Giant magnetorefractive effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films

Complex experimental investigations of the structural, optical, and magneto-optical properties (magnetotransmission, magnetoreflection, and transversal Kerr effect, as well as the magnetoresistance, of La_0.7Ca_0.3MnO₃ epitaxial films indicate that magnetoreflection and magnetotransmission in manganite films can reach giant values and depend strongly on the magnetic and charge homogeneity of the films, their thickness, and spectral range under investigation. It has been shown that the optical enhancement of the magnetorefractive effect occurs in thin films as compared to manganite crystals. In the region of the minimum of the reflectance near the first phonon band, the resonance-like magnetorefractive effect has been observed, which is accompanied by change of the sign of the magnetoreflection. A model based on the theory of the magnetorefractive effect has been proposed to qualitatively explain this behavior.     

Magnetorefractive effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> in the infrared spectral range

The reflection and magnetic reflection spectra, magnetic resistance, electrical properties, and equatorial Kerr effect in La_0.7Ca_0.3MnO₃ crystals have been complexly investigated. The measurements have been performed in wide temperature and spectral ranges in magnetic fields up to 3.5 kOe. It has been found that magnetic reflection is a high-frequency response in the infrared spectral range to the colossal magnetore-sistance near the Curie temperature. Correlation between the field and temperature dependences of the magnetic reflection and colossal magnetoresistance has been revealed. The previously developed theory of the magnetorefractive effect for metallic systems makes it possible to explain the experimental data at the qualitative level. Both demerits of the theory of the magnetorefractive effect in application to the magnets and possible additional mechanisms responsible for the magnetic reflection are discussed.     

Magnetorefractive effect and giant magnetoresistance in Fe(<Emphasis Type="Italic">t</Emphasis><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)/Cr superlattices

The magnetorefractive effect in Fe(t _x , Å)/Cr(10 Å) samples grown by molecular-beam epitaxy with a variable thickness of the iron layer (superlattices, cluster-layered nanostructures) has been studied in the IR region (λ = 2–13 µm) in s and p polarizations of light. The magnetoresistive effect in a dc magnetic field, H ≤ 32 kOe, has been measured on the same samples. The iron layer thickness required for the magnetorefractive response to appear has been found to be t _Fe ≥ 3 Å. The correlation between the magnitude of the magnetorefractive effect in the mid-IR region and magnetoresistance has been discussed.     

Magnetorefractive effect in magnetic nanocomposites

The magnetorefractive effect in ferromagnetic metal-insulator granular nanostructures (CoFeZr)-SiO_n, Co-Al-O, FeSiO_n, and (CoFe)-(Mg-F) is investigated in the infrared spectral region in a wavelength range from 5 to 20 μm. The magnitude of the effect varies from 0.1 to 1.5% for different nanocomposites and strongly depends on the frequency of light and magnetoresistance. It is shown that the reflection coefficient changes in a magnetic field not only due to the magnetorefractive effect, but also due to the even magnetooptical effect. Simple relations describing this effect are given for the case when the reflection from the substrate is insignificant and in the case of a three-layer (insulator-film-substrate) system. The expression for the frequency dependence of the magnetorefractive effect in nanocomposites is derived and its features in the case of high-frequency spin-dependent tunneling are analyzed.     

Features of Magnetorefractive Effect in a [CoFe/Cu]<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> Multilayer Metallic Nanostructure

The features of magnetorefractive effect (MRE) in metallic multilayer film Ni₄₈Fe₁₂Cr₄₀(50 Å)/[Co₉₀Fe₁₀(14 Å)/Cu(22 Å)]₈/Cr(20 Å) nanostructures, which exhibit giant magnetoresistance at room temperature, are investigated experimentally and theoretically. We show that the MRE in these structures reaches 1.5% in an applied magnetic field of 3.5 kOe, in a broad part of the IR region, and can change sign for both transmission and reflection of light. The refraction and extinction coefficients that are calculated for the nanostructures in an external magnetic field are in good agreement with our experimental data. The deduced formulas can be applied to estimating the MRE in multilayer metallic nanostructures.     

Specific features of the magnetoreflection of co-based amorphous ribbons in the IR region

The light reflection spectra and the magnetorefractive effect in the cobalt-based amorphous ribbons in the IR spectral region (λ = 2.5–25 μm) are studied. These amorphous alloys are characterized by the magnetoimpedance effect, which is enhanced after thermal and laser annealing. It is found that the magnetoimpedance and magnetorefractive effects correlate with each other. The magnetorefractive effect reveals a feature at λ = 15–20 μm and the change of sign in the wavelength region 20–25 μm. It is shown that, in the first approximation, the optical properties of the amorphous alloys in the IR spectral region can be explained on the basis of a modified Hagen-Rubens relation. It is found that the change in the reflection coefficient of amorphous metal films in the IR spectral region under action of a magnetic field is caused by the change in both their magnetic permeability and conductivity, which is caused by the degree of polarization of localized electronic states at the Fermi level.     

