Correlated electron materials have been at the forefront of condensed matter research in the past couple of decades. Correlation in materials, especially, with open d and f electronic shells often lead to very exciting and intriguing phenomenon like high temperature superconductivity, Mott metal-insulator transition, colossal magnetoresistance (CMR). This thesis focuses on triple-layered Sr4Ru3O10, Sr substituted double layered (Ca1-- xAx)3Ru2O7 (A = Ba, Sr) and 5d system Sr2IrO4 and Sr3Ir2O7. Triple-layered Sr4Ru3O10 displays interesting phenomena ranging from quantum oscillations, tunneling magnetoresistance, unusual low temperature specific heat, strong spin-lattice coupling to switching behavior. The central feature, however, is the unique borderline magnetism: along the c-axis. Sr4Ru3O10 shows spontaneous ferromagnetism, indicating a strong Coulomb exchange interaction, U and a large density of states at the Fermi surface, g(EF ), hence Ug(EF ) ≥ 1 (Stoner criterion). But within the ab-plane it features a pronounced peak in magnetization and a first-order metamagnetic transition. The coexistence of the interlayer ferromagnetism and the intralayer metamagnetism makes Sr4Ru3O10 a really unique system. Also, in this thesis the spin-valve behavior exhibited by impurity doping at the Ca site by Ba and Sr in the double layered Ca3Ru2O7 is reported. Spin valve effect is a phenomenon only realized in multilayer thin films. Here, spin valve is observed in bulk single crystals of impurity dopedCa3Ru2O7, Ca3(Ru1-xCrx)2O7 and (Ca1- xAx)3Ru2O7 (A = Ba, Sr). 5d Iridates are expected to be more metallic and less magnetic than their 3d and 4f counterparts because of the extended 5d orbitals. In marked contrast, many iridates are magnetic insulators with exotic properties. The focus in this thesis is on Sr2IrO4 which diplays a novel Jeff = 1/2 Mott state. Magnetic, electrical, and thermal measurements on single-crystals of Sr2IrO4, reveal a novel giant magneto-electric effect (GME) arising from a frustrated magnetic/ferroelectric state. The GME and electric polarization hinge on a spin-orbit gapping of 5d-bands, rather than the magnitude and spatial dependence of magnetization, as traditionally accepted.

Orbital degrees of freedom play vital role in prompting novel phenomena in ruthenium based Ruddlesden-Popper compounds through coupling of orbits to spin and lattice. Physical properties are then particularly susceptible to small perturbations by external magnetic fields and/or slight structural changes. Current study pertains to the impact when a more-extended 4d Ruthenium ion is replaced by a less-extended 3d Chromium ion. Perovskite CaRuO3 (n=∞) is characterized by borderline magnetism and non- Fermi liquid behavior – common occurrences in quantum critical compounds. Remarkably, Cr substitution as low as x=0.05 abruptly drives CaRu1−xCrxO3 from a paramagnetic state to an itinerant ferromagnetic state (MS~0.4μB/f.u.), where TC=123K for x=0.22. The Cr-driven magnetism is highly anisotropic suggesting an important role of spin-orbit coupling. Unlike other chemical substitutions in the compound, Cr does not induce any Metal-Insulator transition that is expected to accompany the magnetic transition. The results indicate a coupling of Ru-4d and Cr-3d electrons that is unexpectedly favorable for itinerant ferromagnetism, which often exists delicately in the ruthenates. Bilayered Ca3Ru2O7 (n=2), an abode of huge anisotropy, exhibits a wide range of physical properties – Colossal Magnetoresistance occurring only when the spin polarized state is avoided, Antiferromagnetic-Metallic (AFM-M) state, Quantum Oscillations (periodic in 1/B and in B) that are highly angular dependent, to mention a few. Experimental results obtained so far provide a coherent picture illustrating that orbital order and its coupling to lattice and spin degrees of freedom drive the exotic electronic and magnetic properties in this Mott-like system. Transport and thermodynamic studies on Ca3(Ru1-xCrx)2O7 (0 ≤ x ≤ 0.20) reveal that AFM-M region is broadened with x that ultimately reaches 70K for x=0.20 (~8K for x=0). In this region, electron transport is enhanced and inhibited when B is applied along crystal’s respective axes, confirming an intrinsic half-metallic behavior. Moreover, the difference in coercivities of Ru and Cr magnetic ions pave way for the first-ever observation of a strong spin-valve effect in bulk material, a quantum phenomenon so far realized only in multilayer thin films or heterostructures. This discovery opens new avenues to understand the underlying physics of spin-valves and fully realize its potential in practical devices.