Magnetization and Static Magnetic Susceptibility of Fine-Crystalline High-Temperature YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Superconductors Synthesized by the Sol–Gel Method

A comparative study of the magnetization and static magnetic susceptibility of high-temperature superconductors (HTSC) YBa₂Cu₃O_y synthesized by two variants of the sol–gel method with different average sizes of crystallites 〈 D〉 ranging 0.4–2 μm has been performed in constant magnetic fields (Н ≤ 6 kOe). It has been shown that the different annealing temperatures and times, at which their crystal structure is formed, change both the average sizes of crystallites 〈D〉 and the sizes of the structural homogeneity regions 〈l〉 and, at the same time, the magnetic field penetration depth (λ) and the coherence length (ξ). As a result, such parameters as 〈D〉 ~ λ and 〈l〉 ~ ξ become comparable, leading to a change in the physical characteristics of HTSCs. It has also been shown that the superconducting transition temperature T_c determined from the measurements of magnetic characteristics in constant magnetic fields remains within values optimal for superconductivity (T_c ≈ 92 K) in the case of an optimal number (y) of oxygen atoms, which determine the levels of charge doping for a given compound.     

The voigt-type magneto-kerr effect in cubic semiconductors having ellipsoidal constant energy surfaces

A plane-wave analysis is given for the Voigt orientation of an applied static magnetic field ?B in cubic semiconductors having ellipsoidal constant energy surfaces. The effect is shown to be highly anisotropic with respect to the orientation of ?B (magneto-anisotropy) as well as to the initial polarization direction of the incident wave with respect to ?B (polarization anisotropy). The character and extent of the magneto-anisotropy depends not only on the location of the energy extrema, but on the anisotropy of the energy surfaces and on the energy dependence and anisotropy of the scattering processes.Calculations are given for the cases of propagation along 〈100〉 and 〈110〉 crystallographic directions in n-type germanium. High-field effects and infrared (IR) behavior are considered; however, the low-field quadratic range for microwaves in high-loss material is emphasized. A useful approximation for the high-loss case is given. Particular attention is given to the potential diagnostic applications of the effect.Room-temperature measurements of the Voigt-type Kerr effect for the TE₁₁ circular-waveguide mode at 35 GHz in n-type Ge are presented and compared with plane-wave calculations for propagation along 〈100〉 and 〈110〉 directions. Good quantitative agreement is obtained when the plane-wave calculations are reduced by the same mode-reduction factor as is applicable for the Faraday orientation.     

Magnetic properties of GaAs/δ〈Mn〉/GaAs/In x Ga1 − x As/GaAs quantum wells

The field and temperature dependences of the magnetization of GaAs/δ〈Mn〉/GaAs/In _x Ga_{1 ? x} As/GaAs quantum wells with the δ〈Mn〉 layer separated from the well by a 3-nm GaAs spacer have been studied in the temperature range of 3–300 K in a magnetic field up to 6 T. An external magnetic-field-induced phase transition to a ferromagnetic state with a magnetization hysteresis loop shifted from a zero magnetic field has been found to occur at a temperature below 40 K. A theoretical model is proposed that implies the coexistence of ferromagnetically and antiferromagnetically ordered regions within the GaAs layers.     

Magnetic ordering above room temperature in the sigma-phase of Fe66V34

Magnetic properties of four sigma-phase Fe_100−xV_x samples with 34.4?x?55.1 were investigated by Mössbauer spectroscopy and magnetic measurements in the temperature interval 4.2-300 K. Four magnetic quantities, viz. hyperfine field, Curie temperature, magnetic moment and susceptibility, were determined. The sample containing 34.4 at% V was revealed to exhibit the largest values found up to now for the sigma-phase for average hyperfine field, 〈B〉=12.1 T, average magnetic moment per Fe atom, 〈μ〉=0.89 μ_B, and Curie temperature, T_C=315.3 K. The quantities were shown to be strongly correlated with each other. In particular, T_C is linearly correlated with 〈μ〉 with a slope of 406.5 K/μ_B, as well as 〈B〉 is so correlated with 〈μ〉, yielding 14.3 T/μ_B for the hyperfine coupling constant.     

