Spin Dynamics and Magnetic Properties of Molecular Magnets

Byoung Jin SuhThe Catholic University of Korea

-NMR 1/T1 in AF Coupled Molecular Clusters

-Magnetization in Mn12-Ac, Mn12-PrCl, Mn12-BuCl

-Magnetization in Artificially Engineered Ferritins

Spin Dynamics

Ames Lab. Iowa State Univ. Prof. F. Borsa Prof. M. Luban Dr. D. Procissi Dr. P. Kögerler Dr. A. Shastri I. Rousochatzakis

Pavia Univ. Italy Dr. A. Lascialfari E. Micotti

Korea Prof. D.-Y. Jung Y. J. Kim

Spin Dynamics in AF Rings or Clusters

0 50 1000

1

2

3

4

5

6

1/T

1 (m

s-1)

Temperature (K)

Fe30, 0.75 T Fe10, 1.41 T

0 1 2 3 4

0

1

2

3

4

5

6

1/T

1 (m

s-1)

T* (= T/To)

Fe30, 0.75 T Fe30, 1.49 T Fe10, 0.33 T Fe10, 1.41 T

A strong enhancement

at kBT J

Universal Behavior ?

Universal Scaling Behavior

V6

Polyoxovanadate Compounds

V12 V15

Spin Hamiltonian and Susceptibility

,

)(

42243113

4321

14433221

SSSSSS

SSSSS

SSSSSSSSJH

Constant

)]1()1([2

)1(2

),,(

)(2

24241313

2413

224

213

2

oB HSg

SSSSJ

SSJ

SSSE

SSSJ

H

|2,1,1>

|0,0,0>,|1,0,1>,|1,1,0>

|1,1,1>|0,1,1> Ho

Heisenberg Spin Square: {V12} J

J

JJ

Magnetization measurement of {V12}Magnetization measurement of {V12}

0 50 100 150 200 250 3000.0

0.5

1.0

1.5

J

J

J

J

{V12}H = 5 kG

T

(cm

3 mol

-1 K

)

Temperature (K)

g = 1.96, J = -17.6 K

0

1

2

3

4

0 100 200 3000

1

2

0 50 100 150 200 250 3000

1

2

3

4

0

2

4

6

8

0 100 200 3000.0

0.5

1.0

1.5

0 100 200 3000

1

2

1/T

1 (m

s-1)

V6

T (cm

3 K/m

ol)

T (K)

1/T

1 (m

s-1)

Temperature (K)

H = 1.5 TH = 4.7 T

1/T

1 (m

s-1)

V12

T (cm

3 K/m

ol)

T (K)

V15

T (cm

3 K/m

ol)

T (K)

0 50 100 150 200 250 3000

10

20

30

40

{V12}H

1 ~ 10 Oe

T1

-1 (m

s-1

)Temperature (K)

1/T1 of V12

1 10 100

1E-3

0.01

0.1

1

10

T1-1

(mse

c-1)

Temperature ( K )

1E-4

1E-3

0.01

0.1

1

10

0 1 2 3 4 50

5

10

15

20

25

200 300 400 500 600 7001E-4

1E-3

0.01

0.1

1

10

3.3 1.42.5

T1-1

(m

se

c-1)

5 1.72

(a)

NM

R

H (Tesla)

T (K)

(b)

T1-1

(m

se

c-1)

1000/T (K-1)

1/T1/T11 of {V12} of {V12}

At low T:1/T1 exp(-(H)/kBT)(H) = 0 –gBH 0 = |J|Procissi et al., to be submitted

T-Behavior of 1/TT-Behavior of 1/T11 in Polyoxovanadates in Polyoxovanadates

-In V6, V12: 1/T1 is similar to T. The absence of an enhancement of 1/T1 for kBT J

-In V15: The presence of broad peak of 1/T1: similar to the critical enhancement in other AF clusters with s > 1/2 ? spin triangle of five s = ½ spins

- In 1/T1 of V12: The peak at T 19 K (cf J = -17.6 K) appears to be associated with the critical slowing down of magnetic fluctuations. (very weak enhancement)

- At low temperatures: 1/T1 exp(-(H)/kBT)

0 50 100 150 200 250 300

0

2

4

6

8

10

J = -7.9 K, J' = -27.6 K, g = 2.1

J'

JJ

Mn(III)Mn(III)

Mn(II)

Mn3

T (

cm3 K

/ m

ol)

Temperature (K)

V6

V6 [2 x V3] M. Luban, PRB 66, 054407 (2002)

S1 = S2 = S3 = 1/2g = 1.95J = -64.6 K, J’ = -6.9 K,

= J’ – J = 57.7 K

Mn3 [Mn(II) + 2Mn(III)][Mn3O(O2CCH3)6(C5H5N)3]C5H5N S1 = 5/2, S2 = S3 = 2g = 2.10J = -7.9 K, J’ = -27.6 K = -3J/2 12 K

Mn3: Mgnetic Susceptibility

(4.2 K) = 7.7 x10-26 cm3/Mn(295 K) = 0.29 x10-26

Az (4.2 K) = 3.9 x 1022 cm-3

Az (295 K) = 3.5 x 1022 cm-3

Average distance of H from Mn3O:<r> 3 Å

0 100 200 3000

100

200

300

0 1 2 30

50

100

150

200

250

0 10 20 30 40 50100

200

300

(b)

