Interface Phenomena in Ferromagnetic-Ferroelectric Oxide Thin Films

Complex oxide hetero-structures exhibit an abundance of interesting physical phenomena that involve principally interplay of charge, spin and lattice degrees of  freedom. These physical phenomena become even more interesting when one can design multilayers/heterostuctures with such materials. This is largely due to the presence of interfaces that have unique properties, often not existing in the bulk counterparts. Polarity discontinuities at the interfaces between different crystalline materials (hetero-interfaces) can lead to nontrivial local atomic and electronic structure, owing to the presence of dangling bonds and incomplete atomic coordination. The study of interfacial phenomena, multilayered thin films have an edge over other morphologies since they have relatively large interfaces that can be controlled effectively. One of the interesting developments in the area of oxide electronics in the last decade is the Tunneling Electro-resistance (TER) observed in Ferroelectric tunnel junctions (FTJ). Tunneling in junctions containing FE like Barium Titanate (BaTiO3 or BTO) depends crucially on the nature of potential barrier. Understanding the formation and control of the tunnel barrier and it utilization in application developments is an important issue.

In this study the principal theme is to explore oxide multilayers that have BTO as one of the layers with Nb doped SrTiO3 (Nb:STO) as one of the electrodes. Nb:STO can be made moderately conducting and thus can be used as aconducting substrate on which BTO film can be grown. The other electrodes explored involve conventional metals like Cr-Au and metallic oxide ferromagnet like Strontium Ruthenate (SrRuO3 or SRO) depending the specific physics being explored. In this work we use a FE layer containing multilayer as a "tool" that can be used to understand a specific set of physics issues. Thus, the multilayer devices used in the thesis are not application tools but physics tools.

We use the temperature and electric bias dependent transport in the FE layer containing multilayer devices to explore such issues like effects of phase transition in BTO on Schottky barriers as it is cooled below the ferroelectric transition temperature, control and modulation of Schottky barriers formed at interfaces with BTO which can be a n-type semiconductor, aspects of spin polarized transport when the FM layer SRO is a minority band ferromagnet and last but not the least the effect of oxygen stoichiometry of the BTO layer on the junction transport.

Growing a BTO film with good ferroelectric property, high dielectric constant and controlled oxygen stoichiometry has played the key role in the experimental works. The challenge here was to observe low symmetry FE phase transitions in a thin film of 100 nm grown on a lattice matched substrate and then to separate out their contributions.

In the first set of work we have established that not only BTO films grown on substrate with good lattice match does bear signatures of low symmetry phase transitions but it changes the barriers at the film/substrate or contact/film interfaces as well. Then we show that in a FM-FE multilayer of SRO/BTO the junction transport mechanism from Schottky to a multistep process as the oxygen stoichiometry in BTO is reduced. The density gradient of oxygen vacancies in the sample lowers the depolarizing field in BTO thus emphasizing the effect of FM transition of SRO on the junction transport. The polarity of applied bias play a crucial role here that has been investigated in the third set of work.

We have studied bias dependent dynamic conductance of a FM-FE-NM tunnel junction (SRO/BTO/Nb:STO) as a function of temperature and magnetic field. The observed dynamic conductance curves are strongly asymmetric and have contributions  principally from two effects: bias induced strain in the BTO arising from inverse piezo electric effect and an anomalous asymmetric contribution resulting from SRO that reflects its negative spin polarization in the FM state. The effect of piezotronic strain changes the barrier thickness, the electron effective mass, and also the position of the conduction-band edge. The second effect is proportional to magnetization of SRO and arises from tunneling of electrons to/from minority spin sub-band of SRO, which is a negative spin polarization itinerant FM and leads to anomalous asymmetric contribution that shows up as a negative differential conductance.

This work contributes to a comprehensive study of electrical transport through FM-FE interface of complex oxides focusing on the low symmetry FE phases of BTO as well as the negative spin polarization of SRO that has not been reported before.