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1.
Commercial water dispersions of Fe/Fe-O nanoparticles (∼50-80 nm diameter) were prepared bare and treated with a biodegradable polymer to stabilize the suspension. Hysteresis loops and FC-ZFC curves of samples in liquid and dried form were measured by means of a vibrating sample magnetometer from 77 to 300 K. A marked dependence on the conditioning field is observed in all samples that display the Verwey transition at about 120 K. The role of magnetic interactions on dried samples was also investigated by means of magnetoresistance measurements as a function of temperature.  相似文献   

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The temperature dependence of critical fields is calculated for high field superconductors if the phase transition into the normal state is of first order. The effect of superheating is discussed.  相似文献   

4.
The upper critical field,H c2, is calculated in the temperature range close toT c for arbitrary values of the mean free path 1. The method is to treat the fourth-order term in the linearized differential equation for the gap parameter as a perturbation to the harmonic oscillator equation. The Nambu-Tuan term, which determines the structure of the “clean-limit” and “dirty-limit” parts of the fourth-order term, is calculated by means of the ladder diagram technique for impurity interaction lines. The result is that the magnitude of the slope of the curveH c2/√2H c κ versust=T/T c att=1 decreases monotonically from the value 0.41 to 0.12 as the ratio of the BCS coherence lengthξ 0 to the mean free path 1 increases from 0 to ∞. For 1?ξ 0 this slope is about 0.26.  相似文献   

5.
We measured resistivity and transport critical current density, Jc, as a function of DC magnetic field and the angle (?) between the surface of the film and the magnetic field on ex-situ annealed, c-axis oriented Bi-2223 thin films fabricated by DC sputtering method. Irreversibility field (μ0Hirr) and upper critical field (μ0Hc2) were determined from the resistivity versus the applied magnetic field graph. It is observed that critical temperature (Tc), μ0Hirr,μ0Hc2 and Jc of the films strongly depend on the direction and strength of the field. While Tc of the film without magnetic field is observed to be about 102 K, it is found to decrease to 90 K (85 K) for the applied field perpendicular (parallel) to c-axis of the film. Not only were μ0Hirr(0) and μ0Hc2(0) values determined from the μ0Hirr and μ0Hc2 versus temperature graphs, respectively, but also penetration depths and coherence lengths were interpreted. Anisotropy of the film was also discussed by means of the change of irreversibility as a function of angle. Moreover at 4.2 K, Jc was observed to be 3000 A/cm2 at zero field; however, it was found to abruptly decrease to 1982 (1 1 2 0) A/cm2 under low magnetic field at ?=0° (?=90°), which indicates that anisotropic Jc behavior of the film is intrinsic. Furthermore, we provided a theoretical analysis of the obtained results in the framework of intrinsic pinning theory of superconductors. Microstructural properties of the produced films were also reinvestigated by X-ray diffractometer (XRD) and scanning electron microscopy (SEM) measurements. XRD patterns indicate that the films are c-axis oriented based on the prominent (0 0 l) peaks. SEM images show needle-like grain structures dominate the surface morphology of the films.  相似文献   

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We report on a magneto-luminescence on a double quantum well subject to an in-plane magnetic field. The attention is paid to the properties of interwell excitons, which are indirect in the real space and which become indirect in the reciprocal space as well when a finite in-plane magnetic field is applied. Such indirect exciton states become optically inactive unless some relaxation mechanisms of their momentum appear. The experiment is carried out on a sample where, as reported previously, the radiative recombination of indirect excitons is possible due to their localization or via collisions with structural defects. The experimental data presented here, measured at various temperatures, favour the latter mechanism which is less sensitive to the system temperature in comparison with the former one.  相似文献   

8.
《Physics letters. A》1968,26(8):367-368
An unexpected temperature dependence of the magnetic field periodicity has been observed in flux quantization experiments on superconducting thin-film hollow cylinders of aluminum. Alternate explanations are offered in terms of variations in the thickness or in the order parameter.  相似文献   

9.
The temperature dependence of the surface upper critical field,H c3, nearT c is calculated for arbitrary values of the mean free pathl by taking into account the fourthorder term of the generalized Ginzburg-Landau theory. For finitel the boundary condition is modified such that the normal derivative of the energy gap at the surface becomes positive. The slope of the curveH c3/H c2 versust=T/T c att=1 is found to decrease monotoneously from zero to ?1.040 as one goes from the “dirty” to the “clean” limit.  相似文献   

