Magnetic entropy change in perovskite manganites La0.65Nd0.05Ca0.3Mn0.9B0.1O3 (B=Mn,Cr, Fe)

We have investigated the effect of B-site dopant on magnetic entropy change in perovskite manganite samples of La_0.65Nd_0.05Ca_0.3MnO₃, La_0.65Nd_0.05Ca_0.3Mn_0.9Cr_0.1O₃, and La_0.65Nd_0.05Ca_0.3Mn_0.9Fe_0.1O₃ prepared by sol–gel technique. The maximum ΔS_H is in the order of −1.68 J/kg K and peaks at Curie temperature for La_0.65Nd_0.05Ca_0.3MnO₃ upon 10 kOe applied field change. For the sample with B-site dopant, a decrement of the maximum magnetic entropy change has been observed.     

单相Bi0.9Ba0.1Fe0.85Mn0.15O3陶瓷中的多铁性

溶胶-凝胶法制备了Bi0.9Ba0.1Fe0.85Mn0.15O3陶瓷样品,XRD分析显示其为单相菱方钙钛矿结构,明显的磁滞回线和电滞回线说明其在室温具有弱铁磁性和铁电性,介电常数随外磁场的变化显示双掺样品具有更大的磁容效应.     

Magnetic investigations of Dy(Fe0.9M0.1)2 (M=B,Al, Ga) cubic laves phases

The magnetic properties of the pseudo-binary compounds Dy(Fe_0.9M_0.1)₂ (M=B, Al, Ga) have been investigated. Substitution of M atoms for Fe atoms decreases the Curie temperature and varies the magnetic moment of the Fe atom. The coercivity of the samples increases very rapidly with decreasing temperature. This increase is considerably enhanced by the substitution elements. Pronounced thermomagnetic history effects are observed in the temperature dependence of the magnetization. These effects may be explained as an exponential increase in magnetic hardness on lowering the temperature.     

钙钛矿型稀土锰氧化合物Nd0.5Sr0.4Pb0.1Mn1-xFexO3中Mn位的Fe替代效应

对Nd0 5Sr0 4Pb0 1 Mn1 -xFexO3系列多晶样品的结构 ,磁和转变特性进行了实验研究 .在x =0 0 0— 0 10的范围内获得了单相样品 ,Fe3+ 的替代并没有引起整个系列的结构变化 ,然而Mn位的掺杂却强烈地抑制了Nd0 5Sr0 4Pb0 1MnO3的铁磁性和金属—绝缘体转变 .在低掺杂情况下 (x≤ 0 0 6)Mn被Fe替代 ,金属—绝缘体转变温度TP 平均下降了 19K ,当x≥ 0 0 8铁磁金属态完全过渡为绝缘态 .这主要归因于Fe3+ 的渗入影响了化合物eg 电子浓度和抑制了双交换作用     

A Comparative Study on the Magnetic Properties of Arc-Melted and Ball-Milled Fe0.9−x Mn0.1Al x Alloys

Samples of nominal composition Fe_0.9?x Mn_0.1Al _x (0.1 ≤x≤0.5) were prepared both by mechanical alloying and arc-melting. In order to elucidate the effect of the synthesis method upon the magnetic properties of this system, we have carried out a comparative study involving the use of different experimental techniques (Mössbauer, X-ray diffraction, vibrating sample magnetometry and magnetic susceptibility). Results revealed that independently of the employed method and milling time, the samples exhibit ferromagnetism below ～34 at.% Al. Above this concentration, the preparation method became a determinant factor upon the magnetic properties of the system. The differences are attributed, in the case of the mechanically alloyed samples, to Fe contamination arising from jars material. The results of our study are summarized in a magnetic phase diagram including ferromagnetic, paramagnetic, pure spin glass and reentrant spin glass regions.     

Quantum phase transition in the itinerant antiferromagnet (V0.9Ti0.1)2O3

Quantum-critical behavior of the itinerant electron antiferromagnet (V0.9Ti0.1)2O3 has been studied by single-crystal neutron scattering. By directly observing antiferromagnetic spin fluctuations in the paramagnetic phase, we have shown that the characteristic energy depends on temperature as c1 +c2T3/2, where c1 and c2 are constants. This T3/2 dependence demonstrates that the present strongly correlated d-electron antiferromagnet clearly shows the criticality of the spin-density-wave quantum phase transition in three space dimensions.     

