Electromechanical properties of PMN-PT thin films prepared by pulsed laser deposition technique

Ferroelectric relaxor thin films of the composition 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (PMN-PT) were fabricated by pulsed laser deposition technique on Pt/Ti/SiO₂/Si(100) substrates using a La_0.5Sr_0.5CoO₃ film as a template layer. The films were polycrystalline with random orientation of the grains. Electromechanical properties of the films were evaluated by a sensitive interferometric technique as a function of the dc and ac electric fields and frequency. Experimental results show that the effective piezoelectric coefficient d₃₃ can be tuned over a wide range of magnitudes by varying dc bias fields. A small hysteresis and a low remanent d₃₃ ere observed, which favor the use of PMN-PT films in electric field-controlled micromechanical devices. The maximum d₃₃ ranged from 50 to 100 pm/V, depending on the thickness of the films and frequency. The piezoelectric properties are explained based on the expression for the electrostriction biased by polarization. At low electric field, strain is proportional to the square of electric field, while at higher fields it can be better described as the polarization squared. The asymmetry of the strain response in PMN-PT films is attributed to the polarization offset appearing due to the different electrode materials.