Magnetic Field Dependence of a Capacitance Temperature Sensor

The influence of a magnetic field on the capacitance of a glass-ceramic temperature sensor (Lake Shore CS-501GR) has been measured extensively at temperatures down to 0.09 K and magnetic fields up to 12 T. While the influence of a magnetic field is still negligible at 4 K, the sensor shows an increasing non-monotonic dependence on the magnetic field with falling temperature. The maximum relative change of capacitance with respect to the zero field value remains smaller than 5 · 10⁻⁴. However, with the given sensitivity of the sensor, this small magnetoeffect may not be ignored in the low temperature region.     

Temperature Dependence of Magnetic Parameters in FePt Nanoparticles

Here, particular focus is placed on the atomic alignment and the order–disorder phase transition of the FePt alloy with the aid of a model that can describe realistic phenomena. Here, we present a method to study the order–disorder phenomena of FePt alloys. We will discuss the increase in the coercivity of FePt nanoparticles by an increasing annealing temperature for annealed FePt nanoparticles. According to the experimental evidence, we will present a model for explaining spin-glass-like behavior of these particles during the annealing procedure. In the phase transition from disordered FCC to ordered FCT in FePt nano-particles can be treated by first-order phase transitions. So, the mean field approach can be used in order to model this kind of phase transition. In nanoparticles, which are synthetized by sol–gel methods, the short range parameter is predominant because in these kinds of preparations, the long range order takes a lot of time to happen. By using a short range parameter, we are able to use the mean field approach, which considers the diffusion of atoms to the near neighborhood sites. The effects of random exchange and random magnetocrystalline anisotropy are known as the main results of coercivity reduction in magnetic nanoparticles. These effects are strongly dependent to the annealing temperature of nanoparticles. During disorder–order transition, these two effects are max. As transition continues, these effects vanish and coercivity of nanoparticles increases. Here, we will add these two effects to the Hamiltonian of the FePt system. With increasing the annealing temperature, the fraction of the FePt nanoparticles with a FCC disordered phase vanishes, which leads to decrease in the amount of 〈j〉. During the annealing process, the $\frac{c}{a}$ ratio varies as a function of the annealing temperature. The relation between the $\frac{c}{a}$ ratio and the annealing temperature is derived. The derived $\frac{c}{a}$ ratio and coercivity formulas are compared with experimental results. The results are in a good agreement with experimental data.     

Temperature Dependence of the Magnetic Properties of Amorphous Fe-Me-Si-B Alloys

Alloying of amorphous Fe-Me-Si-B metallic compositions with nickel and molybdenum results in a decrease in the values of saturation magnetization in fields with a magnetic intensity of 0.72 MA/m. The same alloying additions decrease the temperature of formation of new magnetic Fe₃B-type phases, which are stable up to 873 ± 5 K.     

Retardation of the Magnetic Relaxation in High-Temperature Superconductors Near a Ferromagnet

The creep of a magnetic flux trapped in a bulk high-temperature superconductor has been studied. It has been found that the magnetic relaxation is retarded when the superconductor is placed near a ferromagnet. The value of the retardation effect depends on the sequence of magnetization and the approach of the superconductor to a ferromagnet. The magnetic relaxation is fully suppressed when a superconducting sample first is magnetized and then is brought close to a ferromagnet. An interpretation of this effect has been discussed. Being magnetized, a ferromagnet produces its own magnetic field. While penetrating into a disk sample through its planes, the ferromagnet field induces screening currents, which circulate oppositely to the current that arises upon trapping of the magnetic flux. As a result, the stability of the magnetic structure is sharply improved since opposite driving forces can act on different sections of the vortices.     

Toward the Temperature Dependence of the Young Modulus of Magnetic Materials

Journal of Engineering Physics and Thermophysics - Using the example of a ferromagnetic dielectric, an algorithm is proposed for calculating the Young modulus of elastically deformable solid bodies...     

Low Temperature and High Magnetic Field Dependence and Magnetic Properties of Nanocrystalline Cobalt Ferrite

The DC magnetic hysteresis loop measurements were carried out for temperatures varying from 5 to 300 K over a field range of ±10 T on nanocrystalline (～35 nm) cobalt ferrite samples (crystallized to \(Fd\bar {3} m\) space group with cubic symmetry) to validate the law of approach at low temperature for the nanocrystalline cobalt ferrite. A magnetocrystalline anisotropy constant and saturation magnetization have been obtained by analyzing the magnetization curve in saturation using the “law of approach (LA) to saturation.” The magnetocrystalline anisotropy constant is found to be almost constant in the temperature range of 5 to 150 K due to the freezing of spin at low temperature. Also, spin freezing leads to a decrease of coercivity with the increase in the temperature.     

A Change in the Debye Temperature of a Single-Component Substance upon Amorphization

Technical Physics Letters - A method for calculating the Debye temperature of a single-component substance in the amorphous state is proposed based on a nonlinear dependence of first coordination...     

