1. Structure and magnetic properties of Mn11Cr with a half-integer spin
（Nagoya Univ.）　H. Hachisuka, K. Awaga
（Inst. Mol. Sci.) 　T. Yokoyama
（Okayama Univ.) H. Nojiri
（Kyoto Univ.) T. Goto
（Nara Univ. Educ.）T. Kubo
Acknowledgement
S. Miyashita and B. Barbara

2. Jahn-Teller distortion of Mn3+ ion
Magnetic properties of 12-nuclei Mn cluster
1) High-spin ground state : S=10
2) Uniaxial magnetic anisotropy : D=-0.6 K
Jahn-Teller distortion of Mn3+ ion
3) Quantum tunneling of magnetization
Mn4+ (S=3/2 )
Mn3+ (S=2 )
Mn12O12(O2CR)16(H2O)4

3. Structure of Mn12 Site b Mn4+(site a) JT軸 Site c Mn3+(site b)
This shows the structure of Mn12Ac. There are three independent sites for the manganese ions, sites a, b, and c. The site a is occupied by Mn4+. The sites b and c are occupied by Mn3+ with Jahn Teller distortion.
Mn3+(site b)
Mn3+(site c)

4. Chemical modifications on Mn12
JT ion, Mn3+
Ligand, Crystal solvent
SR Molecule
FR Molecule
Chemical
Pressure
(site c)
TB=2.7 K
TB=1.3 K
A lot of chemical modifications have been reported concerning Mn12. There are two approaches. One is the modifications on the outside ligands and crystal solvents. By this modification, we have found Jahn Teller isomerization. Now it is believed that Mn12 has two species, slow and fast relaxation components. In the SR molecule, the structures of the Mn3+ are elongated octahedra and the elongated axes are almost parallel to the molecular axis. In contrast the FR molecule has a compressed Jahn teller distortion at Site c. This brings about the decrease in the blocking temperature, and the tilted magnetic easy axis.
The other approach is the modification of the core part, namely replacement of Mn3+ or Mn4+ with another metal ion. In contrast to the extensive studies in this side, this approaches were quite limited. To my best knowledge, there are only reports.
Uniaxial Anisotropy
Biaxial Anisotropy
Mn3+, Mn Mn+

5. Mn-Fe mixed cluster Fe3+(site c) Ground state is S=0 !?
A. R. Schake et al., Inorg. Chem., 33, 6020 (1994).
55  ºC
Fe(CH3COO)2 ＋ KMnO4
[Fe4Mn8O12(CH3COO)16(H2O)4]
60 % CH3COOH
Fe3+(site c)
Prof. Christou group has reported a Mn-Fe mixed cluster. They obtained Fe4Mn8 by the reaction of Fe acetate and Mn peroxide. They determined the molecular structure and concluded that the cluster molecule have four Fe3+ at site c. They also reported that the ground state is S=0.
Ground state is S=0 !?

6. Mn(CH3COO)2 + K2Cr2O7 [Mn11CrO12(CH3COO)16(H2O)4] (Mn11Cr (S=19/2))
+ Mn12 (S=10), …
A Chinese group reported Mn11Cr. They obtained this by the reaction of Mn acetate and dichromium oxide. Although there was a contamination of Mn12, they determined the structure and the magnetic properties of this material. They found the behavior of single molecule magnet, but there has been no detailed study on it. Therefore, we decided to study this molecule in detail.
Li Jin-Yu, Xu Hao, Zou Jian-Zhong, Xu Zheng and You Xiao-ZengYu Kai-Pei,
Polyhedron, 15, 3325 (1996).

7. Preparation and structural analyses of Mn11Cr*
Mn(CH3COO)2･4H2O + K2Cr2O7
We tried to repeat their chemistry, and easily obtained the beautiful crystals of Mn11Cr*. * indicate that it is a meixed crystal between Mn12 and Mn11Cr. We solved the molecular and crystal structures of this material. There was no significant difference from those of Mn12. This indicates a positional disorder of the Cr ion.
Structure of Mn11Cr*
1) no significant difference from Mn12
2) positional disorder of Cr

8. X-ray Absorption Spectra
1) Existence of Cr in Mn11Cr*
2) Cr ion is in the +3 state
S=3/2
Mn11Cr has S=19/2 spin !?
This shows the X-ray absorption spectrum taken by prof. Yokoyama. Actually the material shows the absorptions of both Cr and Mn, proving the existence of Cr. The absorption edge for the Cr ion is exactly agreement with that of the standard sample of Cr3+. So it is concluded that Cr in the cluster is in the 3+ state. Since the octahedral Cr 3+ usually exhibit S=3/2, so that the Mn11 is expected to have a half integer spin of 19/2. Another important information is Cr3+ is not a JT ion.
Cr3+ is a non-JT ion.

