Youhei Yamamoto

The Control of Cluster-Glass Transition Temperature in Spinel-Type ZnFe2O4-δ Thin Film

The room-temperature cluster glass state has been realized in zinc ferrite (ZnFe2O4-δ) thin films using the pulsed-laser-deposition (PLD) method. When the deposition rate was slow about 7 A/min, the thin film showed antiferromagnetism similar to the bulk zinc ferrite whose Neel temperature was about 20 K. However, by increasing the deposition rate, the magnetization-temperature curve behaved the same as that of cluster glass, and hysteresis between zero-field-cooled and field-cooled magnetization processes was also observed. At higher than 30 A/min, the cluster glass transition temperature (TG) reached approximately room temperature. There are two possibilities for the origin of high-TG cluster glass: 1) spin frustration resulting from the competition of antiferromagnetic interaction between A-B and B-B site iron ions, and 2) random oxygen deficiency. These two factors induced the high-temperature cluster-glass state in zinc ferrite thin films.

Crystal structures of cobalt(II), nickel(II) and zinc(II) dichloro complexes bearing 2-pyridyl-substituted nitronyl nitroxide (NIT2py)

The X-ray analysis of Co(II), Ni(II) and Zn(II) dichloro complexes containing 2-(2-pyridyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide (NIT2py) demonstrated that the Co(II) complex containing two dichloromethane solvate molecules, [CoCl2(NIT2py)2]·2CH2Cl2 (1), crystallizes in a space group P21/c isomorphous to the corresponding Mn(II), Ni(II) (2) and Cu(II) complexes, and that the molecular structure of the complex having Ci symmetry in 1 is isostructural to the above Mn(II), Ni(II) and Cu(II) complexes. A new Ni(II) complex having only one dichloromethane solvate molecule, [NiCl2(NIT2py)2]·CH2Cl2 (2′), could be crystallized from a mixture of dichloromethane, methanol and n-heptane. The (OC-6-12)-[trans(Cl)-trans(py)] geometrical structure of the Ni(II) complex in 2′ is identical to that in 2, but the puckered six-membered chelate rings formed via pyridyl-N and nitroxide-O atoms of NIT2py take the different conformations; i.e. the molecular symmetry is C2 (racemate) with the λ, λ or δ, δ conformations in 2′ and Ci (meso) with the δ, λ conformations in 2. In the case of Zn(II), a reaction of ZnCl2 with NIT2py afforded purple crystals consisting of octahedral bis(NIT2py) and tetrahedral mono(NIT2py) complexes with hydration: [ZnCl2(NIT2py)2]·2[ZnCl2(NIT2py)]·2/3H2O (3). The geometrical structure of the octahedral Zn(II) complex in 3 was found to be (OC-6-22)-[cis(Cl)-trans(py)].

Syntheses, Structures, and Spectroscopic Properties of Cobalt(III) Complexes Containing 3- or 4-Pyridyl-Substituted Nitronyl and Imino Nitroxide Ligands

Several new cobalt(III) complexes incorporating 4,4,5,5-tetramethyl-2-(3- or 4-pyridyl)imidazolin-1-oxyl 3-oxide (NITnpy) (n = 3 or 4) or 4,4,5,5-tetramethyl-2-(3- or 4-pyridyl)imidazolin-1-oxyl (IMnpy) have been prepared. The X-ray crystallographic analyses and the spectroscopic investigations have revealed that the nitronyl or imino nitroxides with the radical character are bound through the pyridyl-N atom to a CoIII center. There are some influences from the radical unpaired electron toward the CoIII complex moiety and those from the coordination of the CoIII ion toward the nature of these radicals which can be detected by 59Co NMR and UV/Vis spectroscopy. Such observed changes coincide with the proposed spin density on the N atom of the pyridyl substituent. Furthermore, it was found that trans-[Co(acac)2(PPh3)(NIT4py)]+ in methanol was gradually converted into the corresponding IM4py complex, while free NIT4py is stable in methanol. The CoII species formed on the partial decomposition of the NIT4py complex in methanol seems to catalyze the deoxygenation of NIT4py to IM4py.

Construction of ZnFe2O4/ZnGa2O4 Spinel-Type Artificial Superlattice by Pulsed Laser Deposition

We constructed spinel-type superlattices consisting of antiferromagnetic ZnFe2O4/diamagnetic ZnGa2O4 along the direction. The stacking periodicity was controlled from 27 to 2 unit cells in order to reduce the dimensionality. X-ray diffraction patterns confirmed that ZnFe2O4/ZnGa2O4 superlattice was formed up to the stacking periodicity of 2 units. Spin-freezing temperatures decreased from 54 K to 36 K and maximum magnetizations shifted from 0.24µB/2Fe to 0.13µB/2Fe with decreasing periodicity from 27 units to 2 units.

Anomalous critical phenomena of a randomly diluted series Mn(1−x−y)Znx>Mgy (HCOO)2 ⋯ 2H2O

Abstract A series of randomly diluted mixtures Mn (1−x−y) Zn x Mg y (HCOO) 2 ⋯ 2H 2 O is investigated near the transition temperature. An anomalous critical phenomenon is found: the peak of susceptibility and heat capacity appears at two successive temperatures while in the pure system Mn(HCOO) 2 · 2H 2 O it does so only at T N .