#### Degree Type

Dissertation

#### Date of Award

1991

#### Degree Name

Doctor of Philosophy

#### Department

Physics and Astronomy

#### First Advisor

John R. Clem

#### Abstract

This dissertation is divided into five sections, each dealing with an aspect of the response of type-II superconductors to applied magnetic fields;In Section I, a generalized variational model of a single vortex is presented, valid in the anisotropic case when the applied magnetic field H is along one of the principal axes. The effects of the anisotropy on pinning forces of individual vortices and on the vortex lattice form are discussed;In Section II, a variational model of the mixed state is proposed, which permits an analytic calculation of the free energy density F in the Ginzburg-Landau regime. A formula for the reversible magnetization is obtained, valid for the entire field region H[subscript]c1 ≤ H ≤ H[subscript]c2. The model is further extended to include anisotropy by introducing an effective mass tensor for the case when H is parallel to one of the principal axes. The method permits an accurate determination of H[subscript]c2 versus temperature T from reversible magnetization measurements, and dH[subscript]c2/dT = -1.65 ± 0.23 T/K is found for a YBa[subscript]2Cu[subscript]3O[subscript]7 single crystal for H parallel to the c axis near T[subscript] c, implying [xi][subscript]ab(0) = 17 ± 1 A;In Section III, the reversible magnetization of anisotropic superconductors is considered for the case that H is along an arbitrary direction with respect to the principal axes. When H is not parallel to one of the principal axes, the average magnetic flux density B is not parallel to H, and a torque associated with the transverse magnetization exists, tending to orient the sample so that the value of ~[kappa] is the largest. Analytic expressions for the magnetization and the torque are obtained for H \gg H[subscript]c1 by making use of the model of Section II and their dependences upon H and T are discussed;In Section IV, it is pointed out that the London model for the reversible magnetization M of high-[kappa] type-II superconductors in the intermediate-field region is quantitatively incorrect. It is also shown that the apparently linear dependence of M versus lnH in the intermediate-field region, a behavior that has been observed experimentally, can be obtained from the model of Section II;In Section V, it is found that the reversible magnetization M can be written to good approximation as -4[pi] M = (H[subscript]spc2/H[subscript]c2)[phi](h) for H \gg H[subscript]c1 and [kappa] \gg 1, where h = H/H[subscript]c2 is the reduced field and [phi] is a function of h. This property can be used to estimate the anisotropy ratio [gamma] from the reversible magnetization measurement, and [gamma] ~eq 18 as a lower bound is found for a Bi[subscript]2Sr[subscript]2CaCu[subscript]2O[subscript]8 single crystal.

#### DOI

https://doi.org/10.31274/rtd-180813-12016

#### Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

#### Copyright Owner

Zhidong Hao

#### Copyright Date

1991

#### Language

en

#### Proquest ID

AAI9202360

#### File Format

application/pdf

#### File Size

122 pages

#### Recommended Citation

Hao, Zhidong, "Model for the reversible magnetization of high-[kappa] type-II superconductors: application to high-temperature superconductors " (1991). *Retrospective Theses and Dissertations*. 10039.

https://lib.dr.iastate.edu/rtd/10039