Development of nanoscale sensors for scanning Hall probe microscopy and investigations of novel vortex phases in highly anisotropic superconductors

  • Hussen Mohammed

Student thesis: Doctoral ThesisPhD


Bismuth Hall effect sensors with active sizes in the range 0.1µm - 2µm have been fabricated by electron beam lithography and lift-off techniques for applications in scanning Hall probe microscopy (SHPM). The key figures of merit of the sensors have been systematically characterised as a function of device size. The miniumum detectable field of 100nm probes at room temperature is found to be Bmin=0.9mT/Hz0.5, with scope for more than a factor of ten reduction by using higher Hall probe currents. This is significantly lower than in similar samples fabricated by focussed ion beam (FIB) milling of continuous Bi films, suggesting that the elimination of FIB damage and Ga+ ion incorporation through the use of lift-off techniques leads to superior figures of merit. A number of ways in which the T=300K performance of our sensors could be improved still futher are discussed.High resolution scanning Hall probe microscopy (SHPM) with semiconductor 2DEG Hall probe devices has been used to search for novel phases of vortex matter in single crystals of the high temperature superconductor Bi2Sr2CaCu2O8+. In the crossing lattices regime of these highly anisotropic superconductors under tilted magnetic fields two orthogonal types of flux structure are formed known as Josephson and pancake vortices (JVs and PVs). SHPM has been used to study interacting JV-PV matter with very high in-plane fields. The spacing of JV chains has been systematically quantified as a function of both in-plane and out-of-plane fields. Surprisingly, it is found that the JV chain spacing is not solely a function of the in-plane field, as previously assumed, and the effective anisotropy, γ_eff, is shown to depend strongly on the out-of-plane field strength. Moreover, for a fixed in-plane field the JV stack spacing shows pronounced sawtooth-like oscillations as a function of the out-of-plane field. These measurements are giving us unique new insights into the properties of crossing vortex lattices in highly anisotropic cuprate superconductors at high Josephson vortex stack densities.
Date of Award1 Apr 2015
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorSimon Bending (Supervisor)


  • scanning Hall probe microscopy, Bi Hall probes, lift-off techniques

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