称号及び氏名

博士（理学）

天谷 直樹

学位授与の日付 平成 29 年 6 月 30 日
論文

名

論文審査委員Study of the magnetic properties of variable spin network formed byorganic radical crystals
主査
副査
副査
副査
副査

細越 裕子
会沢 成彦
久保田 佳基小野 俊雄
Javier CampoStudy of the magnetic properties of variable spin network formed by organic
radical crystals
Abstract
Naoki Amaya
Organic radicals consisting only of light elements have negligibly small spin-orbit coupling andtherefore behave as ideal Heisenberg spin systems bearing an S = 1/2 for each unpaired electron.
Organic radicals have attracted current interests because of the emergence of novel quantum magnetic
phenomena due to the highly isotropic nature of its electronic spin. There are some persistent organic
radical species reported which are stabilized by -conjugation. The -conjugated planar molecules
easily stack themselves with their planes parallel each other and give one-dimensional structures.
Low-dimensional spin systems with small number of nearest neighbors are known to show large
quantum fluctuations. As the simplest quantum spin system, the one-dimensional antiferromagnet has
been extensively studied both theoretically and experimentally. Recently quantum phenomena in more
complicated spin systems have shown a renewed interest.
In this study, new organic radicals were designed and synthesized to realize one-, two- and threedimensional spin network. The author focuses on nitroxide-based materials, which have partial electriccharges on the radical unit. Due to the electrostatic interaction, the intermolecular contact between theradical units is realized which results in the formation of the antiferromagnetic interaction. In order torealize high-dimensional spin network, the polyradical approach was applied. The introduction of twoor three radical units within a molecule increases the number of intermolecular network path. Theauthor also demonstrated that the large dihedral angle in a -conjugated molecule realized theintermolecular network in multiple directions. The magnetic properties at low temperature and inmagnetic field were studied with the aid of molecular orbital and numerical calculations.
This thesis consists of six chapters. Chapter 1 “General introduction” describes the researchbackground of organic magnetic compounds and the purpose of this study. In chapter 2 “Experimental”,
the general techniques of synthesis, crystal structural analysis, magnetic measurements, specific heatmeasurements, molecular orbital calculation, and numerical calculation were described.
Chapter 3 “Magnetic interactions and molecular packing” shows the relation between the magnetic
interactions and molecular packing taking examples on the known compounds of galvinoxyl andPNNBNO. Galvinoxyl is a well-known organic mono-radical whose intermolecular ferromagnetic
interaction was reported in the end of 1960’s. It is also reported that the magnetism disappeared below
about 80 K due to some structural change. The ferromagnetic state of galvinoxyl was studied in themixed crystals and in pressure. In this study, the crystal structure at low temperature was firstly solved.
The differences in the molecular packing between the high and low temperature phases were presented.
The key factors of the appearance of intermolecular ferromagnetic and antiferromagnetic interactions
were discussed with the aid of molecular orbital calculations.
An organic triradical PNNBNO is the first example of organic ferrimagnet but the precise study has
been prevented due to its transition at low temperature (TC = 0.28 K). The preliminary measurementsof neutron scattering were done and the magnetic interactions were examined by molecular orbitalcalculations.
In chapter 4 “Low-dimensional organic magnets”, a quasi-one-dimensional organic magnet isdescribed. Pentafluorophenyl nitronyl nitroxide forms a one-dimensional antiferromagnetic chain andits uniform structure is lost at low temperature. Effect of the stress and behavior in magnetic field isdiscussed. A triangular triradical spin system, giving triangular spin tube coupled in two-dimensions,is also described.
Chapter 5, “Three-dimensional antiferromagnet F4BIPBNN”, describes the quantum properties in athree-dimensional magnet for the first time. A three-dimensional honeycomb network of S = 1/2 isformed. Two nitronyl nitroxide units linked by tetrafluorobiphenyl with a large dihedral angle between
the radical unit and biphenyl due to the electrostatic repulsion between the oxygen and fluorine atoms,
afford the three-dimensional network. Each S = 1/2 is surrounded by four neighboring spins with a J0J1-J1-J2 arrangement, where J0 is the intramolecular interaction whereas J1 and J2 are intermolecularinteractions. The analysis of the numerical calculation gives the parameter set of J0/kB = 6.6 K, J1/kB =4.3K and J2/kB = 5.9K. The temperature dependence of the susceptibility, the magnetization curves
and the transition temperature (TN = 2.7 K at magnetic field B = 0.1 T) are well reproduced by thisthree-dimensional model. The characteristic feature of this material is the concave shape ofmagnetization curve at 0.4 K, which suggests the effect of spin fluctuations. In the temperature-field
phase diagram, the transition temperature decreases for B >3 T and it is almost independent on B for
B < 3 T. This behavior can be related to the quantum effect in this material. This is the first observationof the quantum nature in a three-dimensional spin network.
Chapter 6 “Conclusions” summarizes the concluding remarks of the research. The magnetic propertiesof several organic radicals forming one-, two- and three-dimensional network were examined. The
spin density distribution on the whole molecule is characteristic to organic -conjugated molecule andresponsible for the magnetic interactions. The relation between the molecular packing andintermolecular interactions were revealed. Based on the knowledge, the spin network is controlled bymolecular design. The low-temperature measurements of physical properties and the analysis revealed
the quantum nature of the organic radical crystals even in three-dimensional spin systems.