Hysteresis of magnetoresistance in granular La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at low temperatures

The magnetoresistance of granular La_0.7Ca_0.3MnO₃ is studied experimentally over wide ranges of temperatures and magnetic fields. The emphasis is on anomalously large hysteresis of magnetoresistance at low temperatures (T = 4.2 K). The observed ρ(H) dependence can be qualitatively explained by spin-dependent tunneling of electrons through the dielectric boundaries of conducting granules characterized by a wide spread in the magnetic-moment magnitudes.     

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.     

Inverse magnetoresistance in (FeCoB)-(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) magnetic granular composites

The magnetoresistance, magnetization, and microstructure of granular composites with the general formula (Fe₄₀Co₄₀B₂₀)_x(Al₂O₃)_100?x were studied for contents of the amorphous metallic component both above and below the percolation threshold (x≈43). The low-temperature transverse magnetoresistance of the composites is negative at x=41 and practically zero for x=49. For metal contents below the percolation threshold (x=31), a noticeable (7–8%) positive magnetoresistance, reached in magnetic fields of about 17 kOe, was observed. Possible mechanisms of the generation of inverse (positive) magnetoresistance are discussed.     

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.     

Influence of annealing at temperatures above the solidus temperature on the structure and galvanomagnetic properties of Bi<Subscript>92</Subscript>Sb<Subscript>8</Subscript> solid-solution thin films

The influence of the annealing temperature from the interval between the solidus and liquidus temperatures of Bi₉₂Sb₈ solid solution on its structure and galvanomagnetic and thermoelectric properties has been studied. It has been shown that films of bismuth–antimony solid solution grown by thermal evaporation in a vacuum will have a large-grained structure after annealing at temperatures higher than the solidus temperature of the solid solution. It has been found that these films offer the lowest resistivity, the highest relative magnetoresistance, and the highest mobility of charge carriers. As the annealing temperature approaches the liquidus temperature, the probability that a dendritic structure will form and antimony-enriched regions will appear grows. This causes an increase in the charge carrier concentration and a decrease in the resistivity with a decrease in the relative magnetoresistance and charge carrier mobility.     

Magnetorefractive effect in granular films with tunneling magnetoresistance

The magnetorefractive effect, consisting in a change of reflectivity under magnetization, has been observed in granular Co-Al-O metal-insulator magnetic films with tunneling magnetoresistance in the IR region. It is shown that this effect manifests itself most clearly in interference conditions, is even in magnetization, and reaches as high as 0.8% at frequencies of about 1100 cm^?, which exceeds both the linear equatorial Kerr and the even orientational magneto-optic effects by an order of magnitude.     

Magnetoreflection and Magnetostriction in Ferrimagnetic Spinels CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>

It is shown that magnetoreflectance of natural light up to +4% exists in magnetostrictive ferrimagnetic spinel CoFe₂O₄ single crystal; this effect is associated with a change of the fundamental absorption edge, the impurity absorption band, and the phonon spectrum under the action of a magnetic field. The correlation between the field dependences of magnetoreflectance and magnetostriction has been established. The physical mechanisms responsible for the spectral and field peculiarities of magnetoreflection have been explained. It is shown that the magnetorefractive effect in CoFe₂O₄, which is associated with magnetoelastic properties of the spinel, amounts to +1.5 × 10^–3 in magnetic fields exceeding the saturation field. Analysis of magnetooptical and magnetoelastic data has made it possible to estimate deformation potential as Ξ _u = 20 eV for the valence band of the spinel.     

Giant magnetorefractive effect in magnetic granular CoFe-MgF alloys

This paper reports on an IR study (5–20 µm) of the dispersion and field dependences of the magnetorefractive effect in granular CoFe-MgF (metal-insulator) systems, which exhibit, in compositions close to the percolation threshold, a tunneling magnetoresistance of 7.5% in a field of 1700 Oe. The change in the reflectivity of p-polarized light under magnetization in the IR spectral region 5–7 µm is of the order of 0.1%, while in the 10-to 12-µm region, this change, reaches record-high values of 1.2–1.5%, which exceed the usual odd and even magnetooptic reflectivities in the IR range by two orders of magnitude. The magnetorefractive effect in magnetic systems with tunneling conduction originates from spin-dependent high-frequency tunneling.     

