Structure and magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films grown coherently on (001)NdGaO<Subscript>3</Subscript>

40-to 120-nm-thick (001)La_0.67Ca_0.33MnO₃ films grown through laser evaporation on (001)NdGaO₃ were studied. The lattice parameters of the La_0.67Ca_0.33MnO₃ films measured in the substrate plane (a_∥=3.851 Å) and along the normal to its surface (a_⊥=3.850 Å) practically coincided with that of the pseudocubic neodymium gallate. The unit-cell volume of the La_0.67Ca_0.33MnO₃ film was slightly smaller than that of stoichiometric bulk samples. The position of the maximum in the temperature dependence of electrical resistivity did not depend on the thickness of the La_0.67Ca_0.33MnO₃ film. The negative magnetoresistance (MR≈?0.25, H=0.4 T) of La_0.67Ca_0.33MnO₃ films reached a maximum at 239–244 K.     

Response of the electrical resistivity of La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>(20 nm)/LaAlO<Subscript>3</Subscript>(001) films to magnetic field and variation in temperature

The structure, electrical resistivity, and magnetotransport parameters of 20-nm-thick epitaxial La_0.67Ba_0.33MnO₃ films grown by laser ablation on LaAlO₃(001) substrates are studied. The unit cell volume V _eff = 58.80 ų of the as-grown manganite films is found to be less than that for bulk La_0.67Ba_0.33MnO₃ crystals. Maximum values of the negative magnetoresistance MR(μ₀ H = 1 T) = ?0.27 for La_0.67Ba_0.33MnO₃ films are observed at a temperature of about 225 K. For 5 < T < 100 K, the film magnetoresistance depends only weakly on temperature and is on the order of ?0.1. At temperatures below 100 K and for 3 < μ₀ H < 5 T, the electrical resistivity of the as-grown films decreases linearly with increasing magnetic field.     

Electrical resistivity and magnetotransport in La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films asymmetrically biaxially compressed by an NdGaO<Subscript>3</Subscript>(001) substrate

The La_0.67Ba_0.33MnO₃(40 nm) films are quasi-coherently grown on an NdGaO₃(001) substrate with an orthorhombic unit cell distortion of ～1.4%. The biaxial compressive stresses generated during nucleation and growth lead to a decrease in the unit cell volume of the grown layers. This, in turn, results in a decrease (by ～35 K) in the temperature of the maximum in the dependence of the electrical resistivity ρ of the layers on the temperature. For T < 150 K, the electrical resistivity ρ of the films increases in proportion to ρ₂ T ^4.5 and the coefficient ρ₂ decreases almost linearly with increasing magnetic field H. The negative magnetoresistance (≈?0.17 for μ₀ H = 1 T) reaches a maximum at temperatures close to room temperature. The response of the electrical resistivity ρ of the La_0.67Ba_0.33MnO₃(40 nm) films to the magnetic field depends on the crystallographic direction of the film orientation and the angle between H and I (where I is the electric current through the film).     

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).     

Electrical resistivity and magnetotransport properties of La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films biaxially compressed by the substrate

The structure, electrical resistivity, and magnetoresistance of La_0.67Ba_0.33MnO₃(20 nm) films grown coherently on an La_0.3Sr_0.7Al_0.65Ta_0.35O₃(001) substrate with a lattice misfit of about 1% were studied. The rigid connection of the manganite layer with the bulk substrate brought about the unit cell distortion of the substrate (a _⊥/a _‖ = 1.02) and a decrease in the unit cell volume as compared to that of the corresponding bulk crystals (a _‖ and a _⊥ are the unit cell parameters measured in the substrate plane and along the surface normal, respectively). The temperature T _M ≈ 295 K, at which the electrical resistivity ρ of the (20 nm)La_0.67Ba_0.33MnO₃ films reached a maximum, was 40–45 K lower than that for the corresponding bulk crystals. The negative magnetoresistance (MR ≈ ?0.25 for μ₀ H = 1 T) attained a peak value at T _MR ≈ 270 K. The response of ρ to a magnetic field depended substantially on the angle between the current flow in the film and the direction of the magnetic field.     