The coupling effect between ferroelectric and frustrated antiferromagnetic ordering in hexagonal ferroelectromagnet

Effects of the magnetoelectric coupling between the frustrated antiferromagnetic and ferroelectric ordering in hexagonal ferroelectromagnet are investigated by the soft-mode theory and molecular-field approximation. Applying the Heisenberg model for frustrated triangular antiferromagnets with exchange anisotropy and Diffour model for ferroelectric interaction, we discuss thermodynamic properties of the hexagonal ferroelectromagnetic system, including mean magnetization 〈s_i〉, polarization p, magnetization susceptibility χ_m, and polarization susceptibility χ_p, in a possible coupling form related to a combination of electric polarization and spin correlation. It is found that polarization induced by magnetic coupling leads to an anomaly in χ_p and a cusp in χ_m at low-temperature, which is consistent qualitatively with experimental results in hexagonal ferroelectromagnet YMnO₃.     

Spin polarized field emission from Fe and Co-coated W tips

Spin polarized field emission from thin film Fe- and Co-coated W(001) and W(111) tips has been investigated. A transverse spin polarization component is detected in each case. For W(111) tips, the azimuthal orientation of polarization showed only a weak preference for the set of 〈1?10〉 and 〈112?〉 tip directions due to the competing influence of magnetocrystalline anisotropy and tip morphology on tip magnetization. On the contrary, the polarization direction for W(001) tips exhibited a strong preference for alignment with the transverse low-index crystallographic directions, i.e. the 〈110〉 for Co and 〈100〉 for Fe. Superparamagnetic fluctuations of the tip magnetization are evident in the polarization direction of emitted electrons when film coatings are very thin. At marginally larger thickness, long-term stability of the polarization magnitude and direction is observed at 300 K. A method for changing the stable spin polarization direction is also presented that exploits spontaneous flipping of the tip magnetization at elevated temperature.     

Acoustic NMR in spin-1 systems

Acoustic magnetic resonance, both pulsed and continuous has been discussed in terms of the Bloch-Wangsness-Redfield formulation of the magnetic resonance phenomenon. The quadrupolar mechanism has been taken for the spin-phonon coupling and the mutual interaction has been treated in the ‘effective field approximation’. The expressions for the power absorbed both for Δm=±1 and Δm=±2 have been obtained. It is found that from the measurements of the relaxation parameters for Δm=±1 and Δm=±2 it is possible to estimate the non-secular contributions to the line-width. The power absorbed in pulse excitation comes out to be small for short pulse (short compared to the relaxation parameter) and it reduces to the value obtained in continuous excitation for a long pulse. It is seen that for a given pulse-width the signal decreases with the increase of the relaxation parameter and this happens as temperature is lowered. The saturation of electromagnetic signal in presence of the acoustic excitation has also been studied. The analysis indicates that the relaxation parameter obtained from the plot of the relative signal (〈〉_ω/〈〉₀) vs the acoustic frequency ω, is always less than its true value which can be determined by observing the frequency dependence of the relative fractional signal defined as [(〈〉₀?〈〉_ω)/〈〉_ω]/[(〈〉₀?〈〉_2ω0)/〈〉_2ω0].     

〈S0〉Cs and 〈S0〉Rb relaxation in the presence of buffer gas and the spin exchange between Cs and Rb

〈S⁰〉_Rb and 〈S⁰〉_Cs relaxation is studied in the presence of buffer gas and spin exchange between Cs and Rb. Spin exchange collisions are found to equalize the relaxation rates of Rb and Cs polarization. This result is completely confirmed experimentally for the relaxation of the first diffusion mode.     

Possibility of utilizing the D-3He fuel cycle with 3He production in a spherical tokamak reactor

Possible operating regimes of a spherical tokamak reactor based on the D-³He fuel cycle with ³He production are considered. The parameters of the plasma and magnetic system are calculated for several versions corresponding to the high power efficiency (with a power gain factor in plasma of Q = 20) in a reactor with an aspect ratio of A = 1.5. According to calculations, for an axial magnetic field in vacuum of B ₀ = 2 T, a plasma radius of a = 3 m, an average 〈β〉 value of 0.53, and a plasma temperature of 〈T〉 = 48 keV, the reactor power can reach P _fus = 500 MW. In order to achieve a power of P _fus = 1500 MW in a reactor with a = 2 m, 〈β〉 = 0.36, and 〈T〉 = 40 keV, the magnetic field should be increased to B ₀ = 5 T.     

