Specific features of localization of magnetic TM polaritons in the ferroelectric-nongyrotropic-magnet structure

The conditions are determined under which virtual slot magnetic TM polaritons are formed as a result of the quadratic magnetooptic interaction near a ferroelectric layer embedded in an easy-axis antiferromagnet.     

Topology and symmetry analysis of the Sr2VO(VO4)2 family exhibiting magnetic properties and prediction of structures with different orderings of vanadyl bonds

A topology and symmetry analysis of structures of the Sr-vanadyl-vanadate family Sr₂VO(VO₄)₂ with a quasi-one-dimensional spin system, which is responsible for magnetic properties, is performed. Based on the known structure with space group I2/a, structures belonging to its subgroups with ordered vanadyl bonds in octahedra containing the magnetic V⁺⁴ ion are derived. Structures with a polar orientation of vanadyl bonds in chains of octahedra correspond to space groups P2₁/c, P2/c, and Pc, and structures with a nonpolar orientation of vanadyl bonds correspond to space groups P2/a, P2, and P2/n. Variants of disordering of vanadyl bonds over two directions, namely, along the b and a axes, are considered for all structures. The predicted structures can be used for the diagnostics of phases obtained in this family and as initial information necessary for studies of magnetic properties of crystals.     

I–V characteristic and magnetoresistance of homogeneous materials

A formula providing the current across a slab of homogeneous conducting material under bias and subjected to a perpendicular magnetic field, is presented in both the high and low temperature regime. The current is expressed in terms of the applied voltage, V, the magnetic field, B, temperature, T, mobility and resistivity of the material, μ and ρ_s at zero magnetic field and at temperature T. Furthermore, the sample dimensions enter the expression for the current. The formula in question enables obtaining the magnetoresistance of the slab in terms of the above parameters in the cases under constant bias and under constant current. Several experimental results are compared with theoretical predictions, e.g., switch from positive to negative magnetoresistance. Furthermore, it is shown that there exists a critical value for a certain dimensionless collective parameter, above which for small magnetic fields the magnetoresistance becomes negative, while for values below is positive. The quasi-linear magnetoresistance, observed in experiments at high magnetic fields, is also adequately reproduced.     

Magnetoplastic effect: Basic properties and physical mechanisms

This paper presents a review of the main results of investigations into the magnetoplastic effect, which manifests itself in motion of dislocations in crystals exposed to magnetic fields. The dependences of the mean free path of dislocations on the induction and direction of the magnetic field, the magnetic treatment time, the temperature, and the type and concentration of impurities are studied for NaCl, LiF, CsI, Zn, Al, and InSb crystals. The threshold magnetic field B _c below which the effect is absent, the saturation field B₀ above which the mean free paths of dislocations remain constant with an increase in the magnetic induction B, and the critical frequency v _c of rotation of a sample in the magnetic field above which the effect disappears are examined. The quantities B _c , B₀, and v _c are investigated as functions of the basic physical parameters. It is found that the magnetoplastic effect is highly sensitive to X-ray radiation at low doses and to simultaneous action of an electric field or mechanical loading. The hardening of NaCl(Pb) crystals in the magnetic field is revealed. The theoretical interpretation is proposed for all the findings and dependences observed.     

Role of the poly-dispersity and the dipolar interaction in magnetic nanoparticle systems: Monte Carlo study

In this paper, roles of the poly-dispersity and the dipolar interaction in frozen ferrofluid are studied by Monte Carlo method. A sample containing the uniaxial anisotropy and the random orientation is used to investigate. The temperature dependence of the coercivity is calculated to consider the magnetic phase transition under the influence of the dipolar interaction. We show that in the poly-dispersity and interacting sample, the temperature dependence of coercive field does not follow the classical expression, H_C/H_A = 0.48[1 − (T/T_B)^½]. We find that the transition temperature, which separates the anti-ferromagnetic and ferromagnetic states of strongly interacting sample, is not unique and it strongly depends on the variation of concentration. We also discuss about the concentration dependence of the coercivity at the different size distributions. At the finite temperature, the curve expresses a cusp which is due to the competition between the blocking and super-paramagnetic state at the low concentration. Therefore, we can see that the poly-dispersity also contributes to the complexity of magnetic phase of frozen ferrofluids.     

