TUNING OF MAGNETOELECTRIC COUPLING IN MULTIFERROIC HETEROSTRUCTURES

The work presented in this thesis deals with the optical and thermal control of magnetism in multiferroic heterostructures. The main goal was to investigate the interfacial magnetic modifications of magnetic properties of ferromagnetic (FM) thin films deposited on photo-ferroelectric (FE) substrates, as a function of two external stimuli, specifically visible light excitation and thermal treatments. The photo-induced effects of low-power 405 nm visible light illumination on the ferroelectric and magnetic properties of PMN-0.4PT/Ni heterostructures were investigated by combining electrical, structural, magnetic, and spectroscopic characterizations, in both pristine and polarized states of the PMN-PT substrate. The results show that under light illumination, the photostriction of PMN-PT induces an interfacial strain on the magnetostrictive Ni layer. This leads to an optical modification of the Ni orbital moment, leading up to a 45% reduction in coercive field under illumination, as observed by Magneto-Optic Kerr effect (MOKE). The light-induced variation in the Ni orbital moment was experimentally observed for the first time in multiferroic heterostructures, via sum-rule analysis of x-ray magnetic circular dichroic measurements (XMCD). The observed effect is strongly reduced after polarizing the PMN-PT substrate out-of-plane. This was justified by the larger photostrictive contribution coming from the in-plane FE domains. These results shed light on the delicate energy balance that leads to sizeable light-induced effects in multiferroic heterostructure. The scope is also to highlight the novelty and importance of the presented work, which emphasizes on the photostrictive effect induced by laser illumination in the PMN-PT/FM heterostructure and its effect on the Ni thin film via inverse magnetostriction mechanism. Concerning the thermal treatment effects on multiferroic heterostructures, this aspect was characterized on PMN-0.4PT/Fe heterostructures. The study was done by combining structural, magnetometric, and spectroscopic characterizations. X-ray diffraction (XRD) and micro-Raman spectroscopy were used to characterize the structural properties of PMN-PT substrate, whereas MOKE magnetometry was performed to analyze the in-plane magnetic response of Fe thin films deposited on top. The pristine unannealed substrate exhibited a larger presence of out-of-plane FE domains, with broad and inhomogeneous features in the 2D reciprocal space maps of XRD, signifying the presence of a low crystalline quality of the ferroelectric structure. Once annealed for 15 minutes above the first-order phase transition, the majority of the domain population shifted towards the in-plane direction, showing in addition an overall improved crystalline quality. Further annealing led to a return to a mostly out-of-plane domain population. These structural modifications of PMN-PT upon annealing tailored the magnetic anisotropy of Fe film in different manners, exploiting the magnetoelastic interfacial coupling. Specifically, the angular dependence of the magnetic response of Fe passed from an isotropic one in the pristine case to an anisotropic one after short annealing, due to the larger presence of in-plane tetragonal domains. These results place thermal treatments as an additional parameter to be exploited for tuning the magnetic properties of multiferroic heterostructures. The results obtained with the two presented external stimuli, i.e., light illumination and thermal treatments, exploit the rich set of properties of ferroelectric materials and pave the way for further investigations, either by combining them or by further optimizing the choice of materials.