**2. Materials and Methods**

The precursor solutions of 0.71Pb(Mg1/3Nb2/3)O3-(0.29-x%)PbTiO3-x%PbFeO3 (PMN-PT-xFe, x = 0, 2, 4, 8) thin films were prepared by the CSD method. Trihydrated lead acetate, magnesium ethoxide, niobium ethoxide, titanium iso-propoxide and iron nitrate were used as the raw materials. The solvent was 2-methoxyethanol and acetic acid; about 5 mol% excess of lead was added to compensate the volatilization during heat treatment. The solution was stirred using a magnetic stirrer for 12 h, and further aged for 24 h to promote the uniformity of solution. The concentration of precursor solutions was adjusted to be 0.2 M; the precursor was spin-coated on Pt(111)/Ti/SiO2/Si substrate at 3000 rpm for 30 s. The obtained film was heated at 350 ◦C for 2 min for solvents volatilizing and amorphous films formation, and subsequently annealed at the specified annealing temperature (600~700 ◦C) for 5 min in air atmosphere for film densification and crystallization in a programmed rapid thermal annealing furnace. The desirable thickness was achieved by a 12 time layer-by-layer spin-coating and annealing processes with one layer thickness of about 25 nm.

Crystalline phases were identified by X-ray diffraction (XRD) measurement using Bruker D8 Advanced XRD (Berlin, Germany). The microstructures of thin films were recorded using a field emission scanning electron microscopy (SEM) and atomic force microscope (AFM, MFP-3D Origin+, Concord. MA, USA). The ferroelectric tester (TF Analyzer 3000, Aachen, Germany) was used for polarization-electric field (*P*–*E*) hysteresis loops and leakage current density of thin films. The relative permittivity (*ε*r) and dielectric loss (tanδ) were measured using an impedance analyzer (Agilent 4294A, Santa Clara, CA, USA).
