Role of structure imperfection in the formation of the magnetotransport properties of rare-earth manganites with a perovskite structure

The structure, the structure imperfection, and the magnetoresistance, magnetotransport, and microstructure properties of rare-earth perovskite La_0.3Ln_0.3Sr_0.3Mn_1.1O_3–δ manganites are studied by X-ray diffraction, thermogravimetry, electrical resistivity measurement, magnetic, ⁵⁵Mn NMR, magnetoresistance measurement, and scanning electron microscopy. It is found that the structure imperfection increases, and the symmetry of a rhombohedrally distorted R3?c perovskite structure changes into its pseudocubic type during isovalent substitution for Ln = La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, or Eu³⁺ when the ionic radius of an A cation decreases. Defect molar formulas are determined for a real perovskite structure, which contains anion and cation vacancies. The decrease in the temperatures of the metal–semiconductor (T _ms) and ferromagnet–paramagnet (T _C) phase transitions and the increase in electrical resistivity ρ and activation energy E _a with increasing serial number of Ln are caused by an increase in the concentration of vacancy point defects, which weaken the double exchange 3d ⁴(Mn³⁺)–2p ⁶(O^2–)–3d ³(Mn⁴⁺)–V ^(a)–3d ⁴(Mn³⁺). The crystal structure of the compositions with Ln = La contains nanostructured planar clusters, which induce an anomalous magnetic hysteresis at T = 77 K. Broad and asymmetric ⁵⁵Mn NMR spectra support the high-frequency electronic double exchange Mn³⁺(3d ⁴) ? O^2–(2p ⁶) ? Mn⁴⁺(3d ³) and indicate a heterogeneous surrounding of manganese by other ions and vacancies. A correlation is revealed between the tunneling magnetoresistance effect and the crystallite size. A composition–structure imperfection–property experimental phase diagram is plotted. This diagram supports the conclusion about a strong influence of structure imperfection on the formation of the magnetic, magnetotransport, and magnetoresistance properties of rare-earth perovskite manganites.     

Photoluminescence of the nanosized xerogel Zn<Subscript>2</Subscript>SiO<Subscript>4</Subscript>:Mn<Superscript>2+</Superscript> in pores of anodic alumina

The photoluminescence properties of a composite material prepared by the introduction of the nanosized phosphor Zn₂SiO₄:Mn²⁺ into porous anodic alumina have been investigated. Scanning electron microscopy studies have revealed that Zn₂SiO₄:Mn²⁺ particles are uniformly distributed in 70% of the volume of the pore channels. The samples exhibit an intense luminescence in the range of 2.3–3.0 eV, which corresponds to the emission of different types of F centers in alumina. After the formation of Zn₂SiO₄:Mn²⁺ nanoparticles in the pores, an intense photoluminescence band is observed at 2.4 eV due to the ⁴T₁–⁶A₁ electronic transition within the 3d shell of the Mn²⁺ activator ion. It has been found that the maximum of the photoluminescence of Zn₂SiO₄:Mn²⁺ xerogel nanoparticles located in the porous matrix is shifted to higher energies, and the luminescence decay time decreases significantly.     

Changes in the electronic,optical, and magnetic properties of LaSrMnO films upon transition from the rhombohedral phase to the orthorhombic phase

The properties of LaSrMnO films are investigated in the temperature range of the transition from the rhombohedral phase to the orthorhombic phase (600–650° C). It is shown that, with a variation in the growth temperature T_s, the change in the film properties is governed by the interaction of Mn-O metallic (ferromagnetic) clusters in the dielectric (antiferromagnetic) matrix. At T_s≤600°C, the low density of e _g states and the dielectric gap (E _g =0.3–0.5 eV) are responsible for the following features: (i) the optical transparency in the range ?ω=0.5–2 eV, (ii) the difference between the FC and ZFC magnetizations M(T), (iii) the high resistance, and (iv) the appearance of the portions R(T) ≈ const in the dependence R(T) due to the transformation of clusters into a system of tunnel-coupled quantum dots. At T_s≥650°C, the local increase in the atomic and electronic densities leads to a decrease in the optical transmission and the resistance by three to nine orders of magnitude, the appearance of a maximum and a minimum in the dependences R(T) of the LaSrMnO films, and an increase in the magnetization M(10 K) by one order of magnitude. The inference is drawn that magnetic ordering of the system of tunnel-coupled clusters encourages an increase in the cluster size and in the content of the metallic (ferromagnetic) phase.     

