The results of the investigation of temperature dependences of longitudinal and torsional sound wave velocities and the internal friction of FeGe2 tetragonal single crystal along [100], [110] and [001] crystallographic axes have been presented. At temperatures of magnetic phase transitions of T1≈263 K and T2≈289 K an abrupt decrease of sound velocities is observed. The profound anisotropy of the internal friction is detected in the region of the existence of incommensurate magnetic structure (T1≤T≤T2).

Temperature dependences of longitudinal sound wave velocities and internal friction of ferromagnetic La0.7 – yPryCa0.3MnO3 single crystals (0 ≤ y ≤ 0.3) with the first-order magnetic phase transition have been analyzed. In the paramagnetic region, the temperature dependences of the sound velocity and internal friction exhibit extended temperature hysteresis, which is indicative of inhomogeneity of the paramagnetic state. Structural phase stratification is likely the main reason for the inhomogeneous paramagnetic state of La0.7 – yPryCa0.3MnO3 single crystals (0 ≤ y ≤ 0.3).

The report presents the results of studying the temperature dependences of the velocities of propagation of longitudinal and torsion waves and the internal friction in a tetragonal FeGe2 single crystal along crystallographic axes [100], [110], and [001]. At temperatures of magnetic phase transitions of T1 ≈ 263 K and T2 ≈ 289 K an abrupt decrease of sound velocities is detected. A high anisotropy of the internal friction is observed in the region of existence of an incommensurate magnetic structure (T1 ≤ T ≤ T2).

Results of the investigation of the temperature dependences of the velocity of longitudinal sound and internal friction in the ferromagnetic La 0.7-y Pr y Ca 0.3 MnO3 single crystals (0≤ y≤0.3) with first-order magnetic phase transition are presented. In the paramagnetic state, the temperature dependences of the velocity of sound and the internal friction demonstrate extended temperature hysteresis, which indicates the ingomogeneity of the paramagnetic state. It seems likely that the main reason of this ingomogeneity is structural phase separation.

AbstractThe reflection spectra of FeGe_2 single crystal in a wide spectral region and in the temperature range from 80 to 310 K are studied. The energy of plasma oscillations, the relaxation frequency of charge carriers, and phonon frequencies are determined. Anisotropy of the optical properties is studied. It is shown that the phase transition from the collinear antiferromagnetic structure to the spiral one is accompanied by a significant rearrangement of electronic states.

The influence of structural transitions from high-temperature pseudocubic O* to the Jahn-Teller O' phase and the reverse transition from O' to a low-temperature O* phase on the elastic, magnetic, and transport properties of a La0.82Ca0.18MnO3 single crystal at the temperature decrease from 400 K is studied. Significant anomalies are observed in the temperature dependences of the velocity of longitudinal sound waves and the internal friction. The sound velocity and the internal friction in the Jahn-Teller O' phase are significantly lower than those in the pseudocubic O* phase. The structural transitions take place in the temperature range 30 - 50 K. The majority carriers in the low-temperature O* phase are electrons, whereas the majority carriers in the O' and high-temperature O* phases are holes. Electronic states are characterized by substantially stronger spin-orbital interaction than hole states. The positive magnetoresistance is observed in low magnetic fields in the low-temperature O* phase. At a certain field value, the magnetoresistance changes its sign. Such magnetoresistance has not been observed in the La-Sr and La-Ba manganites. Positive magnetoresistance indicates significant anisotropy of the transport properties of the low-temperature O* phase in the La0.82Ca0.18MnO3 single crystal. It is established that the magnetic and transport properties of the low-temperature ferromagnetic O* phase differ markedly from those of the ferromagnetic Jahn-Teller O' phase.

The reflection spectra of FeGe2 single crystal in a wide spectral region and in the temperature range from 80 to 310 K are studied. The energy of plasma oscillations, the relaxation frequency of charge carriers, and phonon frequencies are determined. Anisotropy of the optical properties is studied. It is shown that the phase transition from the collinear antiferromagnetic structure to the spiral one is accompanied by a significant rearrangement of electronic states.

We review lattice, thermodynamic, transport, high-frequency, and optical properties of Re1 − x Dx MnO3 manganites, where Re is a rare earth and D is usually an alkaline-earth element. Colossal magnetoresistance (CMR) is the most famous effect observed in manganites near the Curie temperature TC. It is shown that the temperature and magnetic field dependences of the magnetocaloric effect, resistivity, magnetoresistance, Hall effect, and thermopower near TC depend on whether the magnetic transition is of first or second order. The possibility of applying the well-known Banerjee criterion to inhomogeneous ferromagnets is discussed. Special attention is paid to various inhomogeneities characteristic of the manganites, including the coexistence of different crystallographic phases, metallic inclusions in the semiconductor matrix, and polarons.

The temperature dependences of the velocity of longitudinal sound waves and the internal friction in a La 0.82 Ca 0.18 MnO 3 single crystal with the Curie temperature T C = 181 K have been studied. As temperature decreases, the single crystal is shown to undergo the transition from the pseudocubic O * to the Jahn–Teller O ’ phase at T ~ 254 K and the reverse transition from O ’ to O * phase at T ~ 84 K. The velocity of sound and the internal friction in the O ’ phase are found to be significantly smaller than those in the O * phase.

