Conductivity,magnetoresistance, and specific heat of oxygen-deficient La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3−α</Subscript> (0≤α≤0.16)

Oxygen-deficient La_0.67Sr_0.33MnO_3?α solid solutions have been studied. A comparison is made with the results obtained in an earlier study of a similar lanthanum-calcium manganite series. The physical characteristics of both series are accounted for as being due to a change in the Mn³⁺/Mn⁴⁺ ratio caused by oxygen removal. The differences between the strontium and calcium series originate from differences in both the bulk properties of the original oxygen-stoichiometric materials and their texture. In the strontium series, the texture manifests itself in intergrain magnetoresistance, which exceeds in magnitude the colossal magnetoresistance caused by bulk properties of the material. Study of the oxygen-deficient La_0.67Sr_0.33MnO_3?α compound revealed specific features in the dependence of the electrophysical parameters on temperature and the Mn⁴⁺ fractional content that were not observed in the La_0.67Ca_0.33MnO_3?α compound studied by us earlier and in La_1?xSr_xMnO₃ samples described in the literature. The physics underlying these differences is discussed. A modified phase diagram relating the phase transition temperature to the Mn⁴⁺ fraction is proposed.     

Oxygen-deficiency and high-pressure effects on the magnetic and crystal structures of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3−<Emphasis Type="Italic">d</Emphasis></Subscript> managanites

The magnetic and crystal structures of anion-deficient La_0.7Sr_0.3MnO_3?d manganites (d = 0.15 and 0.20) are studied by neutron diffraction in the range of high pressures 0–5 GPa and temperatures 10–300 K. It is found that a spin-glass state forms in La_0.7Sr_0.3MnO_2.85 below T _g ～ 50 K, while magnetic phase separation is observed in La_0.7Sr_0.3MnO_2.80, which is characterized by the coexistence of AFM domains of the C type with spin-glass domains. As distinct from the stoichiometric A_0.5Ba_0.5MnO₃ manganites (A = Nd, Sm), in which the high-pressure effect suppresses the spin-glass state and gives rise to ferromagnetism, the spin-glass state in La_0.7Sr_0.3MnO_2.85 is stable under pressure. The bulk modulus of La_0.7Sr_0.3MnO_2.85 is considerably smaller than that for the stoichiometric La_0.7Sr_0.3MnO₃ compound. The causes of the formation of different types of the magnetic structure in La_0.7Sr_0.3MnO_3?d (d = 0.15 and 0.20) and different high-pressure effects on the magnetic structure of stoichiometric and anion-deficient manganites are analyzed.     

Maxwell–Wagner relaxation and magnetodielectric properties of Bi<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> ceramics

The complex permittivity ε = ε′–iε″ of manganite bismuth–lanthanum Bi_0.5La_0.5MnO₃ ceramics has been measured at temperature T = 78 K in the frequency range f = 200–10⁵ Hz and in the magnetic induction range B = 0–5 T. Dielectric relaxation and the pronounced magnetodielectric effect have been detected. The explanation based on the superposition of Maxwell–Wagner relaxation and the magnetoresistance effect has been proposed.     

Hall effect in La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> crystals

A study of the temperature dependences of the ordinary and anomalous Hall coefficients and of the Hall mobility of carriers in single crystals of the ferromagnetic manganites La_1?xSr_xMnO₃ (x=0.15, 0.20, 0.25) was carried out in the temperature interval from 85 to 400 K. The nature of the carriers and the conduction mechanisms in these compounds are discussed.     

Correlations induced orbital ordering and cooperative Jahn–Teller distortion in the paramagnetic insulator KCrF<Subscript>3</Subscript>

We investigate the origin of the orbital ordering in the paramagnetic phase of KCrF₃. All previous studies described structural parameters of the paramagnetic phase using a magnetic ordering in the compound. Our simulations of real paramagnetic KCrF₃ were performed within an approach combining density functional theory and dynamical mean field theory (DFT+DMFT). As a result, it was found that the experimentally observed cooperative Jahn–Teller effect is successfully described in a lattice relaxation calculation for structure without any long-range magnetic ordering. It is established that the existence of the orbital ordering even in undistorted perovskite structure clearly confirms the electronic origin of the orbital ordering in KCrF₃.     

