Dependence Of Hc Research Articles

Investigations on microstructure, electrical and magnetic properties of Ni–Zn–Ga spinel ferrites

Two spinel ferrite samples Ni0.7Zn0.3GayFe2–yO4 (y = 0.5, 1.0) were prepared by both ceramic and citrate methods. X-ray diffraction patterns indicated a single phase of cubic structure for all the samples that have a space group Fd-3m. Rietveld analysis using MAUD software for all samples showed that the lattice parameter (a) decreases, while oxygen parameter (U) and strain (ɛ) increase by increasing the amount of Ga. A marked increase in the frequency bands in both the tetrahedral and octahedral sites was observed in the vibrational frequency bands of Fourier transform infrared spectra. Mossbauer effect (ME) spectra recorded at room temperature for the bulk sample (prepared by ceramic method) with lower concentrations of Ga3+ (y = 0.5) consist of two Zeeman sextets. ME spectra for the other bulk sample (y = 1.0) and the two samples in nanoscale (prepared by citrate method) can be fitted by one and two doublets due to the paramagnetic and superparamagnetic behaviour, respectively. Vibrating sample magnetometer measurements showed that the saturation magnetization (Ms) obtained from the hysteresis loop is decreased by increasing Ga3+ concentration for all the samples. The coercivity (Hc) is inversely proportional to the particle size (D) for bulk samples. Nevertheless, Hc of the nanoscale samples enhanced by increasing the particle size. The size dependence of Hc is an indication of the superparamagnetic characteristics supported by ME. The ac electrical conductivity (ln $$ \sigma $$ ), dielectric constant (e′) and dielectric loss tangent (tan δ) were studied at different frequencies and temperatures for the investigated samples. The obtained results showed that the conductivity for all samples increases with increasing temperature in a behaviour similar to that of the most semi-conductor materials.

General Regularities and Differences in the Behavior of the Dynamic Magnetization Switching of Ferrimagnetic (CoFe2O4) and Antiferromagnetic (NiO) Nanoparticles

In antiferromagnetic (AFM) nanoparticles, an additional ferromagnetic phase forms and leads to the appearance in AFM nanoparticles of a noncompensated magnetic moment and the magnetic properties typical of common FM nanoparticles. In this work, to reveal the regularities and differences of the dynamic magnetization switching in FM and AFM nanoparticles, the typical representatives of such materials are studied: CoFe2O4 and NiO nanoparticles with average sizes 6 and 8 nm, respectively. The high fields of the irreversible behavior of the magnetizations of these samples determine the necessity of using strong pulsed fields (amplitude to 130 kOe) to eliminate the effect of the partial hysteresis loop when studying the dynamic magnetic hysteresis. For both types of the samples, coercive force HC at the dynamic magnetization switching is markedly higher than HC at quasi-static conditions. HC increases as the pulse duration τP decreases and the maximum applied field H0 increases. The dependence of HC on field variation rate dH/dt = H0/2τP is a unambiguous function for CoFe2O4 nanoparticles, and it is precisely such a behavior is expected from a system of single-domain FM nanoparticles. At the same time, for AFM NiO nanoparticles, the coercive force is no longer an unambiguous function of dH/dt, and the value of applied field H0 influences more substantially. Such a difference in the behaviors of FM and AFM nanoparticles is caused by the interaction of the FM subsystem and the AFM “core” inside AFM nanoparticles. This circumstance should be taken into account when developing the theory of dynamic hysteresis of the AFM nanoparticles and also to take into account their practical application.

Tempering temperature control of medium carbon steel according to the parameters of the saturation magnetic hysteresis loop

