Growth and characterisation of Bi-based multiferroic thin films

Resumo

Multiferroic materials, in which both ferroelectric and (anti)ferromagnetic orders coexist in the same phase, have received much interest in the last few years. The possibility of these two ferroic orders being coupled allows new functionalities in these materials as controlling the magnetisation by an electric field or, conversely, controlling the polarisation by a magnetic field. The fulfilment of this magnetoelectric coupling is not only interesting in terms of fundamental research but it would also pave the way for designing novel magnetoelectric applications. For this latter purpose, ferromagnetic multiferroics would have greater advantages over the antiferromagnetic ones because of the net magnetisation. However, it is the antiferromagnetic order which prevails in multiferroic materials. Bi-based perovskite and double-perovskite oxides, BiBO3 and Bi2BB’O6, respectively, where B and B’ are magnetic transition metal ions present an excellent starting point to investigate new ferromagnetic ferroelectric materials. In these compounds ferroelectricity arises from the stereochemical activity of Bi3+ cations. Conversely, magnetism is driven by the superexchange interaction between the magnetic ions through the adjacent oxygen ions (B – O – B). In particular, to date, BiMnO3 and Bi2NiMnO6 systems are the only reported ferromagnetic Bi-based perovskite oxides. Hence, both systems are investigated in the work of this thesis in thin films . First of all, this thesis addresses the synthesis of these compounds. In this process, three main hindrances were met. Firstly, these Bi-based compounds are highly metastable, which implies that they are only possible to be synthesised in bulk under extreme conditions, i.e. under high temperatures and high pressures. The strategy used to circumvent the required high pressures consisted of replacing the mechanical pressure by the epitaxial stress in thin films. For this purpose these Bi-based compounds were grown by pulsed laser deposition (PLD) onto single-crystal (001)-oriented SrTiO3 substrates. Secondly, Bi is a highly volatile element and consequently the synthesis temperature was not a free deposition parameter, forcing the use of low synthesis temperatures in order to prevent non-stoichiometric films or even the no-formation of the compound when the Bi-deficiency was too large. Yet the general metastable character of these compounds demands the use of high temperatures to the synthesis process. These two antagonistic requirements were tried to be balanced by using 10% Bi-rich PLD targets in the case of BiMnO3 system and by partial replacement of Bi3+ cations by La3+ cations (by 10%) in the case of Bi2NiMnO6 system. In the latter approach, La-doping gives rise to a slightly reduced unit cell volume, exerting the socalled chemical pressure which contributes to prevent Bi3+ cations from desorption during the growth process. Thirdly, both in the ternary Bi – Mn – O and quaternary Bi – Ni – Mn – O systems a strong multiphase formation tendency was found, especially in the former, in which apart from the desired BiMnO3 and Bi2NiMnO6 compounds, different parasitic oxide phases appeared in the grown films. As a consequence of all these facts the single-phase stabilisation of either BiMnO3 or (Bi0.9La0.1)2NiMnO6 was greatly hampered and only possible to be achieved under a narrow window of deposition conditions. Once the deposition conditions for single-phase stabilisation of the Bi-based compounds are controlled, structural characterisation proves that both BiMnO3 and (Bi0.9La0.1)2NiMnO6 grow fully coherent on SrTiO3 substrates, thus adopting as the inplane lattice parameter that of the cubic substrate and subsequently a tetragonal-like structure. Importantly enough for the magnetic properties, (Bi0.9La0.1)2NiMnO6 thin films are found to display long-range B-site order and the Ni2+/Mn4+ electronic configuration, which is the required condition for a long-range ferromagnetism. Indeed, ferromagnetic behaviour is recorded but with a reduced Curie temperature probably due to the epitaxial strain of the substrate. Instead, BiMnO3 thin films are found to exhibit similar Curie temperature to that of bulk specimens. Two-dimensional growth mode is obtained for (Bi0.9La0.1)2NiMnO6 thin films, attaining very low rough surface, whereas BiMnO3 thin films were in all cases displaying a clear three-dimensional growth mode, yielding rougher surface morphology. Finally, in order to study the dielectric/resistive, magnetoelectric and ferroelectric properties parallel-plate capacitors were fabricated using single-crystal (001)-oriented Nb doped SrTiO3 substrates as bottom electrode and sputtered Pt as top electrodes. In (Bi0.9La0.1)2NiMnO6 thin films ferroelectric domains switching current is measured, which allows conclusively stating that (Bi,La)2NiMnO6 compounds are indeed ferroelectric up to at least 10% La content. By structural characterisation the ferroelectric transition temperature is inferred to be around 450 K. The second part of this bloc is devoted to study the dielectric properties and the possible magnetoelectric coupling of these compounds. In this work both the dielectric response and the magnetoelectric response was assessed by impedance spectroscopy, the latter using magnetic fields while recording the impedance response, with the final aim of observing any deviation of the dielectric permittivity of these compounds either in the vicinity of the ferromagnetic transition temperature or when applying a magnetic field. Either phenomenon would indicate magnetoelectric coupling. Special attention is given to the conventional artefacts these measurements often produce when performed on dielectric thin films, causing misleading interpretations, like apparent colossal dielectric constants and/or apparent large magnetoelectric couplings. Following these precautions the intrinsic dielectric and magnetoelectric response of BiMnO3 and (Bi0.9La0.1)2NiMnO6 thin films are extracted. Despite the fact that BiMnO3 dielectric data shows clear magnetoelectric signs, results points to a weak magnetoelectric coupling, which is especially emphasised in (Bi0.9La0.1)2NiMnO6 thin films, probably driven by the fact that magnetism and ferroelectricity arise by two independent mechanisms in these Bibased compounds.