Magnetic,electrical, magnetoelectrical,and magnetoelastic properties of La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>MnO<Subscript>3 − <Emphasis Type="Italic">y</Emphasis></Subscript> manganites

This paper reports on a study of the influence of oxygen deficiency on the magnetization, paramagnetic susceptibility, electrical resistivity, magnetoresistance, and volume magnetostriction of the La_0.9Sr_0.1MnO_{3 − y} manganite with y = 0.03, 0.10, and 0.15. The magnetization M(T) behaves in a complex way with temperature; for T < 80 K, it only weakly depends on T, and at 80 ≤ T ≤ 300 K, the M(T) curve shows a falloff. Within the interval 240 K ≤ T ≤ 300 K, the long-range magnetic order breaks up into superparamagnetic clusters. For T < 80 K, the magnetic moment per formula unit is about one-fourth that which should be expected for complete ferromagnetic alignment of Mn ion moments. Although the composition with y = 0.03, in which part of acceptor centers is compensated by donors (oxygen vacancies), the negative magnetoresistance Δρ/ρ and volume magnetostriction ω are observed to pass through maxima near the Curie point, their values are one to two orders of magnitude smaller than those for the y = 0 composition. In compositions with y = 0.10 and 0.15 with electronic doping, the values of Δρ/ρ and ω are smaller by one to two orders of magnitude than those observed for the y = 0.03 composition. They do not display giant magnetoresistance and volume magnetostriction effects, which evidences the absence of ferrons near unionized oxygen vacancies. This allows the conclusion that the part played by both compensated and uncompensated doubly charged donors consists in forming dangling Mn-O-Mn bonds, which lead to a decrease in the Curie temperature with increasing y and to the formation above it of superparamagnetic clusters of the nonferron type.     

Successive structural phase transitions in Pr<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript> in the range 10–1120 K

The crystal and magnetic structures of the Pr_0.5Sr_0.5CoO₃ metallic ferromagnet have been studied using neutron diffraction and synchrotron radiation. Successive structural transitions with the reduction of the crystal symmetry from cubic (space group Pm3m) to rhombohedral (\(R\bar 3c\), ～800 K), orthorhombic (Imma, ～300 K) and, then, to triclinic at ～120 K are detected during cooling from 1120 K. The transition from the orthorhombic system to a phase with a lower symmetry is characterized by a sharp change in the anisotropy of the unit cell, which indicates the partial ordering of the e _g orbitals of cobalt. The accompanying change in the interatomic distances and valence angles give rise to an anomaly in the temperature dependence of the magnetic susceptibility at T ≈ 120 K. The ordered magnetic moment μ_Co ≈ 2μ_B corresponds to the assumption of the intermediate spin state of Co³⁺ ions and the mixture of low- and intermediate-spin states of Co⁴⁺ ions.     

Peculiarities of the magnetic state in the system La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>3−γ</Subscript> (0 ≤ γ ≤ 0.25)

The results of experimental investigation of the chemical phase composition, crystal structure, and magnetic properties of the manganite La_0.70Sr_0.30MnO_3?γ (0 ≤ γ ≤ 0.25) with perovskite structure depending on the concentration of oxygen vacancies are presented. It is found that the mean grain size of the stoichiometric solid solution of La_0.70Sr_0.30MnO₃ amounts approximately to 10 μm, while the grain size for anion-deficient solid solutions of La_0.70Sr_0.30MnO_3?γ is approximately 5 μm. It is found that samples with 0 ≤ γ ≤ 0.13 have a rhombohedral unit cell (with space group \(R\bar 3c\), Z = 2), while samples with γ ≥ 0.20 have a tetragonal unit cell (space group I4/mcm, Z = 2). It is proved experimentally that the magnetic phase state of the manganite La_0.70Sr_0.30MnO_3?γ changes upon a decrease in the oxygen content. It is shown that anion-deficient solid solutions of La_0.70Sr_0.30MnO_3?γ experience a number of successive magnetic phase transformations in the ground state from a ferromagnet (0 ≤ γ ≤ 0.05) to a charge-disordered antiferromagnet (γ = 0.25) via an inhomogeneous magnetic state similar to a cluster spin glass (0.13 ≤ γ ≤ 0.20). The mean size of ferromagnetic clusters (r ≈ 50 nm) in the spin glass state is estimated. It is shown that oxygen vacancies make a substantial contribution to the formation of magnetic properties of manganites. The generalized magnetic characteristics are presented in the form of concentration dependences of the spontaneous magnetic moment, coercive force, and the critical temperature of the magnetic transition. The most probable mechanism of formation of the magnetic phase state in Sr-substituted anion-deficient manganites is considered. It is assumed that in the absence of orbital ordering, a decrease in the magnetic ion coordination number leads to sign reversal in indirect superexchange interactions Mn³⁺-O-Mn³⁺.     