The main magnetic parameter sensitive to the structure of steel is the coercive force (Hc). However, the coercive force of steels with a carbon content more than 0.3% appeared unsuited for control of the tempering temperature (Tt), hardness (HRC), and mechanical properties of hardened products made of such steels due to non-monotonous character of Hc dependence on the tempering temperature after their quenching The goal of the study is to develop a method for control of the structural changes of medium-carbon steels that occur upon heat treatment proceeding from the information parameters generated using the coercive force Hc and the ratio KS of their residual magnetization (Mr) to the magnetization of technical saturation (Ms) in a practically important temperature range. The advantage of the developed approach compared to those based on measuring the relaxation magnetic parameters of steels is that the parameters Hc and KS can be measured using standard methods with the minimal relative errors of measurement (2 and 1%, respectively). We consider three different combinations of the parameters Hc and KS – H0, Hc1, and Hc2 — and analyzed their dependence on the tempering temperature and hardness of medium carbon steels C30 and C45. The parameters H0, Hc1, and Hc2 monotonously change as the hardness of medium-carbon steels decreases. The correlation coefficient between the values of H0, Hc1 and Hc2 and HRC £hardness of steel 30 in the range of 32 HRC 41 is 0.959, 0.965 and 0.978, respectively. The results obtained makes it possible to abandon the complex and inaccurate measurements of the relaxation magnetic parameters of steels when developing the methods and devices for magnetic structuroscopy, and focus on enhancing of the accuracy of measuring£ Hc and Mr/Ms ratio of the material.

Magnetic Properties and Mössbauer Investigations of La0.67Sr0.33FexMn1−xO3 (x = 0 and 0.5) Perovskites

We report magnetic phase separation in pure La0.67Sr0.33FexMn1−xO3 (x = 0 and 0.5) ceramic samples. Rhombohedral crystal structures obtained with R3C space group show no significant changes in the lattice structures due to the close values of ionic radii of Fe3+ and Mn3+ ions. However, significant changes occur in the magnetic properties due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions because of the presence of Fe3+ substitution. The FM double exchange interactions (DE) (Mn3+–O–Mn4+) appears suppressed by AFM superexchange interactions (Fe3+–O–Mn4+, Fe3+–O–Fe3+, Mn4+–O–Mn4+). The saturation magnetization for x = 0 has the highest value of approximately 80 emu/g at 2 K, which dropped significantly to approximately 5 emu/g at 2 K for the x = 0.5 sample. The coercivity (Hc) for x = 0.5 increased from 161 Oe at 300 K to 786 Oe at 2 K compared with that of x = 0 which varied from 110 to 150 Oe. A value of Hc(0) = 1.5 ± 0.04 kOe was extrapolated from the model fit to the temperature dependence of Hc. The temperature-dependent 57Fe Mossbauer spectroscopy measurements show the presence of Fe in two different environments that have an AFM Fe3+–O–Mn4+ and Fe3+/2+–O–Fe3+/2+ coordination. The unequal fraction population of Fe ions at both AFM environments seems to increase the exchange bias effect.

Structural, physical and magnetic evaluations of Ce-Zn substituted SrCo2 W-type hexaferrites prepared via sol gel auto combustion route

SrCo2 W-type hexagonal nanoferrites with composition SrCo2-xZnxCeyFe16-yO27 (whereas x = 0, 0.2, 0.4, 0.6, 0.8, 1.0 and y = 0, 0.02, 0.04, 0.06, 0.08, 0.1) were synthesized by sol-gel auto combustion method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM) and Vibrating sample magnetometer (VSM) were used to find out the structural, vibrational, morphological and magnetic properties of SrCo2 W-type hexagonal ferrites. Physical properties such as bulk density (Dbulk), X-ray density (Dxrd) and porosity (%P) of the SrCo2 W type hexagonal ferrites samples were calculated. Bulk density of the samples decreased whereas X-ray density increased with the Ce-Zn substitution in the W-type ferrites. Single phase samples of all the nanoferrites are achieved at the sintering temperature of 1050 °C. Structural parameters such as lattice parameters ‘a’ and ‘c’ were also calculated. Lattice parameter ‘a’ is found to be increased whereas ‘c’ is found to be decreased with the substitutions. Force constants at tetrahedral and octahedral sites are also evaluated which confirm the phase and substitution in W-type hexagonal ferrite. FESEM images showed homogenous and well crystallized at higher substitution of Ce-Zn in SrCo2 W-type hexagonal ferrites. Detailed magnetic parameters such as magnetic remanence coercivity, saturation, Bohr magneton, initial permeability, magneto crystalline anisotropy constant (K) and magnetic field anisotropy (Ha) were measured. Magnetic remanence decreased whereas coercivity, magneto crystalline anisotropy and field anisotropy increased in the SrCo2 W-type hexagonal ferrites samples. The dependence of Hc and Ha on the SrCo2 W-type hexaferrites may be due to the variations in the K and Ms of the substituted samples. Therefore, the prepared SrCo2 W-type nanoferrites with appropriate amount of metal cations are suitable candidate for the microwave devices and high frequency applications.