Observation of chiral ordering of moments in the magnetic mesostucture of the (Pd0.984Fe0.016)0.95Mn0.05 alloy by means of polarized neutrons

Investigation of the magnetic structure of the diluted (Pd_0.984Fe_0.016)_0.95Mn_0.05 alloy at meso-and nanoscale levels by means of two techniques of small-angle polarized-neutron scattering (within a direct beam and beyond it) has been performed in the temperature range 10<T<60 K. The dependences of the neutron beam depolarization, the polarization rotation angle, and the polarization-dependent magnetic scattering cross section on a weak (0<H<40 A/cm) external magnetic field have been studied. A simple model of neutron beam depolarization by a sample with uniaxial magnetic anisotropy was used to analyze the results obtained. The experiment on polarized-neutron scattering in the so-called direct geometry showed the existence of a polarization-dependent scattering cross section. This scattering has left-right asymmetry and depends on temperature. Comparison of these results with the depolarization data leads to a conclusion about the existence of static chiral fluctuations in large-scale inhomogeneities.     

Superparamagnetism and ferrimagnetism of the small particles of magnetite in a silicate matrix

The magnetic behaviour of a basalt glass and glass-ceramic was studied by magnetization measurements between 4 and 800 K as a function of a wide range of magnetic fields (H) between 0 and 60 kG. For the as-annealed glass it was found that nearly all the iron ions behaved as paramagnetic ions. In the samples heat-treated at 700 and 900°C, the magnetization (M) values showed three magnetic components: paramagnetic Fe²⁺ ions, magnetite in a superparamagnetic and in a ferrimagnetic state. This confirmed our previous Mössbauer results. The superparamagnetic behaviour of the fine particles of magnetite was interpreted by Langevin's theory. From the M (H) and M(T) values we evaluated the percentage of each component as a function of temperature, the magnetization values in the saturated states, the mean particle diameter and the particle size distributions.     

Magnetic properties of TiCx/C nanocomposites

The magnetic properties of nanocrystalline titanium carbide dispersed in a carbon matrix (TiC_x/C) prepared by the non-hydrolytic sol–gel process have been studied by dc magnetization measurements. The superconducting phase of titanium carbide has been observed at low temperatures with the onset of the superconducting transition temperature T_c at about 3.5 K, superimposed on a ferromagnetic component. At T > T_c the magnetic response of TiC_x/C is determined by the interplay of the ferromagnetic contribution with the paramagnetic/diamagnetic signal of the metallic system and the contribution of exchange coupled paramagnetic ions. Moreover, significant differences are observed in the magnetic response for samples of the same preparation batch, indicative of the magnetic/electronic inhomogeneity of nanocrystalline titanium carbide which is important for its practical applications.     

Influence of the number of periods on the magnetization reversal process of antiferromagnetically coupled amorphous CoxSi1−x/Si multilayers

The advantage of using amorphous materials for preparing complex magnetic structures, like magnetic/non-magnetic multilayers, is illustrated for the case of the study of the Co–Si system. In particular, the antiferromagnetic AF coupling recently reported for amorphous Co–Si alloys separated by Si spacers and the associated magnetization reversal processes have been studied in (Co_0.75Si_0.25/Si)_n multilayers with different number of periods n. It has been found that the magnetic field corresponding to the onset of the reversal process increases with the number of periods in a linear way. For n > 2, the magnetization stabilizes not only in the AF state but also in other intermediate configurations between both saturated states. At the same time, as n increases, all the processes of formation and breaking of these states get smoothed.     

Distribution coefficient of boron in Si crystal ingots grown in cusp-magnetic Czochralski process

Silicon single crystals are grown by the Czochralski method with various growing conditions. Effective segregation coefficient of boron is found to depend on the magnetic field in cusp-magnetic Cz method. Effects of zero-Gauss plane (ZGP), ZGP shape and magnetic intensity (MI) on the dopant concentration and its distribution in the crystal are experimentally investigated. The shape of ZGP is not only flat but also parabolic due to the magnetic ratio (MR), which is the ratio of the lower to upper electric-current densities in the configurations of the cusp-magnetic field. Equilibrium distribution coefficient of boron calculated by BPS model is 0.698. With the crystal rotation (w) of 16 rpm and the crucible rotation of ?0.5 rpm, the effective distribution coefficient (k_e) is 0.728 in zero magnetic intensity and increases up to 0.8093 in the parabolic ZGP shape. Although the magnetic strength near the crystal–melt interface decreases with increasing MR, it increases in the bulk melt, and hence k_e increases. Flow stability in the bulk melt influences k_e. At the magnetic field and growing conditions, k_e increases with increasing initial charge size of the silicon melt. There is no significant influence of ZGP on the radial distribution of the boron concentration. Simulation results of melt flow in the presence of a parabolic ZGP are outlined, and the segregation results in the experiments are compared with published experimental data.     