Structural and magnetic phase transitions in La0.9Sr0.1MnO3

A study is made of phase transitions in doped La_0.9Sr_0.1MnO₃ compounds using combined x-ray, electrical, and magnetic measurements. Structural phase transitions are observed accompanied by a change in the cell volume at temperatures of 100–110 K and 300–340 K. These structural changes are found to be related to different contributions of the rhombic Jahn-Teller Q ₂ mode to the formation of the crystal lattice. The structural transition at 100–110 K is accompanied by distinctive magnetic and electrical properties. The data are analyzed in detail. Fiz. Tverd. Tela (St. Petersburg) 41, 1064–1069 (June 1999)     

Magnetic,dielectric, and transport properties of bismuth pyrostannate Bi2(Sn0.9Mn0.1)2O7

Physics of the Solid State - The effect of replacing manganese ions on the structural, dielectric, transport, and magnetic properties of Bi2(Sn0.9Mn0.1)2O7 has been studied and the correlation...     

Preparation and gas sensing characteristics of p-type semiconducting LnFe0.9Mg0.1O3 (Ln = Nd,Sm, Gd and Dy) materials

Nanoparticulate perovskite-type LnFe_0.9Mg_0.1O₃ (Ln = Nd, Sm, Gd and Dy) oxides were prepared by sol–gel method. X-ray diffraction was used to confirm the phase composition of the compounds. The materials exhibit p-type semiconductor behavior. Their sensitivity and selectivity towards ethanol gas were investigated. It was found that SmFe_0.9Mg_0.1O₃-based sensor had the highest response and selectivity. It is a new potential gas sensing material. The great difference of conductance in air and ethanol gas was found and discussed in detail.     

Magnetic and electrical transport properties in the self-doped manganite La0.9Mn0.9M0.1O3 (M=Mn,Zn and Ti)

The magnetic and electrical transport properties of La_0.9Mn_0.9M_0.1O₃ (M=Mn, Zn and Ti) were investigated. The temperature and magnetic field dependence of electrical resistivity (ρ) and dc magnetization were studied. All the compounds are found in rhombohedral structure. The excess oxygen in all three compounds was detected through iodometric titration. A modification in resistivity is observed when M=Mn is replaced by M=Zn and Ti. The high temperature resistivity above T_C follow variable range hopping model for both Zn and Ti compounds. For Zn doping, the observation of large field-cool effect and decrease in resistivity at room temperature and is assumed to be due to the implant of Mn⁴⁺ in Mn³⁺ matrix, which favor Mn³⁺/Mn⁴⁺ double exchange. The ferromagnetic behavior below T_C for the compound with M=Ti is correlated to the excess oxygen in it, which implants Mn⁴⁺ and thus incorporates ferromagnetic interactions. The substitutions lead to a reduction of T_c and magnetization.     

Changes in the optical properties at phase transitions in TEA2MeCl4 (Me=Zn, Mn, Hg, Cu) crystals

Optical properties of TEA₂MeCl₄ crystals, where Me is a divalent metal ion, have been studied under polarization microscope in temperatures ranging from 200 to 300 K. Abrupt changes in the interference colours and strong cracking of the sample have been observed for all crystals except the TEA₂CuCl₄ one, for which only brightening of the plate image has been noted. The cracks are approximately parallel to the cleavage planes. These changes occur in two stages. The brightening of the images of the crystal samples may suggest the ferroelastic deformation in microregions. No classical ferroelastic domain structure has been observed in the low-temperature phase.     

Local electronic structures at selected sites of intermetallic perovskites Mn3MeX (Me=divalent metal,X=N,C)

Summary We have measured the XANES (X-ray absorption near edge structure) spectra on the manganese and metal MeK-edges of the intermetallic perowskites Mn₃MeX (X=N, Me=Mn, Cu, Zn, Ga, Sn; X=C, Me=Ga, Sn) to investigate the distribution on the partial density of the empty states. The information on the local electronic structure at the two differents sites, obtained from the spectra in the first 10 eV above the onset, is discussed in terms of one-electron band model. The relative shape changes in the spectra between the different compounds are explained by the different magnitude of hybridization between the electronic states of different atoms and/or the filling of the empty band by the electrons supplied by the substituted metal. Moreover, information about the local geometrical structure has been obtained from the (10÷50) eV range of the spectra.

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Riassunto La distribuzione della densità parziale degli stati vuoti nelle perovskiti intermetalliche Mn₃MeX (X=N, Me=Mn, Cu, Zn, Ga, Sn; X=C, Me=Ga, Sn) è stata studiata mediante spettroscopia XANES (X-ray absorption near edge structure) alle soglieK del Mn e del metallo Me. Le informazioni sulla struttura elettronica locale di entrambi i siti, ottenute dall’analisi dei primi 10 eV degli spettri XANES, sono discussed in termini di un modello proposto. Il cambiamento relativo della forma negli spetttri dei diversi composti è posto in relazione alla differente ibridizzazione tra stati di atomi diversi e/o dal riempimento degli stati della banda vuota con elettroni forniti dal metallo sostituente. Sono state anche ottenute informazioni sulla struttura geometrica locale analizzando gli spettri nell’intervallo da 10 a 50 eV.

