Magnetic state of the structural separated anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The results of neutron diffraction studies of the La_0.70Sr_0.30MnO_2.85 compound and its behavior in an external magnetic field are stated. It is established that in the 4–300 K temperature range, two structural perovskite phases coexist in the sample, which differ in symmetry (groups R[`3]cR\bar 3c and I4/mcm). The reason for the phase separation is the clustering of oxygen vacancies. The temperature (4–300 K) and field (0–140 kOe) dependences of the specific magnetic moment are measured. It is found that in zero external field, the magnetic state of La_0.70Sr_0.30MnO_2.85 is a cluster spin glass, which is the result of frustration of Mn³⁺-O-Mn³⁺ exchange interactions. An increase in external magnetic field up to 10 kOe leads to fragmentation of ferromagnetic clusters and then to an increase in the degree of polarization of local spins of manganese and the emergence of long-range ferromagnetic order. With increasing magnetic field up to 140 kOe, the magnetic ordering temperature reaches 160 K. The causes of the structural and magnetic phase separation of this composition and formation mechanism of its spin-glass magnetic state are analyzed.     

Phase separation of the spin system in the La<Subscript>0.93</Subscript>Sr<Subscript>0.07</Subscript>MnO<Subscript>3</Subscript> crystal

The magnetic state of the manganite La_0.93Sr_0.07MnO₃ in the range 4.2–290 K was studied using elastic neutron scattering. The magnetic state of this compound was found to occupy a particular place in the La_1?xSr_xMnO₃ solid-solution system, in which the antiferromagnetic type of order (LaMnO₃, T_N=139.5 K) switches to ferromagnetic ordering (La_0.9Sr_0.1MnO₃, T_C=152 K) with increasing x. In the transition state, this compound contains large-scale spin configurations of two types. A fractional crystal volume of about 10% is occupied by regions of the ferromagnetic phase with an average linear size of 200 Å, while the remainder of the crystal is a phase with a nonuniform canted magnetic structure. Arguments are presented for the phase separation of the La_0.93Sr_0.07MnO₃ spin system being accounted for by Mn⁴⁺ ion ordering.     

Phase separation in a manganite La<Subscript>0.95</Subscript>Ba<Subscript>0.05</Subscript>MnO<Subscript>3</Subscript> crystal

The structure and magnetic states of a crystal of lightly doped manganite La_0.95Ba_0.05MnO₃ were studied using thermal-neutron diffraction, magnetic measurements, and electrical resistance data in a wide temperature range. It is shown that, in terms of its magnetic properties, the orthorhombic crystal is characterized by two order parameters, namely, antiferromagnetic (T _N = 123.6 K) and ferromagnetic (T _C = 136.7 K). The results obtained differ in detail from known information on the manganites La_0.95Ca_0.05MnO₃ and La_0.94Sr_0.06MnO₃. Two models of the magnetic state of the La_0.95Ba_0.05MnO₃ crystal are discussed, one of which is a model of a canted antiferromagnetic spin system and another is associated with the phase separation of the manganite. Arguments are advanced in favor of the coexistence in this crystal of the antiferromagnetic phase (about 87%) with a Mn⁴⁺ ion concentration of 0.048 and the 1/16-type charge-ordered ferromagnetic phase (about 13%) with a Mn⁴⁺ ion concentration of 0.0625. The specific features of the manganite studied are due to self-organization of the La_0.95Ba_0.05MnO₃ crystal lattice caused by the relatively large barium ion size.     

Frustrated exchange interactions formation at low temperatures and high hydrostatic pressures in La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>Mn<Subscript>O2.85</Subscript>

The magnetic and thermal properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite are investigated in wide temperature (4–350 K) range, including under hydrostatic pressure (0–1.1 GPa). Throughout the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established, that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic clusters. The magnetic moment freezing temperature T _f of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative value ∼4.5 K/GPa, while the magnetic ordering T _MO temperature dependence is characterized by derivative value ∼13 K/GPa. The volume fraction of sample having ferromagnetic state is V _fer ∼ 13% and it increases under a pressure of 1.1 GPa by ΔV _fer ≈ 6%. Intensification of ferromagnetic properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease. The most likely mechanism of frustrated exchange interactions formation is discussed.     

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.     

