Dissertation committee:
1) Δημοσθένης Σταμόπουλος, Αναπληρωτής Καθηγητής, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
2) Shujun Zhang, Καθηγητής, Institute of Superconducting & Electronic Materials, Australian Institute of Innovative Materials, UoW, Australia
3) Μιχαήλ Πίσσας, Διευθυντής Ερευνών, Ινστιτούτο Νανοεπιστήμης και Νανοτεχνολογίας, ΕΚΕΦΕ ‘Δημόκριτος’
4) Νικόλαος Στεφάνου, Καθηγητής, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
5) Ιωάννα Ζεργιώτη, Καθηγήτρια, Σχολή Μαθηματικών και Φυσικών Επιστημών, Εθνικό Μετσόβιο Πολυτεχνείο
6) Κωνσταντίνα Μεργιά, Διευθύντρια Ερευνών, Ινστιτούτο Πυρηνικών και Ραδιολογικών Επιστημών και Τεχνολογίας, Ενέργειας και Ασφάλειας, ΕΚΕΦΕ ‘Δημόκριτος’
7) Αντώνιος Παπαθανασίου, Επίκουρος Καθηγητής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Summary:
During the last decades, hybrid systems of ‘immiscible’ ingredients have attracted the attention of the international scientific community due to their novel properties. Piezoelectric and ferromagnetic hybrids of appropriate nanostructures allow the control of the piezoelectric and ferromagnetic properties by their heteroconjugate externally applied field, magnetic, Hex and electric, Eex, respectively. Here, we present our systematic study on sample series of two different piezoelectric/ferromagnetic nanostructures studied with a variety of experimental techniques (i) ferromagnetic Co nanolayers (thickness 30-50 nm) in contact with bulk piezoelectric Pb(Mg1/3Nb2/3)O3-0.30PbTiO3 (PMN-PT) single crystals and (ii) ferromagnetic Fe3O4 nanoparticles (diameter 50-100 nm) embedded in bulk piezoelectric Pb(Zr0.52Ti0.48)O3 (PZT) polycrystalline matrix. Regarding the former topology, Co/PMN-PT/Co, in isothermal magnetization measurements, m(Hex,Εex)|Τ, we observed that Eex causes strong modulation of the magnetic coercive field, Hc, (47%) for temperatures lower than a characteristic temperature range Tch=150-170 K. In isofield magnetization measurements, mrem(Τ,Εex)|Η, we observed that Eex causes thermomagnetic instabilities of mrem (16%) for temperatures higher than the same Tch=150-170 K. Concerning the second topology, PZT-xFe3O4, we focused on samples with x=5%, which are strongly insulating and ferromagnetic. From isothermal magnetization measurements, mrem(Hex,Εex)|Τ at Τ=300 Κ, we observed that Eex affects the relaxation of mrem. In addition, from isothermal measurements of the in-plane strain Szi(Εex,Hex)|Τ (i=x,y) at Τ=300 Κ, it was deduced that the respective piezoelectric coefficients, dzi, decrease dramatically when Hex is applied (50-60%). The process is entirely reversible, since the initial values of dzi are completely recovered when Hex is removed. The above results prove that our hybrid nanostructures are magnetoelectric, exhibiting a noticeable performance. We attribute the underlying mechanism to the intense magnetostrictive character of Co and Fe3O4, which, in combination with the piezoelectric behavior of PMN-PT and PZT, promote the bidirectional strain transfer to the interfaces of the relative structural components under the application of Eex and Hex. Finally, by means of simulations, we investigated the limits of reliable application of the optical-microscopy based experimental technique that we used in this study for the measurement of the characteristic Szi(Εex,Hex) curves that was used to estimate the respective piezoelectric coefficients, dzi.
Keywords:
piezoelectrism, ferromagnetism, magnetoelectric materials, composite materials