H = 1.48 T H = 0.42 T

FW

HM

(kH

z)

Temperature (K)

(a)

FW

HM

(kH

z)

Field (T)

T = 4.2 K T = 295 K

H = 1.48 T

(

kHz)

T (K)

int0

302

z

z

A

HrA

H

K 295at kHz 1024

K 4.2at kHz 25129

H

H

1H NMR Linewidth in Mn3

0

1

2

3

4

0 100 200 3000

1

2

0 100 200 300

0 50 100 150 200 250 300

1

10

V6

1/T

1 (m

s-1

)

J = -64.6 K, J' = -6.9 K, g = 1.95

T (c

m3 K

/mol

)

T (K)

J = -7.9 K, J' = -27.6 K, g = 2.1

J'

JJ

Mn(III)Mn(III)

Mn(II)

T (c

m3 K

/ m

ol)

T (K)

Mn3

1/T

1(m

s-1

)

Temperature (K)

Distinct behavior of NMR relaxation 1/T1:

ascribed to the different spin values ??

V6: S1 = S2 = S3 = 1/2 (Quantum Spins ?)

Mn3: S1 = 5/2, S2 = S3 = 2 (Classical Limit ?)

1H Spin-Lattice Relaxation Rate vs. Temperature in Mn3

Strong Enhancement of 1/T1:

Slowing down of magneticfluctuations Building up of AF correlation Low Temperature Side: 1/T1 exp(-U/T)U = NMR : Effective Gap 

U = 19 K at H = 1.5 T

U = 15 K at H = 7.2 T

0.00 0.05 0.10 0.15 0.20 0.25

1

10

0 50 100 150 200 250 300

1

10

1/T

1(m

s-1)

1/T(K-1)

H = 1.48 TH = 4.7 TH = 7.2 T

1/T

1(m

s-1)

Temperature (K)

Spin Dynamics in Mn3

Spin Dynamics in AF ClustersSpin Dynamics in AF Clusters

-T dependence is well understood.

-Critical enhancement and its universal scaling behavior: to be published soon by F. Borsa and M. Luban et al.,

-Absence of critical enhancement or weak critical enhancement in s = ½ systems ???

-At low temperatures: 1/T1 exp(-(H)/kBT) good for Stotal = 0 but for Stotal 0 ???

Prof. S. Yoon, M. Heu, S. W. Yoon, S.-B. Cho, B. J. Kim(The Catholic Univ. of Korea)

Prof. Z. H. Jang(Kookmin Univ. Korea)

Prof. D.-Y. Jung, Y. J. Kim(SKKU Univ. Korea)

Prof. K. S. Kim(Jeonbuk National Univ. Korea)

Magnetization Measurements of Mn12 Clusters and

Artificially Engineered Ferritins

M12-Ac Mn12-PrCl Mn12-BuCl

tetragonal triclinic triclinic

a = 17.319Å 16.355Å 14.556Å

b = 17.319Å 17.635Å 14.582Å

c = 12.388Å 19.243Å 27.265Å

13.72Å 12.92~20.30Å

distance between Mn12 clusters

Mn12-Ac : [Mn12O12(O2CCH3)16(H2O)4]·2CH3CO2H·4H2O

Mn12-PrCl : [Mn12O12(O2CCH2CH2Cl)16(H2O)4]·CH2ClCH2CO2H

Mn12-BuCl : [Mn12O12(O2CCH2CH2Cl)16(H2O)4]·2CH3ClC6H5

-20000 -10000 0 10000 20000

-1.0

-0.5

0.0

0.5

1.0

M(H

) / M

s

Field (Gauss)

Mn12-Ac sample 1 Mn12-Ac sample 2

Mn12-Ac2mm2mm

1mm

-2 -1 0 1 2

-1.0

-0.5

0.0

0.5

1.0

M /

Ms

H (T)

2.00K 2.15K 2.30K 2.50K 2.70K

D/g=0.30K

Mn12-PrCl

Mn12-BuCl

0 5000 10000 15000 20000

0.01

0.1

1

2.9K 2.8K 2.7K

2.6K

2.5K

2.4K

2.3K

2.0K

Mre

d

time (s)

Relaxation at H=0

])/(exp[)0(

)( tMM

MtMM

eq

eqred

Mn12-PrCl Mn12-BuCl

0.4 0.5 0.6100

102

104

106

108

2.0 2.2 2.4 2.6 2.80.6

0.7

0.8

0.9

1.0

1.1

Mn12-BuClU = 60 K

Mn12-PrClU = 57 K

(a)

(s)

1/T (K-1)

n12-PrCl

for Mn12-BuCl

(b)

T (K)

])/(exp[)0(

)( tMM

MtMM

eq

eqred

)/exp(0 TU

Relaxation Time and Activation Energy

0 2 4 60.1

1.0

0 2 4 6

Ac 0 T

0.44 T

PrCl

PrCl

Ac

Ac

(a) on resonance

Mre

d

t (104 s)