List of Publications1) “Crystal dependence of the magnetic properties of an antiferromagnetic alternating chain compound
F5PNN”, Naoki Amaya, Naoya Obata, Hironori Yamaguchi, Toshio Ono, and Yuko Hosokoshi, J.
Phys.: Conf. Ser., 400, 032002/1-4 (2012).
2) “Direct three-dimensional ordering of quasi-one-dimensional quantum dimer system near criticalfields”, Taku Matsushita, Nobuyoshi Hori, Seiya Takata, Nobuo Wada, Naoki Amaya, and Yuko
Hosokoshi, Phys. Rev. B, 95, 020408/1-4 (2017). [Editors’ suggestion]3) “Spin-1/2 quantum antiferromagnet on a three-dimensional honeycomb lattice formed by a neworganic biradical F4BIPBNN”, Naoki Amaya, Toshio Ono, Yuta Oku, Hironori Yamaguchi, AkiraMatsuo, Koichi Kindo, Hiroyuki Nojiri, Fernando Palacio, Javier Campo, and Yuko Hosokoshi, J.
Phys. Soc. Jpn., 86, 074706 (2017).

Supplementary List of Publications
1) “Experimental Realization of a Quantum Pentagonal Lattice”, Hironori Yamaguchi, Tsuyoshi
Okubo, Shunichiro Kittaka, Toshiro Sakakibara, Koji Araki, Kenji Iwase, Naoki Amaya, Toshio Ono,and Yuko Hosokoshi, Scientific report, 5, 15327/1-6 (2015).

Summary of the dissertation screening
Candidate: Naoki Amaya
Title：Study of the magnetic properties of variable spin network formed by organic radical crystals
Organic radicals consisting only of light elements such as carbon, hydrogen, nitrogen, and oxygen, havenegligibly small spin-orbit coupling and behave as ideal quantum spin systems which exhibit novel magnetic
phenomena due to the quantum fluctuation of electron spins. In this study, one-, two- and three-dimensional
spin networks have been realized by molecular designing of nitroxide-based organic radicals, and theirquantum magnetic properties have been examined.
This thesis consists of six chapters. Chapter1 describes general background and chapter 2 gives
experimental techniques. In chapter 3, intra- and intermolecular magnetic interactions in π-conjugated organic
radical molecules are interpreted according to the spin density distribution spread on the whole molecule andthe molecular orbital of unpaired electron. The knowledge obtained in this chapter is the basis of the molecular
design in chapters 4 and 5. Chapter 4 studies low-dimensional quantum antiferromagnets: a one-dimensional
chain and a triangular spin-tube. As for the triangular spin tube, a triradical is designed in which three spins of
S = 1/2 on each corner of a triangle are antiferromagnetically coupled one another and the extended
π-conjugation gives the one-dimensional molecular stacking. This is a quite rare example of an S = 1/2 spin
tube, which is interested in a unique quantum properties by the interplay of low-dimensionality and spin
frustration. This material exhibits no long-range magnetic order down to 0.1 K, due to the effect of quantum
fluctuation. Chapter 5 describes a three-dimensional quantum antiferromagnet. A novel three-dimensional
honeycomb network is realized by the molecular design of a biradical with twisted -planes. Alternate stackingof honeycomb layers gives three-dimensional network of S = 1/2 with four nearest neighbors (z = 4). This zvalue is the smallest among three-dimensional network. The magnitude of the intra- and intermolecularantiferromagnetic interactions is tuned in the same order. Quantum effect of this material is observed in
non-linear magnetization curve. The quantum fluctuation is evaluated by the shrinkage factor of spin, whichshows the intermediate character of this material between a two-dimensional square lattice (z = 4) andthree-dimensional cubic lattice (z = 6). In chapter 6, conclusions of this thesis are mentioned.
Consequently, this study has demonstrated the efficient strategy of molecular designing in order to realize
one-, two-, and three-dimensional spin networks and has succeeded in the observation of the effect of quantum
fluctuations. Mr. Naoki Amaya has presented a thesis of high scientific and technical standard, and most of thecontent can be published in appropriate peer-reviewed international journals. He has demonstrated that he hasdeveloped the intellectual and technical skills to conduct independent scientific research, and the Committee
fully recommends that a Doctor of Philosophy degree should be conferred according to the screening andinterview.
Chairperson

Yuko Hosokoshi
Naruhiko Aizawa
Yoshiki Kubota
Toshio Ono
Javier Campo