Chapter 10
167
has been observed for the first time at the pressure around Pc = 6.5 ± 0.5 kbar below 0. 5 K. The pressure dependence of lattice parameters of its F2dd crystal structure, xamined up to 12.6 kbar at room temperature, shows an anomaly near Pc· At ambient pressure, the effective intermolecular interaction is estimated to be
z] j
kB- 1.8 ± 0.2 K by the analysis of the magnetization up to 5 T. The Curie temperature (7�) in the ferromagnetic state at p < Pc decreases with the increasing pressure. In the antiferromagnetic state at p > Pc, however, the transition temperature starts to increase. The variation of intermolecular interactions under pressure, including their signs, is discussed on the basis of the charge transfer mechanism and the recent aD-initio method for the exchange interactions. It is suggested that the change of the sign of ferromagnetic interlayer interaction may be a direct reason for this transition.
This xperimental result suggests that the organic bulk-ferromagnetism is realized on the basis of the delicate balance between the ferromagnetic and antiferromagnetic exchange paths via various molecular orbitals.
Chapter 7 reports the pressure effect of lD organic ferromagnet p-CDTV. This compound had been taken to be a bulk-ferromagnet with the secondary high or
dering temperature Tc = 0.67 K among the reported organic bulk-ferromagnets.
Our precise inspection, however, suggests that p-CDTV is a quasi-one-dimensional ferromagnetic radical crystal with the dominant ferromagnetic intrachain interac
tions J, the ferromagnetic interchain interaction .F' and another antiferromagnetic interchain interaction J AF'. The bulk-antiferromagnetism of p-CDTV is suggested to originate from the minute antiferromagnetic exchange interactions J AF'. By the pressurization, all of their interactions have been enhanced. The pressure-induced enhanc ment of Tc is smaller than those of other bulk-antiferromagents such as 'I'ANOL, TPV, and p-Cl-BDPA, reflecting the competition of JAF' and J" with opposite sign each other.
In the study of the pressure effect of F 5PNN in chapter 8, we have confirmed the first pressure-induced crossover from the alternating 1 D spin system to the uniform on continuously by the measurement of heat capacity: The alternating chain system of F sPNN at ambient pressure gradually transforms to the uniform one with the increasing pressure; and eventually at p � 6.5 kbar becomes the uniform Heisenberg spin system. The crystal structure of F 5PNN at room temperature and ambient pressure is of the uniform chain, but the magnetism at low temperatures is of the alternating chain. Although its low-temperature crystal structure is unknown, and there is to be the change of the structure from the uniform to the alternating chain structure as the temperature is lowered. The applied pressure works to suppress that structural transition, and therefore the uniform chain structure may become
168 CHAPTER 10. CONCLUDING REMARKS
stable at the pressurized state above 6.5 kbar.
In chapter 9, the pressure effect of the ferromagnetic galvinoxyl radical crystal is reported. This compound occurs a structural transition at 85 K accompany
ing a nonmagnetic phase at ambient pressure. However, the pr surization of 6.4 kbar makes its ferromagnetic behavior maintain down to 0. 7 K, at which the bulk
antiferromagnetic ordering occurs. The ferromagnetism under prcssur is revealed to be explained by a quasi-one-dimensional ferromagnetic model with a slight an
tiferromagnetic interchain interaction. This model can be imaged from the crystal structure at room temperature, in which one-dimensional zig-zag interaction paths with small SOMO-SOMO overlapping are expected.
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174 CHAPTER 10. CONCLUDING REMARKS
List of Publications
1. "The Numerical Comparison of Magnetic Susceptibility and Heat Capacity of TMNIN with the Result of a Quantum Monte Carlo Method for the Haldane System"
M.Ito, M.Mito, H.DBguchi and K.Takeda:
J. Phys.Soc. Jpn. 63(1994) 1123-1129.
2. "T hermodynamic Properties of the Haldane and Non-Haldane Systems with the !so-Structural Compounds (CH3)4NM(N02)3, M=Ni,Co"
K.Takeda, M.Mito, M.Ito, T.Kawae, H.Deguchi and S.Takagi:
J.Magn.Magn. Mater.l40-144(1995)1661.
3. "Low Temperature Excitations in One-Dimensional Heisenb rg Antiferromagnets with Half Integer- and Integer-Spin System"
M.Ito, M.Mito, T.Kawae and K.Takeda:
Technology Reports of Kyushu University.67(1994)477-485.
4. "T hermodynamic Properties of the One-Dimensional Ising Syst m
(CH3)4NCo(N02)3, the !so-Structural Compound with the Haldan System, TMNIN"
M.Mito, M.Ito, T.Kawae, M.Hitaka, H.Deguchi, S.Takagi and K.Takeda:
J.Phys.Soc.Jpn.64(1995)4402-4410.
