Colossal Magnetoresistance of Layered Manganite La<Subscript>1.2</Subscript>Sr<Subscript>1.8</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7</Subscript> and Its Description by a “Spin–Polaron” Conduction Mechanism

The resistance of a La_1.2Sr_1.8Mn_2(1–z)O₇ single crystal has been studied in magnetic fields from 0 to 90 kOe. The magnetoresistance at temperature T = 75 K, near which a colossal magnetoresistance maximum is observed, has been successfully described in terms of the “spin–polaron” electric conduction mechanism. This value of the colossal magnetoresistance is due to a three-fold increase in the polaron size. The method of separating contributions of various conduction mechanisms to the magnetoresistance developed for materials with activation type of conduction is generalized to compounds in which a metal–insulator transition is observed. It is found that, at a temperature of 75 K, the contribution of the “orientation” mechanism is maximum (≈20%) in a magnetic field of 5 kOe and almost disappears in fields higher than 50 kOe.     

Preparation,structure and properties of La<Subscript>0.67</Subscript>Pb<Subscript>0.33</Subscript>(Mn<Subscript>1-x</Subscript>Co<Subscript>x</Subscript>)O<Subscript>3-δ</Subscript>

La_0.67Pb_0.33(Mn_1-xCo_x)O_3-δ ceramics with x=0, 0.01, 0.03, 0.06, 0.1 and 0.15 have been prepared in a two-step procedure. Precursor gels were made by the wet chemical malic acid method. The gels were calcined and then converted into ceramics by heat treatment at 950 °C and 1000 °C in air. X-ray diffraction showed that the compounds were phase pure. The crystal structure symmetry of the compounds was confirmed to be rhombohedral (space group R3̄c) for the whole investigated range of x. All compounds undergo a paramagnetic–ferromagnetic phase transition between 335 K and 225 K. The basic magnetic characteristics such as the Curie temperature , the paramagnetic Curie temperature θ, the effective magnetic moment and the saturated magnetization decrease with increasing Co doping. The ferromagnetic transition is accompanied by an anomaly in the electrical resistance for all compounds. The high-temperature insulator–metal transitions () do not coincide with the relevant . A large magnetoresistance peak of about 15% was observed for all compounds at . PACS 72.80.Ga; 75.47.Lx; 75.60.Ej     

Preparation of dense La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>Ga<Subscript>0.8</Subscript>Mg<Subscript>0.2</Subscript>O<Subscript>3-δ</Subscript> with high ionic conductivity by solid-state synthesis

The results of a systematic investigation on the effects of processing steps, via solid-state reactions, on structural phase characteristics and ionic conductivity of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ solid electrolyte are reported. The main purpose of this work is to establish an optimized route for obtaining good densification and high ionic conductivity of this solid electrolyte. Processing routes with three successive calcinations at 1250 °C followed by attrition milling (R1), and with two sequences of calcination at 1350 °C with intermediary attrition milling (R2) give rise to near full density at 1450 °C sintering temperature. The rate of grain growth is fast when the relative density reaches 95%. Elemental mapping reveals uniform distribution of the constituents in the matrix along with La₄Ga₂O₉, LaSrGa₃O₇ and sub-micrometer MgO grains at grain boundaries. The ionic conductivity of grains remains unchanged with the processing route and sintering profile. The blocking effect of charge carriers at grain boundaries decreases with increasing the dwell temperature.     

Special features of the structure and properties of La<Subscript>0.6</Subscript>Sr<Subscript>0.2</Subscript>Mn<Subscript>1.2−<Emphasis Type="Italic">y</Emphasis></Subscript>Cr<Subscript>y</Subscript>O<Subscript>3</Subscript> magnetoresisitive ceramics

Atomic order and magnetoresistive parameters are studied in La_0.6Sr_0.2Mn_1.2?yCr_yO₃ ceramics, in which Cr substituted for the above stoichiometric Mn content serves to enhance cluster formation in a solid solution. It is shown that in a range of 0.05<y<0.1, La_0.6Sr_0.2Mn_1.2?yCr_yO₃ experiences a phase transition from a partially disordered solid solution with a rhombohedral lattice \((R\bar 3c)\), in the form of planes possessing a long-range order and clusters of mesoscopic order, to a chemically microstratified solid solution, in the form of planes of an orthorhombic (Pnma) crystal lattice having a matrix structure and mesoscopic formations (clusters) coherently combined with a matrix of plane fragments peculiar to oxides of γ-MnO₂ type or, at y>0.1, of Cr₂O₃ type. It is shown that the structural phase transition is accompanied by maximum changes of the magnetoresistive parameters in the same range of Cr concentrations, i.e., 0.05<y<0.10.     

