for half-integer S
Therefore, U < Ueff
Topics for this presentation:
• Crystalline arrays of Mn12 SMMs, and their controlled modification.
• Faster-Relaxing Mn12 SMMs: Jahn-Teller Isomerism
• Electron addition onto Mn12 SMMs, and itseffect on the properties
• New high-quality Mn12 SMMs: the picture comesinto focus.
[Mn12O12(O2CR)16(H2O)4] (Mn12) complexes:
� S = 10� D = -0.40 to -0.50 cm-1 (-0.58 to -0.72 K)� Magnets below 3K
The Mn12 Family of Single-Molecule Magnets (SMMs)
Mn4
O11O7
O12
O19
Mn6
O10O3
Mn2
O4O6
O2
Mn1
O1O2a
O6aO1a O3a
Mn2a
O5a
O4aO7a
O11a
O13a
O14a
Mn4aO17a
O21a
Mn7
O24a
O24
O22
O23
O18
O5
O16
Mn5O15
Mn5a
O15a
O16a
O19aO22a
O12a
O18a
Mn6aO20a
O23a
O20
O14
O21
O17
O10a
O8a
Mn3a
O9a
O9
Mn3
O8
O13
Sessoli, Gatteschi, Caneschi, Novak, Nature 1993, 365, 141Gatteschi, Sessoli, Christou, Hendrickson, et al. JACS, 1993,115, 1804
Mn(O2CMe)2 + MnO4- in 60% acetic acid/H2O
[Mn12O12(O2CMe)16(H2O)4]·2AcOH·4H2O(Lis, Acta Cryst. B,1980) [Mn12Ac]
Identified as a SMM in 1993. Mn12Ac has axial symmetry (tetragonal space group I4(bar)), and has therefore been considered the best to study in detail.
Volume of the Mn12O12 magnetic core ~ 0.1 nm3
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
M/M
s
µµµµ0H (T)
0.002 T/s
3.6 K
3.2 K
3.0 K
2.8 K
2.6 K
2.5 K
2.4 K
2.3 K
2.2 K
2.1 K
1.3 K
2 K
1.8 K
T [K]2 4 6 8 10
χχ χχ'' M
[cm
3 mol
-1]
0
1
2
3
4
5
0 10 20 30 40
0
5
10
15
20
0.1 T0.5 T1 T2 T3 T4 T5 T6 T7 T
H/T (kG/K)
M/N�
B
S = 10D = - 0.50 cm-1
= - 0.72 K
DC magnetization hysteresis loopsDC magnetization fit
AC out-of-phase signal(50 – 1000 Hz)
Arrhenius Plot
10-5
10-3
10-1
101
103
105
107
109
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
DCAC
ττ ττ(s
)
1/T (1/K)
slope = Ueff
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
v=140 mT/sv=70 mT/sv=14 mT/sv=2.8 mT/s
M/M
S
µ 0 H(T)
40 mK
-1 -0.5 0 0.5 1-40
-30
-20
-10
0
Ene
rgy
(K)
µ0 Hz (T)
-10
-9
-8
-7
10
9
8
7
Tunneling Steps in a S = 10 Single-MoleculeMagnet
Field is swept from –1 Tesla to + 1 Tesla. Tunneling transitions are seen as steps, which correspond to asurge in the relaxation rate.
As expected from the Landau-Zener equation, the step size isinversely proportional to thesweep rate.
