Akihiko Machida

Structural Change at Metal-Insulator Transition of Tb2Ba2Co4O11

Temperature variation of the crystal structure has been investigated for Tb 2 Ba 2 Co 4 O 11 , which shows a metal-insulator (MI) transition at T MI = 340 K. The compound has a layered-type double-perovskite structure along the c -axis, with alternating the CoO 6 octahedron and the CoO 5 pyramids along the a -axis. With decrease of temperature below T MI , the distortion of the octahedral and pyramidal sites is fairly released, suggesting that spin state transition of the Co 3+ ions is the origin for the MI transition.

Charge Ordering in the Geometrically Frustrated Spinel AlV2O4

We have found that spinel AlV 2 O 4 shows a phase transition at ∼700 K with anomalies of transport and magnetic properties. It is clarified that this corresponds to a charge-ordering transition, where the V 2.5+ ions are separated into three V 2.5-δ ions and one V 2.5+3δ ion. This three-one type of charge ordering, which is coupled with rhombohedral distortion along the (111) axis, reduces the high degeneracy of ground states inherent in this spinel structure, and thus can be understood in terms of “charge frustration.”

Crystal and magnetic structure of conducting double perovskite Sr2FeMoO6

We have investigated, crystal and magnetic structure of conducting double perovskite transition metal oxide, Sr 2 FeMoO 6 , which has been grown by the floating-zone method. The resisitvity shows T 2 -dependence in a wide temperature range (50–350 K), perhaps due to magnon scattering. We further have determined the crystal and magnetic structure in the ferrimagnetic state by means of synchrotron radiation X-ray powder measurements as well as neutron powder diffraction experiments.

Phase Separation and Destabilization of the Charge-Ordered State in Cr-Doped Manganites

Destabilization of the charge-ordered (CO) state in half-doped manganites by chromium (Cr) doping in the manganese (Mn) sites was investigated. The Cr doping in the half-doped manganites makes the ...

Neutron Investigation of Tb1-xCaxMnO3 (x ≥0.5)

Crystal and magnetic structures of Tb 1- x Ca x MnO 3 ( x ≥0.5) have been investigated by means of neutron powder diffraction experiments. We have derived the magnetic phase diagram against hole concentration x and temperature T : the spin structure changes as CE- → C- → G-type structure with increase of x . In the vicinity of the magnetic phase boundaries, magnetically phase separated state has been observed. At high temperature region, we observe the metallic behavior in a wide x region (0.70 ≤ x ≤0.95), and ascribe it to the enhanced one-electron bandwidth W of the e g state.

Effects of B-Site Disorder in Sr2FeMoO6 with Double Perovskite Structure

Heat treatment effects on the physical properties have been investigated for Sr2FeMoO6 single crystals with double perovskite structure. We have evaluated the occupancy g of the Fe3+ ion on one sublattice of the perovskite-B site by Rietveld refinement of the synchrotron-radiation X-ray powder patterns. Both the saturation magnetization Ms and critical temperature Tc for the ferrimagnetic transition decrease linearly with g in the large-g region (g ≥~ 0.84), but begin to deviate from the empirical relations for g ≤~ 0.84. We interpreted this behavior in terms of variation of the correlation length of the spin ordering.

Room-Temperature Magnetoresistance in Fe-Site-Substituted Sr2FeMoO6

We have observed room temperature magnetoresistance (MR) in Fe-site-substituted Sr2FeMoO6 crystal with a double perovskite structure. These melt-grown crystals have few grain boundaries and show high conductivity (~10-3 Ωcm at 300 K), which is advantageous for a practical application. We will discuss the possible origin on the basis of the substituent concentration dependence and temperature dependence of the magnitude of the MR.

Microdomain structure of Cr-doped manganites : Nd1/2Ca1/2(Mn, Cr)O3

Crystal and magnetic structures of Cr-doped manganites, Nd 1/2 Ca 1/2 Mn 1- y Cr y O 3 ( y =0.00 and 0.03), have been investigated by synchrotron radiation (SR) x-ray powder diffraction as well as neutron powder diffraction measurements. A detailed analysis of the high-resolution x-ray profile has revealed that the Cr-doped compound exhibits broad extra reflections, suggesting the formation of microdomains below the charge-ordering temperature T CO . The origin of the microdomain structure is discussed in terms of the charge separation.

Epitaxial Growth of BaTiO3 Optical Thin Films by Pulsed KrF Laser Deposition and in situ Pulsed Laser Annealing

A simple, hybrid process of pulsed KrF laser deposition (PLD) with in situ pulsed laser annealing has been demonstrated. By this method, epitaxial BaTiO3 optical thin (relatively thick) films with a thickness of up to 2 µ m were successfully fabricated on a MgO substrate for the first time. By in situ pulsed KrF excimer laser irradiation of the surface of the growing film, i.e., in situ laser annealing, good smoothness of the film surface and annealing of the film were achieved, allowing the thick film fabrication. The substrate temperature for the fabrication of epitaxial BaTiO3 films was also reduced to 650° C by incorporating the in situ pulsed laser annealing. Refractive indices of the films were measured to be about 2.30–2.12, which is higher than for films without in situ laser annealing.

Neutron investigation of Ru-doped Nd1/2Ca1/2MnO3: Comparison with Cr-doped Nd1/2Ca1/2MnO3

Lattice and magnetic properties are investigated for 3% Ru- and Cr-doped Nd 1/2 Ca 1/2 MnO 3 . The parent Nd 1/2 Ca 1/2 MnO 3 is a charge-ordered insulator ( T CO = 250 K). With decreasing temperat...