Analysis of the Critical Magnetic Field and the Dielectric Susceptibility for the Smectic A- Smectic C* Transition in DOBAMBC

The temperature dependences of the critical magnetic field Hc and the dielectric susceptibility χ(at constant magnetic fields) are analyzed close to the smectic A- smectic C* transition in DOBAMBC using the experimental data according to a power-law formula. These analyses are performed below Tc in the smectic C* phase. Values of the critical exponent δ for the critical magnetic field are determined in a given temperature interval (Tc- T) with increasing and decreasing Hc. Also, values of the critical exponent γ for the dielectric susceptibility χ (dielectric constant) are determined in the two different temperature intervals (Tc- T) for constant magnetic fields ranging from 0 to 6 T. From our analysis of the temperature dependences of Hc and χ, we extract the values of the critical exponent Φ to describe the magnetic field dependence of the dielectric susceptibility close to the AC* transition in DOBAMBC.

In-Plane Anisotropy of Upper Critical Field and Flux-Flow Resistivity in Layered Organic Superconductor β′′-(ET)2SF5CH2CF2SO3

We report the in-plane anisotropy of the upper critical field (Hc2) and the flux-flow resistivity (FFR) for a layered organic superconductor β′′-(ET)2SF5CH2CF2SO3 with the incoherent nature of the interlayer transport. The in-plane angular dependence of Hc2 showed a fourfold oscillation with maxima in the directions around H || b and a axes. This result is compatible with a \(d_{x^{2} - y^{2}}\) order parameter. The determined nodal structure is consistent with theoretical predictions of superconductivity mediated by charge fluctuations. Moreover, the dependence of the FFR on in-plane field-orientation showed a fourfold symmetry with cusp-like minima in the directions around H || b and a axes, which is very similar to the FFR for κ-(ET)2Cu(NCS)2 with d-wave pairing symmetry. From these results, we claim that the vortex dynamics is strongly affected by the superconducting gap structure for highly two-dimensional superconductors with d-wave pairing symmetry.

N-Dependent variations of coercivity with temperature in [Co/Pt]n multilayers

In a series of [Co(5Å)/Pt(6Å)]n multilayers with perpendicular anisotropy, it has been observed that the coercivity HC shows an rapid increase as the temperature is decreased from 300 to 5K. Above 220K, HC is proportional to the repetition number n, that is to say, HC4>HC3>HC2>HC1 (HCn, n is the repetition number), while at 60K≤T≤210K, n dependence of HC is HC3>HC4>HC2>HC1. At 25K≤T≤55K, HC as a function of the repetition number n can be expressed as HC2>HC1>HC3>HC4, whereas below 20K, the single Co layer of n=1 shows the fastest increase, and the coercivity becomes the largest one, namely, HC1>HC2>HC3>HC4. This observed n-dependent variations of coercivity with temperature can be attributed to surface roughness and temperature dependence of the saturation magnetization in the Co/Pt multilayers.

Coercivity of anisotropic γ-Fe2O3 particles at high temperatures

The coercivity of five different samples of anisotropic γ-Fe2O3 particles is studied in the temperature range 0°–600°C. It is found that their relative coercive force hc = Hc(T)/Hc(0) is almost a linear function of the relative magnetization of the particles ms = Ms(T)/Ms(0), where Hc(0) and Ms(0) are the values of Hc and Ms of the particles at 0°C. It is experimentally found that hc = βms + α, where β = 1.103 ± 0.015 and α = −0.114 ± 0.009. This character of the dependence of hc on ms suggests that, at high temperatures, Hc of anisotropic γ-Fe2O3 particles can depend on both their shape anisotropy and other factors. It is assumed that, as the temperature increases, anisotropic γ-Fe2O3 particles in a zero magnetic field are divided into small structurally and magnetically unstable nanoclusters with magnetization spontaneously changing its direction. As a result, Hc disappears near the Curie temperature, although the saturation magnetization of the particles in a field of 1 T is still retained at this temperature.