Modification of the Mn12 Family of SMMs
1) Carboxylate Substitution-- replacement of acetate with other carboxylate groups.-- add RCO2H to Mn12-Ac
[Mn12O12(O2CMe)16(H2O)4] + 16 RCO2H [Mn12O12(O2CR)16(H2O)4] + 16 MeCO2H
(J. Am. Chem. Soc. 1993, 115, 1804, and J. Am. Chem. Soc. 1995, 117, 301)
• The substitution is an equilibrium that is driven to 100% reaction by removing the generated acetic acid (MeCO2H) as its toluene azeotrope under vacuum
• Many, many different carboxylate R groups have been used (big vs small, polar vs non-polar, isotopically-labelled, element-labelled (e.g. C6F5), etc
• Allows control of Mn12 solubility, crystallinity, redox potentials, etc
The Faster-Relaxing Variants of Mn12 SMMs
-- even in pure Mn12 crystals, additional signals from a faster-relaxing (LT) species are almost always seen-- in Mn12-Ac, the LT species is ~ 5%. In others, it can be much higher, and in some it is the majority species.
Temperature (K)2 4 6 8 10
0
1
2
31000 Hz250 Hz50 Hz
0
�M
´´(c
m3
mol
-1)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
R = Me R = C6F5
The Ueff typically differ by a factor of two
“Single-molecule magnets: control by a single solvent moleculeof Jahn-Teller isomerism in [Mn12O12(O2CCH2But)16(H2O)4]”
Soler et al. Chem. Commun. 2003, 2672
T (K ) 0 2 4 6 8 1 0 1 2
32 days
18 days
4 days
40 days
Crystals of HT form
Wet crystals of Pure LT form ! The compound crystallizes
from a MeNO2/CH2Cl2 solvent mixture as Mn12.MeNO2.CH2Cl2
The compound crystallizes from a MeCN/CH2Cl2 solvent mixture as Mn12.MeCN.CH2Cl2
triclinic, P�, a = 15.814(2), b = 16.42(2), c = 27.434(3) Å� = 76.900(1), � = 78.220(1), � = 78.210(1)º, Z = 2, V = 6699.08 Å3
triclinic, P�,a = 15.757(1), b = 16.763(1), c = 27.183(1) Å, � = 77.444(1), � = 77.490(1), � = 78.315(1)º, Z = 2, V = 6750.17 Å3
Mn6
O18
O85
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
M/M
s
µ0Hz (T)
0.004 T/s
3.0 K3.2 K
3.4 K
2.8 K
2.6 K
2.4 K
2.2 K
2.0 K
3.6 K1.8 K
1.6 K
1.4 K
-1
-0.5
0
0.5
1
-1.2 -0.8 -0.4 0 0.4 0.8 1.2
M/M
s
µ0Hz (T)
0.008 T/s
1.8 K
1.7 K
1.6 K
1.5 K
1.4 K
1.3 K
1.2 K
1.05 K
0.9 K0.8 K
0.7-0.04 K
Pure LT formMn12.MeNO2.CH2Cl2
Pure HT formMn12.MeCN.CH2Cl2
2. Site-Selective, Partial Carboxylate Substitution
[Mn12O12(O2CR)16(H2O)4] + x HL [Mn12O12(O2CR)16-x(L)x (H2O)4]
Examples: [Mn12O12(O2CR)12(NO3)4 (H2O)4] [Mn12O12(O2CR)8(O2CR’)8 (H2O)4]
[Mn12O12(O2CR)8(O3SPh)8 (H2O)4] [Mn12O12(O2CR)8(O2PPh2)8 (H2O)4]
Inorg. Chem. 2001, 40, 4199 Dalton Trans. 2003, 2243-2248