5. "Magnetic Susceptibility of a Spin 1/2 Heisenberg Antiferromagnetic
Chain: (3,3'-Dimethyl-2,2'-Thiazolinocyanine]-TCNQ"
S.Takagi, H.Deguchi, K.Takeda, M.Mito and M.Takahashi:
J. Phys.Soc. Jpn.65(1996) 1934-1937.
6. "Pressure Induced Enhancement of Ordering Temperature of a Ferromagnetic Organic Radical Crystal: p-CDTV"
M.Mito, H.Nakano, M.Ito, M.Hitaka, T.Kawae, K.Konishi, K.Mukai and K.Takeda:
Czech.J. Phys.46(1996)Suppl S4. 2107-2108.
7. "Magnetic Properties of Haldane System TMNIN of Finite Chain Length"
T.Kawae, M.Ito, M.Mito, M.Hitaka and K.Takeda:
Czech.J.Phys.46(1996)Suppl S4.1935-1936.
8. "Doping Effect of Non-Magnetic and Magnetic Impurities an Magnetic Susceptibility of the Haldane System (CH3)4NNi(N02) 3"
M.lto, M.Mito, T.Kawae, M.Hitaka and K.Takeda:
J. Phys.Soc. Jpn. 65(1996)261 0-2613.
9. "Magnetism of a Two-Dimensional Weak-Ferron1agnetic Organic Radical Crystal 1 ,3,5-Tripheny l-6-0xoverdazyl"
M.Mito, II.Nakano, T.Kawae, M.Hitaka, S.Takagi, H.Deguchi, K.Suzuki, K.Mukai and K.Takeda:
J. hys.Soc.Jpn.66(1997)2147-2156.
10. "Magnetic Properties of an S=1 Heisenberg Antiferromagnet TMNIN with Bond Impurity"
T.Kawae, M.Ito, M.Mito, M.Hitaka and K.Takeda:
J. Phys.Soc.Jpn.66(1997) 1892-1895.
175
11. "Magnetism of an Organic Radical Crystal with Weak Ferromagnet Moment
Below 5.4 K"
K.Takeda, M.Mito, H.Nakano, T.Kawae, M.Hitaka, S.Takagi, H.Deguchi, S. Kawasaki and K. Mukai:
Mol. Cryst. Liq. Cryst. 306(1997)431-438.
12. "Pressure Effects on Intermolecular Interactions in Genuine Organic Radical Crystal"
K.Takeda, M.Mito, M.Ito, T.Kawae, M.Hitaka, M.Matsubara and K.Mukai:
Moi.Cryst. Liq. Cryst. 306(1997) 177-184.
13. "Magnetic Properties and Crystal Structure of 1,5-Diphenylverdazyls with El ctron Acceptor Groups in 3-Position"
M.Mito, K.Takeda, K.Mukai, N.Azuma, M.R.Gleiter, C.Krieger and F. A. Neugebauer:
J.Phys.Chem.Bl01(1997)9517-9524.
176 CHAPTER 10. CONCLUDING REMARKS
14. "Pressure Induced Ferro- to Antiferromagnetic Transition in a Purely Organic Compound, ,B-phase Para-Nitrophenyl Nitronyl Nitroxidc"
M.Mito, T.Kawae, M.Takumi, K.Nagata, M.Tamura, M.Kinoshita and K.Tak da : Phys.Rev.B56.Rapid Commun. (1997) in press.
15. "Pressure Effects of a Genuine Organic Ferromagnet p-NPNN"
M.Mito, T.Kawae, M.Takumi, K.Nagata, M.Tamura, M.Kinoshita and K.Takeda : J.Magn.Magn.Mater.177-181(1998) in press.
16. "Magnetism of Some Genuine Organic Radical Crystals"
K.Takeda, M.Mito, T.Kawae, H.Nakano, M.Hitaka, K.Mukai and K.Suzuki:
J.Magn.Magn.Mater.177-181(1998) in press.
17. "Pressure Dependence of Intermolecular Interactions in the G nuine Organic ,B-Phase p-Nitrophenyl Nitronyl Nitroxide Crystal
Accompanying a Ferro- to Antiferromagnetic Transition"
K.Takeda, M.Mito, T.Kawae, M.Takumi, K.Nagata, M.Tamura and M.Kinoshita : J.Phys.Chem.B(1998) in press.
18. "2(2'-Halophenyl)-a-Nitronyl Nitroxides"
S.Nakatsuji, M.Saiga, N.Haga, A.Naito, T.Hirayama, M.Nakagawa, Y.Oda, H.Anzai, K.Suzuki, T.Enoki, M.Mito and K.Takeda:
New J.Chem.(1998) in press.