Asymmetric magnetic dots: A way to control magnetic properties

We have used Monte Carlo simulations to investigate the magnetic properties of asymmetric dots as a function of their geometry. The asymmetry of round dots is produced by cutting off a fraction of the dot and is characterized by an asymmetry parameter α. This shape asymmetry has interesting effects on the coercivity (Hc), remanence (Mr), and barrier for vortex and C-state formation. The dependences of Hc and Mr are nonmonotonic as a function of α with a well defined minima in these parameters. The vortex enters the most asymmetric part and exits through the symmetric portion of the dot. With increasing α the vortex formation starts with a C-state which persists for longer fields and the barrier for vortex exit diminishes with increasing asymmetry, thus providing control over the magnetic chirality. This implies interesting, naively unexpected, magnetic behavior as a function of geometry and magnetic field.

TRANSPORT ANISOTROPY IN ErNi2B2C ALONG c-AXIS

The resistivities along c-axis ρc(H, T) of ErNi 2 B 2 C have been measured with H⊥ and H‖c-axis for 2 < T < 300 K and the superconducting upper critical field Hc2(T) curves of ErNi 2 B 2 C were constructed for each magnetic fields. Our Hc2(T) curves have been compared and discussed with those from ρab(H, T) measurements which explain the anisotropy and its temperature dependence of Hc2(T) are thought to arise from magnetic pair breaking and the anisotropic field dependence of Néel temperature TN originated from Er +3 sublattice.

Crystal Structure and Magnetic Properties of Sm2 Fe17Nδ Thin Films Deposited on Si (100) Substrates

This work focuses on the crystal structure and magnetic properties of the hard magnetic Sm2Fe17Nδ films prepared by dc magnetron sputtering and the subsequent nitriding process. The XRD, EDS, M-H and M-T data show that N enters the cell structure and the films with the single Th2Zn17 phase are obtained when the nitriding temperature varies from 300 to 400° C, thus the maximum value of the coercivity HC reaches 2561.7 Oe. However, the Sm2Fe17 phase decomposes to the SmN nonmagnetic phase and the α-Fe soft magnetic phase with further increasing nitriding temperature, which corresponds to the decreasing HC. Furthermore, the easy magnetization direction (EMD) is found to locate randomly in the film plane. This texture can not give an excellent MR/MS higher than the Stoner-Wohlfarth limitation (MR/MS = 0.5), which agrees well with the observed low MR/MS (0.58). It is suggested that the magnetization reversal process is dominated by the nucleation mechanism according to the initial magnetization curve and the dependence of HC on the field H.

Magnetic properties of Fe-rich Fe–V alloy films

Polycrystalline Fe100−xVx with x=0 to 14 at.% V films on a glass substrate were made by the dc magnetron sputtering method. The deposition temperature was 250 °C. The magnetic properties, i.e., saturation magnetization 4πMs, bulk-pinning field for a domain wall Hcw, coercive force Hc, initial permeability μi, maximum permeability μm, saturation magnetostriction λs, and electrical property, such as the electrical resistivity ρ of the Fe–V films, were measured. We find that 4πMs is a decreasing function of x, Hc, and/or Hcw has a minimum at x≈x0=8 at. % V, μi, μm, and/or μc has a maximum at x=x0, λs≈0 at x=x0, and ρ is an increasing function of x. These new magnetic results are consistent with earlier magnetostriction measurements [Jen and Chen, J. Magn. Magn. Mater. 204, 165 (1999)] and theoretical calculations [Ostanin, Staunton, and Razee, Phys. Rev. B 69, 064425 (2004)]. Moreover, all the magnetic features in the x dependence of Hc, Hcw, μi, μm, μc, and λs, respectively, can be explained within the framework of the bulk-pinning (or surface-pinning) and the end-pinning (or nucleation) models. Briefly, the pinning model analysis of the magnetic hysteresis data confirms the inference that near zero λs implies maximal permeabilities and smallest Hc and Hcw. Among all the Fe–V films, the Fe92V8 film has the optimal magnetic and electrical properties for application.

