Quantum (spin) magnetism and charge transport (also depending on outermost orbital shape and lattice environment), when coupled strongly, in complex magnetic oxides trigger for vast technological applications. However, many fundamental and technical aspects are still a challenge for anticipated practical utilization of these fascinating phenomena. My doctoral research primarily investigated the magnetic ferroelectricity (M-FE) in non-antiferromagnetic (AFM) or say, uncommon ferro-/ferri-magnetic (FM/FiM) systems for applications at low magnetic fields. Alongside, I investigated the 5d spin-orbital magneto-electric layered Iridate Sr2IrO4, isostructural to the parent compound of celebrated 3d High-TC Superconducting (HTSc) Cuprates. The key outcomes of the doctoral thesis research work are discussed in two categories as follows:

Magnetic ferroelectricity in “orbital-quenched” 3d oxides from “non-magnetic” d0 (type-I) to“spin-frustrated” dn (type-II) origin: I started with FM double perovskites, wherein a remarkably “large” low-field (H ≤ 5 kOe) “intrinsic” magneto-dielectric coupling, ∆ε′/ε′ ≃ 6 % (near TC) is achieved in high-quality epitaxial thin films of d0-based (type-I) M-FE Bi2NiMnO6, driven by inherent exchange magnetoelectric interactions [A. Rathi et al., J. Phys. D: Appl. Phys. 50, 135006 (2017)]. In the family, I then revealed exchange-striction (type-II) M-FE in “collinear” FM Nd2CoMnO6 with a high spontaneous polarization, PS ∼ 1.3 µC/cm2 from an “intrinsic” spin-frustrated ↑↑↓↓ spin-chain and an “extrinsic” antisite disorder [A. Rathi et al., J. Phys. D: Appl. Phys. 50, 465001 (2017)]. I next explored the spin- and orbital-frustrated (Jahn-Teller-active) “non-collinear” FiM Spinel NiCr2O4 and observed signatures of “spiral” (type-II) magnetic ferroelectricity with magnetic transitions (FiM, AFM & “lock-in” of spiral moment), similar to that in (non-Jahn-Teller-active) “spiral” type-II M-FE CoCr2O4, despite the relieving of spin frustration via structural transitions (concurrently with magnetic transitions) caused by orbital-frustration and magnetoelastic effect in the system. In the sharp contrast, the M-FE signatures is suppressed in NiCr2O4 at the nanoscale with significant enhancement in the magnetic “collinearity” (than in bulk form), while no ordering of “spin-frustrated” AFM interactions is observed down to 3 K, indicating a rare co-existence of FiM ordering and possible “spin-liquid” state [A. Rathi et al., J. Magn. Magn. Mater. 474, 585 (2019) & Phys. Rev. B (2020), submitted].

Competing magnetic interactions and “coupled” Insulating state in 5d “spin-orbital” Iridate: The detailed investigation on Sr2IrO4 firstly revealed the signatures of non-analytic “Griffiths singularity” above TN at low magnetic fields and a crossover of the dominant magnetic interactions from weak FM to AFM at low temperatures. The critical exponent, β = 0.18 obtained from Bray analysis in Griffiths regime show highly “anisotropic” 2D XYh4 interactions, in corroboration with a crossover from low-field “highly anisotropic” to high-field “isotropic” 2D magnetic interactions in specific heat study. Importantly, the study suggest a continuous metal-insulator transition with “Slater” gap formation, followed by a Griffiths singularity and Anderson localization, unlike strong correlation physics in 3d parent Cuprates. The study emphasizes on the inclusion of local disorder (like irregular IrO6 octahedral rotation), competing intralayer/interlayer magnetic interactions to understand magnetism and insulating state in Sr2IrO4 [A. Rathi et al., J. Magn. Magn. Mater. 468, 230 (2018) & J. Magn. Magn. Mater. (2020), ready for communication].

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