Crystal and magnetic structures and physical properties of the Sm<Subscript>0.37</Subscript>Sr<Subscript>0.63</Subscript>MnO<Subscript>3</Subscript> manganite

The ¹⁵²Sm_0.37Sr_0.63MnO₃ manganite is investigated using neutron diffraction. The parameters of the crystal and magnetic structures of the manganite are determined. The diffraction data are compared with the transport and magnetic characteristics of this compound. A comparison is performed between the ¹⁵²Sm_0.37Sr_0.63MnO₃ and ¹⁵²Sm_0.45Sr_0.55MnO₃ manganites. Although these compounds differ insignificantly in the strontium doping level, are homogeneous antiferromagnets, and do not exhibit a colossal negative magnetoresistance, they have different crystal symmetries (tetragonal I4/mcm and orthorhombic Pnma), differ in the type of spin ordering (C-type antiferromagnetic and A-type antiferromagnetic ordering), are characterized by different orbital polarizations (\(d_{3z^2 - r^2 } \) and \(d_{x^2 - y^2 } \)), and possess one-and two-dimensional magnetic and transport properties, respectively. The critical concentration range in which samarium strontium manganites undergo a concentration structural transition from the orthorhombic to tetragonal crystal symmetry with a change in the type of orbital and magnetic order is revealed.     

Giant positive magnetoresistance in heterostructure (La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>) coated with YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> composites

(La_0.7Sr_0.3MnO₃) _x /(YBa₂Cu₃O₇) _y composites were prepared by mixing La_0.7Sr_0.3MnO₃ powders and the sol–gel-derived YBa₂Cu₃O₇ matrix, followed by high-temperature calcinations. Their structural, magnetic properties and magnetoresistance effect have been investigated systematically. A giant positive magnetoresistance (PMR) at low magnetic field is observed at low temperatures. In the case of (La_0.7Sr_0.3MnO₃)₁/(YBa₂Cu₃O₇)₉ composite, the PMR achieves 260% under a magnetic field of 5800 Oe. However, the PMR value sharply decreases with increasing temperature and no magnetoresistance effects are found above metal-insulator transition temperature. The enhancement of spin-dependent scattering at the grain boundaries should be responsible for the observed PMR. In addition, the temperature dependence of resistance under magnetic field could be explained by the competition between diamagnetism and paramagnetism in YBCO phase. At low temperature, the diamagnetism is predominant over paramagnetism and the interface scattering between LSMO grains is enhanced correspondingly. As a result, the low-temperature resistance increases and large PMR appears.     

Peculiarities of magnetic phase separation in anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The temperature and field dependences of the specific magnetic moment of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite have been measured. It is established, that the magnetic state of the sample studied is a cluster spin glass and it is the result of frustration of exchange Mn³⁺-O-Mn³⁺ interactions due to the redistribution of oxygen vacancies. The increase of the magnetic field leads to an increase in the degree of polarization of local spins of manganese. It is established using the magnetic criterion that a phase transition into the paramagnetic state for the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite is a thermodynamic second order phase transition. The causes and mechanism of the magnetic phase separation are discussed.     

Effect of the substrate ferroelastic transition on epitaxial La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films grown on LaAlO<Subscript>3</Subscript>

Thin films of La_0.7Sr_0.3MnO₃ were grown by molecular beam epitaxy on (001)LaAlO₃ crystals. High resolution X-ray diffraction analysis proves the presence of twins in the films at room temperature, showing that the twin structure of the substrate which forms at the ferroelastic transition at T_F = 813 K served as a template for the film microstructure. Magnetic measurements indicate a thermomagnetic irreversibility which is ascribed to the quenched disorder related to twinning and discussed in terms of coexisting ferromagnetic and spin disordered regions connected with the undeformed domain cores and strained domain walls respectively.     

