V. E. Arkhipov

Red Shift of Absorption Edge and Nonmetal–Metal Transition in Single Crystals La1—xSrxMnO3 (x = 0.1, 0.2, 0.3)

The red shift of the fundamental absorption edge (mo.16 eV) of a La 0.9 Sr 0.1 MnO 3 single crystal occurs as the crystal is cooled into the ferromagnetic phase. This result shows that there exist spin-dependent states. The reflection and absorption spectra of La 1-x Sr x MnO 3 single crystals (x = 0.1, 0.2, 0.3) indicate that the red shift may be the reason for the appearance of free charge carriers participating in the colossal magnetoresistance phenomenon. The nonmetal-metal type transition below T C confirms this.

Centers of charge nonuniformity in absorption spectra of lanthanum manganites

In order to study the microscopic nature of the phase separation in lanthanum manganites, experimental investigations were made of the optical, electrical, and magnetic properties of La1−δMnO3 and La0.9Sr0.1MnO3 lanthanum manganite single crystals. The infrared absorption spectra revealed two bands at 0.14 eV and 0.35 eV whose intensity was sensitive to the magnetic order. The different temperature dependence of the electrical resistivity (semiconducting) and the transmission (metallic) below the Curie ferromagnetic temperature indicates that an insulator-metal transition takes place in various regions of the insulating matrix, i.e., phase separation occurs. Characteristic features of the properties and the nature of the phase separation in these compounds can be explained using a model of polar (hole [MnO68−]JT and electron [MnO610−] JT) pseudo-Jahn-Teller clusters which form centers of charge nonuniformity in the crystal.

Spin susceptibility of Ga-stabilized δ-Pu probed by 69Ga NMR

Spin susceptibility of stabilized \delta phase in the Pu-Ga alloy is studied by measuring {69,71}^Ga NMR spectra and nuclear spin-lattice relaxation rate {69}T_{1}^{-1} in the temperature range 5 - 350 K. The shift ({69}^K) of the {69,71}^Ga NMR line and {69}^T_{1}^{-1} are controlled correspondingly by the static and the fluctuating in time parts of local magnetic field arisen at nonmagnetic gallium due to transferred hyperfine coupling with the nearest f electron environment of the more magnetic Pu. The nonmonotonic with a maximum around 150 K behavior of {69}^K(T) \chi_{s,5f}(T) is attributed to the peculiarities in temperature dependence of the f electron spin susceptibility \chi_{s,5f}(T) in \delta phase of plutonium. The temperature reversibility being observed in {69}^K(T) data provides strong evidence for an electronic instability developed with T in f electron bands near the Fermi energy and accompanied with a pseudogap-like decrease of \chi_{s,5f}(T) at T<150 K. The NMR data at high temperature are in favor of the mainly localized character of 5f electrons in \delta phase of the alloy with characteristic spin-fluctuation energy \Gamma(T) T^{0.35(5)}, which is close to

Magnetooptical faraday effect in La0.7Sr0.3MnO3−δ films

\Gamma(T) T^{0.5} predicted by Cox et al. [J. Appl. Phys. 57, 3166 (1985)] for 3D Kondo-system above T_Kondo}. The dynamic spin correlations of 5f electrons become essential to consider for {69}^T_{1}^{-1}(T) only at T<100 K. However, no NMR evidences favoring formation of the static magnetic order in \delta-Pu were revealed down to 5K .

Effects of pressure and magnetic field on the electrical resistivity of La0.8Ba0.2MnO3 in rhombohedral and orthorhombic phases

The Faraday effect spectra were measured in La0.7Sr0.3MnO3−δ films. A band attributed to d-d transitions 4A2g-4T2g in Mn4+ ions or octahedral complexes (MnO6)8− was observed in the spectral region of ∼2.7 eV. The position and value of the maximum of the Faraday rotation band and the figure of merit were found to depend on the degree of charge and magnetic uniformity of the films. The films can be used to develop mag-netooptical modulators.

