V. P. Dyakonov

Canted spin structure in clusters of the (La0.7Ca0.3)1−xMn1+xO3 perovskites

Abstract In this paper, we report on the effect of excess manganese on the magnetic properties of the (La0.7Ca0.3)1−xMn1+xO3 perovskites. All samples display a colossal magnetoresistance effect which becomes considerably enhanced with increasing manganese content. The magnetic behaviour of manganites studied was shown to depend strongly on the values of manganese doping and applied magnetic field. For the samples with x=0.1 and 0.2, in addition to an anomaly connected with paramagnet–ferromagnet transition, there are anomalies in the field-cooled (FC) and zero-field-cooled (ZFC) magnetization at low temperatures. In low magnetic fields (H 200xa0Oe, a jump upwards takes place in the MFC(T) and MZFC(T) curves at the same temperature. A low-temperature anomaly in AC susceptibility also indicates a change in magnetic ordered state. From M(T,H) and χ(T) dependencies, one can assume the existence of regions with different magnetic order at T

Influence of cobalt on the structural and magnetic Inhomogeneities, phase transitions, and magnetoresistive properties of La0.6Sr0.2Mn1.2 − x Co x O3 ± δ

AbstractThe structure and properties of magnetoresistive ceramics La0.6Sr0.2Mn1.2 − xCoxO3 ± δ (x = 0−0.3) sintered at a temperature of 1200°C are investigated using x-ray diffraction, resistance, and magnetic (χac, M, 55Mn NMR) measurements. It is shown that the samples contain the rhombohedral (R

Noncollinear cluster ferromagnetism in lanthanum manganite perovskites with an excess of manganese

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Acoustical investigations of a La0.75Sr0.25MnO3 single crystal

c) perovskite (90%) and tetragonal (I41/amd) hausmannite (10%) phases. The lattice parameters of these phases decrease with an increase in the cobalt content x. The real perovskite structure involves point defects (anion and cation vacancies) and nanostructured defects of the cluster type. An analysis of the asymmetrically broadened 55Mn NMR spectra confirms the high-frequency electron-hole exchange between Mn3+ and Mn4+ ions and a local inhomogeneity of their environment by other ions and defects of the point and cluster types. An increase in the Co content leads to an increase in the electrical resistivity, an enhancement of the magnetoresistance (MR) effect, and a decrease in the magnetic susceptibility and the temperatures of the metal-semiconductor (Tms) and ferromagnetic-paramagnetic (TC) phase transitions due to the suppression of the exchange interaction between Mn3+ and Mn4+ ions by vacancies and clusters. The introduction of cobalt results in a decrease in the ferromagnetic component and the activation energy. The magnetoresistance effect in the vicinity of the phase transition temperatures Tms and TC is associated with the scattering of charge carriers from intracrystallite inhomogeneities of the lattice, and the low-temperature magnetoresistance effect is governed by the tunneling at the intercrystalline boundaries.

Magnetic phase transition in KGd(WO4)2 double tungstate

The EPR spectrum of a KDy(WO4)2 monoclinic crystal is investigated. It is found that the EPR spectrum of magnetically concentrated materials at a low frequency (9.2 GHz) undergoes a substantial transformation in addition to the well-known broadening of the EPR lines. At low Dy3+ concentrations (x<10−2), the EPR spectrum of an isomorphic crystal, namely, KY(1−x)Dyx(WO4)2, is characterized by the parameters gx=0, gy=1.54, and gz=14.6. For a magnetically concentrated crystal KDy(WO4)2, the g values are as follows: gx=0, gy=0.82, and gz=2.52. It is demonstrated that the difference in the parameters is associated with the specific spin-spin interaction between Dy3+ ions, including the Dzyaloshinski interaction, which is not observed at high frequencies.

Anomalous acoustoelectric effect and the transport properties of thin La0.67Ca0.33Mno3 films

A theoretical substantiation is provided for realization of the spin structure in a weakly doped subsystem based on LaMnO3, which is observed in compounds with an excess of manganese, (La1−yCay)1− xMn1+xO3, where y=0 and 0.3, x=0–0.4. It is proved experimentally that samples with x>0.1 exhibit an anomalous behavior of magnetization at T<45 K. The magnetization decreases in fields H<100 Oe and increases for H>200 Oe. It is assumed that this is associated with the emergence of a canted phase in clusters of manganese ions with difference valences. A theoretical analysis indicates that a competition between double and superexchange interaction is in principle possible in such clusters, since the charge carrier concentration in them is considerably lower than in the host matrix. It is shown that the inclusion of quantum properties of spin leads to a considerable modification of the thermodynamic behavior of a magnet with collectivized electrons. The results of analysis are compared with analogous results in the de Gennes classical theory. The possibility of formation of a state with a canted magnetic sublattice in the weakly doped subsystem in the low-temperature region is substantiated on the basis of calculations. An analysis of the thermodynamic behavior of the weakly doped subsystem based of LaMnO3 taking into account quantum-mechanical properties of spin shows that relaxation phenomena determined, to a considerable extent, by the relation of parameters of intra-and interplanar indirect exchange, as well as the electron transport energy, can take place in the region of phase transition to the canted state. The microscopic parameters of interactions are estimated quantitatively. The results of calculations are in qualitative agreement with experimental data.

Influence of structure defects on functional properties of magnetoresistance (Nd0.7Sr0.3)1−xMn1+xO3 ceramics

The results of investigations of the magnetization, susceptibility, and magnetic-field-induced changes in the entropy of polycrystalline manganite (La0.6Ca0.4)0.9Mn1.1O3 near the magnetic phase transition have been presented. Magnetic measurements have been carried out at temperatures in the range from 210 to 310 K in magnetic fields of up to 9 T. The magnetocaloric effect has been revealed by measuring the magnetic-field dependences of magnetization. The magnitude of the magnetocaloric effect is compared with similar results obtained for other manganites.

EPR of Yb3+ ions in a monoclinic KY(WO4)2 single crystal

The acoustical, resistive, and magnetic properties of a La0.75Sr0.25MnO3 lanthanum manganite single crystal are investigated in the temperature range involving the second-order magnetic phase transition. The acoustical measurements are performed by the pulse-echo method in the frequency range 14–90 MHz. It is found that, as the temperature decreases, the velocity of a longitudinal acoustic wave propagating along the [111] axis in the single crystal drastically increases at temperatures below the critical point of the magnetic phase transition. No dispersion of the acoustic velocity is revealed. A sharp increase in the acoustic velocity is accompanied by the appearance of an acoustical absorption peak. The observed effects are discussed with due regard for the interaction of acoustic waves with the magnetic moments of the manganese ions.