Oxygen vacancies effects on phase diagram of epitaxial La<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> thin films

We investigated the effects of oxygen vacancies on the structural, magnetic, and transport properties of La_1–x Sr _x MnO₃ (x=0.1, 0.2, 0.33, 0.4, and 0.5) grown around a critical point (without/with oxygen vacancies) under low oxygen pressure (10 Pa) and high oxygen pressure (40 Pa). We found that all films exhibit ferromagnetic behavior below the magnetic critical temperature, and that the films grown under low oxygen pressures have degraded magnetic properties with lower Curie temperatures and smaller magnetic moments. These results show that in epitaxial La_1–x Sr _x MnO₃ thin films, the magnetic and transport properties are very sensitive to doping concentration and oxygen vacancies. Phase diagrams of the films based on the doping concentration and oxygen vacancies were plotted and discussed.     

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.     

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.     

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.     

Structural and magnetoelectric properties of MFe<Subscript>2</Subscript>O<Subscript>4</Subscript>–PZT (M = Ni,Co) and La<Subscript>x</Subscript>(Ca,Sr)<Subscript>1-x</Subscript>MnO<Subscript>3</Subscript>–PZT multilayer composites

Thick-film layered magnetoelectric composites consisting of ferromagnetic and ferroelectric phases have been synthesized with nickel ferrite (NFO), cobalt ferrite, La_0.7Sr_0.3MnO₃ (LSMO), or La_0.7Ca_0.3MnO₃ (LCMO) and lead zirconate titanate (PZT). Structural, magnetic, and ferromagnetic resonance characterization shows evidence for defect-free ferrites, but deterioration of manganite parameters. The resistivity and dielectric constants are smaller than expected values. The magnetoelectric effect (ME) is stronger in ferrite–PZT than in manganite–PZT. The ME voltage coefficient _E at room temperature is the highest in NFO–PZT and the smallest for LCMO–PZT. The transverse ME effect is an order of magnitude stronger than the longitudinal effect. The magnitude of _E correlates well with magnetic permeability for the ferrites. PACS 75.80.+q; 75.70.Gg; 75.60.-d     

Resistance-switching properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films with Ag–Al alloy top electrodes

A novel Ag–Al alloy electrode has been prepared on the La_0.67Ca_0.33MnO₃ (LCMO) film grown by pulsed laser deposition, with the aim to improve its resistance-switching properties. Nonlinear, asymmetric, and hysteretic current–voltage characteristics and reversible polarity-dependent switching properties are achieved in the Ag–Al alloy/LCMO/Pt structure. Detailed current–voltage characteristics analysis indicates that the resistance-switching behavior can be well explained by the mechanism of trap-controlled space charge limited conduction at the Ag–Al alloy/LCMO interface. The LCMO film with an Ag–Al alloy top electrode exhibits much better resistance-switching properties than that with an Al top electrode, including the shorter switching time and more stable switching process, demonstrating that the Ag–Al alloy electrode is a promising electrode materials of manganite films for resistance random access memory applications.     

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     

Magnetocaloric properties of manganites La<Subscript>1−x</Subscript>(Ag,K)<Subscript>x</Subscript>MnO<Subscript>3</Subscript>

The temperature and magnetic-field dependences of the magnetocaloric effect in manganites La_1−x Ag _x MnO₃ (x = 0.1; 0.15; 0.2) and La_1−x K _x MnO₃ (x = 0.1; 0.11; 0.13; 0.15; 0.175) were studied by a direct method. Large changes in the sample temperature were detected as a magnetic field changed by ΔH = 10 kOe. Temperatures of the magnetocaloric effect maxima are near room temperatures. Field dependences of the magnetocaloric effect show no signs of saturation in fields to 30 kOe.     

