Electronic inhomogeneities play an important role in the definition of physical properties of correlated systems. To study these inhomogeneities one has to use local probe techniques which can distinguish electronic, magnetic and structural variations at the nanoscale. In the present work the high-field electron spin resonance technique (HF-ESR) is used to probe electronic and magnetic inhomogeneities in two transition-metal element based systems with very different properties. The first system is an iron based hightemperature superconductor, namely a member of a so called 1111-family, the (La,Gd)O1−xFxFeAs compound. Our HF-ESR spectroscopy study on Gd3+ ion has revealed that this material exhibits anisotropic interaction between Gd and Fe layers, which is frustrated in the absence of an external magnetic field. Moreover, the study of the superconducting samples has shown a coexistence of a static short range magnetic order with superconductivity up to high doping levels. The second system is a lightly hole doped cubic perovskite LaCoO3. Here, our HF-ESR investigation, complemented with static magnetometry and nuclear magnetic resonance techniques, has established that the hole doping induces a strong interaction between electrons on neighboring Co ions which leads to a collective high-spin state, called a spin-state polaron. These polarons are inhomogeneously distributed in the nonmagnetic matrix. This thesis is organized in three chapters. The first chapter gives basic ideas of magnetism in solids, focusing on the localized picture. The aim of the second chapter is to introduce the method of ESR. The third chapter is dedicated to the study of 1111-type iron arsenide superconductors. In the first part X-band (9.5 GHz) ESR measurements on 2% and 5% Gd-doped LaO1−xFxFeAs are presented. In the second part a combined investigation of the properties of GdO1−xFxFeAs samples by means of thermodynamic, transport and high-field electron spin resonance methods is presented. The last, fourth chapter presents the investigation of the unexpected magnetic properties of lightly hole-doped LaCoO3 cobaltite by means of the electron spin resonance technique complemented by magnetization and nuclear magnetic resonance measurements.

Electronic inhomogeneities play an important role in the definition of physical properties of correlated systems. To study these inhomogeneities one has to use local probe techniques which can distinguish electronic, magnetic and structural variations at the nanoscale. In the present work the high-field electron spin resonance technique (HF-ESR) is used to probe electronic and magnetic inhomogeneities in two transition-metal element based systems with very different properties. The first system is an iron based hightemperature superconductor, namely a member of a so called 1111-family, the (La,Gd)O1−xFxFeAs compound. Our HF-ESR spectroscopy study on Gd3+ ion has revealed that this material exhibits anisotropic interaction between Gd and Fe layers, which is frustrated in the absence of an external magnetic field. Moreover, the study of the superconducting samples has shown a coexistence of a static short range magnetic order with superconductivity up to high doping levels. The second system is a lightly hole doped cubic perovskite LaCoO3. Here, our HF-ESR investigation, complemented with static magnetometry and nuclear magnetic resonance techniques, has established that the hole doping induces a strong interaction between electrons on neighboring Co ions which leads to a collective high-spin state, called a spin-state polaron. These polarons are inhomogeneously distributed in the nonmagnetic matrix. This thesis is organized in three chapters. The first chapter gives basic ideas of magnetism in solids, focusing on the localized picture. The aim of the second chapter is to introduce the method of ESR. The third chapter is dedicated to the study of 1111-type iron arsenide superconductors. In the first part X-band (9.5 GHz) ESR measurements on 2% and 5% Gd-doped LaO1−xFxFeAs are presented. In the second part a combined investigation of the properties of GdO1−xFxFeAs samples by means of thermodynamic, transport and high-field electron spin resonance methods is presented. The last, fourth chapter presents the investigation of the unexpected magnetic properties of lightly hole-doped LaCoO3 cobaltite by means of the electron spin resonance technique complemented by magnetization and nuclear magnetic resonance measurements.