I. K. Kamilov

Magnetocaloric effect in La1?xAgyMnO3 (y ? x): direct and indirect measurements

For the first time the magnetocaloric properties of La0.9Ag0.1MnO3, La0.8Ag0.2MnO3, La0.85Ag0.15MnO3, La0.8Ag0.15MnO3 and La0.8Ag0.1MnO3 manganites have been investigated by direct and indirect measurement techniques. All samples showed almost the same relative cooling power (RCP). Temperatures of maxima of the magnetocaloric effect (MCE) are between a few degrees below freezing and the room temperature region. The compounds showed RCP values of about 100 J kg−1 at a field change of 2.6 T, which is about half the RCP of gadolinium. Because of considerable MCE and the Curie temperatures ranging from 269 to 303 K, these materials could be used as magnetic refrigerants for magnetic refrigeration in the sub-room and room temperature range.

Kinetic effects in manganites La{sub 1-x}Ag{sub y}MnO{sub 3} (y {<=} x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La1 − xAgyMnO3 (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρFM(T) = ρ0 + AT2 + BT4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.

Critical behavior of a cubic-lattice 3D Ising model for systems with quenched disorder

A Monte Carlo method is applied to simulate the static critical behavior of a cubic-lattice 3D Ising model for systems with quenched disorder. Numerical results are presented for the spin concentrations of p = 1.0, 0.95, 0.9, 0.8, 0.6 on L × L × L lattices with L = 20–60 under periodic boundary conditions. The critical temperature is determined by the Binder cumulant method. A finite-size scaling technique is used to calculate the static critical exponents α, β, γ, and ν (for specific heat, susceptibility, magnetization, and correlation length, respectively) in the range of p under study. Universality classes of critical behavior are discussed for three-dimensional diluted systems.

Magnetoelectric interactions in BiFeO3, Bi0.95Nd0.05FeO3, and Bi0.95La0.05FeO3 multiferroics

The technology of ceramic BiFeO3, Bi0.95Nd0.05FeO3, and Bi0.95La0.05FeO3 multiferroics is described. The room-temperature magnetization, magnetoelectric (ME), and magnetodielectric (MDE) effects in these compounds have been studied. It is established that even a small fraction (x = 0.05) of rare-earth additives (La, Nd) to bismuth ferrite not only enhance its magnetic properties, but also significantly influence the ME and MDE effects. The dependence of the ME effect on the frequency of modulation of the alternating magnetic field in Bi0.95Nd0.05FeO3, and Bi0.95La0.05FeO3 is more pronounced than in pure BiFeO3.

Finite-size scaling and critical exponents of the real antiferromagnetic model

The static critical properties of the Cr2O3 real antiferromagnetic model are investigated by the Monte-Carlo method. The systems with periodic boundary conditions containing N=500; 864; 1372; 2048; 2916; 4000 spins are studied. Using the theory of finite-size scaling, the values of critical exponents α, β, γ are calculated.The comparison of data with the results of theoretical and experimental research is carried out.

Heat capacity and magnetocaloric properties of La1-xKxMnO3 manganites

The magnetocaloric effect and the heat capacity of La1 − xKxMnO3 (x = 0.1, 0.15, 0.175) ceramic samples have been studied at temperatures in the range 77–350 K and in magnetic fields of up to 27 kOe. The technique for preparing the samples has been described. The heat capacity anomalies related to the ferromagnetic-paramagnetic magnetic phase transition have been revealed and interpreted. It has been demonstrated that the change in the magnetic entropy ΔS calculated from the data on the heat capacity Cp and direct measurements of the magnetocaloric effect ΔT reaches values that are of practical interest.

Electrical and thermal properties of the manganite La0.8Ag0.15MnO3

A comprehensive investigation of the electro- and thermophysical properties of the manganite La0.8Ag0.15MnO3 is carried out over a wide temperature interval (4.2–350K) and in magnetic fields up to 26kOe. It is shown that the colossal magnetoresistance in a magnetic field of 11kOe amounts to 57%, and the effect is maximum at room temperature. The dominant mechanisms of current carrier scattering in the ferromagnetic and paramagnetic phases are established. An analysis of the data on the low-temperature heat capacity provides estimates of the electronic density of states at the Fermi level, NF=6.82×1024eV−1mol−1, and of the Debye temperature, θD=370K. The results of thermal expansion measurements are used to find the spontaneous magnetostriction. It is found that the phonon mechanism of heat transfer is dominant, and the local Jahn–Teller distortions are considered as the main mechanism of phonon scattering.

Critical properties of the three-dimensional frustrated Ising model on a cubic lattice

The critical properties of the three-dimensional fully frustrated Ising model on a cubic lattice are investigated by the Monte Carlo method. The critical exponents α (heat capacity), γ (susceptibility), β (magnetization), and ν (correlation length), as well as the Fisher exponent η, are calculated in the framework of the finite-size scaling theory. It is demonstrated that the three-dimensional frustrated Ising model on a cubic lattice forms a new universality class of the critical behavior.

Effect of a magnetic field on the thermal and kinetic properties of the Sm0.55Sr0.45MnO3.02 manganite

The temperature and magnetic-field dependences of the heat capacity, thermal conductivity, thermopower, and electrical resistivity of the Sm0.55Sr0.45MnO3.02 ceramic material are studied in the temperature range 77–300 K and in magnetic fields up to 26 kOe. It is revealed that the quantities under investigation exhibit anomalous behavior due to a magnetic phase transition at the Curie temperature TC. An increase in the magnetic field strength H leads to an increase in the Curie temperature TC and a jump in the heat capacity ΔCp at TC. The temperature dependences of the measured quantities are characterized by hystereses that are considerably suppressed in a magnetic field of 26 kOe and depend neither on the thermocycling range nor on the rate of change in the temperature. The thermal conductivity K at temperatures above TC shows unusual behavior for crystalline solids (dK/dT>0) and, upon the transition to a ferromagnetic state, drastically increases as a result of a decrease in the phonon scattering by Jahn-Teller distortions. It is demonstrated that the hystereses of the studied properties of the Sm0.55Sr0.45MnO3.02 manganite are caused by a jumpwise change in the critical temperature due to variations in the lattice parameters upon the magnetic phase transition.

Cluster algorithms of the Monte Carlo method, finite-size scaling, and critical exponents of complex lattice models

For the first time, cluster algorithms of the Monte Carlo method are used to investigate critical properties of microscopic models of real ferromagnetic gadolinium. On the basis of the finite-size scaling theory, the critical exponents of the heat capacity α, magnetization β, susceptibility γ, and Fisher index η are calculated. Specific features, character, and the degree of influence of two types of weak relativistic interactions on the critical properties of gadolinium models are determined when both these interactions are taken into account. It is shown that cluster algorithms of the Monte Carlo method provide an effective tool for studying critical properties of complex models involving crossovers.