Interpretation of multiscale characterization techniques to assess ferroelectricity: The case of GaFeO3

In this paper, we propose a thorough experimental procedure to assess the ferroelectricity of thin films, and apply this procedure to Pulsed Laser Deposition grown GaFeO3 thin films at the macroscale by means of Polarisation-Voltage hysteresis and at the nanoscale by Piezoresponse Force Microscopy. GaFeO3 is a serious candidate for the multiferroicity at room temperature, being ferrimagnetic and possibly ferroelectric. However, the non-ambiguous measurement of ferroelectric polarisation of such thin films remains a challenge. We show that although doped to decrease the leakage currents, the samples remain too leaky to allow any detection of a polarisation current, whereas Piezoresponse Force Microscopy images are indeed obtained in certain conditions. Nevertheless, the images obtained from scanning probe methods must be questioned in that context. This is why we propose to obtain PFM images at much higher frequencies to discriminate between artefactual images and true ferroelectric behaviour. The application of the method combined with the comparison with results obtained on a PbZrTiO3 sample allow to rule out the ferroelectricity of our GaFeO3 samples. Beyond the problem of GaFeO3, our objective is to propose a method which enables to assess objectively the ferroelectricity of any leaky film.