• R RANJITH

      Articles written in Bulletin of Materials Science

    • Tailoring the bandgap and magnetic properties by bismuth substitution in neodymium chromite

      VENKATESWARA RAO MANNEPALLI M M SAJ MOHAN R RANJITH

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      The intrinsic distortions present in rare-earth orthochromites (RCrO$_3$) observed from lanthanum to lutetium (inR-site) can influence the magnetic properties like Neel transition and weak ferromagnetic coupling. A nonmagnetic cationwith similar ionic radius would be a suitable candidate to engineer the inherent distortions of particular orthochromite. In thisstudy, bismuth (Bi$^{3+}$) with a 6s$^2$ lone pair was chosen to substitute in neodymium (Nd$^{3+}$) site of NdCrO$_3$ (NCO) to tailor the intrinsic structural distortions. The variation of optical absorption edge evidently suggests that Bi (6s$^2$) substituted in the magnetic rare-earth Nd$^{3+}$ influences the Cr–O overlap integral. The interaction of Bi cation with oxygen bonds influencesthe structural distortions through Cr–O polyhedral, which are evident from Raman scattering studies. The observed structuraland magnetic properties of similar ionic radius of Bi$^{3+}$ in Nd$^{3+}$ reveal that intrinsic structural distortions are interrelatedto enhanced weak ferromagnetic component and change in Neel and spin reorientation temperatures in our compounds. Inaddition, a reduction in the optical bandgap of NCO from 3.1 to 2.6 eV was observed.

    • Effect of non-180° polarization invariants on the exchange bias of tetragonal ⟨001⟩ and rhombohedral ⟨111⟩ orientations of bismuth ferrite epitaxial thin films

      M M SAJMOHAN R RANJITH J A CHELVANE

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      BiFeO₃ (BFO) is the only room temperature multiferroic material that has been extensively studied due to its multifunctional properties. BFO with a canted G-type antiferromagnetic (AFM) ordering exhibits strong exchange bias characteristics with NiFe which offers the potential to design and utilize devices working based on multiferroic features. In the past, it was known that the presence of 180° domain walls of BFO hinders a plausible exchange bias interaction. To understand the role of the strain-induced effects on such 180° domain walls and its effect on the exchange bias, NiFe, a soft ferromagnetic layer, was grown on the epitaxial BFO AFM layers. An approximately 80 nm BFO layer was grown epitaxially in both the tetragonal (001) phase on the LaAlO₃ (001) substrate and the rhombohedral (111) phase on the SrTiO₃ (111) substrate, with a thin (10 nm) layer of Ni₈₀Fe₂₀ on top of it. An exchange bias of 510 Oe was observed in the tetragonal phase of BFO with a $c/a$ ratio of 1.22, which is comparable with the exchange bias shown by the (111) oriented rhombohedral phase (360 Oe). Both the tetragonal (001) and rhombohedral (111) layers possess ferroelectric polarization normal to the sample surface and so the domain walls are mostly 180° oriented which is expected to have a minimum effect on the exchange bias. However, the weak strain-induced structural variants in the (111) oriented rhombohedral BFO and the monoclinic distortion present in the tetragonal BFO introduce non-180° domain walls in the system. These variants arising due to the structural distortion are expected to play a key role in defining the ferroelectric domain wall nature, thereby exhibiting exchange bias characteristics.

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