Structure and phase composition of thin a-C:H films modified by Ag and Ti

The structure and phase composition of thin a-C:H and a-C:H〈M〉 films (M = Ag, Ti, or Ag + Ti) have been studied by Raman and X-ray photoelectron spectroscopy. The a-C:H〈M〉 films were prepared by ion-plasma magnetron sputtering of a combined target of graphite and metal in an Ar–CH₄ gas mixture. The Raman spectra of these films indicate that their structure is amorphous. The a-C:H〈Ag + Ti〉 films have a more graphitized structure in comparison with pure a-C:H films and films containing only one metal. It is established that carbon in the a-C:H〈Ag + Ti〉 films is in the sp ², sp ³, and C=O states, which are characteristic of the a-C:H, a-C:H〈Ag〉, and a-C:H〈Ti〉 films. In addition, there are also ether (–C–O–C–) or epoxy (?C?O–) carbon groups in the a-C:H〈Ag + Ti〉 films. It has been revealed that silver atoms in the a-C:H〈Ag〉 and a-C:H〈Ag + Ti〉 films form no chemical bonds with carbon, oxygen, and titanium. Titanium in the a-C:H〈Ti〉 and a-C:H〈Ag + Ti〉 films exists in the form of titanium IV oxide (TiO₂).     

Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.     

Effect of mean free path on the strength of the Ruderman-Kittel-Kasuya-Yoshida interaction in a dilute (Ag0.6Au0.4)Mn alloy

From measurements of magnetic susceptibility from 1.2 to 200 K and magnetization (0–50 kG) from 1.2 to 4.2 K, the strength of the RKKY interaction between Mn spins in a Ag_0.6Au_0.4 + 810 ppm Mn alloy has been determined to be V₀ = (2.4±0.5) x 10^-37 erg cm³. It is shown that this value for V₀ is consistent with an RKKY interaction strength which has has been reduced by mean free path effects by a factor exp (— 〈 r〉/l), where 〈 r 〉 is the average separation between Mn spins and l is the electron mean free path in the alloy.     

Superparamagnetism and spin-glass-like behavior in (La1−xTbx)2/3Ca1/3MnO3 Perovskite

The magnetic and magnetotransport properties of a series of (La_1−xTb_x)_2/3Ca_1/3MnO₃ oxides have been investigated. Substitutions ranging from x=0.0 to 0.4 have been used. In the largest 〈r_A〉 region, only ferromagnetism is observed. As 〈r_A〉 decreases, there is a transition from ferromagnetic to spin-glass order. As 〈r_A〉 decreases further, the ferromagnetism will completely disappear and a direct transition from superparamagnetic to spin-glass order will occur on cooling.     

On the transfer of line radiation in random magnetic fields

An analysis is made of the influence of a magnetic field of constant magnitude but with random orientations, described by the spherical coordinates θ and φ, on the polarization transfer in spectral lines. The assumption that the statistical properties of the field are determined by the one- and two-point probability densities P₁(x, θ, φ) and P₂(x₁, θ₁, φ₁; x₂, θ₂, φ₂) where x is the coordinate along the ray, implies that the variations of θ and φ can be considered as being due to a Markovian process in x. The expectation values of the Stokes parameters 〈Iγ〉, γ = 1, …4, are calculated functions of x or the optical depth τ. The formalism includes the limiting cases of infinite and zero correlation length. Assuming an artificial triplet and simple conditions, the results suggest that isotropic magnetic fields of the type considered have no influence on the polarization characteristics during the transfer.     

Investigation on the magnetocaloric effect in DyNi2, DyAl2 and Tb1−nGdnAl2 (n=0, 0.4, 0.6) compounds

The magnetocaloric effect (MCE) in the DyNi₂, DyAl₂ and Tb_1−nGd_nAl₂ (n=0, 0.4, 0.6) was theoretically investigated in this work. The DyNi₂ and DyAl₂ compounds are described considering a model Hamiltonian which includes the crystalline electrical field anisotropy. The anisotropic MCE was calculated changing the magnetic field direction from 〈1 1 1〉 to 〈0 0 1〉 in DyNi₂ and from 〈1 0 0〉 to 〈0 1 1〉 in DyAl₂. The influence of the second- and first-order spin-reorientation phase transitions on the MCE that occurs in these systems is discussed. For the calculations of the MCE thermodynamic quantities in the Tb_1−nGd_nAl₂ systems we take into account a two sites magnetic model, and good agreement with the available experimental data was obtained.     