Thermodynamically Consistent Calculation of the Debye Temperature Using Thermophysical Data

Equations are obtained which must be satisfied by the temperature dependence of the Debye temperature ((T)) so that the thermodynamic functions calculated on its basis would satisfy both the third principle of thermodynamics and the law of equipartition of energy over degrees of freedom. A general expression for the function (T) is obtained, which satisfies these thermodynamic regularities.     

Magnetic Field Dependence of Blocking Temperature in Oleic Acid Functionalized Iron Oxide Nanoparticles

We report the synthesis of phase pure, mono-dispersed Fe₃O₄ nanoparticles of size ??10?nm via chemical co-precipitation of ferrous and ferric ions, under controlled pH and temperature. The nanoparticles are oleic acid functionalized and hence dispersible in organic medium. The structure and morphology of nanoparticles are determined by analyzing XRD pattern and TEM micrographs, confirming the formation of phase pure Fe₃O₄ nanoparticles. The magnetization studies reveal the superparamagnetic behavior of the nanoparticles at room temperature. The changes in blocking temperatures (T _B) of magnetic nanoparticles with applied magnetic fields (H _ap), noted from the cusp of the zero-field-cooled magnetization, the indicate effects of dipole interactions. A decrease in blocking temperature from 95?K to 15?K has been observed on varying the magnetic field from 50?Oe to 5000?Oe. T _B versus H relation follows the equation T _B(H)=T _o(1?(H/H _o)) ^m , i.e. the Néel?CBrown model of magnetic relaxation in nanoparticles.     

The Superconducting Ferromagnet UCoGe

The correlated metal UCoGe is a weak itinerant ferromagnet with a Curie temperature T _C=3 K and a superconductor with a transition temperature T _s=0.6 K. We review its basic thermal, magnetic—on the macro and microscopic scale—and transport properties, as well as the response to high pressure. The data unambiguously show that superconductivity and ferromagnetism coexist below T _s=0.6 K and are carried by the same 5f electrons. We present evidence that UCoGe is a p-wave superconductor and argue that superconductivity is mediated by critical ferromagnetic spin fluctuations.     

Calculation of Magnetic Properties and Specific Heat for the Nuclear Enhanced Ferromagnet PrNi5

A general model hamiltonian for hyperfine enhanced magnetic materials is presented. The use of the model is not limited only to the nuclear regime, but the model treats also situations when the strength of the exchange interaction is close to and above the threshold value. The model is solved numerically for PrNi₅ and the results are compared with recent ultra-low temperature experiments. In particular, the behavior of magnetization, susceptibility, and specific heat of PrNi₅ is studied for two external field geometries and anisotropic properties are pointed out. The effect of the quadrupole interaction is examined in detail. The question of the correlations among interacting ions is also addressed.     

Temperature Dependence of Flexural Strength of a CF-SMC Composite

This paper presents the temperature dependence of predicting the flexural strength of a carbon fiber-reinforced sheet molding compound (CF-SMC). First, three-point flexural tests were performed to measure strength and elastic modulus at a variety of temperatures for CF-SMC specimens. Next, simple equations for predicting flexural strength were derived based on the fracture mechanics approach. The predicted flexural strength was in reasonably good agreement with the experiment results. The scatter of flexural strength was ascribed to the variation of location and size of the initial damage. In addition, the effect of temperature on flexural strength and delamination behavior was explained in association with the temperature dependence of the elastic modulus and fracture toughness.     

The Temperature Dependence of Elastic Nonlinearity in Metal-Matrix Composites

Abstract

Thermal stresses are very important in determining the strength of composites. In metal-matrix composites, these stresses are generated at the matrix-reinforcement interface as a result of the difference in thermal expansion coefficients of matrix and reinforcement during solidification. In order to evaluate these stresses, we studied the effect of temperature on the second- and third-order elastic constants in two metal-matrix composites consisting of the aluminum alloys 8091 and 7064 and silicon carbide particles up to 20% volume fraction. The elastic constants were determined at the temperatures 0, 25 and 55°C using measurements of absolute as well as changes of ultrasonic velocities as a function of applied stress. The values of these constants are used to calculate the acoustic nonlinearity parameters. In both composites, the acoustic nonlinearity parameters increase with the amount of reinforcement, which is opposite to that previously observed in aluminum alloys containing second-phase precipitates. Also, the temperature behavior of the nonlinearity parameters in the composites are opposite to those in the aluminum matrices. These differences in behavior are interpreted as due to the presence of thermal stresses at the matrix-reinforcement interface, and give promise to the possibility of using these parameters in the nondestructive evaluation of these stresses in metal-matrix composites.     

Temperature Field of a Turbulent Flow in a Well with Allowance for the Temperature Dependence of Heat Capacity

The article discusses a method to solve a field-matching problem that describes a nonstationary heat exchange between an upward flow of fluid in a pipe and an ambient medium with allowance for the variability of the coefficients due to the turbulence of a multiphase flow and the nonlinear dependence of the oil heat capacity on temperature and precipitation of paraffins. The method combines asymptotic methods of small and formal parameters. Analysis of the experimental data on the dependence of heat capacity on the temperature makes it possible to approximate it by a linear function that contains a small parameter in the form of the first terms of the Taylor series. Expansion of the problem by the small parameter in the zeroth approximation leads to a linear problem that is solved by the asymptotic method for a formal parameter for uncoupling of the first coefficient of the expansion. Expressions determining the temperature field in the well and surrounding rocks that take into account the orthotropy of the thermophysical properties of the media are obtained.     