Magnetic field, temperature and frequency dependence of complex conductance of ultra thin YBa2Cu3O7-x films

We have studied the magnetic field, temperature and frequency dependencies of the complex sheet conductance, σ(ω,T), of 1–3 unit cell (UC) thick YBa2Cu3O7-x films sandwiched between semiconducting Pr0.6Y0.4Ba2Cu3O7-x layers. Experiments have been carried out at frequencies between 2 MHz -1 GHz, with one-spiral coil technique and at 250–500 GHz, by aid of a quasi optical spectrometer based on backward-wave oscillators. We have found a very large shift of the superconducting transition onset temperature and of the maximum Reσ(T) position to higher temperatures with increasing ω. The dependence of Hc2(T) is exponential close to Tc, with a break in a slope at the Berezinski-Kosterlitz-Thouless (BKT) temperature TBKT, due to the vortex-antivortex pair unbinding, and linear below TBKT because the Abrikosov vortex formation. We have compared our experimental results with the extended dynamic theory for the BKT transition and found that the research explored the dynamics of bound vortex-antivortex pairs with short separation lengths.

Critical magnetic field H c2 and electron scattering in MgB2

The correlation between the upper critical field Hc2 and the residual resistivity ρ of the MgB2 compound is studied in a wide range of ρ values. The slope −dHc2/dT of the temperature dependence of Hc2 near Tc is found to increase steadily with increasing residual resistivity to 100 μΩ cm. Over the range from 0 to 50 μΩ cm, the dependence of −dHc2/dT on ρ is fitted well by a linear function, which is typical of a single-band superconductor. The fit is performed using the electronic parameters of the two bands (π and σ) that form the Fermi surface. The field Hc2 is assumed to depend on the electronic parameters of the σ band only. Using this approximation, the following quantities are estimated: the relative contribution of σ electrons to the total conductivity along the boron planes (which turns out to be about 1/2), the ratio of the mean free paths of electrons in the σ and π bands lσ/lπ ≈ 1.5, and the ratios between some other parameters describing electron scattering.

Comparison of the scaling analysis of mixed-state magnetizationdata with direct measurements of the upper critical field forYBa2Cu3O7−x

By comparison of recent direct measurements of the temperature dependence of the upper critical fieldHc2 ofan YBa2Cu3O7−x high-Tc superconductor with the scaling analysis of magnetization data, collected in fields , we demonstrate that the temperature dependence of the Ginzburg–Landau parameterκ is negligible. Another conclusion is that the normalized temperature dependence ofHc2 is independent of the orientation of the magnetic field with respect to the crystallographicaxes of the sample. We also discuss the fact that isotropy of the temperature dependence ofHc2 straightforwardly follows from the Ginzburg–Landau theory ifκ does not depend on the temperature.

Rigorous solution of a Hubbard model extended by nearest‐neighbour Coulomb and exchange interaction on a triangle and tetrahedron

AbstractThe Hubbard model extended by both nearest‐neighbour (nn) Coulomb correlation and nearest‐neighbour Heisenberg exchange is solved rigorously for a triangle and tetrahedron. All eigenvalues and eigenvectors are given as functions of the model parameters in a closed analytical form. For fixed electron numbers we found a multitude of level crossings, both in the ground state and in the excited states in dependence on the various model parameters. By coupling an ensemble of clusters to an electron bath we get the cluster gas model or the cluster gas approximation, if an extended array of weak‐interacting clusters is considered. The grand‐canonical potential Ω (μ, T, h) and the electron occupation N (μ, T, h) of the related cluster gases were calculated for arbitrary values (attractive and repulsive) of the three interaction constants. For the cluster gases without the additional interactions we found various steps in N (μ, T = 0, h = 0) higher than one. The reason is the degeneration of ground states differing in their electron occupation by more than one electron. For the triangular cluster gas we have one such degeneration point. For the tetrahedral cluster gas two. As a consequence, we do not find areas with one electron in the μ‐U ground‐state phase diagram of the triangular cluster gas or with one, two and five electrons in the case of the tetrahedral cluster gas. The degeneration point of the triangular cluster gas can not be destroyed by an applied magnetic field. This holds also for the lower degeneration point of the tetrahedral cluster gas. Otherwise, the upper degeneration point breaks down at a critical magnetic field hc. The dependence of hc on U shows a maximum for strong on‐site correlation. The influence of nn‐exchange and nn‐Coulomb correlation on the ground‐state phase diagrams is calculated. Whereas antiferromagnetic nn‐exchange breaks the degeneration points of the tetrahedral cluster gas partially only, a repulsive nn‐Coulomb correlation lifts the underlying degeneracies completely. Otherwise both ferromagnetic nn‐exchange and attractive nn‐Coulomb interaction stabilise the degeneration points. The consequences of the cluster gas results for extended cluster arrays are discussed.

