Magnetic ordering and charge transport in electron-doped La₁-yCeyMnO₃ (0.1 ≤ y ≤ 0.3) films

Microstructure, magnetic and transport properties of the as-deposited La₁−yCeyMnO₃(0.1 ≤ y ≤ 0.3) films, prepared by a pulse laser deposition, have been investigated in wide region of temperature and magnetic field. The microstructure analysis reveals that all films have a high c-oriented texture, the orthorhombic crystal lattice and the negligible quantity of CeO₂ inclusions. The observed strip-domain phase with a periodic spacing of about 3c, the crystal lattice of which is the same to the basic film phase, reveals the magnetic behavior typical for the Griffiths phase. The regions of the double-period modulated phase was found at room temperature in the y = 0.1 film, which are treated as the Mn³⁺/Mn²⁺ ordering with the partial ferromagnetic → antiferromagnetic transition at TN ≤ 80 K. At the same time, the carried out investigation manifests that the magnetic and transport properties of the electron-doped La₁-yCeyMnO₃ films, driven by a cation doping, are similar to that for the hole-doped La/Ca manganites. Therefore, one can conclude, that does not exist of a principle difference between the mechanisms of spin-ordering and charge-transport in the hole- and the electron-doped manganites

Evidence for non-Dzyaloshinskii–Moriya ferromagnetism in epitaxial BiFeO₃ films

BiFeO₃ films have been prepared by dc magnetron sputtering on LaAlO₃ (001) single-crystalline substrate. X-ray diffraction analysis and high-resolution electron-microscopy study reveal that the films have a highly coriented orthorhombic crystal structure. It was found that the magnetic properties of the BiFeO₃ films are typical for the ensemble of interacting superparamagnetic clusters rather than for the Dzyaloshinskii–Moriya weak ferromagnet. Appearance of the extrinsic nanoscale superparamagnetic clusters is explained by the oxygen deficiency in certain regions of the film, where the ferromagnetic ordering is realized through the double-exchange mechanism by Zener

Flux pinning and vortex dynamics in MgB₂ doped with TiO₂ and SiC inclusions

The mixed-state superconducting properties of bulk MgB₂ + 2 at.% TiO₂ and + 8 at.% SiC, prepared by the in situ solid state reaction, have been investigated. The analysis on the mixed-state parameters, such as the upper critical field, the coherence length and the Ginzburg–Landau parameter, proves that the MgB₂ + 2 at.% TiO₂ is a high-к type-II superconductor in the dirty limit while the MgB₂ + 8 at.% SiC corresponds to that in the moderately clean limit. It was shown that the anisotropic grain-boundary pinning is realized in the fine-grained doped MgB₂ polycrystals rather than the electron scattering one. The field-cooled temperature dependences of magnetic moment exhibit a transition of the samples to the paramagnetic state at certain applied magnetic fields, which is treated as manifestation of the paramagnetic Meissner effect. The experimental results are discussed on the base of modern theoretical approaches

The origin of paramagnetic magnetization in field-cooled YBa2Cu3O7 films

Temperature dependences of the magnetic moment have been measured in YBa_2Cu_3O_{7-\delta} thin films over a wide magnetic field range (5 <= H <= 10^4 Oe). In these films a paramagnetic signal known as the paramagnetic Meissner effect has been observed. The experimental data in the films, which have strong pinning and high critical current densities (J_c ~ 2 \times 10^6 A/cm^2 at 77 K), are quantitatively shown to be highly consistent with the theoretical model proposed by Koshelev and Larkin [Phys. Rev. B 52, 13559 (1995)]. This finding indicates that the origin of the paramagnetic effect is ultimately associated with nucleation and inhomogeneous spatial redistribution of magnetic vortices in a sample which is cooled down in a magnetic field. It is also shown that the distribution of vortices is extremely sensitive to the interplay of film properties and the real experimental conditions of the measurements.Comment: RevTex, 8 figure

Perovskite heterostructures grown by MOCVD

We have deposited epitaxial perovskite heterostructures including nickelates RNiO3 (R=Pr, Nd) on the top of CMR manganites (La1-xPrx)0.7Ca0.3MnO3, (x=0-1) and antiferromagnetic ferrites RFeO3 (R=Nd, Eu). The heterostructures were characterised by XRD, SEM, EDX, HREM, electric and magnetic measurements. Particular attention is paid to the lattice strain in the layers and the structure of their interfaces. A 60 K shift of the metalinsulator transition temperature as well as suppression of the electrical resistivity hysteresis were found for the nickelate film on the ferrite layer. The effects are believed to be due to the strain and magnetic coupling of the layers

Magnetic and transport properties controlled by structural disorder in La₀.₇Ca₀.₃MnO₃ films