Structure-Dependent Magnetoresistance in the Zn<Subscript>0.1</Subscript>Cd<Subscript>0.9</Subscript>GeAs<Subscript>2</Subscript> + MnAs Hybrid Nanocomposite

The effect of high pressure on electron transport and on the field dependence of the transverse magnetoresistance has been studied in a hybrid nanocomposite based on the Zn0.1Cd0.9GeAs2 matrix and MnAs clusters. A record high negative magnetoresistance of ~74% is formed near a pressure-induced structural transition (P≈ 3.5 GPa). The considered scattering mechanisms include both the contribution from MnAs clusters at relatively low pressures (up to 0.7 GPa) and spin-dependent scattering by localized magnetic moments in the Mn-substituted structure of the matrix in the region of the structural transition. The presence of the positive magnetoresistance region associated with the two-band transport model in the high-pressure phase, as well as the large negative magnetoresistance, is described in the framework of the semiempirical Khosla–Fischer expression.     

Electrophysical parameters of <Emphasis Type="Italic">c</Emphasis>-oriented Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> films with a low concentration of antistructural defects

Epitaxial c-oriented Bi₂Te₃ films 1.2 μm in thickness are grown by the hot wall method for a low supersaturation of the vapor phase over the surface of mica substrates. The hexagonal unit cell parameters a = 4.386 Å and c = 30.452 Å of the grown films almost coincide with the corresponding parameters of stoichiometric bulk Bi₂Te₃ crystals. At T = 100 K, the Hall concentration of electrons in the films is on the order of 8 × 10¹⁸ cm^?3, while the highest values of the thermoelectric coefficient (α ≈ 280 μV K^?1) are observed at temperatures on the order of 260 K. Under impurity conduction conditions, conductivity σ of the films increases upon cooling in inverse proportion to the squared temperature. In the temperature range 100–200 K, thermoelectric power parameter α²σ of Bi₂Te₃ films has values of 80–90 μW cm^?1 K^?2.     

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.     

Optical properties and mechanisms of current flow in Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> films prepared by spray pyrolysis

Thin films Cu₂ZnSnS₄ (up to 0.9 μm thick) with p-type conductivity and band gap E_g = 1.54 eV have been prepared by the spray pyrolysis of 0.1 M aqueous solutions of the salts CuCl₂ · 2H₂O, ZnCl₂ · 2H₂O, SnCl₄ · 5H₂O, and (NH₂)₂CS at a temperature T_S = 290°C. The electrophysical properties of the films have been analyzed using the model for polycrystalline materials with electrically active grain boundaries. The energy and geometric parameters of the grain boundaries have been determined as follows: the height of the barriers is E_b ≈ 0.045–0.048 eV, and the thickness of the depletion region is δ ≈ 3.25 nm. The effective concentrations of charge carriers p₀ = 3.16 × 10¹⁸ cm^–3 and their mobilities in crystallites μ_p = 85 cm²/(V s) have been found using the technique for determining the kinetic parameters from the absorption spectra of thin films at a photon energy hν ≈ E_g. The density of states at grain boundaries N_t = 9.57 × 10¹¹ cm^–2 has been estimated.     

Effect of thermal treatment on the electrotransport properties of thin-film In<Subscript>2</Subscript>O<Subscript>3</Subscript>, ZnO materials and the multilayer (In<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZnO)<Subscript>83</Subscript> heterostructure

The effect heat treatment has on the electrotransport mechanisms in films of ZnO and In₂O₃, and in a multilayer (In₂O₃/ZnO)₈₃ structure obtained via ion-beam sputtering, is studied. It is shown that there is a mechanism of weak electron localization in the In₂O₃ and (In₂O₃/ZnO)₈₃ samples. The relaxation processes that occur during the heat treatment of In₂O₃ films are found to increase the length of elastic electron scattering, but to reduce this parameter in multilayer heterostructures.     

Magnetorefractive effect in (Co<Subscript>50</Subscript>Fe<Subscript>50</Subscript>)<Subscript>x</Subscript>(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> granular films

The reflectivity spectra and the magnetorefractive effect (MRE) of (Co₅₀Fe₅₀)_x(Al₂O₃)_1?x metal-dielectric granular films (0.07<x<0.52) are analyzed in the IR spectral range λ=2.5–25 µm. It is revealed that the specific features observed in the spectra at λ≈8.5 and 20 µm are associated with the excitation of phonon modes in the dielectric matrix. The magnetorefractive effect in the films is observed below the percolation thresh-old only in p-polarized light and above the percolation threshold for both the p and s polarizations. It is demonstrated that the optical properties of (Co₅₀Fe₅₀)_x(Al₂O₃)_1?x films in the IR spectral range, to a first approximation, can be interpreted in the framework of the effective-medium theory and the magnetorefractive effect can be explained in terms of the modified Hagen-Rubens relation.