A SYSTEMATIC STUDY OF THERMODYNAMIC AND TRANSPORT PROPERTIES OF LAYERED Ca\u3csub\u3en+1\u3c/sub\u3e(Ru\u3csub\u3e1-x\u3c/sub\u3eCr\u3csub\u3ex\u3c/sub\u3e)\u3csub\u3en\u3c/sub\u3eO\u3csub\u3e3n+1\u3c/sub\u3e

Abstract

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