Response of the electrical resistivity and magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films to biaxial tensile strains

The structure, electrical resistivity, and magnetoresistance of (50-nm)La_0.67Ca_0.33MnO₃ epitaxial films grown on a [(80 nm)Ba_0.25Sr_0.75TiO₃/La_0.3Sr_0.7Al_0.65Ta_0.35O₃] substrate with a substantial positive lattice misfit have been studied. The tensile biaxial strains are shown to account for the increase in the cell volume and in the relative concentration of Mn⁺³ ions in the manganite films as compared to those for the original material (33%). The peak in the temperature dependence of the resistivity ρ of La_0.67Ca_0.33MnO₃ films was shifted by 30–35 K toward lower temperatures relative to its position in the ρ(T) graph for a manganite film grown on (001)La_0.3Sr_0.7Al_0.65Ta_0.35O₃. For T < 150 K, the temperature dependences of ρ of La_0.67Ca_0.33MnO₃/Ba_0.25Sr_0.75TiO₃/La_0.3Sr_0.7Al_0.65Ta_0.35O₃ films could be well fitted by the relation ρ = ρ₀ + ρ₁T^4.5, where ρ₀ = 0.35 mΩ cm and the coefficient ρ₁ decreases linearly with increasing magnetic field. In the temperature interval 4.2–300 K, the magnetoresistance of manganite films was within the interval 15–95% (μ₀H = 5 T).     

Magnetic phase separation in the (La<Subscript>0.3</Subscript>Sr<Subscript>0.7</Subscript>)<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>FeO<Subscript>3</Subscript> solid solution with a perovskite structure

The crystal structure and magnetic properties of the (La_0.3Sr_0.7)_0.5Ca_0.5FeO₃ solid solution with a perovskite structure have been investigated. The solid solution has been synthesized according to the high-pressure technique. The unit cell parameters have been refined using the Rietveld full-profile analysis under the assumption of the single-phase crystalline state and the two-phase model corresponding to the parent compositions. It follows from the calculations that the best agreement between the experimental data and the theoretical curve is observed for the two-phase model. The measurement of the magnetic properties also indicates the coexistence of two magnetic phases.     

Energy level alignment at interfaces between organic semiconductors and clean ferromagnetic La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> thin film contacts for spin injection

We have studied the energy level alignment at interfaces between clean ferromagnetic La_0.7Sr_0.3MnO₃ and two archetype organic semiconductors, α-sexithiophene and copper-phthalocyanine, by combined X-ray and ultraviolet photoelectron spectroscopy. We observe the formation of a large interface dipole at both studied interfaces and small hole injection barriers. In addition, our results indicate a small chemical interaction between the organic materials and the La_0.7Sr_0.3MnO₃ surface which leads to a pinning of the Fermi level and the relatively small hole injection barriers.     

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⁺.     

Study of crystal and magnetic structures of manganite Pr<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressure

The crystal and magnetic structures of manganite Pr_0.7Ba_0.3MnO₃ have been studied at high pressures of up to 5.1 GPa and temperatures from 10 to 300 K by means of the neutron diffraction. At normal pressure and a temperature T _C = 200 K, a ferromagnetic state forms in Pr_0.7Ba_0.3MnO₃. At high pressures P ≥ 1.9 GPa and T < T _N ≈ 153 K, a new antiferromagnetic state of A-type have been observed. Under high pressure, the Curie temperature T _C increases with the characteristic quantity dT _C/dP ≈ 2.4 K/GPa. A possible reason for the appearance of an A-type antiferromagnetic phase in Pr_0.7Ba_0.3MnO₃ at high pressures may be anisotropic uniaxial compression of oxygen octahedra along the b axis of the orthorhombic structure.     

Size effect in nanocrystalline manganites La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript><Emphasis Type="Italic">A</Emphasis><Subscript>x</Subscript>MnO<Subscript>3</Subscript> (<Emphasis Type="Italic">A</Emphasis>=Ag,Sr)

Nanocrystalline samples of the manganites La_0.9Ag_0.1MnO₃, La_0.7Ag_0.3MnO₃, and La_0.7Sr_0.3MnO₃ were synthesized through pyrolysis and isothermally annealed. The atomic, subatomic, and magnetic structures of these manganites were studied using magnetic, x-ray, and neutron diffraction measurements. Increasing the annealing temperature from 600 to 1200°C coarsens the grains from 30–40 to 600–700 nm in size. All the samples studied have rhombohedral structure and are ferromagnets. The Curie temperature decreases for the samples doped by silver and increases for the samples doped by strontium as the anneal temperature is increased. The magnetization of the Mn ions increases with nanoparticle size in all the three systems, which indicates the presence of a size effect.     

