N. V. Kostromitina

Charge segregation and a nonuniform magnetic state in donor-and acceptor-doped LaMnO3

A coordinated study of the magnetic, electrical, optical, and EPR properties of LaMnO3 single crystals doped by donors (7 at. % Ce) and acceptors (7 at. % Sr) revealed that in all cases, except undoped LaMnO3, charge segregation associated with large-scale crystal-field fluctuations occurs and the magnetic properties originate from the existence of ferromagnetic phase inclusions and localized ferrons in the matrix with a canted magnetic structure.

Magnetorefractive effect in La{sub 0.7}Ca{sub 0.3}MnO{sub 3} in the infrared spectral range

The reflection and magnetic reflection spectra, magnetic resistance, electrical properties, and equatorial Kerr effect in La0.7Ca0.3MnO3 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.

Intercluster conduction in lightly doped La1 − xCaxMnO3 manganites in the paramagnetic temperature range

The magnetotransport and magnetic properties of La 1 − xCaxMnO3 polycrystalline samples (x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T* ≈ 300 K, which is about 2–3 times higher than their Curie temperature TC. The temperature dependence of the electrical resistivity measured from T* down to nearly T ≈ TC is fitted by the relation logρ ∼ T−1/2, which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T*, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.

Multiphase magnetic state of Ca1 − x La x MnO3 − δ single crystals (x = 0.03, 0.05, 0.07) containing oxygen vacancies

Magnetic properties (magnetization, dynamic and static susceptibility) and transport properties (resistance and magnetoresistance) have been studied in a temperature range of 2–600 K in magnetic fields to 90 kOe for single crystals of Ca1 − xLaxMnO3 − δ with a weak electron doping (x ≤ 0.07) grown in argon and oxygen atmospheres. The magnetic state of Ca1 − xLaxMnO3 − δ single crystals is multiphase. Below T = TN(G) ∼ 110 K, in all the crystals there coexists an AFM G phase with an FM contribution and an AFM C phase. In crystals with x = 0.07, a transition from the paramagnetic phase into the AFM C phase occurs in part of their volume below T ∼ 130–150 K. In crystals with x = 0.05 annealed in oxygen, an anomaly of paramagnetic susceptibility is observed near T* ∼ 270 K, which is related to the formation of FM clusters near defects. At x = 0.05 and 0.07, AFM correlations are retained in the paramagnetic state (to 600 K). The differences in the magnetic and transport properties of single crystals grown in argon and oxygen are explained by the various content of oxygen vacancies and by their possible ordering.

Magnetic, electrical, and optical properties of Ca1 − xCexMnO3 (x ≤ 0.12) single crystals

The magnetic, electrical, and optical properties of Ca1 − xCexMnO3 (x≤0.12) manganite single crystals are investigated with the aim of revealing the specific features of the multiphase electronic and magnetic state as a function of the cerium concentration and the atmosphere used for growing single crystals. It is found that the concentration dependence of the low-temperature magnetization M(x) of the single crystals is shifted toward the high-concentration range as compared to the corresponding dependence of the polycrystals, which is explained by the predominant cation deficiency. The electrical resistivity and the reflection spectra of the single crystals in the infrared spectral range indicate that charge carriers exhibit a band nature at temperatures close to room temperature. The temperature dependence of the electrical resistivity of the single crystal with x = 0.08, which has the maximum magnetization in the studied series of Ca1 − xCexMnO3 compounds, unlike polycrystals, exhibits a metallic behavior over the entire temperature range. The G-type antiferromagnetic phase with the Néel and Curie temperatures TN(G) = TC = 100 K is characterized by maxima of the electrical resistivity ρ and the magnetoresistance Δρ/ρ = |(ρ0 − ρH)/ρ0| = 38% in the magnetic field H = 90 kOe. The magnetoresistance Δρ/ρ of the single crystals at cerium concentrations x = 0.10 and 0.12 with variations in temperature exhibit three specific features: near the temperature of charge ordering Tco, near the temperature of the magnetic phase transition to the C-type antiferromagnetic phase TN(C), and near the temperature of the phase transition to the magnetic charge-ordered phase TN(MCO). An anomalous temperature dependence of the magnetization is revealed for a single crystal with x = 0.10 grown in oxygen at a pressure of 5 atm, which is explained by the presence of regions with hole conductivity due to cation deficiency. The inhomogeneous electronic and magnetic state is associated with the interrelation of the charge, orbital, and spin orderings.

Crossover from polaron to band conduction upon doping CaMnO3 with La ions

In studying the specular reflection IR spectra of manganite polycrystals with electron doping Ca1−xLaxMnO3 (0 ≤ x ≤ 0.050) at room temperature, a crossover from polaron to band conduction is observed at x = 0.030. It has been shown that the observed changes in the electronic subsystem is associated with the crossover in the behavior of the magnetization and magnetoresistance in the magnetically ordered and paramagnetic phases that occurs at the same concentration and is described in [C. Chiorescu et al., Phys. Rev. B 73, 014406 (2006)].

Infrared and X-Ray Absorption Spectra of Cu2O and CuO Nanoceramics

We present the results of the study of middle infrared absorption (MIR) and X-ray absorption (XAS) spectra of nanostructured CuO and Cu2O ceramics prepared by the high pressure torsion method from coarse-grain oxide powders. We have found that the MIR bands at ~140 meV in CuO and Cu2O nanoceramics are associated with the presence of Cu+ and Cu2+ ions, respectively. X-ray absorption spectra indicate an appearance of extra Cu2+ and Cu+ ions in nanoCu2O and nanoCuO ceramics, respectively.

Effect of Electron Irradiation on the Non-Stoichiometric and Doped Lanthanum Manganites

The effect of point lattice defects on the magnetic properties of manganites LaMnO3+δ, La0.67Ca0.33MnO3 and La0.67Ba0.33MnO3 has been investigated. Temperature dependences of magnetic susceptibility of the initial and electron-irradiated (F9×1018 e/cm2) samples were measured in the temperature range 80K

Structure and Properties of Ca1-xEuxMnO3 Single Crystals

The crystal growth of electron-doped manganites Ca1-xEuxMnO3 by the floating zone method is reported. The special features of the growth parameters are discussed. Results of x-ray analysis, magnetic and transport characteristics are shown.

Optical Anisotropy of RMnO3 Manganites in the Range of 4f-4f Transitions

Optical absorption of orthorhombic and hexagonal manganites RMnO3 (R= Pr3+, Gd3+, Sm3+, Eu3+, Ho3+, Tm3+, Er3+, and Yb3+) were studied in polarized light in the near- and mid-infrared range for the temperature range 80 - 300 K. Several spectral features associated with 4f-4f transitions of R3+ ions were found in the transparency window of RMnO3. Anisotropy of the absorption spectra in the range of multiplet 4f-4f transitions and changes in populations of the multiplet levels with temperature were observed.