Quasi-two-dimensional ferroelectricity in KNbO<Subscript>3</Subscript>/KTaO<Subscript>3</Subscript> superlattices

First-principles density functional theory is used to calculate the phonon spectrum in the paraelectric phase, the ground-state structure and polarization distribution in the polar phase, and energies of ferro- and antiferroelectrically ordered phases of free-standing (KNbO₃)₁(KTaO₃) _n ferroelectric superlattices with n = 1–7. It is established that quasi-two-dimensional ferroelectricity with polarization oriented in the layer plane, which weakly interacts with polarization in neighboring layers, appears in potassium niobate layers with a thickness of one unit cell in the superlattices. The possibility of using of such ferroelectric superlattices as a medium for three-dimensional information recording is shown.     

Polarization 1/<Emphasis Type="Italic">f</Emphasis> noise in ferroelectric PbFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> with a diffused phase transition

The dispersion of complex permittivity in ferroelectric PbFe_1/2Nb_1/2O₃ with diffused phase transition at frequencies of 20 Hz-10 kHz was studied. Dielectric measurements revealed that polarization 1/f noise occurs in the region of coexistence of polar and nonpolar phases. It was found that the temperature dependence of parameter α exhibits a singularity when the temperature of the local order parameter rises. Our analysis of the experimental data enables us to say that the source of the noise is the fluctuations of spontaneous polarization.     

Retardation of polarization in mixed K<Subscript>0.88</Subscript>(NH<Subscript>4</Subscript>)<Subscript>0.12</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystal

The time dependences of polarization of K_0.88(NH₄)_0.12H₂PO₄ mixed crystal have been studied within the temperature range of 74–100 K. Two mechanisms of polarization relaxation were found. The first mechanism is caused by domain walls lateral motion and their interaction with point lattice defects. The second one supposedly is due to polar regions infiltration through the regions of frustrated paraelectric phase.     

The evolution of electronic configuration and magnetic characterization of Fe<Subscript>9</Subscript>Ni<Subscript>1</Subscript>, Fe<Subscript>8</Subscript>Ni<Subscript>2</Subscript> alloy in theoretical calculation

To analyze the origin of the magnetic enhancement of Fe-Ni alloy, the electronicconfigurations and magnetic properties were investigated using density functional theorybased on the first-principle. The supercell (5 × 1 × 1) of Fe,Fe₉Ni₁ and Fe₈Ni₂ were constructed. Thedefect formation energy, band structure, density of states and electron density differencewere calculated. The results showed that Ni doping changed the electronic configuration ofFe atoms, resulting in the enhancement of spin polarization of Fe and the larger Bohrmagnetic moment in Fe-Ni alloys (Fe₉Ni₁). The results showed thatthe charge transfer and the atomic spacing between Fe atoms and the dopant Ni atoms playedan important role in determination of magnetic moment. The value of Fe supercell(5 × 1 × 1), Fe₉Ni₁ and Fe₈Ni₂ were 23.14,23.34 and 22.61μ _B, respectively.     

Far-infrared optical excitations in multiferroic TbMnO<Subscript>3</Subscript>

We provide a detailed study of the reflectivity of multiferroic TbMnO₃ for wave numbers from 40 cm^-1 to 1000 cm^-1 and temperatures 5 K < T < 300 K. Excitations are studied for polarization directions E || a, the polarization where electromagnons are observed, and for E || c, the direction of the spontaneous polarization in this material. The temperature dependencies of eigenfrequencies, damping constants and polar strengths of all modes are studied and analyzed. For E || a and below the spiral ordering temperature of about 27 K we observe a transfer of optical weight from phonon excitations to electromagnons, which mainly involves low-frequency phonons. For E || c an unusual increase of the total polar strength and hence of the dielectric constant is observed indicating significant transfer of dynamic charge probably within manganese-oxygen bonds on decreasing temperatures.     

Electro-optical polarization mode dispersion in PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript> single crystals

The polarization mode dispersion induced by an external ac electric field in PbMg_1/3Nb_2/3O₃ single crystals is studied experimentally at frequencies of 10⁴–10⁷ Hz. It is established that the discovered electro-optical polarization mode dispersion (EPMD) displays anomalous properties related to the orientation of the induced dipole moments in microregions of the crystal. Analytical expressions describing the peculiarities of the physical properties of relaxors giving rise to the EPMD are obtained.     

