Scanning probe microscopy on ferroelectrics and ferromagnets

Abstract

This thesis deals with the investigation of exotic properties of ferroelectrics and ferromagnets by using scanning probe microscopy. The first part consists of the study of superconductivity induced in δ doped SrTiO3 system by laser irradiation. The second part discusses the inves tigation of ferroelectricity in the CsPbBr3 perovskite system by using temperature-dependent Piezo Force Microscopy (PFM). And the third part discusses the fabrication and investigation of magnetic properties of Permalloy (Ni0.8Fe0.2) ferromagnetic nanoislands. SrTiO3 (STO) is known to be an incipient ferroelectric which undergoes a structural phase transition from a cubic to a tetragonal phase below 105K. We have used a fast laser irradiation technique at room temperature to modify the surface such that the low temperature tetragonal phase is locally created and frozen at room temperature. Upon laser irradiation, multiple do mains of the tetragonal SrTiO3 form where each domain has a different long-axis direction. The domain boundaries are found to be highly conducting at room temperature. The irradiated system as a whole undergoes a superconducting transition below 150 mK. We will discuss a mechanism of the emergence of superconductivity in dilute electronic systems like d-doped STO in the light of our experiments. Novel ground states can also be obtained in low dimensional ferroelectric systems. We have investigated the ferroelectric domain structures in the nanosheets of the perovskite system CsPbBr3bytemperaturedependentpiezo-response force microscopy (PFM).Thedomainstruc ture is dependent on the temperature and the thickness of the nanosheets. Our PFM experi ments revealed the emergence of topological defects in the ferroelectric domains in the form of nanometre scale bubbles. The results raises the possibility of controlling photovoltaic effects in perovskite halides through ferroelectric domain imaging. Novel magnetic ground states can be realized in artificial lattices of magnetic nanoislands. We have fabricated square lattices of ellipsoidal nanoislands of ferromagnetic permalloy by elec tron beam lithography and investigated their magnetic configurations that emerged as a result of inter-island interactions, by magnetic force microscopy (MFM). We controlled the inter site interactions by engineering the materials and the lattice parameters. We varied the lattice constant and investigated the domain structures by using MFM. The MFM results revealed the emergence of two different types of domain structures: single dipole like and bloch like domains. Over a long range, the magnetic domains organize themselves in a complex archi tecture. We investigated origin of all such features by micromagnetic simulations using Object Oriented Micro Magnetic Framework (OOMMF).