Magnetic interactions in frustrated Mn3Fe4(VO4)6

Magnetic interactions in a site-disordered multicomponent vanadate Mn₃Fe₄(VO₄)₆ are studied using DC magnetization and multifrequency Electron Paramagnetic Resonance (EPR). The static magnetic susceptibility χ shows antiferromagnetic interactions between Fe³⁺ and Mn²⁺ spins with a Curie–Weiss temperature Θ = ?165(5) K. EPR measurements indicate a strong dependence of χ on the frequency and temperature. The EPR spectra due to iron and manganese ions are observed in the X-band. It is mostly manganese ions that are observed at 80 GHz while two kinds of magnetic centers are identified at frequencies above 160 GHz. The observed shifts of the resonance lines for Fe³⁺ ions at low frequencies differ from those at high frequencies. The observed features may be due to different magnetic sublattices which modify the magnetic ground state, while competing magnetic interactions may lead to magnetic frustration. It appears that the very high magnetic fields employed in our high-frequency EPR measurements may affect the spin-flop transitions anticipated below Neel temperature T_N.     

Evidence of hopping of charge carriers in the clustered state of manganites

The electrical resistivity and magnetization measurements as a function of both temperature and magnetic field provide clear evidence for three temperature intervals in which the magnetic and transport properties are altered in the La_0.55Y_0.15Ca_0.3MnO₃ compound. The intermediated-T regime, called clustered state, involves the presence of both ferromagnetic and paramagnetic phases and is shown to be closely related to the colossal magnetoresistivity effect. An extra hopping conducting process is proposed to exist in this clustered state. We use both the Abeles’ model and microstructural parameters to estimate the hopping activation energy between ferromagnetic clusters. The size (density) temperature dependence and the shape of the ferromagnetic metallic clusters are revealed to play a major role in the clustered state of this manganite.     

Magnetic, magnetocaloric, and lattice properties of the La(Fe x Si1 − x )13 ferromagnets

The magnetic and lattice properties of a sample of La(Fe_0.86Si_0.14)₁₃ ferromagnet have been measured. The influence that neutron irradiation has on the physical properties of this ferromagnet is studied. It is shown that the irradiation of this sample by a fluence of 3 × 10¹⁹ n/cm² increases the lattice constant a and the Curie temperature (T _C ) as the volume magnetostriction decreases. A model of ferromagnet is proposed which satisfactorily describes the dependence a(T) of the initial and irradiated samples and their magnetic properties. The temperature dependence of the change in entropy when switching the magnetic field on and off is calculated. It is established that the change in both the magnetic and lattice parts of the total entropy at the magnetic phase transition must be taken into account for La(Fe _x Si_{1 ? x} )₁₃ compounds.     

Complex formation and background impurity of Oxygen in the PbTe:Eu doped crystals grown from melt by the Bridgman method

The magnetic field dependence of magnetization M(B) at the temperature 1.72 K in magnetic fields up to 5 T and the temperature dependence of magnetic susceptibility (M_S) χ(T) in the temperature range 1.7–400 K of six PbTe:Eu samples with the concentration of Eu impurity of the order of 1×10¹⁹–1×10²⁰ cm^?3, prepared from the doped crystals grown from the melt by the Bridgman method, have been investigated. It is shown that the dependence of M(B) and χ(T) can be quantitatively explained by the contribution of the single centers of Eu ions, their pairs, and the matrix of the doped crystals using the same set of parameters for each sample. This is true provided we use in our analysis the values of the exchange integrals between Eu ions in EuO normalized with the lattice constant of PbTe, i.e., J₁/k_B=0.056 K for the ferromagnetic interaction of the NN (nearest neighbor) pairs and J₂/k_B=?0.13 K for the antiferromagnetic interaction of the NNN (next nearest neighbor) pairs, as well as different values of the M_S of crystal χ_matrix. It is revealed that the probability of the formation of complexes based on the magnetic impurity pairs is higher in the incipient section of a doped ingot, and it decreases towards the ingot end where the single centers of Eu ions become the only centers of the impurity. We conclude that the pairs of Eu²⁺ ions, which are formed during the growth of the PbTe:Eu ingots from melt by the Bridgman method, are the constituents of the complexes of the magnetic impurities with the background Oxygen impurities in the crystal matrix of the doped lead telluride. It is shown that the formation of the complexes leads to an increase of the M_S of crystal matrix χ_matrix and can even cause the change of its sign from minus to plus, i.e., it can convert the crystal matrix from the diamagnetic to paramagnetic state. The possible causes of this effect are analyzed.     