Metamagnetic effects in epitaxial BaFe<Subscript>1.8</Subscript>Cr<Subscript>0.2</Subscript>As<Subscript>2</Subscript> thin films

Epitaxial BaFe_1.8Cr_0.2As₂ thin films with the tetragonal c-axis perpendicular to the thin film surface were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) single crystalline substrates using pulsed laser deposition (PLD). Resistive measurements indicate the existence of two transitions at temperatures of about 80 K and 40 K. The transition at 80 K is attributed to the structural transition from the high temperature tetragonal phase to the low temperature orthorhombic phase accompanied with the magnetic transition from a paramagnetic to an antiferromagnetic state as known for doped bulk systems. Below T ≈ 40 K the magnetization curves measured perpendicularly to the orthorhombic c-axis in fields up to 9 Tesla show two inflexion points indicating metamagnetic transitions.     

Magnetic and electric properties of Eu<Subscript>0.62</Subscript>Bi<Subscript>0.38</Subscript>MnO<Subscript>3</Subscript> and Eu<Subscript>0.53</Subscript>Bi<Subscript>0.32</Subscript>Sr<Subscript>0.15</Subscript>MnO<Subscript>3</Subscript> crystals

Single crystals of Eu_0.62Bi_0.38MnO₃ and Eu_0.53Bi_0.32Sr_0.15MnO₃ solid solutions crystallizing in an orthorhombically distorted perovskite structure were prepared. At temperatures above 120 K, Eu_0.62Bi_0.38MnO₃ exhibits the properties of structural glass while remaining a dielectric at all temperatures. There is no long-range magnetic order in this compound. Eu_0.53Bi_0.32Sr_0.15MnO₃ behaves as a semiconductor above 120 K and exhibits a jump in conductivity at T = 175 K associated with a metal-insulator transition occurring within limited regions of the crystal. In these regions, there appears a ferromagnetic moment (due to double exchange mediated by charge carriers) and local electric polarization.     

Positive magnetoresistance of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>/C Composites at room temperature

Two-phase composites xLa_0.7Sr_0.3MnO₃/(100–x)C (x = 5–85 mass %) have been synthesized. The magnetoresistive properties of these materials in magnetic fields from 0 to 15 kOe have been investigated. It has been shown that, at room temperature, the positive isotropic magnetoresistance for samples with x = 50–60 mass % reaches 15%.     

X-ray studies of the (La,Sr)MnO<Subscript>3</Subscript> perovskite manganite structure

X-ray studies of perovskite manganites (La_0.9Sr_0.1)_0.9MnO₃ and La_1?xSr_xMnO₃ (x = 0.1, 0.15, 0.2, 0.25) are reported. The atom positions and interatomic distances and angles are calculated as a function of Sr doping at room temperature using the FullProf software. The temperature dependences of the crystal lattice parameters and unit cell volume are investigated. The effects of structural and magnetic phase transitions on the crystal lattice parameters are studied in detail. The bulk magnetoelastic contribution to thermal expansion is studied experimentally and calculated.     

Magnetotransport characteristics of strained La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> epitaxial manganite films

The electrical and magnetic characteristics of La_0.7Sr_0.3MnO₃ (LSMO) epitaxial manganite films are investigated by different methods under conditions when the crystal structure is strongly strained as a result of mismatch between the lattice parameters of the LSMO crystal and the substrate. Substrates with lattice parameters larger and smaller than the nominal lattice parameter of the LSMO crystal are used in experiments. It is shown that the behavior of the temperature dependence of the electrical resistance for the films in the low-temperature range does not depend on the strain of the film and agrees well with the results obtained from the calculations with allowance made for the interaction of electrons with magnetic excitations in the framework of the double-exchange model for systems with strongly correlated electronic states. Investigations of the magneto- optical Kerr effect have revealed that an insignificant (0.3%) orthorhombic distortion of the cubic lattice in the plane of the NdGaO₃(110) substrate leads to uniaxial anisotropy of the magnetization of the film, with the easy-magnetization axis lying in the substrate plane. However, LSMO films on substrates (((LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7)(001)) ensuring minimum strain of the films exhibit a biaxial anisotropy typical of cubic crystals. The study of the ferromagnetic resonance lines at a frequency of 9.76 GHz confirms the results of magnetooptical investigations and indicates that the ferromagnetic phase in the LSMO films is weakly inhomogeneous.