PrCl

0.22 T

0 T 0.66 T

Ac

PrCl

(b) off resonance

t (104 s)

Relaxation in PrCl is faster at on-resonance fields

-50 0 50 100 150 200 250 300 350 4000

2

4

6

8

10

12

H=1T

M()

[104

em

u/m

ol]

Angle ( degree )

5K 10K 15K 20K Fit

Triclinic : S=10, D=0.56K, g=1.90, E=0.15K

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.06

8

10

12

14

16

18

M/N

H/T (T/K)

1T 2T 3T 4T 5T 6T 7T Fit

)()sincos(Η 222yxxzBz SSESSHgDS

)/(exp

)/(exp)(

kTE

kTEHE

NMn

n

nn

n

ddkTE

kTEdHdE

NM

nn

nn

n

sin)/exp(

)/exp()(

4

2

Mn12-PrCl

-20000 -10000 0 10000 20000

-1.0

-0.5

0.0

0.5

1.0

M(H

) / M

s

Field (Gauss)

Mn12-Ac sample 1 Mn12-Ac sample 2

After being measured, sample 1 was heated up to 50 oC (1 oC/min) and quenched

2mm2mm

Heat Treatment

0 100 200 300 400 500 60050

60

70

80

90

100

Mn12

O12

= 41.3%

2CH3COOH : 5.8%

4H2O : 3.5%

W

eig

ht(

%)

Temperature(0C)

MnO+MnO2

Thermogravimetric Analysis (TGA)

Mn12-Ac

[Mn12O12(O2CCH3)16(H2O)4]·2CH3CO2H·4H2O

S=10, D=0.67K, g=1.97

0 90 180 270 3600.0

0.2

0.4

0.6

0.8

1.0

H = 1 TT = 20 K

M(

) / M

( =

0)

Rotation Angle, (degree)

S0 S1

)sincos(Η 2 xzBz SSHgDS

)/(exp

)/(exp)(

kTE

kTEHE

NMn

n

nn

n

Uniaxial symmetry is not changed

K0.034 and 028.0 ],)/(exp[)2()(

s103.1 ,97.1 , K65.0

resonances-off and -onat relaxation ionMagnetizat

21

160

EEEEEf

gD

0 1x104 2x104 3x104 4x1040.3

0.4

0.5

0.6

0.7

0.8

0.9

1

S0 S1H

z = 0.66 T

[M(t

) -

Ms]

/ [M

in-

Ms]

Time (s)

Hz = 0 S0 S1

S0: as preparedS0: as preparedS1: after heat treatmentS1: after heat treatment

Mn12-Ac

QTM in Mn12 Clusters

-Strongly related to bulk and/or local structure

-Local structure means:

? Local distortion ? Dislocation ? Jahn-Teller Isomerisom

- Two types of protein chains: H-Chain L-Chain

Ferritin

- Recombinant human ferritin homopolymers were successfully produced in E Coli transformed with human ferritin H or L-chain genes, respectively.

- Apoferritins were then reconstituted with Fe atoms under the variable experimental conditions.

- HF (reconstituted ferritin with H-chain only): 900 Fe/molecule- LF (reconstituted ferritin with L-chain only): 800 Fe/molecule

- Molecular based device: V

I

0 10000 20000 30000 40000 50000 600002

4

6

8

10

12

14

16

18

20

Blo

ckin

g Tem

pera

ture

, TB (K

)Field (Oe)

H-Ferritin L-Ferritin

Blocking Temperarture, TB

0 5000 1 104

1.5 104

2 104

0

5

10

1515

0

Tb H 100 50000( )

200000 H

molecules between int. Dipolar :10 HH

HUU eff

K

])/(exp[)2()( 20

10 UUUUf

0 50 100 150 200 250 300 350 4000

1

2

3

4

5

6

7

M0 (

emu /

Fe

1g)

Temperature (K)

M0, H-ferritin

M0, L-ferritin

0 50 100 150 200 2500

100

200

300

400

500

600

700

800

eff (in

the

unit o

f B

ohr

mag

net

on)

Temperature (K)

L-Ferritin H_ferritin

Mo(T)=M*(TN-T)/TN

Big Difference in eff0 10000 20000 30000 40000 50000 60000

0.0

0.5

1.0

1.5

2.0

L Ferritin 20 K 30 K 40 K 50 K

M (em

u /

g)

Field (Oe)

Modified Langevin Function:

M = MoL(effH/kBT) + H

M vs H

0 50 100 150 200 250

1.80x10-4

2.25x10-4

2.70x10-4

Temperature (K)

H-Ferritin L -Ferritin

(em

u/g

-Fe-

Oe)

T-behavior of the linear susceptibility

Ferritin ? The unusual behavior of TB(H) with a broad peak: Distribution of the energy barrier which depends on the applied field

? However, the distribution is not simply due to size distribution.

? eff(L) > eff(H) even for the similar size: - not only from the uncompensated spins on the surface - but also from a kind of random defects and the number of defects can be distributed even for the same size of molecules

? The strong T-dependence of linear susceptibility