Multiferroic materials exhibit simultaneously, magnetic and electric order. In a magnetoelectric
composite structure, a coupling is induced via an interfacial elastic interaction between
magnetostrictive and piezoelectric materials enabling the control of the magnetisation by applying
an electric field and vice versa. However, despite the potential of such coupling, experimental limits
of theoretical models were observed. This work sheds some light on these limits by focusing the
research on the chemistry of nanocomposite CoFe2O4 and BaTiO3, particularly at the interfaces
where the coupling predominates.
A comparison of the most common conductive oxides, Nb doped SrTiO3 and SrRuO3, was made for
the bottom electrode application. The variation of conductive properties in Nb-SrTiO3 thin films at
high temperature has been quantified when artificially strained and 60 nm SrRuO3 film was found to
be the best bottom electrode choice for room temperature use.
Epitaxial growth of magnetic CoFe2O4 was achieved on various metal oxide substrates despite large
lattice mismatches. Crystallographic properties and strain evaluation were investigated and a
Stranski-Krastanov growth mechanism, arising from the PLD deposition, was predominant. A notable
drop of magnetisation was observed depending on the growth template, particularly on BaTiO3
substrates, the piezoelectric counterpart of the magnetoelectric structures. However, an
encouraging magnetoelectric coupling induced by thermal phase transition of BaTiO3 was revealed.
For BaTiO3, a control of the growth direction was realised by varying the deposition pressure, and
the existence of both 180° and 90° ferroelectric domains was observed for films up to 300 nm in
thickness. However, both the ferroelectric and piezoelectric properties were reduced in the thin
films due to the clamping effect of the substrate.
Finally, highly crystalline multilayers of CoFe2O4 and BaTiO3 were prepared on SrRuO3 buffered
SrTiO3 substrates. It was found that the degradation of both magnetic and ferroelectric properties
was proportional to the increase in the number of interfaces. A thorough microscopic study revealed
interdiffusion and chemical instability occurring between CoFe2O4 and BaTiO3 at the interface. This
undesired effect was partially recovered by the insertion of an ultra thin layer of SrTiO3, acting as a
barrier layer at every interface. This research shows how interfacial chemistry need to be
understood to achieve high magnetoelectric coupling in these types of epitaxial engineered
structures.