@misc{oai:ir.soken.ac.jp:00000246, author = {沼田, 陽平 and ヌマタ, ヨウヘイ and NUMATA, Youhei}, month = {2016-02-17, 2016-02-17}, note = {Molecule-based magnets have been investigated for several decades, because not only theirmolecular structures but also crystal structures can be controlled. For example, a lot of compoundsconsisting of the magnetic metal ions bridged by the polydentate ligands, such as metal complexesand high spin organic radicals are reported. These compounds have various dimensions from zero tothree dimensional networks. In these materials, some low dimensional magnets show interestingmagnetic properties. For example, single molecule magnets and single chain magnets show uniquemagnetic hysteresis and relaxation properties based on their molecular structures and single ionmagnetic anisotropies. So we try to construct and study low dimensional molecule-based magnets using of highly magnetically anisotropic Co(Ⅱ) ion and bisnitroxide radicals as bridging ligands.
　Another useful features of such materials, multifunctional materials can be constructed by usingthe functionalized molecules. For example, some materials, which have magnetism and conductivity, optical properties are cited.
　On the other hand, chiral or non-centrosymmetric magnets are expected to show interesting and novel physical properties. In such asymmetric structural compounds, electronic dipole fields stabilize ferroic Dzyaloshinsky-Moriya (DM) interactions. Results of these situations, the chiralmagnetic structures are expected. Many chiral structural molecule-based magnets by means of thestructural chirality inducing from chiral substituents are prepared. But one dimensional chiralmagnets are still rare. And there are no reported about the magnetic interaction between Co(Ⅱ) ionand nitroxide radical. Then first, he synthesizes and studies for simple Co (Ⅱ) complex in chapter 2. Next, in chapter 3 and 4, he reports magnetic properties of one dimensional chiral structural molecule-based magnets.
　He prepared new paramagnetic cyclic dimmer [{Co(hfac)2}・BNO^t-Bu]2 (hafac =1,1,1,5,5,5-hexafluoroacetylacetone, BNO^t-Bu= 1,3-bis (N-t-butyl-N-oxylamino)-5-t-butylbenzene) (1)by the reaction between [Co(hfac)2](S=3/2) and bisnitroxide radical : BNO^t-Bu(S=1). Here, hfac ishexafluoroacetylacetonato. In chapter 2, he reports preparation, crystal structure and magneticproperties of 1. The compound constructed by two pairs of [Co(hfac)2] and BNO^t-Bu and has cyclicdimer structure of two [Co(hfac)2] units bridged by the BNO^t-Bu radicals. This compound was paramagnet in all temperature regions. The estimated curie constant, C was 2.162 (emu K/Oe mol) from the fitting of temperature dependences of 1/xm values using Curie-Weiss equation and geff value of 2.854 was obtained from the equation of C=0.125(geff)²S(S＋1). And the high temperaturelimit magnetization values estimated from room temperature data was 5μB and this value is furthersmall than theoretical value 10μB. This result indicates that 1 has very strong antiferromagnetic interaction between Co(Ⅱ) and nitroxide radical. 　From these results, though this cyclic dimer was paramagnet, Co-radical system has strong intramolecular magnetic interactions and high magneticanisotropies from 1/xm vs T data.
　In chapter 3, he describes about preparation, crystal structure and magnetic properties of novelchiral molecule-based magnet: [Co(hfac)2]・BNO*　(BNO*=1,3-bis (N-t-butyl-N-oxylamino)-5-1’-methy l-1’- { 2’’-(S)-methylbutoxy}ethylbenzene)　(2).　Thiscompound is consisting of Co(Ⅱ) ions and the chiral bisnitroxide radicals:　BNO*. This compoundforms one dimensional alternate helical chain structure with R-helicity. Space group of 2 is chiralp1(No.1). Intrachain Co-Co distances are 8.709 and 8.721　Å and the nearest interchain Co-Codistance is 10.704 Å. Resulting this, intrachain magnetic interactions may be much stronger thaninterchain ones. Temperature dependences of the xT　values have a minimum at 220K and this isindicative that this material displays ferromagnetic alignment within one dimensional chain. Thiscompound has two magnetic phase transitions. These magnetic phases are paramagnetic (Para)phase above 20 K, antiferromagnetic (AF) phase (20-4 K), and field induced ferromagnetic (FiFi)phase below 4K. At 20K, three dimensional AF magnetic ordering achieved and below thistemperature, this material shows spin flip transition. At 4K, the crossover from three dimensionalHeisenberg type AF state to three dimensional Ising type AF state is occurred. In FiFi phase, thismaterial shows large coercivity (ca. 2.5 T at 2K) and its saturated magnetization value at of 1.16μB.This value is slightly larger than expected value of 1μB (3/2 －1= 1/2). This is supposed by the influence of magnetic anisotropy of Co(Ⅱ).
In 1999, preparation, crystal structure and magnetic properties of first chiral moleculer-basedmagnet: [Mn(hfac)2]・BNO* (3)were reported by Kumagai et. al., (Angew. Chem. Int. Ed. 1999, 38, 1601.). This compound has one dimensional chain structure with R-helicity and chiral space group(P1, No.1). This is metamagnet TN=5.4 K) and spin flip is occurred at around 500 Oe at 2 K inpower sample. Its saturated magnetization value is 2.7μB at 2K and this value corresponds toexpected value of 3μB (5/2－1=3/2). This compound has been expected that interesting magneticproperties based on the influence of chirality may be observed in single crystal. He succeeded to grow large single crystal of 3. In chapter 4, he reports magnetic properties of singlecrystal of 3 and constructs its magnetic structure model.　Normally, in the single crystal ofmetamagnets and antiferromagnets, magnetization curves show the angular dependent behaviors.These behaviors depend on the direction of applying magnetic field and spin direction. If appliedmagnetic field and spin direction are parallel (H//) bellow TN, spin flip transition is observed (easy axis). On the other hand, if applied magnetic field and spin direction are perpendicular (H∞),magnetization values increase gradually and saturate (hard axis). But this compound showsspin-flip transition in all crystallographic axes. This is indicative that this compound has antiferromagnetic muiltisublattices. From the magnetization measurements of single crystal of 3,The calculated angles of total spin components from crystallographic axes and the magnetic spinstructure of 3. From these results, it is indicative that this compound has antiferromagnetic multisublattices and the magnetic structure is constructed by the influence of chirality.
In conclusion, the author prepared and studied some molecule-based low dimensional magneticsystems. In the discrete cyclic dimer 1 indicates Co-radical system has strong intramolecular magnetic interactions and strong anisotropy. In the basis on these results, the author preparedCo(Ⅱ) and chiral triplet bisnitroxide based one dimensional magnet 2. This compound showremarkable magnetic properties based on its strong anisotropy and low dimensionality. Finally,from the magnetic measurements of single crystal of Mn based chiral structural magnet 3, theauthor revealed its interesting magnetic behaviors and calculated its magnetic structure. Herevealed that this novel magnetic structure is considered of the cooperation with magnetic propertyand structural chirality., 総研大甲第1024号}, title = {Magnetic Properties of Low-Dimensional Molecule-based Magnets Consisting of Transition Metals and Organic Radicals}, year = {} }