Solid State Phenomena Vols. 152-153

Abstract: We report the study of the magnetoresistance (MR) and resistivity as a function of temperature in epitaxial thin films of the La0.67Ca0.33MnO3 type (LCMO). The films deposited onto Y-cut of single-crystalline LiNbO3 (LNO) substrates by pulsed-laser deposition in an on-axis geometry. For comparison, the same manganite film deposited onto LaAlO3 (LAO) substrate is studied. The MR of LCMO/LNO film is found to be anisotropic and exhibits a hysteresis loops in low magnetic fields. When magnetic field is parallel to the film plane a number of jumps are observed in the MR below magnetic fields H~6 kOe. These jumps reflect the presence of pinning centers for domain walls. Evidently, a lattice deformation produced by a mismatch of lattice constants LCMO film and LNO, which lead to distortion of the film, is the origin of such strong pinning centers.

Abstract: Peculiarities of penetration of electromagnetic waves through the bulk and thin film samples of the La0.67Sr0.33MnO3 manganite were investigated for the frequency range from 50 kHz to 20 MHz. Strong variations of penetration coefficient in the applied dc magnetic field were observed both for ferromagnetic and paramagnetic states. Local dynamic magnetic ordering in the paramagnetic temperature range was observed. The experimental data reveal weak dependence of electromagnetic properties on microstructure of the films. The upper frequency limit of the dynamic polaron correlations in the paramagnetic phase was estimated.

Abstract: The effect of 16О → 18О isotope substitution on the properties of Pr0.5Ca0.5MnO3 manganites doped by Cr and Ru is studied. In these compounds, chromium and ruthenium favor (i) the suppression of a charge-ordered state and (ii) the formation of a ferromagnetic metallic phase. The 16О → 18О isotope substitution leads to the growth of the charge-ordering transition temperature (TCO), and to the lowering of ferromagnetic transition temperature (TFM) accompanied by a decrease in the content of ferromagnetic phase. The difference in the behavior of the Cr- and Ru-substituted samples is analyzed.

Abstract: Investigation of crystal structure and magnetic properties of the diamagnetically- substituted Bi1-xAxFeO3-x/2 (A= Ca, Sr, Pb, Ba; x= 0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A2+ substitution result in the formation of oxygen vacancies in the host lattice. The solid solutions have been found to possess a rhombohedrally distorted perovskite structure described by the space group R3c. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO3 giving rise to the appearance of weak ferromagnetism.

Abstract: Nanocrystalline La0.50Ba0.50MnO3 manganite was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size increases with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously with the average crystallite size. The most significant lattice compression occurs in the sample with an average crystallite size of ~ 30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curie temperature and in the spontaneous magnetic moment. The anomalous behavior of the magnetic properties of the La0.50Ba0.50MnO3 manganites obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn3+ – O – Mn4+ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.

Abstract: The influence of oxygen deficiency on magnetization, paramagnetic susceptibility, electrical resistivity, magnetoresistance and magnetostriction of La1-xSrxMnO3-y (x = 0, 0.2, 0.4; y = 0, 0.13, 0.2) is studied. These properties demonstrate that the ferro – antiferromagnetic magnetic two-phase state are in the oxygen deficiency samples. In samples with y  0 in lattice sites, in which the oxygen ions are absent, electrons in twos are disposed from maintenance of a crystal electroneutrality. They partially (in x = 0.4, y = 0.13) or fully (in x = 0.4, y = 0.2; x = 0.2, y = 0.13, 0.2) compensate single-charge acceptors (Sr2+ ions). In non-compensated donors two electron spins are antiparallel and do not form of the ferromagnetic nanoregions from the s-d exchange gain (ferrons). In sites of lattice with the oxygen vacancies the broken Mn-O-Mn connections present which break the ferromagnetic exchange interactions in them decreasing the middle magnetization in samples with x = 0.2, 0.4 and y = 0.13 and provoking a transition to superparamagnetic behavior in samples with x = 0.2, 0.4 and y = 0.2.

Abstract: The room temperature magnetoelectric effect was observed in epitaxial iron garnet films that appeared as magnetic domain wall motion induced by electric field. The films grown on gadolinium-gallium garnet substrates with various crystallographic orientations were examined. The effect was observed in (210) and (110) films and was not observed in (111) films. Dynamic observation of the domain wall motion in 800 kV/cm electric field pulses gave the domain wall velocity in the range 30÷50 m/s. Similar velocity was achieved in magnetic field pulse about 50 Oe.

Abstract: We present resonant x-ray scattering experimental data from YVO3. By scattering at the vanadium K edge we are able to observe diffraction from the anisotropic tensor of susceptibility at the Bragg forbidden (010). The resonant energy spectra from these reflections are unusually complex, giving an indication of the crystal field distortions around the vanadium site.

Abstract: In this work we studied magnetorheological suspensions, which are produced by dispersion of magnetic particles in liquid polymer matrix, based on natural and synthetic rubber. Such suspensions are the raw materials for creation of new high-elastic magneto-controlled composite materials (magnitoelastics[1-4]). Processes of aggregation and structurization of magnetic particles in suspension are also examined. We discovered that motion of magnetic clusters in oligomer solution has interrupted character. Such behavior can be explained by interaction of magnetic clusters moving in not uniform magnetic field with polymer net fragments. Evaluation of polymer net’s elastic properties was calculated.

Abstract: In this work are considered new magneto-controlled materials – magnetoelastics, their elastic and magnetic properties. Much attention is devoted to influence of distribution of magnetic particles by their size on these properties. It was found that the application of a magnetic field on magnetoelastics with larger particles leads to a considerable rise of the shear modulus. It was shown that damping of torsional vibrations strongly depends on both value of magnetic field and concentration of large particles. A comparison between shear modulus, obtained from classic method and from torsional vibrations was made and a good coincidence of results was found. It was found out that hysteresis loops for considered polymer materials have unusual character.