Magnetoresistance of La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films grown on a substrate with low lattice mismatch

The structure, electrical resistivity, and magnetoresistance of La_0.67Sr_0.33MnO₃ heteroepitaxial films (120-nm thick) practically unstrained by lattice mismatch with the substrate were studied. A strong maximum of negative magnetoresistance of ≈27% (for μ₀H = 4 T) was observed at T ≈360 K. While the magnetoresistance decreased monotonically in magnitude with decreasing temperature, it was still in excess of 2% at 150 K. For T < 250 K, the temperature dependence of the electrical resistivity ρ of La_0.67Sr_0.33MnO₃ films is fitted well by the relation ρ = ρ₀ + ρ ₁(H)T^2.3, where ρ₀ = 1.1×10^?4 Ω cm, ρ₁(H = 0) = 1.8×10^?9 Ω cm/K^2.3, and ρ₁(μ₀H = 4 T)/ρ₁(H = 0) ≈0.96. The temperature dependence of a parameter γ characterizing the extent to which the electrical resistivity of the ferromagnetic phase of La_0.67Sr_0.33MnO₃ films is suppressed by a magnetic field (μ ₀H = 5 T) was determined.     

Transport parameters of granular La0.67Ca0.33MnO3 films grown on an R-plane sapphire

Precisely (100)-oriented, 200-nm thick La_0.67Ca_0.33MnO₃ films have been grown by laser ablation on a sapphire (R-plane) substrate covered by a (100)SrTiO₃/(001)Bi₂SrNb₂O₉/(001)CeO₂ trilayer buffer. The azimuthal misorientation of crystal grains (50–300 nm) in the La_0.67Ca_0.33MnO₃ films decreased by about 40% as the condensation temperature was increased ered from 760 to 810° C. The lattice parameter of the grown manganate films was reduced to 3.81–3.82 Å by enriching them with oxygen. The maximum in the temperature dependence of the electrical resistivity of the La_0.67Ca_0.33MnO₃ films grown was shifted toward lower temperatures by 20–50 K relative to its position for bulk ceramic samples of a stoichiometric composition. The largest magnetoresistance (MR=42% at H=0.4 T) was found in La_0.67Ca_0.33MnO₃ films with a Mn⁴⁺ concentration on the order of 50% (T=166 K).     

Magnetoresistance of La0.67Ca0.33MnO3 films the coherent growth of which is disturbed by mechanical stress relaxation

Because of a large (m = 1.8%) lattice mismatch between La_0.67Ca_0.33MnO₃ and LaAlO₃, manganite films grown on a lanthanum aluminate substrate experience biaxial mechanical compression stresses. Strong adhesion to the substrate causes a substantial tetragonal distortion (γ ≈ 1.04) of the unit cell in a 20-nm-thick layer of the manganite film coherently grown on (001)LaAlO₃, while in the remaining part (≈75%) of the manganite film, stresses partially relax. The stress relaxation decreases γ and increases the effective volume of the unit cell of the La_0.67Ca_0.33MnO₃ film. The relaxed part of the La_0.67Ca_0.33MnO₃ film consists of crystallites 50–200 nm across azimuthally misoriented by approximately 0.3°. The temperature dependences of the resistivity and negative magnetoresistance of the manganite films exhibit maxima at 240 and 215 K, respectively. At temperatures below 50 K, the dependence of the resistivity on the magnetic induction taken with the induction varying from 0 to 14 T and vice versa becomes hysteresis.     