Scientific opportunities for the study of amorphous solids using pulsed neutron sources

The use of pulsed-neutron-scattering techniques for the investigation of amorphous solids is reviewed in the light of current problems in amorphous solid-state physics and chemistry. The broad areas in which neutron scattering can contribute are those of structure and dynamics, both atomic and magnetic. Possible applications for the new generation of pulsed neutron sources are discussed, and it is concluded that not only will these play a vital role in future studies but also that an important step forward will be made with the extension of measurements into hitherto inaccessible regions of ω-Q space.     

Interplay between the magnetic field and the dipolar interaction on a magnetic nanoparticle system: A Monte Carlo study

We have studied the influence of the applied magnetic field on the blocking temperature (T_B) of a fine magnetic particle system. By means of a Monte Carlo technique we have simulated zero field cooling (ZFC) curves under different applied fields, obtaining the respective T_B as a function of H. We have focused our study on the limit H → H_K (where H_K is the anisotropy field), since the results found in the literature usually lack a detailed study of this range. The simulations were done at different sample concentration of the nanoparticles, with the purpose of observing how the magnetic dipolar interaction affects the field dependence of T_B. The classical expression predicts T_B to disappear for H ? H_K, independently of the dipolar interaction strength. Our simulations show that at strong interacting conditions T_B exists even for fields H > H_K.     

Magnetic moments of Fe and Y in the FeBY glass forming system

The new Fe_71.2B₂₄Y_4.8 glassy alloy has been prepared in amorphous ribbon form via melt spinning and splat cooling. The high degree of sample homogeneity and structural quality of the ribbon is revealed by the observation of narrow linewidths in the spectra of the field derivatives of the absorbed microwave power in ferromagnetic resonance experiments. Combination of vibrating sample magnetometry and element-specific X-ray magnetic circular dichroism experiments at the L_3,2-edges of Y revealed an induced magnetic moment of 0.065 μ_B/atom for Y antiparallel oriented to an Fe magnetic moment of 1.83 μ_B/atom.     

New family of LnBaMn2O6 − γ manganites (Ln = Nd, Sm, and Gd)

A new family of manganites, LnBaMn₂O_{6 ? γ}(Ln = Nd, Sm, and Gd), is obtained by the method of topotactic reactions. In these compounds, rare-earth and barium ions are ordered, which results in a dramatic increase in the temperature of transition to the paramagnetic state. Thus, a SmBaMn₂O₆ compound with a disordered arrangement of Sm and Ba ions is a spin glass with the freezing temperature of magnetic moments T _f = 40 K, whereas in the state with an ordered Sm and Ba arrangement, the Curie temperature T _C, is ～ 280 K. Below T _C, a maximum in resistivity and magnetoresistance is observed. The possible models of magnetic ordering are discussed.     

Microscopic nature of the extremely high specific heat of rare earth intermetallic compounds at low temperatures and the possibility of its application in technical superconductivity

The presence of an unfilled f-electron shell in rare earth intermetallic compounds under conditions of strong electron correlation between localized and delocalized electrons is responsible for the formation of local magnetic moments. According to the data of neutron, synchrotron, and other investigations of a number of such systems, the interaction of these moments with the local crystalline environment, hybridization with conduction electrons, f-f correlations (i.e., both one-site and cooperative phenomena), and combinations of these main effects form the physical base for the reconstruction of the excitation spectrum of an f-electron system and appearance of pronounced specific features of thermodynamic characteristics. The range of characteristic temperatures of these anomalies is determined by the interaction energy, which generally corresponds to the range 1–100 K. For some intermetallic compounds, the additional component of the specific heat (of electron origin) may greatly (by two to three orders of magnitude) exceed the specific heat of conventional structural materials. This feature makes it possible to consider such systems as promising functional materials (a kind of thermodynamic dampers) capable of compensating for various thermal perturbations in low-temperature super-conducting magnetic systems.     

Structural,compositional and annealing effects on magnetic properties in R1−xCox(R = Er,Tb, Sm) amorphous thin film alloys

Amorphous alloys of the rare earths and transition metals series possess unique magnetic properties which are highly dependent on the alloy compositions and the microstructure. The magnetic properties have been investigated in various sputtered R_1−xCo_x amorphous thin film alloys presenting highly anisotropic magnetic systems. Samples have been annealed for different temperatures. The magnetic changes, at various composition and various temperatures, have been measured by vibrating sample magnetometer (VSM); the structural and compositional properties have been investigated utilizing Rutherford back scattering (RBS) and scanning electron microscopy (SEM) analysis. The most significant results are: (1) Curie temperatures T_c above room temperature and coercivity H_c are strongly dependent on the alloys composition; moreover, the shapes of hysteresis loops are affected and (2) field annealing (before temperature of crystallisation) makes the samples magnetically softer (reducing H_c) and reduce the internal stress induced during the deposition by inducing a well defined anisotropy.