Magnetotransport in Ferroelectric Conducting Domain Walls

  • Matthew Colbear

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

Magnetotransport, the transport of electrons within a conductor in a magnetic field, provides insight into the electronic properties of materials. This thesis aims to reveal aspects of conductive behaviour in charged ferroelectric domain walls through magnetotransport: investigating firstly the possibility of spin-polarised electron propagation in domain walls and its detection with conductive atomic force microscopy and a ferromagnetic probe, and secondly the carrier mobility in lithium niobate, using geometric magnetoresistance.

A moving electron in a static electric field will perceive, in its frame of reference, a magnetic field. Electric fields thus have the potential to sustain spin polarised currents. Conducting ferroelectric domain walls are usually associated with polarisation discontinuities which generate extreme electric fields and electric field variations across the walls. As a result, `charged' conductive domain walls might be ideal as spin-current conduits as spin propagation lengths should be very large indeed.

However, sensing spin current is non-trivial and normally involves the creation of spin-valve detectors. We decided to explore a potentially revolutionary alternative using a magnetically coated tip in conjunction with conductive atomic force microscopy to create a mobile spin-sensitive probe. To explore the capabilities of this measurement technique, ferromagnetic thin-films were created with perpendicular magnetic anisotropy. Currents emanating from such thin-film surfaces would be spin-polarised, with the sense of polarisation dependant on domain orientation. Spin-polarised conducting atomic force microscopy was explored by seeing if current contrast reflected surface domain contrast. While progress was made, it proved difficult to unequivocally establish a spin-valve like response. Nevertheless, some domain-like current contrast was seen and this is encouraging for future work.

Greater success was achieved in using geometric magnetoresistance to probe the mobility of carriers in conducting lithium niobate domain walls. Domains in thin-film lithium niobate form as truncated cones. When projected onto the plane normal to the conical axis, such truncated cones and their electrical contacts together constitute the perfect Corbino disc. The Corbino disc is the ideal geometry for geometric magnetoresistance measurements. With concentric electrodes, the unperturbed current is radial. The Lorentz force is hence azimuthal and, as there is no surface in the azimuthal direction, the perturbed current cannot generate any charge build up or accompanying Hall fields. Making the most of this geometry we were able to use the magnetoresistance response of lithium niobate domain walls to reveal the mobility of active carriers. This mobility (μ) was found to be up to ≈ 3700 cm2V-1s-1 at room temperature. This is the highest mobility reported in any oxide system at room temperature and should prompt follow-up mapping of the low temperature behaviour.
Date of AwardDec 2023
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsEngineering & Physical Sciences Research Council
SupervisorMarty Gregg (Supervisor)

Keywords

  • Ferroelectrics
  • domain walls
  • lithium niobate
  • magnetotransport
  • magnetoresistance

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