The problem of determining the magnitude of the atomic magnetic moments in compounds with rare-earth and transition elements using the X-ray magnetic circular dichroism (XMCD) is investigated. The standard sum rules approach usually gives the moment values by several times smaller than their bulk values obtained from the direct magnetic measurements. We attribute this to the strong spin fluctuations in the surface layer which actually formed the soft XMCD signal. A method of determining the value of the local magnetic moments in the presence of strong fluctuations is proposed. The magnetic interactions in La0.5Pr0.2Ca0.3MnO3 are investigated by x-ray absorption spectroscopy, XMCD and bulk magnetization measurements. The proposed procedure is used to determine the values of atomic magnetic moments of transition and rare-earth elements as well as net magnetization on the surface and in the bulk from experimental Mn L2,3 and Pr M4,5 XMCD spectra.

Приведены результаты исследования температурных зависимостей скорости продольных звуковых волн и внутреннего трения монокристалла La0.82Ca0.18MnO3 с температурой Кюри TC=181 K. Показано, что при понижении температуры переход из псевдокубической O* в ян-теллеровскую O'-фазу происходит при T~254 K, а возвратный переход из O'- в O*-фазу --- при T~84 K. Установлено, что скорость звука и внутреннее трение в O'-фазе значительно меньше, чем в O*-фазе. Работа выполнена в рамках государственного задания ФАНО России (тема Спин" N 01201463330) при частичной поддержке проекта N 3.2076.2014/K госзадания Минобрнауки. DOI: 10.21883/FTT.2017.02.44047.276

The temperature dependences of the velocity of longitudinal sound and internal friction in the ferromagnetic La 0.5 Pr 0.2 Ca 0.3 MnO 3 single crystal with magnetic first-order phase transition were studied. It was found that the sound velocity decreases by ≈20% in transition from the ferromagnetic to paramagnetic state. In the paramagnetic region, the extended temperature hysteresis of the sound velocity and the internal friction was observed. It was shown that La 0.5 Pr 0.2 Ca 0.3 MnO 3 has two paramagnetic phases with different sound velocities.

Results of the investigation of optical density spectra of Cr5S6 single crystals in the infrared region have been presented. A comparative analysis of the temperature dependences of the magnetization and the light absorption has been performed. Physical mechanisms to explain the specific features of the spectral and temperature dependences of the optical density have been proposed.

Influence of the substitution of Hg by Cd on Kerr effect, magnetoreflection, and magnetotransmission of unpolarized light for Hg 1-x Cd x Cr 2 Se 4 (0≤x≤1) single-crystals is studied. The changes in the magnetooptical properties of the ferromagnetic spinels are shown to occur in compositions with x>0.1. The giant values of magnetooptical effects in Hg 1-x Cd x Cr 2 Se 4 spinels can be used in the magnetooptoelectronics.

The concentration, temperature, and magnetic-field dependences of the magnetoreflection and magnetotransmission of natural light in the infrared spectral range and the Kerr effect in single crystals of ferromagnetic Hg 1- x Cd x Cr 2 Se 4 (0 ⩽ x ⩽ 1) spinels have been studied. A relationship of the magneto-optical properties to the electronic band structure of spinels has been established. The most significant changes in the spectra of magnetoreflection, magnetotransmission, and the Kerr effect are shown to be observed for 0.1 < x < 0.25 and are attributable to a rearrangement of the band structure as the composition changes.

Magnetocaloric effect in single crystals of spinels CdCr2Se4 and HgCr2Se4 is studied. The maximum value of the magnetic-field-induced change in entropy in the field of 15kOe is about 1.4–1.5J/(kgK). At Curie temperature the entropy change obeys the relation: −ΔS=αH^(2/3)–β. The magnetic transitions in the spinels are found to be noticeably smeared. The experimental data have been compared with the theoretical results obtained for Heisenberg model in the mean field approximation (MFA). It is found that MFA overestimates the change of entropy in a magnetic field and underestimates the influence of inhomogeneity on ΔS.

The magnetic, transport and elastic properties of CaMn0.96Mo0 04O3 polycrystal are reported. The magnetic data of the paramagnetic region point to the magnetic polarons. The transition from the paramagnetic state into the antiferromagnetic state of G-type occurs near 93 K, the crystal orthorhombic structure being unchanged. The transition from the paramagnetic orthorhombic phase into the C-type antiferromagnetic monoclinic phase is revealed at about 130-140 K, the transition taking place in small regions of the sample. (C) 2014 Elsevier B.V. All rights reserved.

The magnetic properties of ferromagnetic single crystals of La0.7−xPrxCa0.3MnO3 with x≤0.3 are studied. It is shown that the transition temperature TM determined from the position of an inflection point in the magnetization-versus-temperature curve is linear in magnetic field: TM(H)=TM(0)+BMH with TM(0)=(229−270x)K, BM=0.8 K/kOe for x=0.0, 0.1, 0.2 and BM=1.2 K/kOe for x=0.3. The magnetic moments of Pr and Mn are shown to be parallel to each other. The data obtained are employed in order to analyze when Banerjee criterion for distinguishing first and second order transitions can be used in inhomogeneous ferromagnets. Phenomenological theory for the Arrott–Belov plots is also developed. It is found experimentally and theoretically that in inhomogeneous ferromagnets, the slope of the Arrott–Belov plots is not a universal tool for determining the type of transition, in fact, Banerjee criterion may be used only in the paramagnetic state beyond the transition region.

The article is devoted to the theoretical study of magnetocaloric effect in imhomogeneous ferromagnets in vicinity of magnetic phase transitions of first and second orders as well as in vicinity of tricritical and critical points. In the frame of Landau theory the formulae for the magnetic field induced entropy change are derived. The theory is compared with the experimental data obtained on single crystals of La0.7Ba0.3MnO3 (second order transition) and La0.7Ca0.3MnO3 (first order transition).