O4O13
O1
O38
O39
S8O2
O12
S3
Mn2
O3
O6O11
Mn3
O7
O8
O10 S2
O14O9
O57
Mn10
O45O49
O42
O37
O36
O29O30 S5
Mn1
O46
O31
Mn7
Mn8O40
O56
O5Mn9
O43
O34O32
O48S7O33
O28
O25
O35 O26
O27
O24
O20
O21O23
O19
O15
S4
O51
O16
Mn4O18Mn11
O50
O55O53 O44
Mn12
O52
S6
Mn5
Mn6
S1
10-5
10-3
10-1
101
103
105
107
109
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
DCAC
ττ ττ (s
)
1/T (1/K)
τ0 = 6.6 x 10 -9 s
Ueff = 67 K
3. Electron Addition (Spin Injection)Carboxylate variation alters the Mn12 redox potentials (E),
(related to the thermodynamic Gibbs free energy by �Gº = -nFEºi.e. ease of addition (or removal) of
electrons)-- determined by electrochemistry
0.300.60CH2Cl
-0.500.02CH2CH3
0.460.64C6F5
0.450.74C6H3(NO2)2-2,4
0.610.91CHCl2
E2(V)bE1 (V)aR
DPV
in CH2Cl2 solution vs. ferrocenePotential (V)
-0.40.00.40.81.21.6
50 µA
0.860.56
0.24
-0.03
0.950.64
0.34
0.91 0.61 0.29
10 µA
Cur
rent
CV
a. First e- additionb. Second e- addition
Electrochemical data for[Mn12O12(O2CR)16(H2O)4]
[Mn12O12(O2CR)16(H2O)4] + I- [Mn12O12(O2CR)16(H2O)4]- + ½ I2[Mn12]-
[Mn12O12(O2CR)16(H2O)4] + 2 I- [Mn12O12(O2CR)16(H2O)4]2- + I2[Mn12]2-
-- products can be made in multi-gram amounts, and isolated as pure, crystalline solids.
Electron Addition to Mn12 : Bulk Isolation of Products
-- using iodide (I-) as a one-electron donor
JACS 1995, 117, 301, and JACS, 2003, 135, 3576
Crystal Structures of Mn12 and [Mn12]-
H2O ( )H2O ( )
Eppley et al. JACS, 1995, 117, 301
Mn4
O11O7
O12
O19
Mn6
O10O3
Mn2
O4O6
O2
Mn1
O1O2a
O6aO1a O3a
Mn2a
O5a
O4aO7a
O11a
O13a
O14a
Mn4aO17a
O21a
Mn7
O24a
O24
O22
O23
O18
O5
O16
Mn5O15
Mn5a
O15a
O16a
O19aO22a
O12a
O18a
Mn6aO20a
O23a
O20
O14
O21
O17
O10a
O8a
Mn3a
O9a
O9
Mn3
O8
O13
Crystal Structure of [Mn12]2-
--- added electrons are localized on opposite sides of the Mn12 molecule
--- the neutral H2O ligands bind preferentially to the MnII atoms
1H NMR Spectra of [Mn12]2- in CD3CN Solution
60 50 40 30 20 10 0 -10
ax(III-III)
eq
eq
eqax
(III-IV)
ax(III-IV)
ax(III-IV)ax
(III-III)
ax(III-III)
C
C
S
S
*
S
*
*a
*
00101020203030404050506060
00101020203030404050506060
00101020203030404050506060
ax(III-III)
ax(III-IV)
*
ax(III-IV)
ax(III-IV)
eq(III-III)
eq(III-III)
eq(III-III)
ax(III-III)
ax(III-III)
C
*
*
* *
S
S
*
[Mn12O12(O2CCHCl2)16(H2O)4]z
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[Mn12O12(O2CCH2Cl)16(H2O)x]z
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sc
Effective solution symmetry D2d
