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Title: Atomically thin stacks of polar insulators : a route to atomic-scale multiferroics
Author: Martinez Castro, J.
ISNI:       0000 0004 8497 7360
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2016
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The continuous demand for smaller electronic components is forcing industry to consider devices at the atomic scale. One of the side effects of size reduction in electronic devices is the appearance of quantum effects, which preclude engineers from simply scaling down the functionality of existing components. In the case of data storage, different approaches have been followed to increase of information density, for instance a reduction of the physical bit size or a combination of different properties (like magnetic and ferroelectric) to achieve more than two states in which to store data. Nonetheless, combining ferroelectric and magnetic properties has been difficult since there is a contra-indication between the conventional mechanism for cation off-centring in ferroelectrics and the formation of magnetic moments. Moreover, the observation of ferroelectric and magnetoelectric effects at the atomic scale has remained difficult due to physical constraints. Using scanning tunnelling microscopy and spectroscopy we study the effects of stacking sodium chloride bilayer and monolayer (NaCl), that is, an atomically thin ionic insulator, on top of copper nitride (Cu2N), which is another atomically thin insulating layer. We observe that NaCl exhibits inverse piezoelectric behaviour - displacement of the atoms driven by an applied electric field - that can reverse electric dipole orientations. Small detected hysteresis suggests that is an incipient ferroelectric behaviour, further confirmed when the same experiment is performed on top of defects of the NaCl layer, where the dipole reversal is pinned and the ferroelectric cycle broadened. Combined capabilities of the microscope - single atom evaporation and high energy resolution - allow us to study spin excitation of single magnetic impurities when adsorbed on certain substrates. Co atoms adsorbed on top of NaCl defects give us the chance to perform inelastic electron tunnelling spectroscopy (IETS) and ferroelectric cycles at the same time; with this, we observe a change in the magnetic excitation spectrum as a function of the polarization state of the supporting substrate. These results suggest a new way to create piezoelectric effects at the interface between ultra-thin insulating polar materials near conducting electrodes, pushing the feasible size of functional polar materials down to the atomic scale. Incipient ferroelectric manifestations in combination with STM capabilities allow us to study the fundamental interactions between magnetocrystalline anisotropy, magnetic moment and electric polarization, providing in practice the proof of concept of a multiferroic device at the atomic scale.
Supervisor: Hirjibehedin, C. F. ; Serrate, D. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available