μSR study of Eu<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> and EuMn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

A comparative μSR study of ceramic samples of the EuMn₂O₅ and Eu_0.8Ce_0.2Mn₂O₅ multiferroics is performed in the temperature range from 15 to 300 K. It is found that the Ce doping of the EuMn₂O₅ sample slightly reduces the temperature of the magnetic phase transition from T _N = 45 K for the EuMn₂O₅ sample to T _N = 42.5 K for the Eu_0.8Ce_0.2Mn₂O₅ sample. Below the temperature T _N for both samples, there are two types of localization of a thermalized muon with different temperature dependences of the precession frequency of the magnetic moment of the muon in an internal magnetic field. The higher frequency in both samples refers to the initial antiferromagnetic matrix. The behavior of this frequency in Eu_0.8Ce_0.2Mn₂O₅ follows the Curie–Weiss law with the exponent β = 0.29 ± 0.02, which differs from the value β = 0.39 standard for 3D Heisenberg magnetics and is observed in EuMn₂O₅, because of the strong frustration of the doped sample. The temperature-independent low frequency is due to the presence of Mn³⁺–Mn⁴⁺ ferromagnetic pairs located along the b axis of the antiferromagnetic matrix and in the regions of phase separation, which contain such ion pairs and e _g electrons recharging them. In both samples, polarization losses are the same (about 20%) and are associated with the formation of Mn⁴⁺–Mn⁴⁺ + Mu complexes near Mn³⁺–Mn⁴⁺ ferromagnetic pairs. In the temperature interval from 25 to 45 K, the separation of the Eu_0.8Ce_0.2Mn₂O₅ structure into two fractions where the relaxation rates of polarization of muons differ by an order of magnitude is revealed. This effect is due to a change in the state of regions of phase separation (1D superlattices) at the indicated temperatures. Such effect in EuMn₂O₅ is significantly weaker.     

Magnetic and magnetoresistive properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Sr<Subscript>2</Subscript>FeMoO<Subscript>6–δ</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoheterostructures

A method has been developed for fabricating nanoporous matrices based on anodic aluminum oxide for the deposition of ferromagnetic nanoparticles in them. The modes of deposition of strontium ferromolybdate thin films prepared by the ion-plasma method have been worked out, and the magnetic and magnetoresistive properties, structure, and composition of the films have been investigated. It has been revealed that the microstructure and properties of the strontium ferromolybdate films deposited by ionplasma sputtering depend on the deposition rate and the temperature of the substrate. Based on the measurement of the electrical resistivity of nanoheterostructures in a magnetic field, it has been found that the magnetoresistance reaches 14% at T = 15 K and B = 8 T, which is due to the manifestation of tunneling magnetoresistance.     

Thermal expansion of (Sr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript>x</Subscript>)<Subscript>3</Subscript>Ru<Subscript>2</Subscript>O<Subscript>7</Subscript> single crystals at low temperatures

For single-crystal samples of the (Sr_1?xLa_x)₃Ru₂O₇ ruthenates, the temperature dependence of the thermal expansion coefficient α(T)) is measured in the range 4.2–80 K. The effect of magnetic fields H ≤ 3.5 T on thermal expansion is analyzed. It is found that the (Sr_1?xLa_x)₃Ru₂O₇ ruthenates exhibit an anomalous (negative) thermal expansion coefficient in the temperature range T ≤ 18 K. The position and width of the anomaly revealed in the temperature dependence of the thermal expansion coefficient α(T)) depend substantially on the magnetic field. The origin of the thermal expansion anomaly in ruthenates, the correlation of this anomaly with the stability of the crystal lattice, and the common nature of the anomalies in the thermal properties of ruthenates and high-temperature superconductors are discussed.     

