Daisuke Akahoshi

Magnetic and M-I Transitions in YBaCo 2O 5+ x (\({\bf 0} \leQQ \mib{x} \leQQ {\bf 0.5}\))

Three compounds, orthorhombic YBaCo 2 O 5.5 , tetragonal YBaCo 2 O 5.5 and YBaCo 2 O 5.0 , have been synthesized and their structural, magnetic and electrical properties have been investigated. The structures of all three compounds have a CoO 2 –Y(O)–CoO 2 block layer sandwiched between the BaO layers. Magnetic and metal-insulator transitions have been observed in orthorhombic YBaCo 2 O 5.5 which has a one-dimensional ordering of oxygen atoms along the b -axis in the Y(O) layers. Tetragonal YBaCo 2 O 5.5 and YBaCo 2 O 5.0 , which are both semiconductive, show magnetic transitions. The physical properties of YBaCo 2 O 5+ x strongly depend on the structure and oxygen content.

Charge/Orbital Ordering Structure in Ordered Perovskite Sm1/2Ba1/2MnO3

In an A-site ordered perovskite manganite Sm 1/2 Ba 1/2 MnO 3 , a new charge/orbital ordering pattern was found at room temperature. Electron diffraction studies revealed a series of superlattice reflections with modulation vectors at q 2 =(1/2, 1/2, 1/2) as well as at q 1 =(1/4, 1/4, 0) in the tetragonal setting ( a p × a p ×2 a p , a p being the cubic perovskite lattice parameter). Together with the results of the resonant X-ray scattering and the charge-transport and magnetization measurements, a new model for the three-dimensional charge/orbital ordering in the ordered perovskite is proposed.

Local magnetic properties and spin state of YBaCo2O5.5:59Co NMR study

Abstract 59 Co NMR studies have been made to clarify the spin state and local magnetic properties of YBaCo2O5.5, a new cobalt oxide with an oxygen-deficient and A-site ordered perovskite structure. In the antiferromagnetic state we observed four Co NMR spectra with internal fields of 216, 24, 19, and 0 kOe. This means that the crystal structure at low temperatures does not have the 2×1×1 superstructure observed at room temperature.

Order–Disorder Effect of

We have synthesized A -site ordered ( A O)- and A -site disordered ( A D)-Y 1/4 Sr 3/4 CoO 3-δ (YSCO) with various oxygen deficiencies δ, and have made a comparative study of the structural and physical properties. We have found that A -site (Y/Sr) ordering produces the unconventional oxygen-vacancy ordered (OO) structure, and that the magnetic and transport properties of YSCO strongly depend on the A -site ordering and/or the oxygen-vacancy (or excess oxygen) ordering patterns. A O-YSCO with a stoichiometric δ of 0.5 has an unconventional OO structure reflecting Y/Sr ordering pattern. With decreasing δ from 0.5, the overall averaged OO structure is essentially unchanged except for an increase of occupancy ratio for the oxygen-vacant sites. At δ= 0.34, excess oxygen atoms are ordered to form a novel superstructure, which is significant for the room-temperature ferromagnetism of A O-YSCO. In A D-YSCO, on the other hand, the quite different OO structure, which is of a brownmillerite-type, is found only in t...

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We have investigated electron- and hole-doping effects on A -site ordered perovskite manganite NdBaMn 2 O 6 , which has the A -type (layered) antiferromagnetic (AFM) ground state. Electrons (holes) are introduced by partial substitution of Ba 2+ (Nd 3+ ) with Nd 3+ (Ba 2+ ). Electron-doping generates ferromagnetic (FM) clusters in the A -type AFM matrix. With increasing the electron-doping level, the volume fraction of the FM phase or the number of the FM clusters is abruptly increasing. In contrast, the A -type AFM phase is robust against the hole-doping, and no FM correlation is observed in the hole-doped NdBaMn 2 O 6 .

-site and Oxygen-Vacancy on Magnetic and Transport Properties of Y1/4Sr3/4CoO3-δ

We have systematically investigated the magnetic properties of (Fe 1- y Zn y ) 2 Mo 3 O 8 (0≤ y ≤1). The ground state of the parent compound, Fe 2 Mo 3 O 8 ( y = 0), is an antiferromagnetic (AFM) insulator. The substitution of magnetic Fe 2+ with nonmagnetic Zn 2+ gives rise to ferromagnetic (FM) correlation. With increasing y , the FM correlation evolves and becomes the most enhanced with y = 0.5. One of the possible explanations for the origin of the FM behavior is that Zn 2+ ions prefer the octahedral oxygen coordination to the tetrahedral one. In addition, the magnetizations when y = 0.125 and 0.25 show a stepwise behavior at low temperatures, implying that several magnetic phases are competing with one another.

Electron- and Hole-Doping Effects on A-Site Ordered NdBaMn2O6

We have investigated the magnetic and dielectric properties of orthorhombic Eu 1- y Y y MnO 3 (0 ≤ y ≤0.6) single crystals without the presence of the 4 f magnetic moments of the rare-earth ions. In y ≥0.2, the magnetic-structure driven ferroelectricity is observed. The ferroelectric transition temperature is steeply reducing with increasing y . In y ≥0.52, two ferroelectric phases ( P ∥ a and P ∥ c ) are coexistent at low temperatures. In these phases, ferroelectricity has different origin, which is evidenced by the distinctive poling-electric-field dependence of electric polarization. Namely, the electric polarization along the c -axis ( P c ) is easily saturated by a poling electric field, therefore P c is caused by the b c -spiral antiferromagnetic order. On the other hand, the electric polarization along the a -axis ( P a ) is probably attributed to the collinear E -type antiferromagnetic order, because P a is unsaturated even in a poling field of 10 6 V/m.

Ferromagnetic Behavior of (Fe1-yZny)2Mo3O8 (0≤y≤1) Induced by Nonmagnetic Zn Substitution

We have grown single crystals of layered perovskite oxides Sr 3 Fe 2- x Co x O 7-δ with x =0.2 (δ=0.11) and x =0.5 (δ=0.15), and investigated the anisotropic magnetic and transport properties. The x =0.2 crystal undergoes a semiconductor-insulator transition around 40 K, below which the magnetization exhibits cluster-glass like behavior. On the other hand, the x =0.5 crystal shows two-dimensional (2D) ferromagnetic (FM) behavior below ∼150 K, and quasi metallic transport is observed in the FM a b -plane. The negative magnetoresistance (MR) is observed both parallel and perpendicular to the FM plane. A contribution of tunneling process, like the case of the isostructural 2D ferromagnet La 2-2 x Sr 1+2 x Mn 2 O 7 with x =0.3, might be present in the out-of-plane MR.