Acoustic emission and thermal expansion of Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> and Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> crystals

The temperature dependence of the elongation per unit length for Pb(Mg_1/3Nb_2/3)O₃ crystals unannealed after growth and mechanical treatment is investigated in the course of thermocycling. It is revealed that this dependence deviates from linear behavior at temperatures below 350°C. The observed deviation is characteristic of relaxors, is very small in the first cycle, increases with increasing number n of thermocycles, and reaches saturation at n≥3. In the first cycle, a narrow maximum of the acoustic emission activity is observed in the vicinity of 350°C. In the course of thermocycling, the intensity of this maximum decreases and becomes zero at n>3. For (1?x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ crystals, the dependence of the temperature of this acoustic emission maximum on x exhibits a minimum. It is assumed that the phenomena observed are associated with the phase strain hardening due to local phase transitions occurring in compositionally ordered and polar nanoregions.     

Influence of cooling mode on the electrochemical properties of Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> anode materials for lithium-ion batteries

Li₄Ti₅O₁₂ (LTO) was synthesized with two different cooling methods by solid-state method, namely fast cooling and air cooling. The samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), galvanostatic charge–discharge test, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), respectively. XRD revealed that the basic LTO structure was not changed. FESEM images showed that fast cooling effectively reduced the particle sizes and the agglomeration of particles. Galvanostatic charge–discharge test showed that the air cooling sample exhibited a mediocre performance, having an initial discharge capacity of 136.3mAh?·?g^?1 at 0.5 C; however, the fast cooling sample demonstrated noticeable improvement in both of its discharge capacity and rate capability, with a high initial capacity value of 142.7 mAh?·?g^?1 at 0.5 C. CV measurements also revealed that fast cooling enhanced the reversibility of the LTO. EIS confirmed that fast cooling resulted in lower electrochemical polarization and a higher lithium-ion diffusion coefficient. Therefore, fast cooling have a great impact on discharge capacity, rate capability, and cycling performance of LTO anode materials for lithium-ion batteries.     

Structural and electrical properties of 0.7Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>–0.3PbTiO<Subscript>3</Subscript> thin films grown on Ir/MgO buffered Si(100) substrates

Thin films of 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) have been grown on Ir/MgO buffered Si(100) substrates at different substrate temperatures by pulsed laser deposition. Crystalline phases as well as preferred orientations in the PMN-PT films were investigated by X-ray diffraction analysis (XRD). The microstructure, dielectric and ferroelectric properties of PMN-PT film prepared at 650 °C were studied. The results show that the film prepared at 650 °C exhibits pure perovskite phase and single c-axis orientation. The dielectric constant and dissipation factor of the single c-axis oriented film are 1000 and 0.04 at a frequency of 1 kHz, while the remnant polarization and coercive field are about 13.0 μC/cm² and 100 kV/cm under an electric field of 480 kV/cm, respectively. PACS 81.15.Fg; 77.80.-e; 77. 22.Ej; 77.55.+f; 85.50.Gk     

Colossal magnetodielectric effect in SmFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The colossal (more than threefold) decrease in the dielectric constant ɛ in the easy-plane SmFe₃(BO₃)₄ ferroborate in a magnetic field of ∼5 kOe applied in the basal ab plane of the crystal has been found. A close relation of this effect to anomalies in the field dependence of the electric polarization has been established. It has been shown that this magnetodielectric effect is due to the contribution to ɛ from the electric susceptibility, which is related to the rotation of spins in the ab plane, arises in the region of the antiferromagnetic ordering T < T _N = 33 K, and is suppressed by the magnetic field. A theoretical model describing the main features of the behavior of ɛ and electric polarization in the magnetic field has been proposed, taking into account the additional anisotropy in the basal plane induced by the magnetoelastic stresses.     

Study of ferroelectric phase transitions in disordered PbSc<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> and PbSc<Subscript>1/2</Subscript>Ta<Subscript>1/2</Subscript>O<Subscript>3</Subscript> solid solutions

Ferroelectric phase transitions and thermodynamic properties of disordered PbSc_1/2Ta_1/2O₃ and PbSc_1/2Nb_1/2O₃ solid solutions have been investigated by the Monte Carlo method. The parameters of the effective Hamiltonian were calculated within the generalized Gordon-Kim model. The obtained values of the phase-transition temperatures and spontaneous polarization are in satisfactory agreement with the experimental data.     