Promising performances for a La<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>Co<Subscript>0.8</Subscript>Fe<Subscript>0.2</Subscript>O<Subscript>3-δ</Subscript> cathode with a dense interfacial layer at the electrode-electrolyte interface

As for the commonly studied La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (6428), here, a very low area-specific resistance (ASR) was measured for La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (6482) cathode deposited on a Ce_0.9Gd_0.1O_2-δ (GDC) electrolyte with addition of a thin (1 μm) dense LSCF film deposited by spin coating at the interface between the GDC electrolyte and a 40-μm-thick screen-printed electrode. The ASR ranged from 1 Ω.cm² at 500 °C, 0.11 Ω.cm² at 625 °C and value as low as 0.03 Ω.cm² at 700 °C. Impedance spectra collected in between 500 and 700 °C were carefully studied. They could all be modelled with two R//CPE in series which are likely associated to the oxygen reduction reaction itself (dissociation/adsorption/ionization) at low frequency and to the oxide ion transfer at the electrode/electrolyte interface at high frequency.     

Negative magnetoresistance in binary distorted perovskites Ca(Cu<Subscript>x</Subscript>Mn<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript>)Mn<Subscript>4</Subscript>O<Subscript>12</Subscript>

The substitution of copper for manganese in one position of the double distorted perovskite Ca(Cu_xMn_3?x)Mn₄O₁₂ leads to a change in the valence of manganese atoms in the other position. This is accompanied by a sharp increase in the temperature of magnetic ordering and by a change in the conductivity type from semiconductor to metallic. These perovskites also exhibit a negative magnetoresistance in a broad temperature range, with the [ρ(H)?ρ(0)]× 100%/ρ(0) ratio (at liquid nitrogen temperature) reaching 10% in a field of 1 T.     

Synthesis of cubic Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Zr<Subscript>2</Subscript>O<Subscript>12</Subscript> by modified sol–gel process

Polycrystalline cubic Li₇La₃Zr₂O₁₂ (LLZ) with garnet-related type structure has been synthesized at 700 °C by modified sol–gel processes using citric acid as organic complexing agent and butan-1-ol or propan-2-ol as surface active agent. Thermal analysis (thermogravimetric/differential thermal analysis) indicated that the gel must be annealed at around 700 °C to completely remove the organic solvent. X-ray powder diffraction, X-ray fluorescence, and scanning electron microscopic investigations revealed that Al may not be essential to form cubic-phase LLZ; however, the addition of Al₂O₃ led to enhanced sintering of LLZ.     

Structural and Mössbauer studies of nanocrystalline Mn<Superscript>4+</Superscript>-doped Li<Subscript>0.5</Subscript>Fe<Subscript>2.5</Subscript>O<Subscript>4</Subscript> particles prepared by mechanical milling

The structure and magnetic properties of spinel-related Mn⁴⁺-doped Li_0.5Fe_2.5O₄ nanocrystalline particles of the composition Li_0.5Fe_2.25Mn_0.1875O₄, prepared by milling a pristine sample for different times, were investigated. The average crystallite and particle size, respectively, decreased form ～40 nm to ～10 nm and ～2.5 μm to ～10 nm with increasing milling time from 0 h to 70 h. Rietveld refinement of the XRD data of the non-milled sample show the Mn⁴⁺ dopant ions to substitute for Fe³⁺ at the octahedral B-sites of the spinel-related structure. The Mössbauer spectra of the milled ferrites indicate that more particles turn superparamagnetic with increasing milling time. The Mössbauer data collected at 78 K suggest that while in the non-milled sample the Mn⁴⁺ ions substitute for Fe³⁺ at the octahedral B-sites, this is reversed as milling proceeds with doped Mn⁴⁺ ions, balancing Fe³⁺ vacancies and possibly Li⁺ ions progressively migrate to the tetrahedral A-sites. This is supported by the slight increase observed in the magnetization of the milled samples relative to that of the non-milled one. The magnetic data suggest that in addition to the increasing superparamagentic component of the milled particles, thermal spin reversal and/or spin canting effects are possible at the surface layers of the nanoparticles.     