Spin-wave resonance in Ge<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films exhibiting percolation ferromagnetism

Magnetic properties of Ge_{1 ? x} Mn _x (x = 0.02, 0.04, 0.08) thin films obtained by ion-implanting Mn⁺ ions into single-crystal Ge plates are investigated. The contributions of the subsystem of dispersed Mn²⁺ ions, Ge³Mn⁵ ferromagnetic clusters, and Mn-enriched ferromagnetic domains of Ge to the magnetic moment of Ge_{1 ? x} Mn _x films are distinguished. In the subsystem of dispersed Mn²⁺ ions in Ge_{1 ? x} Mn _x films at temperatures below 10 K, a spin-wave resonance is observed in the magnetically ordered state due to percolation ferromagnetism. It is established that, in the films with percolation ferromagnetism, the exchange integrals determined from static measurements correspond to those determined by dynamic measurements.     

Studying the occupied and unoccupied electronic structure of LaCoO<Subscript>3</Subscript> by using DFT+embedded DMFT method with the calculated value of <Emphasis Type="Italic">U</Emphasis>

In this work, we present a systematic study of the occupied and unoccupied electronic states of LaCoO₃ compound using DFT, DFT+U and DFT+embedded DMFT methods. The value of U used here is evaluated by using constrained DFT method and found to be ~6.9 eV. It is found that DFT result has limitations with energy positions of PDOS peaks due to its inability of creating a hard gap although the DOS distribution appears to be fine with experimental attributes. The calculated value of U is not an appropriate value for carrying out DFT+U calculations as it has created an insulating gap of ~1.8 eV with limitations in redistribution of DOS which is inconsistent with experimental spectral behavior for the occupied states mainly. However, this value of U is found to be an appropriate one for DFT+embedded DMFT method which creates a gap of ~1.1 eV. The calculated PDOS of Co 3d, La 5d, La 4f and O 2p states are giving a remarkably good explanation for the occupied and unoccupied states of the experimental spectra in the energy range ~–9.0 eV to ~12.0 eV.     

Reciprocal and nonreciprocal magnetic linear birefringence in the γ-Dy<Subscript>2</Subscript>S<Subscript>3</Subscript> sesquisulfide

A study has been made of the spectral dependence of the Cotton-Mouton effect (CME) quadratic in magnetic field, nonreciprocal birefringence (NB) linear in magnetic field, and the Faraday effect (FE) in the cubic magnetic semiconductor γ-Dy₂S₃. Unlike the FE, the CME and the NB in this crystal are anisotropic, with the pattern of the anisotropy being dependent on the photon energy. The dependence of the CME and NB dispersion on the direction of the magnetic field B indicates contribution from a variety of electronic transitions and mechanisms to these phenomena. It is shown that the resonant contributions to the CME and NB in the transparency region originate from electronic transitions near E?3.4 eV (beyond the band edge E _g=2.8 eV), which are likely transitions from the localized ground state of the Dy³⁺ ion to states derived from mixing of the band and 4f ^N?1 5d states of the dysprosium ion. The character of the CME anisotropy in the transparency region and near the local electronic transition ⁶ H _15/2 → ⁶ F _3/2 connecting states of the unfilled 4f shell of the Dy³⁺ ion suggests the presence of a strong axial component of the crystal field acting on the rare earth ion.     

Variable-range hopping conduction in LaMnO<Subscript>3 + δ</Subscript>

The temperature dependence of the electrical resistivity ρ(T) for ceramic samples of LaMnO_{3 + δ} (δ = 0.100–0.154) are studied in the temperature range T = 15–350 K, in magnetic fields of 0–10 T, and under hydrostatic pressures P of up to 11 kbar. It is shown that, above the ferromagnet-paramagnet transition temperature of LaMnO_{3 + δ}, the dependence ρ(T) of this compound obeys the Shklovskii-Efros variable-range hopping conduction: ρ(T) = ρ₀(T)exp[(T ₀/T)^1/2], where ρ₀(T) = AT ^9/2 (A is a constant). The density of localized states g(?) near the Fermi level is found to have a Coulomb gap Δ and a rigid gap γ(T). The Coulomb gap Δ assumes values of 0.43, 0.46, and 0.48 eV, and the rigid gap satisfies the relationship γ(T) ≈ γ(T _v)(T/T _v)^1/2, where T _v is the temperature of the onset of variable-range hopping conduction and γ(T _v) = 0.13, 0.16, and 0.17 eV for δ = 0.100, 0.125, and 0.154, respectively. The carrier localization lengths a = 1.7, 1.4, and 1.2 Å are determined for the same values of δ. The effect of hydrostatic pressure on the variable-range hopping conduction in LaMnO_{3 + δ} with δ = 0.154 is analyzed, and the dependences Δ(P) and γ_v(P) are obtained.     