Magnetic Properties of [Mn12]1- , 2-
32 57 66 Ueff (K)
-0.39-0.49-0.65D (K)
1019/210S
[Mn12]2-[Mn12]1-[Mn12]R = CHCl2
H/T (kG/K)0 10 20 30 40 50
10
12
14
16
18
20
7T6T5T4T3T2T1T
M/N
µ B
DC Reduced Magnetization
S = 10D = - 0.27 cm-1
= - 0.39 K
AC Susceptibility
T(K)0 2 4 6 8 10 12 14
0
1e-5
2e-5
3e-5
4e-5
5e-5
χ” (e
mu)
[Mn12]2-
)
0.1 0.2 0.3 0.4 0.5
10-4
10-3
10-2
10-1
Ueff = 53 K
Ueff = 28 K
Ueff = 64 K
�(s
)
1/T (K-1)
[Mn12][Mn12]1-
[Mn12]2-
R = C6F5
χ M"
(cm
3m
ol-1
)
Mn12
χM" peak at 6 - 8 K
[Mn12]-
χM" peak at 4 - 6 K
[Mn12]2-
χM" peak at 2 - 4 K
T (K)1.5 2.0 2.5 3.0 3.5 4.0 4.5
1000 Hzχ" (e
mu)
Out-of-Phase AC Susceptibility for Mn12, [Mn12]- and [Mn12]2-
- dried solids
wet crystals
Soler, et al., JACS, 2003, 135, 3576
T (K)
1.5 2.0 2.5 3.0 3.5 4.0 4.5
χ" (e
mu)
8e-5
1e-4
1e-4
1e-4
2e-4
2e-4
2e-4
2e-4
2e-4
3e-4
1000 Hz
Out-of-phase AC Magnetic Susceptibility for(PPh4)2[Mn12O12(O2CCHCl2)16(H2O)4]
(PPh4)2[Mn12]•4CH2Cl2•H2O (PPh4)2[Mn12]•6CH2Cl2triclinic P�V = 6969.79 Å3
Monoclinic P21/cV = 7468.51Å3
Arrhenius → Ueff = 18.5 K Arrhenius → Ueff = 30.2 K
Peak at 3.6 KPeak at 2.4 K
(PPh4)2[Mn12]•4CH2Cl2•H2O (PPh4)2[Mn12]•5CH2Cl2
(PPh4)2[Mn12]•4CH2Cl2•H2O (plates)
• SMM below TB =2.0 K• Tunneling steps • D = - 0.28 cm-1
• Ueff = 21.0cm-1 = 30.2K
(PPh4)2[Mn12]•6CH2Cl2 (needles)
Hysteresis Loops for (PPh4)2[Mn12O12(O2CCHCl2)16(H2O)4]
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
M7065M6055
M/M
s
µ 0 Hz (T)
1.5 K0.07 T/s
plates
needles
• SMM below TB = 1.5 K• Tunneling steps • D = - 0.17 cm-1
• Ueff = 12.9cm-1 = 18.5K
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
M/M
s
µ0 Hz (T)
0.07 T/s
0.6 K
0.7 K
0.8 K
1.2
1.5 K
0.5 K
0.9
1.0
1.10.4 K
0.3 K0.2 K0.04 K
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
M/M
s
µ0 Hz (T)
0.07 T/s
0.6 K
0.7 K
0.8 K
1.2
2.0 K
0.5 K
0.9
1.0
1.8 K
0.4 K0.3 K0.2 K0.04 K
1.6 K
1.4 K
Topics for this presentation:
• Crystalline arrays of Mn12 SMMs, and their controlled modification.
• Faster-Relaxing Mn12 SMMs: Jahn-Teller Isomerism
• Electron addition onto Mn12 SMMs, and itseffect on the properties
• New high-quality Mn12 SMMs: the picture comesinto focus.
A Mn12 Complex with Tetragonal (Axial) Symmetry:[Mn12O12(O2CCH2Br)16(H2O)4] (Mn12-BrAc)
[Mn12O12(O2CMe)16(H2O)4] + 16 BrCH2CO2H [Mn12O12(O2CCH2Br)16(H2O)4] + 16 MeCO2H
Mn3
Mn3a
Mn3b
Mn3c
Mn2c
Mn2b
Mn2aMn2
Mn1 Mn1b
Mn1a
Mn1c
crystallizes as [Mn12BrAc]·4CH2Cl2
tetragonal space group I42d
--- single type of molecule in the crystal.