Comparing the physical properties of Pr/Gd and Pr/Ce substitutions in Ru(Gd<Subscript>1.5</Subscript>Ce<Subscript>0.5</Subscript>)Sr<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10−<Emphasis Type="Italic">δ</Emphasis></Subscript>

We have compared the electrical and magnetic properties of Ru(Gd_1.5−x Pr _x )Ce_0.5Sr₂Cu₂O_10−δ (Pr/Gd samples) with x = 0.0, 0.01, 0.03, 0.033, 0.035, 0.04, 0.05, 0.06, 0.1 and RuGd_1.5(Ce_0.5−x Pr _x ) Sr₂ Cu₂O_10−δ (Pr/Ce samples) with x = 0.0, 0.01, 0.03, 0.05, 0.08, 0.1, 0.15, 0.2 prepared by the standard solid-state reaction technique. We obtained the XRD patterns for different samples with various x. The lattice parameters versus x for different substitutions have been obtained from Rietveld analysis. To determine how the magnetic and superconducting properties of these layered cuprate systems can be affected by Pr substitution, the resistivity, and magnetoresistivity, with H _ext varying from 0.0 to 15 kOe, have been measured at various temperatures. Superconducting transition temperature T _c and magnetic transition T _irr , have been obtained through resistivity and ac susceptibility measurements. The T _c suppression due to Pr/Gd and Pr/Ce substitutions show competition between pair breaking by magnetic impurities, hole doping due to different valances of ions, difference in ionic radii, and oxygen stoichiometry. Pr/Gd substitution suppresses superconductivity more rapidly than for Pr/Ce, showing that the effect of hole doping and magnetic impurity pair breaking is stronger than the difference in ionic radii. In Pr/Gd substitution, the small difference between the ionic radii of Pr^3+,4+ and Gd³⁺, and absorption of more oxygen due to the higher valence of Pr with respect to Gd, decreases the mean Ru-Ru distance, and as a result, the magnetic exchange interaction becomes stronger with the increase of x. However, Pr/Ce substitution has the opposite effect. The magnetic parameters such as H _c , obtained through magnetization measurements versus applied magnetic field isotherm at 77 K and room temperatures, become stronger with x in Pr/Gd and weaker with x in Pr/Ce substitution.     

Effects of deposition temperature and post-annealing on structure and electrical properties in (La<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>)CoO<Subscript>3</Subscript> films grown on silicon substrate

(La_0.5Sr_0.5)CoO₃ (LSCO) thin films have been fabricated on silicon substrate by the pulsed laser deposition method. The effects of substrate temperature and post-annealing condition on the structural and electrical properties are investigated. The samples grown above 650°C are fully crystalline with perovskite structure. The film deposited at 700°C has columnar growth with electrical resistivity of about 1.99×10⁻³ Ω cm. The amorphous films grown at 500°C were post-annealed at different conditions. The sample post-annealed at 700°C and 10⁻⁴ Pa has similar microstructure with the sample in situ grown at 700°C and 25 Pa. However, the electrical resistivity of the post-annealed sample is one magnitude higher than that of the in situ grown sample because of the effect of oxygen vacancy. The temperature dependence of resistivity exhibits semiconductor-like character. It was found that post-annealing by rapid thermal process will result in film cracks due to the thermal stress. The results are referential for the applications of LSCO in microelectronic devices.     

Magnetic characteristics of MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles obtained by glycine–nitrate synthesis

The magnetic properties of magnesium–iron spinel (MgFe₂O₄) powdered nanoparticles obtained by glycine–nitrate synthesis are investigated by X-ray phase analysis and the NMR method. According to the results of X-ray phase analysis, the average size of the crystalline part of nanoparticles of the powder under investigation is 45 ± 4 nm. Magnetization J is determined using the formula J = (B/μ0)–H, where B and H are the induction and strength of the magnetic field in the sample, which are measured by the NMR method. The magnetic characteristics of MgFe₂O₄ are as follows: specific saturation magnetization J_sat = 17.52 A m²/kg, specific residual magnetization J_r = 5.73 A m2/kg, coercive force H_c = 4600 A/m, and magnetic moment P_sat = 371 × 10^–20 A m² in the magnetic saturation state and P_r = 121 × 10^–20 A m² in the residual magnetization state.     

Piezĭlectric and elastic properties of Sr<Subscript>3</Subscript>NbGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> (SNGS) single crystals

The elastic, piezoelectric, and dielectric constants of new piezoelectric single crystals Sr₃NbGa₃Si₂O₁₄ (SNGS) are measured. The elastic moduli C ₁₁ and C ₆₆ and the piezoelectric coefficient e ₁₁ are determined over a wide temperature range. It is demonstrated that the piezoelectric activity of the crystal is retained up to the highest temperatures (900°C).     