The electronic structure and optical and magnetooptical properties of the CaCo<Subscript>2</Subscript> compound synthesized at a high pressure: Experiment and theory

The optical properties (the real ε₁ and imaginary ε₂ permittivity parts, optical conductivity σ, and reflectivity R) of the new ferromagnetic compound CaCo₂ in the Laves cubic phase (C15) synthesized at a pressure of 8.0 GPa were studied over the spectral range ?ω = 0.2–9 eV. The field and spectral (?ω = 0.5–4.2 eV) dependences of the equatorial Kerr effect were determined. The electronic structure and optical characteristics of CaCo₂ were calculated using the electron density functional theory by the linearized augmented-plane-wave method. The main band structure parameters of the compound were determined. The experimental and theoretical σ(ω) and R(ω) dependences were in satisfactory agreement with each other. The formation of the main absorption bands was found to be caused by the (p,d → d,p)-type electronic transitions related to the cobalt and calcium atoms. The exchange splitting of the 3d band of CaCo₂ was estimated, 2Δ_exc ～ (1–1.3) eV.     

Linear and nonlinear optical spectroscopy of gadolinium iron borate GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The optical spectra and the second-harmonic generation (SHG) are studied in a noncentrosymmetric GdFe₃(BO₃)₄ magnet. In the region of weak absorption (α～20–400 cm^?1) below ～3 eV, three absorption bands are distinguished, which can be unambiguously assigned to forbidden electronic transitions from the ground ⁶A₁ state of the Fe³⁺ ion to its excited states ⁴T₁(～1.4 eV), ⁴T₂(～2 eV), and ⁴A₁, ⁴E(～2.8 eV). Intense absorption begins in the region above 3 eV (α～2–4×10⁵ cm^?1), where two bands at ～4.0 and 4.8 eV are observed, which are caused by allowed electric dipole charge-transfer transitions. The spectral features of SHG in the 1.2–3.0-eV region are explained by a change in the SHG efficiency caused by a change in the phase mismatch. It is shown that in the weak absorption region, phase matching can be achieved for SHG.     

Peculiarities of antiferromagnetic ordering in orthorhombic LiMnO<Subscript>2</Subscript>

Data on the antiferromagnetic ordering in orthorhombic lithium manganite LiMnO₂ are obtained from magnetic-susceptibility, calorimetry, and nuclear magnetic resonance studies. The minimal hysteresis and the absence of jumps in the temperature dependences of the sublattice magnetization M(T) and the magnetic susceptibility near T _N indicate that the ordering occurs through a continuous second-order phase transition. Within the critical temperature range, the M(T?T _N) variation is satisfactorily described by a power-law dependence with a critical exponent β = 0.25(4), which is substantially smaller than that predicted for 3D magnetic systems with isotropic Heisenberg exchange. The band structure of orthorhombic LiMnO₂ is calculated using the LMTO-ASA method. Taking into account the spin states of manganese ions, an adequate pattern is obtained for the density-of-states distribution with an energy gap near the Fermi level (～0.7 eV), which is in agreement with the measured electrical parameters of lithium manganite. The calculations demonstrate that the exchange interactions between Mn³⁺ ions leading to antiferromagnetic ordering are significantly anisotropic. It is found that small paramagnetic regions persist in the manganite below the Néel temperature, and it is concluded that the reason for this is partial structural disordering of LiMnO₂. As a result, a certain fraction of the manganese positions is occupied by lithium ions (Li_Mn) and vise versa (Mn_Li). These defects are not involved in the formation of the ordered magnetic structure and compose a paramagnetic fraction.     