9. Fourier transfer forms
of the EXAFS spectra
Mn4+(a)
Mn3+(b)
Mn3+(c)
The red and blue curve in this figure show the Fourier transfer forms for the Cr and Mn ions. The peaks indicate the presence of the interatomic distances. For instance, the peak at 1.3 A corresponds to metal oxide distance. The peaks in this range indicate the metal-metal distances. The green, blue and pink curve show the theoretical ones, when we assume that metal ions are located at site a, b, or c, respectively. The blue curve can be explained as a superposition of the three curves. This is natural, because the Mn ions occupy the three sites. In contrast, the red curve can be explained only by the orange curve, where the metal ion is assumed to be at site c. It was concluded that Cr#+ occupies the position of site c.
3) Cr3+ occupies site(c)

10. High-Field EPR Spectra
Mn11Cr*
High-Field EPR Spectra
S=19/2
S=10
This shows the high field EPR spectra taken by Prof. Nojiri at the four temperatures. The top and bottom spectra are those of usual Mn12. It is clear that the absorption lines for Mn11 is the double of the lines for Mn12. This puzzled us seriously, but we finally recognized that this can be well explained by assuming that Mn11Cr* is a nearly 1:1 mixed crystal of Mn11Cr (S=19/2) and Mn12 (S=10) and They have nearly the same D. This shows the zeeman energy diagram for the two species with the same D. Under a radiation of microwave, transitions for the two appear alternantly.
Mn11Cr* is a nearly 1:1 mixed crystal of
Mn11Cr (S=19/2) and Mn12 (S=10)
2) They have nearly the same D

11. AC Susceptibilities Mn12 Mn11Cr* Mn11Cr Mn12
We have another evidence for the mixed crystal. This shows a comparison between the AC response, imaginary component, of Mn12 and Mn11CRr*. Mn12 shows a symmetric peak as a function of temperature, but the peak of Mn11 has a shoulder on the high temperature side. This peak can be decomposed into the two contribution. The high temperature one is probably caused by Mn12 and the low temperature contribution is due to Mn11. By decomposing the every peak, we obtained the temperature dependence of the relaxation time for the two species in Mn11Cr* separately.
Temp. depend. of relax. time

12. Arrhenius Plots pure Mn12 Mn11Cr* Mn11Cr (S=19/2) Mn12 (S=10)
Blue and green show the arrheniou plots for the Mn11 and Mn12 in the mixed crystal. The red shows the data for pure Mn12. red and green agree with each other. The activation energies for the two are obtained to be 56.8 and 68.8 K respectively. The theoretical ratio is 90 %. This disagreement could be explained by a tunneling effect at the thermal excited states.
Mn11Cr (S=19/2)
Mn12 (S=10)
⊿ = 56.8 K
⊿ = 68.8 K　
= 83 %
(19/2)2
102
= 90 %

13. Mn12 and Mn11Cr have nearly the same D value !?
Magnetization Curves at 1.8 K
Mn12
Mn11Cr
Mn12
Mn11Cr*
This shows a comparison between the magnetization curves for Mn12 and Mn11Cr* at 1.8 K. There is a significant difference in the range below 1 T. This shows the derivative. You can see a beautiful intensity alternation. At first we thought about a parity effect, but it was not the case. Finally we concluded that the magnetization change below 1 T is caseud by Mn11 and the change above it is caseund by Mn12. I would like to show another evidence.
Mn12 and Mn11Cr have nearly the same D value !?

14. Magnetization Manipulation
Mn11Cr*
Mn11Cr
Mn12
I would like to make sure that Mn11Cr* is a 1:1 mixed crystal of Mn11 and Mn12. We have many evidences. To understand the magnetization curve, we carried out minor loop measurements. After saturation, we scanned the field down to -1 T, and went back to zero. From here we scanned the field down to -3 T. Please take a look at this part. The magnetization is constant. The sample is under the negative field in this process, but nothing happens. This is the firm evidence that the behavior in this range is caused by Mn11 and the behavior above 1 Tis caused by Mn Now please take a look at the four state at the zero field. The spin alignment of the two species are expected up-up, uo-down, down-up and down down. In the other word we can manipulate the magnetizations of the two separately.
Mn11Cr* is ca. 1:1 mixed crystal of Mn11Cr and Mn12.
H/T

15. Measure of Internal Field
Behavior between 0 – 1 T
Mn11Cr
QTM of Mn11Cr
Mn12
Mn11Cr
Mn12
Mn11Cr
Measure of Internal Field
0.05 T
We carried out another minor loop measurements. Please look at the blue curve and red curve. As I said the change in this range occurs only on the Mn11. So the small steps shows the QTM for Mn11 from down to up. However the crucial difference is the magnetization of Mn12. At the zero field the two spins are opposite on the blue curve, that’s why the magnetization is zero. On the red curve, the two magnetizations are both negative. This show the derivative of the two curves. We found a significant shift of about 50 mT between the curves. This is probably cased by an internal field, probably dipole field, created by the neighboring Mn12. This is a kind of measure of the dipole field. Another important thing is that the tunneling probability, namely the peak height is not affected by the shift completely.

16. Angular Dependence of QTM
Resonance Fields
Mn12
Mn11Cr
We examined the angular dependence of the magnetization curve at 1.8 K. This show the resonance fields for QTM. The red and orange plots show the filed for Mn12 and green and, light and dark blue plots show the field of Mn12. They can well explained by the same equation. This means that Mn12 and Mn11Cr have nearly the same anisotropy.
Mn12 and Mn11Cr have nearly the same anisotropy !?

17. Summary 1) Crystal of Mn11Cr* is 1:1 mosaic of Mn11Cr and Mn12.
2) Mn12 and Mn11Cr have nearly the same anisotropy !?
Site C
JT isomer
non-JT ion
(site c)
TB=1.3 K
Biaxial Anisotropy
3) Crystal of Mn11Cr* can store four spin structures.
4) Internal field is 0.05 T.