Electrical resistivity and magnetoresistance of nonuniformly mechanically strained films (30 nm) of the La0.67Ca0.33MnO3 manganite

The structure, electrical resistivity, and magnetoresistance of predominantly oriented La_0.67Ca_0.33MnO₃(30 nm)/LaAlO₃ films are investigated after partial relaxation of biaxial mechanical stresses. The negative magnetoresistance MR of the films reaches a maximum at T = 235–240 K. The full width at half-maximum of the peak in the curve MR(T) for a film is five to six times greater than that for a manganite layer grown on a substrate with a small lattice mismatch. For T < 150 K, the temperature dependence of the electrical resistivity ρ of the films is fitted well by the relationship ρ = ρ₀ + ρ₁ (H)T ^4.5, where ρ₀ ≡ ρ(T = 4.2 K) and ρ₁(H) is a parameter that is independent of temperature but dependent on the magnetic field H. The parameter ρ₁(H = 0) for the La_0.67Ca_0.33MnO₃(30 nm)/LaAlO₃ films is several times larger than that for thin manganite layers only weakly strained by the substrate. The electrical resistivity ρ₁ decreases almost linear as the quantity μ₀ H increases in the field range 1–5 T.     

Structural and electrical transport properties of nanosized La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> sample synthesized by a simple low-cost novel route

We have synthesized nanosized La_0.67Ca_0.33MnO₃ by a simple low-cost novel synthesis route without calcination at high temperature. The study of these nanoparticles indicates excellent properties similar to colossal magnetoresistance (CMR) materials sintered at ∼1600°C for 20 h. The resulting particle size is in the range of 50–160 nm as determined by scanning electron microscopy. Resistivity measurement has been carried out down to 12 K. The sample shows metal-to-insulator (M–I) transition at 205 K.      

Effect of an interface boundary on the magnetooptical and magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> heterostructures

The optical, magnetooptical (Kerr effect and magnetotransmission), and magnetotransport properties of La_2/3Ca_1/3MnO₃/La_2/3Sr_1/3MnO₃ and La_2/3Ca_1/3MnO₃/SrTiO₃/La_2/3Sr_1/3MnO₃ heterostructures on SrTiO₃ substrates are studied. The contribution of the interface boundary to the magnetotransmission is typical of a material with a transitional composition. It is found that a 2-nm-thick SrTiO₃ spacer does not influence the shape and position of the magnetotransmission peak in a field normal to the surface of the heterostructure but increases the contribution of the upper layer to the magnetotransmission in the Voigt geometry and also enhances the magnetoresistance that is due to the tunneling of spin-polarized carriers through the spacer. The Kerr spectra taken of the heterostructures are typical of single-layer single-crystal films.     

Metastable structure and properties of (La0.73Bi0.27)0.67Ca0.33MnO3

The electrical transport and magnetic properties of high Bi doped (La_0.73Bi_0.27)_0.67Ca_0.33MnO₃ are studied at the temperature and magnetic field ranges from 10 to 300 K and 0 to 3 T. Significant temperature and magnetic field hystereses are observed in both resistivity and magnetization measurements. Meanwhile, an enhanced magnetoresistance effect, within a wide temperature window, is obtained in the (La_0.73Bi_0.27)_0.67Ca_0.33MnO₃. The hysteresis and enhanced magnetoresistance are discussed based on an inhomogeneous metastable structure related to the Bi dopant.     

Intrinsic metastability of low doped manganites: La<Subscript>0.8</Subscript>Ca<Subscript>0.2</Subscript>MnO<Subscript>3</Subscript> case

Transport properties of phase separated La_0.8Ca_0.2MnO₃ crystals in the aged highly resistive metastable state were studied. It was found that the coexistence of different ferromagnetic phases at low temperatures is sensitive to electric current/field. In a contrast with the previously studied low resistivity metastable states the high resistivity state exhibits positive magnetoresistance and significant current dependence of the resistivity even at temperatures much higher than the Curie temperature. Application of current pulses results in appearance of zero bias anomaly in the current dependent conductivity. Similarly to the low resistivity metastable states the memory of the resistivity can be erased only after heating of the sample to T_e ≈360 K. After one year storage at room temperature the La_0.8Ca_0.2MnO₃ samples show clear signatures of aging. The aged samples spontaneously evolute towards high resistivity states. The results are discussed in the context of a coexistence of two ferromagnetic phases with different orbital order and different conductivity. The metallic ferromagnetic phase seems to be less stable giving rise to the experimentally observed electric field effects and aging.     