“Negative” gap in the spectrum of localized states of (In<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.9</Subscript>(SrO)<Subscript>0.1</Subscript>

The reflection R(?ω), transmission t(?ω), absorption α(?ω), and refraction n(?ω) spectra of polycrystalline In₂O₃–SrO samples with low optical transparency, which contain In₂O₃ and In₂SrO₄ crystallites with In₄SrO_{6 + δ} interlayers, are examined. In the region of small ?ω values, the reflection coefficient decreases as the resistance of samples saturated with oxygen increases. Spectral dependences n(?ω) and α(?ω) are calculated using the classical electrodynamics relations. The results are compared to the data based on the t(?ω) spectra. The calculated absorption spectra are interpreted within the model with an overlap of tails of the density of states in the valence band and in the conduction band. A “negative” gap E _gn in the density of states with a width from–0.12 to–0.47 eV is formed in highly disordered samples in this model. It is demonstrated that the high density of defects and the band of deep acceptor states of strontium in the major matrix In₂O₃ phase are crucial to tailing of the absorption edge and its shift toward lower energies. The direct gap E _gd = 1.3 eV corresponding to the In₂SrO₄ phase is determined. The energy band diagram and the contribution of tunneling, which reduces the threshold energy for interband optical transitions, are discussed.     

Experimental Study and Analysis of Absorption Spectra of Ni<Superscript>2+</Superscript> Ions in Nickel Orthoborate Ni<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>2</Subscript>

The results of studies of the absorption spectra of nickel orthoborate Ni₃(BO₃)₂ in the range of electronic d–d-transitions are reported. The obtained data are analyzed in the framework of the crystal field theory. The Ni²⁺ ions are located in two crystallographically nonequivalent positions 2a and 4f with point symmetry groups C_2h and C₂, respectively, surrounded by six oxygen ions forming deformed octahedra. The absorption spectra exhibit three intense bands corresponding to spin-resolved transitions from the ground state of nickel ion ³A_2g (³F) to the sublevels of the ³T_2g (³F), ³T_1g (³F) and ³T_1g (³P) triplets split by the spinorbit interaction and the rhombic component of the crystal field. At temperatures below 100 K, the spectra exhibit a thin structure, in which phonon-free lines can be distinguished. Comparison of the calculated frequencies of the zero-phonon transitions with the experimental data allows estimating parameters of the crystal field acting on the nickel ions in the 2a- and 4f-positions, as well as the parameters of electrostatic interaction between the 3d electrons and spin-orbit interaction constants.     

Giant nonlinear absorption in the NiO antiferromagnet

A study of the spectrum of nonlinear two-photon and two-step absorption in NiO single crystals, carried out in the energy region ?ω₁ + ?ω₂ = 2.45–4.575 eV, showed it to have a complex shape and consist of very strong peaks (from 0.05 to 2.7 cm/MW). Within the energy interval 2.45–3.3 eV, the spectrum is due to d-d transitions in the Ni²⁺ ion. The band gap width was determined to be E _g =3.466 eV. The spectral features seen above this energy originate from interband transitions from three valence subbands to the conduction band bottom.     

Europium luminescence in fluorite upon high-energy excitation

The reflection and luminescence excitation spectra of CaF₂ crystals containing europium ions in divalent (Eu²⁺) and trivalent (Eu³⁺) states were measured in the range from 4 to 16 eV. It was established that, in CaF₂ : Eu³⁺ crystals, luminescence of Eu³⁺ ions (the f-f transitions) is effectively excited both in the charge-transfer band (at ～8 eV) and in the region of the 4f–5d transitions (at ～10 eV) but is virtually not excited in the fundamental region of the crystal (at an energy higher than 10.5 eV). Luminescence of Eu²⁺ ions (the 427-nm band) in CaF₂ : Eu³⁺ is effectively excited in the fundamental region of the crystal; i.e., luminescence of divalent europium ions occurs through the trapping mechanism. Emission of Eu²⁺ ions in CaF₂ : Eu²⁺ crystals is characterized by the excitation band at an energy of 5.6 eV (the 4f → 5d,t _2g transitions), as well as by the exciton and interband luminescence excitations. The results obtained and data available in the literature are used to construct the energy level diagram with the basic electron transitions in the CaF₂ : Eu crystals.     