Properties of InGaAs/GaAs quantum wells with a δ〈Mn〉-doped layer in GaAs

A method of formation of two-dimensional structures containing a δ〈Mn〉-doped layer in GaAs and an In_xGa_1?x As quantum well (QW) separated by a GaAs spacer of thickness d = 4–6 nm is developed using laser evaporation of a metallic target during MOS hydride epitaxy. It is shown that, up to room temperature, these structures have ferromagnetic properties most likely caused by MnAs clusters. At low temperatures (T _m ～ 30 K), the anomalous Hall effect is revealed to occur. This effect is related to hole scattering by Mn ions in GaAs and to the magnetic exchange between these ions and QW holes, which determines the spin polarization of the holes. The behavior of the negative magnetoresistance of these structures at low temperatures indicates the key role of quantum interference effects.     

Anomalous shape of cyclotron resonance line in n-GaP in high magnetic fields

Cyclotron resonance in n-GaP has been observed at 119 μm in pulsed magnetic fields up to 410 kG. From the experiments with the magnetic field parallel to the 〈100〉, 〈110〉 and 〈111〉 axes, it is concluded that the transverse effective mass for electron is and that the anisotropy factor of the conduction band is K = 7.9^+3.2_?2.0. An anomalous shape of the absorption curve was found in the magnetic field directions parallel to the crystal axes 〈110〉 and 〈111〉.     

Neutron diffraction study of single crystal europium in an applied magnetic field

A single crystal of europium has been studied in applied magnetic fields up to 41.7 koe. At T_N = 90.5 ± 0.5°K a first order magnetic transition is observed. At 4.2°K the effect of an applied magnetic field in either a 〈100〉 or 〈110〉 direction is to stabilize a helix structure having τ along the field direction. This field stabilized structure remains even if the field is reduced to zero. The hysteresis associated with these transformations has been investigated and is discussed. No ferromagnetic components have been detected in fields up to 41.7 koe.     

Fluctuation interaction of Ising subsystems

A phase diagram of two Ising subsystems σ and s has been constructed on a Bethe lattice with a coordination number 4 (a simplified analog of a square lattice). In contrast to the known Ashkin-Teller model, the interaction between these two subsystems is of a purely fluctuational nature; i.e., it does not manifest itself in the ground state and nullifies the sums of the products of average spins 〈σ〉〈s〉 (interactions of this type are realized in lattice-type adsorbed systems with dipolelike intermolecular interactions and strong azimuthal angular dependence of the adsorption potential of symmetry C₄). Apart from conventional states, i.e., a high-temperature disordered state (〈σ〉=〈s〉=0) and a low-temperature ordered state (〈σ〉 and 〈s〉 =? 0), this system can also exist in a correlated state (〈σs〉 =? 0 at 〈σ〉=〈s〉=0). In the theory of orientational phase transitions, this state corresponds to a fundamentally different, intermediate (on the temperature axis) phase in which a preferred direction of long molecule axes arises in the absence of spontaneous polarization. The results of Monte Carlo simulation on a square lattice agree with the conclusions obtained on a Bethe lattice. The characteristics of the orientational phase transition in a 2 × 1 monolayer of CO molecules adsorbed on the NaCl(100) surface are discussed.     

Möβbauereffekt am61Ni

With the Möβbauer technique the hyperfine splittings of the 67.4 keV transition of⁶¹Ni in compounds and alloys have been studied. A magnetic momentΜ _61,4=+ (0,477 ±0.031)Μ _n of the first excited state has been determined. From isomer shifts an order of magnitude estimate of the change in radius ofδ〈r ²〉/〈r ²〉=?6· 10^?4 can be inferred. The electric quadrupole interaction of the first excited level has been observed in Ni-J-boracite. In addition the hyperfine fields at the nickel sites were determined in the alloy series Ni _x Fe_1-x (0≦x≦1). In one case an effect of polarization in a high external field is measured. The fields in some rare earth (RE) intermetallic compounds of the form RENi₂ were deduced from linewidth to be less than 15 kOe.