Modulation of ultrafast laser-induced magnetization precession in BiFeO<Subscript>3</Subscript>-coated La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films

The ultrafast laser-excited magnetization dynamics of ferromagnetic (FM) La_0.67Sr_0.33MnO₃ (LSMO) thin films with BiFeO₃ (BFO) coating layers grown by laser molecular beam epitaxy are investigated using the optical pump-probe technique. Uniform magnetization precessions are observed in the films under an applied external magnetic field by measuring the time-resolved magneto-optical Kerr effect. The magnetization precession frequencies of the LSMO thin films with the BFO coating layers are lower than those of uncoated LSMO films, which is attributed to the suppression of the anisotropy field induced by the exchange interaction at the interface between the antiferromagnetic order of BFO and the FM order of LSMO.     

Effect of phase stratification into electro- and magnetotransport in heteroepitaxial La2/3Ca1/3MnO3 films

The La_2/3Ca_1/3MnO₃ films 25-nm thick biaxially compressed in the substrate plane, grown quasi-coherently on the (001) surface of the LaAlO₃ single crystal, were studied using laser vaporization. It was found that mechanical stresses acting during nucleation and growth promoted calcium enrichment of the cation sublattice of the manganite layer, which caused a decrease in its unit cell volume. Crystalline grains in manganite films were distinctly oriented along the normal to the substrate plane; the grain size in the substrate plane was within 20–40 nm, and the relative grain misorientation in the substrate plane did not exceed 0.2°. In zero magnetic field, the maximum in the temperature dependence of the resistivity ρ of La_2/3Ca_1/3MnO₃ films was observed at temperatures close to 210 K. At T < 100 K and μ₀ H = 2 T, the magnetoresistance of manganite films was negative, weakly depended on temperature, its value was about–0.45. The magnetic field caused transformation of nonferromagnetic phase inclusions to ferromagnetic ones, which resulted in a decrease in the La_2/3Ca_1/3MnO₃ film resistivity with increasing magnetic field. At low temperatures (T < 100 K), a hysteresis was observed in the dependences of the film resistivity on the magnetic field.     

Magnetoresistance and transport properties of different impurity doped La0.67Ca0.33MnO3 composite

An enhanced magnetoresistance and a two-fold effect result from impurity dopant were observed in composites of La_0.67Ca_0.33MnO₃/YSZ and La_0.67Ca_0.33MnO₃/Fe₃O₄. Where YSZ represents yttria-stabilized zirconia and the doping level of both YSZ and Fe₃O₄ is 1 mol%. Different electrical and magnetic transport properties, in particular a lower field magnetization behavior, were observed between pure La_0.67Ca_0.33MnO₃ and the impurity doped La_0.67Ca_0.33MnO₃ composites. Compared with pure La_0.67Ca_0.33MnO₃, a possible interpretation is presented by considering the influences of YSZ and Fe₃O₄ on the structure of grain boundaries and/or surfaces of La_0.67Ca_0.33MnO₃grains.     

Galvanomagnetic transport properties of nitrogenated (La,Sr)MnO<Subscript>3-δ</Subscript> and LaMnO<Subscript>3-δ</Subscript>

Epitaxial thin films of nitrogenated La_0.65Sr_0.30MnO₃ were grown on MgO(100) substrates by pulsed laser deposition (PLD). The nitrogenation was achieved by a continuous nitrogen flow in the PLD chamber with pressures of up to 0.12 mbar. The chemical analysis of the samples regarding the exchange of oxygen by nitrogen was achieved by time of flight secondary ion mass spectrometry, sputtered neutral mass spectrometry (SNMS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and yielded a content of incorporated nitrogen ranging from 0.6% to 3.8%. Without nitrogenation the electrical resistivity of La_0.65Sr_0.30MnO₃ exhibited a metal–insulator (MI) transition at about 180 K. The magnetoresistance (MR) effect (ΔR/R(0)) was about -50% at the transition temperature. Our nitrogen contents affected the MI transition so as to completely disappear and resulted in a resistivity increase of more than three orders of magnitude as well. By carefully reoxidizing the samples with subsequent heat treatments in air the MI transition reappeared at lower temperatures and we found a continuously enhanced MR ratio for decreasing temperatures. MR ratios of more than -99% were observed for a magnetic field of 10 T. The results are interpreted as a percolation phenomenon of ferromagnetic–metallic domains within an antiferromagnetic–semiconducting matrix. PACS 75.47.Gk; 75.47.Lx; 74.62.Dh; 81.15.Fg     