Peculiarities of magnetic field-induced ferromagnetic ordering in narrow-band manganites

The temperature dependences of the residual magnetization in narrow-band manganites (Pr_0.67Ca_0.33MnO₃, Sm_0.55Sr_0.45Mn¹⁸O₃, Sm_0.55Sr_0.45Mn¹⁶O₃, and (NdEu)_0.55Sr_0.45Mn¹⁸O₃) have been studied. All compounds studied are characterized by a fairly high residual magnetization M _R (about 0.5 μ_B/Mn) at 4.2 K, which vanishes upon sample heating to the temperature T _RE ≈ 30–35 K, which is much lower than the temperature T _C of the ferromagnetic transition. However, upon magnetization of the samples at T _RE < T < T _C , the residual magnetization (smaller in magnitude) remains up to T _C . For the composition (NdEu)_0.55Sr_0.45Mn¹⁸O₃, the residual magnetization remains at T < T _C , independent of the temperature of magnetization. The disappearance of the residual magnetization found at intermediate temperatures is apparently related to the destruction of the magnetic field-induced ferromagnetic ordering (which contains an additional contribution of the rare-earth sublattice).     

Effect of the orbital structure and symmetry of electronic states on nonradiative <Emphasis Type="Italic">S-T</Emphasis> intersystem crossing: Dibenzofuran

The nonradiative S-T intersystem crossing S ₁(ππ*) ? T ₁(ππ*) in dibenzofuran (DB(O)) molecules has been theoretically investigated within the model of vibronically induced spin-orbit (VISO) coupling of electronic states, where the vibronic perturbation takes into account all out-of-plane vibrational modes of a molecule. It is established that the S-T intersystem crossing S ₁(¹ A ₁) ? T ₁(³ B ₂) involves also the intermediate (T _m )T ₂(³ A ₁) and T ₃(³ B ₂) triplet states. The calculated rate constant K _ST = (4.5–4.7) × 10⁷s^?1 of the nonradiative transition is in agreement with the known experimental data. The manifestation of approximate (belonging to the D _2h group) symmetry of singlet and triplet molecular states in VISO couplings has been studied. An effect of the heavy (oxygen) atom of a DB(O) molecule on K _ST is established.     

Der Zerfall des Mn52m

Mn^52m(T _1/2=21 m) was produced by irradiating iron-foiles with deuterons ofE _d =9 MeV. Coincidences ofγ-rays with the main 1434 keV-transition had been sought for with the aid of a scintillation fast-slow-coincidence circuit. There was no evidence ofγ-lines which had been found earlier byKatoh et al. atγ-energies 700, 940, 1020, 1150, 1370, 1520 keV. If these transitions exist, their relative intensities are less than 0.7% (700 keV) and 0.3% (all others) per decay of Mn^52m.     

Electronic and optical properties of Fe<Subscript>2</Subscript>SiO<Subscript>4</Subscript> under pressure effect: ab initio study

We report first-principles studies the structural, electronic, and optical properties of the Fe₂SiO₄ fayalite in orthorhombic structure, including pressure dependence of structural parameters, band structures, density of states, and optical constants up to 30 GPa. The calculated results indicate that the linear compressibility along b axis is significantly higher than a and c axes, which is in agreement with earlier work. Meanwhile, the pressure dependence of the electronic band structure, density of states and partial density of states of Fe₂SiO₄ fayalite up to 30 GPa were presented. Moreover, the evolution of the dielectric function, absorption coefficient (α(ω)), reflectivity (R(ω)), and the real part of the refractive index (n(ω)) at high pressure are also presented.     

Causes of the Metamagnetism in a Disordered EuMn<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>O<Subscript>3</Subscript> Perovskite

The magnetic properties of the EuMn_0.5Co_0.5O₃ perovskite synthesized under various conditions are studied in fields up to 140 kOe. The sample synthesized at T = 1500°C is shown to exhibit a metamagnetic phase transition, which is irreversible below T = 40 K, and the sample synthesized at T = 1200°C demonstrates the field dependence of magnetization that is typical of a ferromagnet. Both samples have T_C = 123 K and approximately the same magnetization in high magnetic fields. The metamagnetism is assumed to be related to a transition from a noncollinear ferromagnetic phase to a collinear phase, and the presence of clusters with ordered Co²⁺ and Mn⁴⁺ ions leads to ferromagnetism. The noncollinear phase is formed due to the competition between positive Co²⁺–Mn⁴⁺ and negative Mn⁴⁺–Mn⁴⁺ and Co²⁺–Co²⁺ interactions, which make almost the same contributions, and to the existence of a high magnetic anisotropy.