Electronic structure and half-metallic property of Si<Subscript>3</Subscript>CaC<Subscript>4</Subscript>

The electronic structures and magnetic properties of Si₃CaC₄ in zinc-blende phase has been studied by employing the first-principles method based on density functional theory (DFT). The calculations predict stable ferromagnetic ground state in Si₃CaC₄, resulting from calcium substitution for silicon. The calculated total magnetic moment is 2.00 μ _B per supercell, which mainly arises from the Ca and neighboring C atoms. Band structures and density of states studies show half-metallic (HM) ferromagnetic property for Si₃CaC₄. The ferromagnetic coupling is generally observed between the Ca and C atoms. The ferromagnetism of Si₃CaC₄ can be explained by the hole-mediated double exchange mechanism. The sensitivity of half-metallicity of Si₃CaC₄ as a function of lattice constant is also discussed, and the half-metallicity can be kept in a wider lattice constant range.     

Magnetoelectric effect in ytterbium aluminum borate YbAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The anisotropic magnetoelectric properties of an ytterbium aluminum borate YbAl (BO single crystal having noncentrosymmetric crystal structure (space group R32) are studied, including the orientational, field, and temperature dependences of the polarization in magnetic fields up to 5 T in the temperature range of 2–300 K. It has been shown experimentally for the first time that the symmetry of the observed magnetoelectric effects exactly corresponds to the trigonal structure of the crystal and is characterized by two quadratic magnetoelectric constants. The polarization in the basal plane P _{a, b} is a quadratic function of the field at low fields and reaches 250–300 μC/m² in a field of 5 T at a temperature of 2 K, almost an order of magnitude exceeding the previously reported values. A theoretical model based on the spin Hamiltonian of the ground Kramers doublet of Yb³⁺ ions in the crystal field is proposed including magnetoelectric interactions allowed by the symmetry. This model makes it possible to quantitatively describe all observed magnetic and magnetoelectric properties of YbAl₃(BO₃)₄.     

Magnetoelectric phenomena in manganites R<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript>(R = Pr,Nd) with charge ordering suppressed by a magnetic field

A change in electric polarization (up to 300 μC/m²) upon magnetic-field suppression of a charge-ordered antiferromagnetic state upon a transition to the ferromagnetic conducting phase (H _cr ～ 65–80 kOe at 4.2 K) is discovered in Pr_0.6Ca_0.4MnO₃ and Nd_0.6Ca_0.4MnO₃ single crystals. The transition is also accompanied by a jump in magnetization and magnetostriction. The dependence of the induced polarization sign on the polarity of the electric field in which the sample was preliminarily cooled indicates the existence of spontaneous electric polarization. The effect is the strongest in Nd_0.6Ca_0.4MnO₃ and is weaker by a factor of 5–10 in Pr_0.6Ca_0.4MnO₃, for which the tolerance factor is higher. The observed effect may be associated with recently predicted noncentrosymmetric structures in doped manganites with x ～ 0.5 (see D.V. Efremov, J. van den Brink, and D.I. Khomskii, Nature Materials 3, 853 (2004)), in which e _g electrons are not localized upon charge and orbital ordering at one manganese ion, but are distributed among neighboring ions, thus forming an ordered polar dimer structure.     

Dielectric properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)(Nb<Subscript>0.93</Subscript>Sb<Subscript>0.07</Subscript>)O<Subscript>3</Subscript> ferroelectric ceramics modified with BaTiO<Subscript>3</Subscript>

Processes of the polarization and repolarization of ferroelectric ceramics based on potassium sodium niobate in the region of infralow frequencies are discussed. The effect aging and subsequent annealing in a strong alternating electric field have on the nonlinearity of the dielectric response of a sample at different temperatures is determined.     