--- in contrast to Mn12Ac, where each molecule has strong H-bonding to 0,1, 2, 3 or 4 acetic acid molecules of crystallization (Cornia et al., Phys. Rev. Lett. 2002, 89, 257201)
Nicole Chakov
A New Mn12 Complex with Tetragonal (Axial) Symmetry:[Mn12O12(O2CCH2But)16(MeOH)4] (Mn12-tBuAc)
• Better even than Mn12BrAc
• Less solvent of crystallization
• Bulky R group : well separated molecules
• Well aligned with cell axes
I42d
Mn12BrAc
I4(bar)I4(bar)
Mn12ButAcMn12Ac
Muralee Murugesu
H/T [kG/K]
0 10 20 30 40
M/N
µµ µµB
0
2
4
6
8
10
12
14
16
18DC Magnetization fit
S = 10D = - 0.51 cm-1
= - 0.73 K
AC Susceptibility
(1/T)/K-1
0.14 0.16 0.18 0.20 0.22
ln( ττ ττ
−1−1 −1−1// //s
−1−1 −1−1)) ))
3
4
5
6
7
8
9
10
T [K]2 4 6 8 10
χχ χχ'' M
[cm
3 mol
-1]
0
1
2
3
4
5
Magnetic Properties of [Mn12O12(O2CCH2But)16(MeOH)4](Mn12ButAc) Dr. Muralee Murugesu
Arrhenius Plot
Ueff = 67.8 K�0 = 5.58 x 10-8s
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
M/M
s
µµµµ0 H (T)
0.002 T/s
3.6 K
3.2 K
3.0 K
2.8 K
2.6 K
2.5 K
2.4 K
2.3 K
2.2 K
2.1 K
1.3 K
2 K
1.8 K
Single-Crystal 55Mn NMR Spectra of Mn12Ac vsMn12BrAc with S4 (Axial) Symmetry
Mn3
Mn3a
Mn3b
Mn3c
Mn2c
Mn2b
Mn2aMn2
Mn1 Mn1b
Mn1a
Mn1c
Mn12-Ac: tetragonal space group I4(bar) Mn12-BrAc: tetragonal space group I42d
Mn12BrAcMn4+
Mn3+Mn3+
N. S. Dalal, S. O. Hill, G. Christou, et al.Inorg. Chem. 2005, 44, 2122.
JACS, 2006, 128, 6975
A New Mn12 Complex with Tetragonal (Axial) Symmetry:[Mn12O12(O2CCH2But)16(MeOH)4]·MeOH (Mn12-tBuAc)
I4(bar)I4(bar)
Mn12-tBuAcMn12-Ac
Wernsdorfer, Murugesu, and Christou, P. R. L., 2006, 96, 057208
--- no symmetry-lowering contacts with the solvent molecules in the crystal --- bulky R group : well separated molecules
Mn12-Ac Mn12-tBuAc
-1
-0.5
0
0.5
1
2.5 3 3.5 4 4.5 5 5.5
0.1 K0.6 K0.7 K0.8 K0.9 K1.0 K1.1 K1.2 K1.3 K1.4 K1.5 K1.6 K1.7 K1.8 K1.9 K2.0 K2.1 K2.2 K2.3 K2.4 K
M/M
s
µµµµ0 H (T)
0.002 T/s
Ground statetunneling
Excited statetunneling
-100
-80
-60
-40
-20
0
-5 -4 -3 -2 -1 0 1 2 3 4 5
Ene
rgy
(K)
µµµµ0 Hz (T)
Ground statetunneling
Excited statetunneling
The Sharpness of the Hysteresis Loops in Mn12-tBuAc allows Steps due to Excited State Tunneling to be seen
Summary: Researchers have thought for over 13 years that axial Mn12-Ac is the best one to study, but it is not. More interesting
physics is now being discovered with cleaner, truly axial Mn12 SMMs
Wernsdorfer, Murugesu and Christou, Phys. Rev. Lett., 2006, 96, 057208