Variation of the electrophysical properties of La0.67Ca0.33MnO3 epilayers during secondary recrystallization

La_0.67Ca_0.33MnO₃ epitaxial films are grown by laser evaporation on (LaAlO₃)_0.3 + (Sr₂AlTaO₆)_0.7 substrates. The films have single-crystalline mosaic blocks (grains), which are distinctly oriented in both the azimuth direction and the substrate plane. During the heat treatment of the manganite films in the oxygen atmosphere, their microstructure is improved both in the bulk and in the intergranular regions. The maximal value of the temperature coefficient of resistance for the heat-treated La_0.67Ca_0.33MnO₃ films increases by nearly two times, while the magnetoresistance increases by no more than 10%.     

Dielectric response of a (1000 nm)SrTiO<Subscript>3</Subscript> layer epitaxially grown on (001)La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> to temperature variation and electric field

The structure and dielectric characteristics of the (1000 nm)SrTiO₃ spacer in a (001)SrRuO₃ ‖ (001)SrTiO₃ ‖ (001)La_0.67Ca_0.33MnO₃ trilayer heterostructure grown on a (001)(LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7 substrate have been studied. Both oxide electrodes, as well as the strontium titanate layer, were cube-on-cube epitaxially grown. The unit cell parameter in the SrTiO₃ layer measured in the substrate plane (3.908±0.003 Å) practically coincided with that determined along the normal to the substrate surface (3.909±0.003 Å). The temperature dependence of the real part of the permittivity ?′ of the SrTiO₃ layer in the range 70–180 K fits the relation (?′)^?1 ～ ? ₀ ^?1 C ₀ ^?1 (T-T _C ) well, where C₀ and T_C are the Curie constant and the Curie-Weiss temperature, respectively, for bulk strontium titanate crystals and ?₀ is the free-space permittivity. The data obtained on the temperature dependence of the permittivity of SrTiO₃ films enabled us to evaluate the effective depth of electric field penetration into the manganite electrode (L _e ≈ 0.5 nm) and the corresponding capacitance (C _e ≈1×10^?6 F/cm²) of the interface separating the (001)SrTiO₃ layer from the (001)La_0.67Ca_0.33MnO₃ bottom electrode.     

Magnetoresistance and electrical conductivity of manganite bicrystal contacts

Magnetic bicrystal contacts in epitaxial films of manganite La_0.67Ca_0.33MnO₃ prepared on bicrystal substrates with a misorientation of the NdGaO₃(110) basal planes rotated through an angle of ±14° around the bicrystal boundary line were studied. The temperature dependence of the contact electrical resistance was studied, and the magnetoresistance was measured in fields of up to 1.5 kOe. It is shown that the suppression of ferromagnetic correlations near the boundary leads to the formation of a layer having a substantially lower Curie temperature. Magnetoresistance of 150%, which is record-braking for bicrystal contacts, was measured at T = 4.2 K in a weak magnetic field of about 500 Oe and at a characteristic electrical resistance of the boundary of 3 × 10⁻⁶ Ω cm². It is found that slight orthorhombic distortions of a La_0.67Ca_0.33MnO₃ film due to a lattice mismatch with the NdGaO₃(110) substrate crystal structure lead to the formation of biaxial magnetic anisotropy in the La_0.67Ca_0.33MnO₃ film.