Features of the magnetic state of ƒ electrons in the stabilized δ phase of the Pu0.95Ga0.05 alloy

A structural transition from the orthorhombic phase to the rhombohedral one is observed and investigated on a La0.8Ba0.2MnO3 single crystal. In the region of the coexistence of two phases, the effects of magnetic field (up to 12 T) and hydrostatic pressure (up to 5 kbar) on the temperature dependences of the electrical resistivity are studied. It is shown that magnetic field and pressure reduce the transition temperature and produce qualitatively similar effects on the electrical resistivity in the region of the structural transition.

(139La, 55Mn) NMR and magnetic susceptibility data on microscopic phase separation in La0.9MnO3 single crystal

For the Pu0.95Ga0.05 alloy belonging to the stabilized δ phase of the Pu-Ga system, the 69Ga NMR spectra are measured in the temperature interval 10–650 K and the static magnetic susceptibility χ is measured in the interval T = 20–350 K. In the region T > T* = 235 K, the temperature-dependent magnetic part of the shift K(T) of the Ga NMR line reproduces the χ(T) dependence and follows the Curie-Weiss law K(T) ∼ (T + Θ)−1 with Θ = 280(40) K, which is typical of incoherent spin fluctuations of localized ƒ electrons in concentrated nonmagnetic Kondo systems. The estimate of the effective magnetic moment μeff, 5ƒ(ge = 2) = 0.15(5)μB per Pu atom testifies to a strong suppression of the spin magnetism in the alloy, where the configuration of the ƒ shell of a Pu ion is close to atomic-like ƒ6. The difference between the K(T) and χ(T) dependences observed for the alloy in the temperature range T < T* is analyzed in terms of the two-fluid description of Kondo lattices developed by Pines et al. [15, 16] in application to the coherent state of a heavy-fermion liquid. Possible causes for the anomalous increase in long-wave contributions in the spatial dispersion of the localized spin susceptibility component of ƒ electrons χƒƒ at temperatures below 50 K are discussed.

Charge ordering in crystals of Pr1−x Sr x MnO3 (x = 0.22, 0.24) ferromagnetic manganites

Magnetic susceptibility of the La0.9MnO3 single crystal was measured and its (139La, 55Mn) NMR spectra were recorded. The data obtained indicate that the areas with an A-type antiferromagnetic order (TN=140 K) and magnetic moments aligned with the b axis occupy a major part of the sample volume in manganite with a considerable concentration of cationic vacancies; simultaneously, the clusters with a canted magnetic sublattice and ferromagnetic interaction between magnetic moments are formed near the vacancies. Charge distribution in these clusters is materially different from that in the antiferromagnetic areas. Magnetic state and relative concentration of the clusters are discussed.

Polaron hopping conduction in La0.85Sr0.15MnO3 single crystal

The structural and magnetic states of Pr1−xSrxMnO3 (x = 0.22, 0.24) manganite crystals were studied over a wide temperature range. Measurements of the magnetic susceptibility and electrical resistivity demonstrated that these manganites belong to the class of ferromagnetic semiconductors. Thermal neutron elastic scattering patterns revealed that, in the temperature range 4.2–350 K, the manganites have an orthorhombic structure (space group Pnma) with a well-pronounced cooperative Jahn-Teller effect. Major emphasis was placed on the nuclear magnetic superstructure with a wave vector q = (2π/2a, 0, 2π/2c). It was shown that this superstructure suggests 1/4-type charge ordering in the manganites under investigation.

Phase transitions in crystals with a BCC structure

Temperature dependences of electrical resistance and thermopower of a La0.85Sr0.15MnO3 single crystal were measured in the paramagnetic and ferromagnetic dielectric phases. It was shown that charge transfer in both phases is due to the variable-range polaron hopping over the localized states. The activation energies in both phases linearly depend on T3/4 but differ from each other by a constant which is approximately equal to the exchange energy kTc. The results obtained are in compliance with the concept of percolation character of metal-insulator transition in manganites.