10.
The effect of an axial magnetic field on the Poiseuille flow of nematicp-azoxyanisole (PAA) has been computed using the Ericksen-Leslie continuum theory. The apparent viscosity decreases appreciably in the presence of the magnetic field. Orientation and velocity profiles for different shear rates and magnetic fields are presented.  相似文献   

11.
The temperature dependence of the asphericity of the 3d magnetic moment distribution has been calculated by the two-band CPA technique. It is shown that this asphericity in the case of Co-Ni and Co-Fe alloys is almost constant, and this behaviour agrees with recent measurements.  相似文献   

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Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (B hf ) at Ce impurity in Co using 140La→ 140Ce probe. The radioactive 140La produced by neutron irradiation of lanthanum metal with thermal neutrons was introduced in Co by arc melting in argon atmosphere. The present measurements cover the temperature range from 4.2– 1300 K. Two pure magnetic interactions were observed at impurity sites, corresponding to a ferromagnetic ordering of Co moments in hcp and fcc phases. The temperature dependence of B hf for both phases, however, shows a sharp deviation from an expected standard Brillouin-like behavior for the host magnetization. The results are discussed in terms of a simple molecular-field model where the localized moment at impurity ions as well as the conduction electron contributions to the hyperfine field are taken into account.  相似文献   

14.
We have derived the so-called gap equation, which determines the upper critical magnetic field, perpendicular to conducting chains of a quasi-one-dimensional superconductor. By analyzing this equation at low temperatures, we have found that the calculated angular dependence of the upper critical magnetic field is qualitatively different than that in the so-called effective mass model. In particular, our theory predicts a non-analytical angular dependence of the upper critical magnetic field, H c2(0) − H c2(α) ∼ α3/2, when magnetic field is close to some special crystallographic axis and makes an angle α with it. We discuss possible experiments on the superconductor (DMET)2I3 to discover this non-analytical dependence.  相似文献   

15.
We study pairing correlations in ultrasmall superconductor in the nanoscopic limit by means of a toy model where electrons are confined in a single, multiply degenerate energy level. We solve the model exactly to investigate the temperature and magnetic field dependence of number parity effect (dependence of ground state energy on evenness or oddness of the number of electrons). We find a different parity effect parameter to critical temperature ratio (4 rather than 3.5) which turns out to be consistent with exact solution of the BCS gap equation for our model. This suggests the equivalence between the parity effect parameter and the superconducting gap. We also find that magnetic field is suppressed as temperature increases.  相似文献   

16.
R V Upadhyay 《Pramana》1997,49(3):309-316
The time dependent magnetization measurements of the fine particles (100Å) of Zn0.1Fe0.9Fe2O4 ferrite system at 68 K are reported. The time decay of magnetization is found to be logarithmic. The time dependent co-efficient varies with the applied field and is maximum at coercive field. The value of anisotropy obtained from the variation of time dependent co-efficient with field agrees well with earlier results.  相似文献   

17.
In Ni2+x Mn1?x Ga shape-memory ferromagnetic alloys with coincident magnetic and structural phase transitions, a reversible structural field-induced phase transition was observed at constant temperature and pressure in magnetic fields of about 10 T. Computational results are in qualitative agreement with experiment.  相似文献   

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The influence mechanisms of temperature on the band gap properties of the magnetic fluids based photonic crystals are elaborated. A method has been developed to obtain the temperature-dependent structure information (Asol/A) from the existing experimental data and then two critical parameters, i.e. the structure ratio (d/a) and the refractive index contrast (Δn) of the magnetic fluids photonic crystals are deduced for band diagram calculations. The temperature-dependent band gaps are gained for z-even and z-odd modes. Band diagram calculations display that the mid frequencies and positions of the existing forbidden bands are not very sensitive to the temperature, while the number of the forbidden bands at certain strengths of magnetic field may change with the temperature variation. The results presented in this work give a guideline for designing the potential photonic devices based on the temperature characteristics of the magnetic fluids based photonic crystals and are helpful for improving their quality.  相似文献   

20.
In contrast to the experimentally widely used isentropic demagnetization process for cooling to ultra-low temperatures we examine a particular classical model system that does not cool, but rather heats up with isentropic demagnetization. This system consists of several magnetite particles in a colloidal suspension, and shows the uncommon behavior of disordering structurally while ordering magnetically in an increasing magnetic field. For a six-particle system, we report an uncommon structural transition from a ring to a chain as a function of magnetic field and temperature. Received 5 September 2000  相似文献   

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