Electronic structure,lattice dynamics,and magnetoelectric properties of double perovskite La<Subscript>2</Subscript>CuTiO<Subscript>6</Subscript>

The results of ab initio calculations of the electronic structure, vibrational properties, and the magnetoelectric effect in the La₂CuTiO₆ crystal with double perovskite structure are presented. The lattice dynamics calculation shows the presence of unstable modes in the phonon spectrum of the high-symmetry cubic phase with space group \(Fm\overline 3 m\). Condensation of two most unstable modes belonging to the center and the boundary point X of the Brillouin zone leads to the formation of a nonpolar stable phase with space group P2₁/n. The calculation taking into account spin polarization shows that the magnetic ground state is E*-type antiferromagnetic with doubled magnetic cell and with the two spin-up and two spin-down configuration of magnetic moments of copper ions along the [010] crystallographic direction. Such ordering of magnetic moments leads to polar space group and polarization formation. The polarization magnitude is estimated as 71 μC/m².     

Methods for spin injection managing in inGaAs/GaAs/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/CoPt spin light-emitting diodes

Spin injection in CoPt/Al₂O₃/(Al)GaAs spin light-emitting diodes (SLEDs) was studied. The oscillations of the degree of circular polarization upon variation of a distance between the active region of the SLED and a CoPt ferromagnetic injector were observed. The oscillations depend neither on a SLED material (GaAs or AlGaAs), nor on the type of injected spin-polarized carriers (electrons and holes) and are related to the action of a perpendicular magnetic field on the injected spin-polarized carriers that causes their precession. During the transfer to the active region through a distance of 50–100 nm from the injector, a z–component of a spin changes a phase that is detected experimentally as the change in sign of the degree of circular polarization of luminescence. Conceivably, a source of the internal magnetic field leading to spin precession is the magnetic field of the nonuniformly magnetized CoPt contact.     

Specificity of magnetoelectric effects in a new GdMnO<Subscript>3</Subscript> magnetic ferroelectric

A complex study of the magnetic, electric, magnetoelectric, and magnetoelastic properties of GdMnO₃ single crystals has been performed in the low-temperature region in strong pulsed magnetic fields up to 200 kOe. An anomaly of the dielectric constant along the a axis of a crystal has been found at 20 K, where a transition from an incommensurate modulated phase to a canted antiferromagnetic phase, as well as electric polarization along the a and b axes of the crystal induced by the magnetic field H‖ b (H_cr ～ 40 kOe), is observed. Upon cooling the crystal in an electric field, the magnetic-field-induced electric polarization changes its sign depending on the sign of the electric field. The occurrence of the electric polarization is accompanied by anisotropic magnetostriction, which points to a correlation between the magnetoelectric and magnetoelastic properties. Based on these results, it has been stated that GdMnO₃ belongs to a new family of magnetoelectric materials with the perovskite structure.     

Magnetic and electric properties of Eu<Subscript>0.62</Subscript>Bi<Subscript>0.38</Subscript>MnO<Subscript>3</Subscript> and Eu<Subscript>0.53</Subscript>Bi<Subscript>0.32</Subscript>Sr<Subscript>0.15</Subscript>MnO<Subscript>3</Subscript> crystals

Single crystals of Eu_0.62Bi_0.38MnO₃ and Eu_0.53Bi_0.32Sr_0.15MnO₃ solid solutions crystallizing in an orthorhombically distorted perovskite structure were prepared. At temperatures above 120 K, Eu_0.62Bi_0.38MnO₃ exhibits the properties of structural glass while remaining a dielectric at all temperatures. There is no long-range magnetic order in this compound. Eu_0.53Bi_0.32Sr_0.15MnO₃ behaves as a semiconductor above 120 K and exhibits a jump in conductivity at T = 175 K associated with a metal-insulator transition occurring within limited regions of the crystal. In these regions, there appears a ferromagnetic moment (due to double exchange mediated by charge carriers) and local electric polarization.     

Magnetic properties of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> surface

The magnetic properties of the magnetite Fe₃O₄(110) surface have been studied by spin resolved Auger electron spectroscopy (SRAES). Experimental spin resolved Auger spectra are presented. The results of calculation of Auger lines polarization carried out on the basis of electronic state density are presented. Problems related to magnetic moments of bivalent (Fe²⁺) and trivalent (Fe³⁺) ions on the Fe₃O₄(110) surface are discussed. It is established that the deposition of a thin bismuth film on the surface results in significant growth of polarization of iron Auger peaks, which is due to additional spin-orbit scattering of electrons by bismuth atoms.