Role of A-site deficiency in the magneto-transport properties of Pr<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> relaxed films

The magneto-transport properties of thick relaxed A-site deficient films having the composition Pr Sr_0.3MnO₃ (P_□SMO with the □ symbol for the Pr vacancy) and Pr_0.7Sr MnO₃ (PS_□MO) are studied. A direct comparison with a Pr_0.7Sr_0.3MnO₃ (PSMO)completely relaxed film, deposited under the same growth conditions, shows a reduction of the in-plane parameter a₁₀₀ associated to an enhancement of the out-plane parameter. The strains (bulk strain ε_B and biaxial Jahn-Teller strain ε_J-T) do vary with the nature of the cationic vacancy. For example, an enhancement of ε_B of 9% in the PS_□MO film (Sr deficient) produces a decrease of T_C of 30 K, whereas the Pr deficient P_□SMO film exhibits a large reduction of both ε_B (-16%) and ε_J-T (divided by a factor of 5), which enhances T_C of 12 K, similarly to previous observations on bulk ceramics. With a reduced resistivity (ρ<0.02 Ω cm), the obtained Pr-deficient film, P_□SMO, exhibits the best magneto-transport properties with a decreasing magnetoresistance sensitivity at low field.     

Mössbauer and magnetic studies of the phase state of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/La<Subscript>0.9</Subscript>Ca<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> composites

The formation of an intermediate phase in SrFe₁₂O₁₉/La_0.9Ca_0.1MnO₃ composites was demonstrated for the first time using only Mössbauer spectroscopy. The SrFe₁₂O₁₉/La_0.9Ca_0.1MnO₃ composite was prepared by the two-stage (sol–gel and hydrothermal) synthesis with varying initial conditions. The X-ray diffraction studies showed that the composite consisted of two phases: well-formed structures of manganite La_0.9Ca_0.1MnO₃ and hexagonal ferrite SrFe₁₂O₁₉. It was found that nanocrystalline La_0.9Ca_0.1MnO₃ particles with size d ? 150 nm formed in the composites at the surface of plate-like SrFe₁₂O₁₉ crystallites. The Mössbauer studies showed that the composite contained additional (intermediate) phase La_0.9Ca_0.1Mn(Fe)O₃ that formed at the interface between SrFe12O19 and La_0.9Ca_0.1MnO₃ phases. The intermediate phase concentration increased with the molar content of La_0.9Ca_0.1MnO₃; in this case, the fraction of the surface of SrFe₁₂O₁₉ crystallites coated with La_0.9Ca_0.1MnO₃ increased, which led to the increase in the total area of the interface surface and the intermediate phase concentration.     

Influence of a magnetic two-phase state on the magnetocaloric effect in the La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

The magnetocaloric effect ΔT_ex and the magnetization in La_1?xSr_xMnO₃ single crystals (x=0.1, 0.125, 0.175, 0.3) have been experimentally studied. The magnetic entropy and the magnetocaloric effect ΔT_th were computed from magnetization curves. All the samples exhibited a maximum in the ΔT_th(T) curve at T=T _max ^′ . A step was observed on the ΔT_ex(T) curve in the region of T _max ^′ , with the value of ΔT_ex on this step being substantially smaller than ΔT_th. The step on the ΔT_ex(T) curve was followed by a maximum, which appeared at a temperature 20–40 K above T _max ^′ . This anomalous behavior of ΔT_ex and ΔT_th is assigned to the coexistence of two magnetic (ferro-and antiferromagnetic) phases in the crystal. The calculated value of ΔT_th is determined primarily by the ferromagnetic part of the crystal and disregards the negative contribution from the antiferromagnetic part of the crystal to ΔT_ex.     

Mass and kinetic energy distribution of the species generated by laser ablation of La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript>

The mass distributions of the species generated by laser ablation from a La_0.6Ca_0.4MnO₃ target using laser irradiation wavelengths of 193 nm, 266 nm and 308 nm have been investigated with and without a synchronized gas pulse of N₂O. The kinetic energies of the species are measured using an electrostatic deflection energy analyzer, while the mass distributions of the species were analyzed with a quadrupole mass filter. In vacuum (pressure 10⁻⁷ mbar), the ablation plume consists of metal atoms and ions such as La, Ca, Mn, O, LaO, as well as multiatomic species, e.g. LaMnO⁺. The LaO⁺ diatomic species are by far the most intense diatomic species in the plume, while CaO and MnO are only detected in small amounts. The interaction of a reactive N₂O gas pulse with the ablation plume leads to an increase in plume reactivity, which is desired when thin manganite films are grown, in order to incorporate the necessary amount of oxygen into the film. The N₂O gas pulse appears to have a significant influence on the oxidation of the Mn species in the plume, and on the creation of negative ions, such as LaO⁻,O⁻ and O₂⁻.