**1. Introduction**

Lead-based materials have been researched for a series of device applications, such as actuators, non-volatile memories, transducers and sensors [1–4]. The Pb(Mg1/3Nb2/3)O3- PbTiO3 (PMN-PT), as relaxor ferroelectrics, with outstanding dielectric, ferroelectric and piezoelectric properties, has been widely investigated in terms of bulk ceramics and single crystals [5–8]. However, the single crystals and ceramics have been unable to meet the requirements of integrated and miniaturized devices with the development of microelectromechanical systems (MEMS) in recent years. Advances have been made in synthesis and modification of the PMN-PT thin films because of unique advantages, such as low synthesis temperature, small size, easy integration [9,10]. However, it is still a big challenging to prepare PMN-PT thin films with single phase, crack-free and dense microstructure. In addition, the performances of thin films are far weaker than the bulk materials because of the thickness effect of film and the clamping effect of the substrates.

In order to improve the quality of crystallization and enhance electrical properties of PMN-PT thin film, researchers have done a great deal of works on topics including site engineering, regulating of annealing process and so on. For example, rare earth element doping can effectively enhance electrical properties of PMN-PT thin films [11]; Gabor et al. have reported the process window can be expanded in order to obtain pure phase PMN-PT thin film with the help of LaNiO3 layer [12]; Keech et al. prepared (001) orientation PMN-PT films with a PbO buffer to account for Pb loss [13]; Shen et al. found the additive methanamide can enhance electrical properties of PMN-PT thin films with reduced residual stress of the film and dense microstructure [14]. Deposition methods of various kinds have been reported to synthesize PMN-PT thin films, including pulsed laser deposition, magnetron sputtering, metalorganic chemical vapor deposition and chemical

**Citation:** Feng, C.; Liu, T.; Bu, X.; Huang, S. Enhanced Ferroelectric, Dielectric Properties of Fe-Doped PMN-PT Thin Films. *Nanomaterials* **2021**, *11*, 3043. https://doi.org/ 10.3390/nano11113043

Academic Editor: Jürgen Eckert

Received: 14 October 2021 Accepted: 6 November 2021 Published: 12 November 2021

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solution deposition (CSD) [15–18]. CSD is believed to be a further industrial method with easy composition control of precursor solutions, excellent reproducible, low synthesis temperature and low cost. For the CSD route, it is critical to control crystallization through regulating annealing process parameters because annealing treatment is necessary to crystallize the amorphous films. Due to the electrical properties of PMN-PT thin films being strongly dependent on their grain orientation, the realization of preferential orientation is also expected to improve the electrical properties of ferroelectric thin films. Therefore, the synthesis of an ideal PMN-PT thin film should include two aspects: appropriate annealing parameters and the preferential orientation resultant thin film.

In this research, 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 thin films, with different doping concentrations of the Fe element used as acceptor doping, were prepared on Pt(111)/Ti/SiO2/Si substrate by the CSD method. The effects of annealing temperature and doping concentration on the phase, microstructure, dielectric and ferroelectric properties of PMN-PT thin films were systematically researched. The highly (111) preferred orientation and dense microstructure were achieved in the 2% Fe-doped PMN-PT thin film annealed at 650 ◦C, which lead to the enhanced ferroelectric with high saturation polarization (*P*<sup>s</sup> = 78.8 μC/cm2) and remanent polarization (*P*<sup>r</sup> = 23.1 μC/cm2) as well as dielectric with a high dielectric constant (*ε*r~1300 at 1 kHz).