Magnetic Properties and Phase Transitions in a Diluted Ferromagnet: Ising,XY, and Heisenberg Models

The magnetic properties and phase transition of a ferromagnetic spin-S disordered diluted thin film with a face-centred cubic lattice are investigated using the high-temperature series expansion technique extrapolated with Padé approximant method for Heisenberg, XY and Ising models. The reduced critical temperature of the system t_c=frack_BT_c2S(S+1)J_btau_{mathrm{c}}=frac{k_{mathrm{B}}T_{mathrm{c}}}{2S(S+1)J_{mathrm{b}}} is studied as the function of the thickness of the thin film and the exchange interactions in the bulk, and within the surfaces J _b,J _s and J _⊥, respectively. It is found that τ _c increases with the exchange interactions of surface. The magnetic phase diagrams (τ _c versus the dilution x) and the percolation threshold are obtained. The shifts of the critical temperatures T _c(L) from the bulk value (fracT_c(￥)T_c(L)-1)(frac{T_{mathrm{c}}(infty )}{T_{mathrm{c}}(L)}-1) can be described by a power law L ^−λ , where λ is the inverse of the correlation length exponent.     

Magnetic Field Dependence of the Energy Gap in Nanotubes

The dependence of the energy gap in the band structure of a carbon nanotube on the flux of an external uniform magnetic field is studied in the framework of the zero-range potential method. Taking into account a renormalization procedure for the coupling constant of the zero-range potential, we obtain an estimate for the maximal gap between the conductivity band and the valence one; this estimate is closer to experimental data when compared to the value given by the tight-binding approximation. It is shown numerically that decreasing the diameter of nanotubes breaks down the flux periodicity of the gap structure.     

Temperature Dependence of Hydrogen Adsorption Isotherms

In the past it has already been shown that adsorption isotherms of liquid or solid films are not described completely by the Frenkel–Halsey–Hill theory. Substrate roughness as well as thermal fluctuations have to be taken into account in understanding the adsorption behavior. The inclusion of thermal fluctuations into the adsorption theory has already been addressed and proven to provide an explanation for the deviations found in many experiments. However, a resulting temperature dependence of such isotherms has not yet been verified. In our investigations we have addressed this issue with a series of adsorption isotherms of hydrogen on gold in a temperature range from 11 K to 19.5 K (i.e., below and above the triple-point temperature of hydrogen). Our measurements are compared with existing theories and the nature of the remaining discrepancies is discussed.PACS numbers: 68.08 Bc, 68.15 +e, 68.43 −h     

Thermal Transport in High-T c Crystals: Some Aspects on Its Temperature and Magnetic Field Dependence

We have measured the temperature (10 KT150 K) and magnetic field (B9 T) dependence of the in-plane thermal conductivity (T, B) of YBa ₂ Cu ₃ O _7–x (Y123), YBa ₂ Cu ₄ O ₈ (Y124), and Bi ₂ Sr ₂ CaCu ₂ O ₈ (Bi2212) high-temperature superconducting crystals, at different orientations of the magnetic field B c and c ). Our results show that depends on field below and above the critical temperature due to the contribution of superconducting fluctuations and of vortices and orbital effects to the scattering of quasiparticles or electrons. Within experimental resolution no dimensional crossover of the superconducting fluctuations can be clearly recognized. Qualitative and quantitative differences in the field dependence between the crystals are observed and discussed. We argue that the anisotropy plays a mayor role in the relative weight between the different scattering processes and the field effect on the occupation number of quasiparticles, determining the field dependence of the thermal conductivity in these cuprates. In particular we found that k increases with field in a Bi2212 crystal for B c and at T<20 K.     

Dependence of Magnetization on Temperature, Magnetoresistance Effect and Magnetic Phase Diagram for the Layered Gd5Ge4 Compound

The dependence of dc magnetization and electrical transport on temperature were systemically studied for the layered Gd₅Ge₄ compound. A complex coexistence behavior of ferromagnetism (FM) with antiferromagnetism (AFM) phase and significant magnetoresistance effect were found under the inducement by the applied magnetic field. The results show an unusual magnetoresistance (MR) effect with opposite behavior at the high and low temperatures, which is positive at high temperature region and negative at low temperature region for the layered Gd₅Ge₄ alloy. And its MR maximum is close to 50%, which can be a very large numerical value e.g. for giant magnetoresistance (GMR) materials. From the magnetization loop under different temperatures, also a magnetization step behavior was found below ∼6.2 K. The results proved an existence of reentrance AFM phase at low temperature. From experimental data, also the magnetic phase diagram is obtained. This kind of complex magnetization is analyzed and discussed in the frame of phase separation. The experimental results opened also the possibility of application of the layered Gd₅Ge₄ compound in the fields of information record and sensor technique.