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.