The magnetic properties of an amorphous, a partially-disordered, and a lattice-strained crystalline La₀.₇Ca₀.₃MnO₃ film are investigated. It is shown that the amorphous film exhibits Curie- Weiss-type paramagnetism with the effective magnetic moment of 4.2 µB/Mn ion and a small ferromagnetic contribution governed by the formation of a quasi-two-dimensional crystalline interfacial inclusions. The crystalline film with nanocrystalline randomly-oriented inclusions demonstrates a superposition of ferromagnetic (in the crystalline matrix) and superparamagnetic (in the inclusions) nature. The fitted average size of the superparamagnetic particles in the case of a Langevin function is coincident with that of the nanocrystalline clusters reveated in high-resolution electron-microscopy images. An increase in the applied magnetic field leads to a reduction in the average magnetic moment of superparamagnetic particles, which is due to an enhancement of the ferromagnetic coupling between the individual randomly-oriented crystallites. The completely crystalline film undergoes only a ferromagnetic transition with a saturation magnetization at 5 K of 2.73 µB /Mn ion

Influence of structural disorder on magnetic and transport properties of (La₀.₇Sr₀.₃)₀.₅(Pr₀.₆₅Ca₀.₃₅)₀.₅MnO₃ films

Magnetic and transport properties of (La₀.₇Sr₀.₃)₀.₅(Pr₀.₆₅Ca₀.₃₅)₀.₅MnO₃ films prepared by a «co-deposition» utilizing the laser-ablation technique are investigated in a wide temperature range. The film deposited at 300 °C has a nano-crystalline disordered structure and exhibits a paramagnetic temperature dependence of the magnetization with a narrow peak (ΔT ≃ 10 K) at TG ≃ 45 K, which can be interpreted as a paramagnetic → superparamagnetic transition. A short-term annealing of the as-deposited film at 750 °C leads to the formation of a high-textured polycrystalline microstructure and to the appearance of ferromagnetic (FM) and metal—insulator (MI) transitions at TC ≃ 240 K and TP ≃ 140 K, respectively. The observed discrepancy between TP and TC values can be ascribed to a percolating nature of the MI transition, with an exponent of 5.3 for the percolating conductivity. The film deposited at Tsub ≃ 740 °C is composed of the lattice strain-free and the lattice-strained crystallites with different lattice parameters and TC‘s, and is consistently described in the framework of the Millis model [A.J. Millis, T. Darling, and A. Migliori, J. Appl. Phys. 83, 1588 (1998)]. For a single-phase crystalline film obtain TC ≃ 270 K and TP ≃ 260 K

Perovskite heterostructures grown by MOCVD

3d-metal perovskites are promising thin film materials with a great variety of electrical and magnetic properties. We have deposited the epitaxial heterostructures including different combinations of CMR manganites (La1-xPrx)0,7Ca0,3Mn0,3, (x=0-l), metallic nickelates RNiO3 (R=Pr, Nd, Sm) with sharp metal-insulator transition and antiferromagnetic insulators RFeO3 (R=Nd, Eu) . The heterostructures were characterised by XRD, SEM, EDX, HREM, RBS, electric and magnetic measurements. Particular attention is paid to the lattice strain in the layers and the structure of their interfaces. The prototype electronic devices based on the heterostructures are discussed

Magnetic proximity effect in La₀.7Ca₀.₃MnO₃/La₀.₉Ca₀.₁MnO₃ multilayered film with diffusive interfaces

The structural, the magnetic and the transport properties of La₀.7Ca₀.₃MnO₃/La₀.₉Ca₀.₁MnO₃ multilayer film, prepared by rf-magnetron sputtering, have been investigated. The high-resolution electron-microscopy studies reveal the formation of different crystal structures in the constituent sublayers, but without sharp and well-defined interfaces. At the same time, the small regions of double-period modulated phase exist in the La₀.₉Ca₀.₁MnO₃ sublayers at room temperature, manifesting the formation of charge-ordered antiferromagnetic state. The magnetic measurements reveal a significant enhancement of the ferromagnetic ordering in the La₀.₉Ca₀.₁MnO₃ layers due to a strong magnetic coupling between the constituent sublayers. The multilayer film shows the anisotropic saturation magnetization at low temperature and the alternating shape of the temperature-dependent anisotropic magnetoresistance near the metal–insulator transition

Observation of the strain-driven charge-ordered state in La₀.₇Ca₀.₃MnO₃₋δ thin film with oxygen deficiency

The magnetic and transport properties of La₀.₇Ca₀.₃MnO₃₋δ films with an oxygen deficiency (δ≃ 0.1) and a La₀.₉Ca₀.₁MnO₃ film with the stoichiometric oxygen content are investigated in a wide temperature range. It is shown that the charge-ordered insulating (COI) state is observed for a La₀.₇Ca₀.₃MnO₂.₉ film with thickness d ≤ 30 nm, which manifests mainly a cubic crystal structure with an anomalously small lattice parameter for this composition. An increase in the film thickness (d ≃ 60 nm) leads to a structural transition from the lattice-strained cubic to the relaxed rhombohedral phase, is accompanied by a shift of the Curie point (TC) to lower temperature and a frustration of the COI state. The magnetic and transport properties of the La₀.₇Ca₀.₃MnO₂.₉ film with d ≃ 60 nm are similar to those exhibited by the optimally oxygen-doped La₀.₉Ca₀.₁MnO₃ film. It is concluded that the formation of the COI state in the La₀.₇Ca₀.₃MnO₃₋δ compound is governed by a compression of the crystal lattice rather than accumulation of oxygen vacancies, the low doping of the substituted divalent ions, or electronic phase separation