     

Specific features of the electrical resistivity of half-metallic ferromagnets Fe2MeAl (Me = Ti,V, Cr,Mn, Fe,Ni)

The transport properties of half-metallic ferromagnetic Heusler alloys Fe₂MeAl (where Me = Ti, V, Cr, Mn, Fe, and Ni are 3d transition elements) have been measured in the temperature range of 4–900 K. The specific features in the behavior of the electrical resistivity have been considered in terms of the two-current conduction model, which takes into account the presence of an energy gap in the electron spectrum of the alloys near the Fermi level.     

Catalytic precursors: (Fe,Mn)3O4 and γ-(Fe,Mn)2O3

Iron-manganese oxide catalysts are known to influence the production of short chain hydrocarbons in the Fischer-Tropsch-synthesis process. XRD, XRF, gravity measurements and Mössbauer-effect spectroscopy has been used to study a catalytic precursor made by coprecipitation of iron and manganese oxides. The sample is characterized as a mixture of two compounds with the same iron to manganese ratio. The two compounds are cubic spinels with the same crystallographic parameters, however, one compound is a defect spinel while the other is not. In the Mössbauer studies the defect spinel shows superparamagnetic relaxation behavior—fluctuations of the magnetization vector along two opposite easy directions, while the non-defect spinel shows evidence of collective excitations—fluctuations of the magnetization vector in directions close to the easy direction. These properties are related to the particle sizes.     

Phase transitions in [NH(CH3)3]2MeCl4 (Me = Cd,Zn,Cu) crystals

Dielectric properties of the new [NH(CH₃)₃]₂ZnCl₄ and [NH(CH₃)₃]₂CdCl₄ crystals from the [(CH₃) _n NH_4-n ]₂MeCl₄ group have been investigated in a wide temperature range (4.2–320 K). A series of phase transitions has been discovered at T₃ = 325 K,T₄ = 251 K,T₅ = 193 K, for [NH(CH₃)₃]₂CdCl₄ and at T₃ = 309 K, T₄ = 282 K, T₅ = 269 K for [NH(CH₃)₃]₂ZnCl₄. A ferroelectric phase has been discovered in the temperature interval T₄—T₅ from the temperature and frequency dependence of the dielectric permittivity ε(T, v). According to optical investigations the existence of ferroelastic phases in the temperature interval T₁ = 349 K–T₂ = 391 K and below T₅ for [NH(CH₃)₃]₂CdCl₄ and both above T₃ and below T₅ for [NH(CH₃)₃]₂ZnCl₄ has been ascertained.     

Ferroelectric and structural instability in double perovskites Me1+Bi3+Me3+Nb5+O6 (Me1+ = Na, K, Rb; Me3+ = Sc, Ga, In, Lu)

Within the Gordon-Kim generalized model with regard to the polarizabilities of ions, the lattice constants, the high-frequency permittivity, the Born dynamic charges, and the vibration constants of the crystal lattice are calculated for cation-ordered double perovskites Me¹⁺Bi³⁺Me³⁺Nb⁵⁺O₆. The vibration spectra of all the compounds exhibit two types of instabilities: instability associated with the rotation of the oxygen octahedron and ferroelectric instability. Various combinations of distortions with respect to the rotation mode yield five energetically most favorable distorted phases. The symmetry and the energy characteristics of these phases are discussed. In four of the five phases, the distortions associated with the oxygen octahedron rotation lead to polar phases, thus allowing one to speak of improper ferroelectricity in these compounds. One phase turns out to be nonpolar; however, it contains unstable polar modes such that a displacement along the eigenvectors of these modes gives rise to polarization in the crystal.     

Magnetic properties of SrRu0.9Fe0.1O3 thin films grown on different surfaces of SrTiO3 substrates

We stabilized SrRu_0.9Fe_0.1O₃ single-crystalline films on SrTiO₃ (001) and SrTiO₃ (110) substrates using epitaxial strain during thin-film growth. X-ray diffraction (XRD) θ–2θ scans showed strong peaks demonstrating single-crystal quality. Fe doping in SrRuO₃ had negative effects on the ferromagnetic properties, such as decreasing the T_c and saturated magnetic moment, as well as weakening the ferromagnetism. The negative effects were reduced when a suitable surface of the cubic substrate was selected for thin-film SrRuO₃ growth. We found that the ferromagnetic properties, such as the T_c and saturated magnetic moment, differed depending on the substrate surface. The observed differences are discussed in terms of Ru–Ru nearest-neighbor distance.