Synthesis and characterization of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>–Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> magnetoelectric composites with dielectric and magnetic properties

Composite structures consisting of (001)-oriented SrTiO₃ (STO)/La_0.7Sr_0.3MnO₃ (LSMO) films of 30 nm thickness, grown on an (001) Pb(Mg_1/3Nb_2/3)TiO₃– 28 mol.% PbTiO₃ piezoelectric relaxor-ferroelectric single-crystalline wafer were investigated by means of Wide-Angle X-ray Diffraction (WAXRD) in situ under influence of a d.c. electric field with strength E up to ±18 kV/cm. The WAXRD measurements of the films and substrate reflection profiles resulted in a determination of the strain s in the films and the substrate separately. The strained state of the STO/LSMO films is effectively controlled by a huge converse piezoelectric effect of the PMN-PT substrate. The coefficients of coupling between electric-field-induced out-of-plane strain in the films and in the substrate for the composite system STO/LSMO/PMN-PT are obtained.     

Specific features of the magneto-optical properties of α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> in the IR region

The spectra of reflection and magnetoreflection of light from the crystalline insulator α-Al₂O₃ in the IR spectral region (λ = 2.5–25 μm) are investigated. Some features in the magnetoreflection spectra in the wavelength range corresponding to the excitation of optical phonon modes in α-Al₂O₃ are found. A significant increase in magnetoreflection is observed near these wavelengths. The amplitude and shape of the magnetore-flection spectra for the p and s polarizations of probe light are determined. It is shown that the optical and magneto-optical properties of α-Al₂O₃ in the IR region can be described on the basis of the theory of polaron excitation. A satisfactory correlation between the theoretical and experimental spectra is obtained, which indicates that polarons play an important role in determining the optical characteristics of nonmagnetic insulators and make the dominant contribution to the magnetoreflection spectra.     

Evidence of Jahn–Teller distortion in Li<Subscript>x</Subscript>Mn<Subscript>2</Subscript>O<Subscript>4</Subscript> by thermal diffusivity measurements

Thermal diffusivities of Li_xMn₂O₄ (x=0.9, 1.0 and 1.1) polycrystalline pellets are determined by photoacoustic technique in the reflection configuration, at two temperatures, 298 K and 280 K, above and below their Jahn–Teller phase transition temperature (290 K). The diffusivities of Li_xMn₂O₄ at 280 K show a drastic reduction from their corresponding room temperature values (298 K) and the percentage of reduction in the thermal diffusivity for Li_xMn₂O₄ increases with their Li content. These effects are associated with the reduction in crystal symmetry due to structural deformation by Jahn–Teller distortion observed in Li_xMn₂O₄ below its transition temperature. PACS 78.20.Nv; 82.47.Aa; 63.20.Mt     

Transport properties in La<Subscript>0.7−<Emphasis Type="Italic">x</Emphasis></Subscript>Gd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> system

The influence of Gd doping at La-site on the electrical transport properties and the colossal magnetoresistance of La_0.7−x Gd _x Sr_0.3MnO₃ (x=0.00, 0.10, 0.15, 0.20, 0.30, 0.40, 0.50, 0.60, and 0.70) is studied. The experimental results indicate that the transport properties exhibit abnormal behavior under high doping condition. Forx=0.50, we find that a transition from metal to insulator occurs after the occurrence of insulator-metal transition nearT _c, which seldom occurs in ABO₃ structure. For samplesx=0.60 and 0.70, it exhibits insulator behavior far aboveT _c. These abnormal behaviors are attributed to different magnetic background, i.e. the system undergoes a transition from long range ferromagnetic order to the cluster-spin glass state and further to antiferromagnetic order.     

Inhomogeneous magnetic states in the Nd(Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript>x</Subscript>)O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Nd(Mn_?xCr_x)O₃ system (x≤0.85) have been studied. Substitution of chromium for manganese was shown to induce a transition from the antiferromagnetic to ferromagnetic state (x≈0.2) and a decrease in the critical temperature followed, conversely, by an increase in the Néel temperature and decay of spontaneous magnetization. At low temperatures, the magnetization was found to behave anomalously as a result of magnetic interaction between the ferromagnetic and antiferromagnetic phases. The formation of the ferromagnetic phase is attributed to destruction of cooperative static orbital ordering, while the coexistence of different magnetic phases is most probably due to internal chemical inhomogeneity of the solid solutions.