Influence of light waves on the thermoelectric power under large magnetic field in III-V, ternary and quaternary materials

The Hubbard model extended by both nearest-neighbour (nn) Coulomb correlation and nearest-neighbour Heisenberg exchange is solved rigorously for a triangle and tetrahedron. All eigenvalues and eigenvectors are given as functions of the model parameters in a closed analytical form. For fixed electron numbers we found a multitude of level crossings, both in the ground state and in the excited states in dependence on the various model parameters. By coupling an ensemble of clusters to an electron bath we get the cluster gas model or the cluster gas approximation, if an extended array of weak-interacting clusters is considered. The grand-canonical potential Ω (μ, T, h) and the electron occupation N (μ, T, h) of the related cluster gases were calculated for arbitrary values (attractive and repulsive) of the three interaction constants. For the cluster gases without the additional interactions we found various steps in N (μ, T = 0, h = 0) higher than one. The reason is the degeneration of ground states differing in their electron occupation by more than one electron. For the triangular cluster gas we have one such degeneration point. For the tetrahedral cluster gas two. As a consequence, we do not find areas with one electron in the μ-U ground-state phase diagram of the triangular cluster gas or with one, two and five electrons in the case of the tetrahedral cluster gas. The degeneration point of the triangular cluster gas can not be destroyed by an applied magnetic field. This holds also for the lower degeneration point of the tetrahedral cluster gas. Otherwise, the upper degeneration point breaks down at a critical magnetic field hc. The dependence of hc on U shows a maximum for strong on-site correlation. The influence of nn-exchange and nn-Coulomb correlation on the ground-state phase diagrams is calculated. Whereas antiferromagnetic nn-exchange breaks the degeneration points of the tetrahedral cluster gas partially only, a repulsive nn-Coulomb correlation lifts the underlying degeneracies completely. Otherwise both ferromagnetic nn-exchange and attractive nn-Coulomb interaction stabilise the degeneration points. The consequences of the cluster gas results for extended cluster arrays are discussed.

Microscopic theory of superconductivity in MgB2-type systems in a magnetic field: A neighborhood of H c2

We calculate the upper critical magnetic field H c2 in the framework of a microscopic superconductivity theory with two energy bands of different dimensions on the Fermi surface with the cavity topology typical of the compound MgB2 taken into account (an anisotropic system). We assume an external magnetic field parallel to the crystallographic z axis. We obtain analytic formulas in the low-temperature range (T/Tc ≪ 1) and also near the critical temperature ((T-Tc)/Tc ≪ 1). We compare the temperature dependence of Hc2 for a two-band anisotropic system with that of H c2 0 corresponding to a two-band isotropic system (with Fermi-surface cavities of the same topology). We determine the role of the band-structure anisotropy, the positive curvature of the upper critical field near the critical temperature, and the important role of the ratio v1/v2 of the velocities on the Fermi surface in determining Hc2. We also obtain the values of the parameters Δ1 and Δ2 along the line of the critical magnetic field.

Correlation between parameters of the Fermi surface and the upper critical magnetic field Hc2 of magnesium diboride

The temperature dependence of the upper critical magnetic field Hc2 of two-band MgB2 superconductor samples with various imperfection was studied. It was shown that the dependence of Hc2 on the residual resistivity can be described using theoretical concepts on the upper critical magnetic field of single-band superconductors. The parameters of the σ band rather than the entire Fermi surface consisting of two bands (π and σ) were used in the description.