A SYSTEMATIC STUDY OF THERMODYNAMIC AND TRANSPORT PROPERTIES OF LAYERED Ca_n+1(Ru_1-xCr_x)_nO_3n+1

Author

Vinobalan Durairaj, University of KentuckyFollow

Date Available

12-14-2011

Year of Publication

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Arts and Sciences

Department/School/Program

Physics and Astronomy

Faculty

Dr. Gang Cao

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 CaRu_1−xCr_xO₃ from a paramagnetic state to an itinerant ferromagnetic state (M_S~0.4μ_B/f.u.), where T_C=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(Ru_1-xCr_x)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.

Recommended Citation

Durairaj, Vinobalan, "A SYSTEMATIC STUDY OF THERMODYNAMIC AND TRANSPORT PROPERTIES OF LAYERED Ca_n+1(Ru_1-xCr_x)_nO_3n+1" (2008). University of Kentucky Doctoral Dissertations. 661.
https://uknowledge.uky.edu/gradschool_diss/661