Dependence Of Critical Field Research Articles

Magnetoresistance of Hexagonal FeGe Single Crystals between 4.2 K and 30 K

The low field transverse magnetoresistance of hexagonal FeGe has been measured for the field in the basal plane with current both parallel and perpendicular to the c-axis. At a temperature dependent critical field (Δϱ/ϱ0) // c shows a sharp peak, then decreases rapidly until it levels off at a second critical field. (Δϱ/ϱ0) ⊥ c exhibits a sharp dip at the first critical field followed by rising magnetoresistance with increasing field until the second critical field, where it levels off. The observed anomalies are related to changes in the spin structure and magnetic superzone effects. The possibilities of itinerant antiferromagnetism are also considered.

Superconductivity of Niobium Films

Niobium thin films are prepared by evaporation in ultra-high vacuum system with an electron beam and their superconducting properties are examined as a function of film thickness and temperature. An inflection point appearing in the temperature dependence of the parallel critical field H c // is explained by the temperature dependence of the coherence length ξ( t ). The perpendicular critical field H c ⊥ varies as 1- t independently of their thickness. The coherence length ξ(0) at 0°K of some films are estimated by three methods. Anomalous results that the minimum occurs on the angular dependence of the critical field and the ratio H c // / H c ⊥ becomes lower than 1.69 are discussed by considering the isolated columnar structure.

Experimental Studies of Magnetic Properties of a Metal in the Normal and Superconducting States

Experiments have been designed for the measurement of the magnetoresistance of a normal metal and of the critical field dependence of a superconductor. The equipment is relatively cheap and simple to construct and operate, and the experiments yield accurate results in times short enough for inclusion in an undergraduate laboratory class.

The temperature dependence of the critical field H c2 in solid solution Nb-Ta alloys

The temperature dependence of the upper critical field H c2 is studied on the whole composition range of Nb-Ta alloys. The results are in accord with the theoretical prediction made by Maki.

Temperature dependence of critical fields in high field superconductors

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.

Fluxoid Quantization and Critical Fields in Small Superconducting Samples

This paper presents a theoretical treatment of the critical fields of thin superconducting films and related microgeometries. This treatment is based on the Ginzburg-Landau theory and the quantization of the fluxoid. A symmetric superconducting surface state is found which contains fluxoid vortices. This surface state may exist in type-II films in parallel fields or in small samples of thin films in normal fields. For a sample which is infinite in one transverse dimension a critical-field dependence is found which is similar to that previously found theoretically by Saint-James and de Gennes. The model is extended to treat samples which are limited in both transverse dimensions. Excellent qualitative and semiquantitative agreement is found with the experiments of Parks, Mochel, and Surgent which determine the critical field in a microscopic superconducting bridge.

Critical Fields of Thin Superconducting Films. I. Thickness Effects

A theoretical model is presented with which the critical magnetic fields of thin superconducting films can be calculated from any theory of superconductivity for which the kernel of the current-vector potential relationship is known. The model is worked out in detail for the nonlocal theory of Pippard with specular boundary conditions, and the critical field is shown to be a function of film thickness and the nonlocal parameters $\ensuremath{\xi}$ and ${\ensuremath{\xi}}_{0}{{\ensuremath{\lambda}}_{L}}^{2}$. The results are compared to critical-field data for pure indium films and are found to predict very well the observed thickness dependence of critical field. On the basis of reasonable assumptions, ${\ensuremath{\xi}}_{0}$ and ${\ensuremath{\lambda}}_{L}(0)$ are calculated from the indium critical field data to be 2600\ifmmode\pm\else\textpm\fi{}400 \AA{} and 350\ifmmode\pm\else\textpm\fi{}30 \AA{}, respectively.

Properties of superconducting colloids and emulsions

The characteristic feature of a large superconductor is that it cannot be penetrated appreciably by an external magnetic field lower than the critical field; in other words, the permeability of a superconductor is practically zero and its susceptibility strongly diamagnetic. It is reasonable, however, to expect that actually there is a slight penetration of the field, and theoretical predictions as to the details of this penetration have been made by F. and H. London (1935). Judging from the accuracy with which the permeability has been found zero for large specimens, the depth of penetration cannot be greater than 10 -3 cm., and so to obtain appreciable penetration effects very small specimens must be used. One method of experiment which has been used successfully is that of resistance measurements on thin wires (Pontius 1937) and thin films (Shalnikov 1938; Appleyard and Misener 1938; Appleyard, Bristow and London 1939; Appleyard, Bristow, London and Misener 1939) of superconductors; with this method, however, all that can be learnt is the size dependence of critical field, and thermodynamic arguments (which, as will be seen, involve certain assumptions) are necessary to infer anything about the details of the field penetration. A much more direct method is to measure susceptibilities of small specimens, since if there is any appreciable penetration it must reduce the diamagnetic susceptibility characteristic of the superconductor in bulk; thus, from the size dependence of the susceptibility, information can be obtained about the penetration law. Moreover, by measuring magnetic moments, the critical field can be deduced as well as the susceptibility, so that results of the same nature as from resistance measurements can also be obtained.