arXiv subjects

Shekhar Tyagi

Publications and source records attributed to Shekhar Tyagi.

6 recordsLinked to original sources

arXiv2020

Imaging of Strain Driven Magnetic Domains and Strong Spin-Phonon Coupling in Epitaxial Thin Films of SrRuO3

Epitaxial thin films of SrRuO3 with large strain disorder were grown using pulsed laser deposition method which showed two distinct transition temperatures in Magnetic measurements. For the first time, we present visual evolution of magnetic domains across the two transitions using Magnetic force microscopy on these films. The study clearly showed that the magnetic anisotropy corresponding to the two transitions is different. It is observed that the perpendicular magnetic anisotropy is dominating in films which results in domain spin orientation preferably in out of plane direction. The Raman studies showed that the lattice is highly influenced by the magnetic order. The analysis of the phonon spectra around magnetic transition reveals the existence of strong spin-phonon coupling and the calculations resulted in spin-phonon coupling strength (λ) values of λ ~ 5 cm-1 and λ ~ 8.5 cm-1, for SrRuO3 films grown on LSAT and SrTiO3 single crystal substrates, respectively.

cond-mat.mtrl-sci↗

arXiv2019

Raman spectroscopic investigation of relaxor behavior in Pr doped SrTiO3 and Origin of Fano resonance

Detailed Raman spectroscopy studies on polycrystalline Sr1-xPrxTiO3 (x=0.01, 0.025, 0.05, 0.075, 0.09, 0.13, 0.15, 0.17) samples are reported elucidating the microscopic mechanism of relaxor ferroelectrics. The polar mode was observed upto very high temperature ~1000 K suggesting that the dipoles exists at temperatures well above the characteristic relaxor temperatures and they develop a short range correlation at 505 K leading to formation of PNRs. The TO2 polar mode showed anomalous softening in cooling below T~505 K supporting the growth of PNRs. Fano resonance is reported in the Pr doped compounds which decreases with increasing doping. Our work on Pr doped SrTiO3 reveals that the local TiO6 octahedral tilt scales with the intensity of the polar mode and hence can be used as an order parameter for relaxor transition. The study showed that the paraelectric to relaxor ferroelectric phase transition in this compound is random polarizability instability driven phenomenon which is correlated with the local octahedral tilt angle. The study supports competition and cancelation between lattice and polar instabilities at global length scale while cooperation between the two at local length scale. The modulation in local structure of the material in temperature interval related to dielectric anomaly has been observed which is not been investigated previously by any structural tool.

cond-mat.mtrl-sci↗

arXiv2019

Phonon Invisibility Driven by Robust Magneto-Elastic Coupling in AlFeO3 Thin Film

The thin films of lead free magneto-electric compound AlFeO3 have been deposited using pulsed laser deposition technique. X-ray diffraction, X-ray absorption spectroscopy and reflectivity measurements established the orthorhombic structure and material density of 4.5g/cc which is comparable with bulk AlFeO3. The Raman mode corresponding to AlFeO3 was found to vanish when magnetic field of 800 Oe was applied at room temperature. Additionally, it was observed that the Raman phonon mode present at the room temperature becomes invisible in the temperature window of 280K-236K and reappears below it. The detailed analysis of magnetization showed a change in magnetic order in this temperature interval. The invisibility of Raman phonon mode corresponding to AlFeO3 have been attributed to the lattice deformation caused by the magnetoelastic effect. The presence of strong spin-lattice coupling is also validated by the renormalization of phonon frequencies below 200 K.

cond-mat.mtrl-sci↗

arXiv2019

Strain Healing of Spin-Orbit Coupling: A Cause for Enhanced Magnetic Moment in Epitaxial SrRuO3 Thin Films

Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed the high spin state for the enhancement. Our magnetization results show high magnetic moment of 3.3 Bohr-magnetron/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 Bohr-magnetron/Ru ion observed in bulk compound. Contrary to the expectation the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin-orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment. Strain disorder is shown to be an efficient mean to control the spin-orbit coupling.

cond-mat.str-el↗

arXiv2018

Spin-lattice coupling mediated giant magnetodielectricity across the spin reorientation in Ca2FeCoO5

The structural, phonon, magnetic, dielectric, and magneto dielectric responses of the pure bulk Brownmillerite compound Ca2FeCoO5 are reported. This compound showed giant magneto dielectric response (10%-24%) induced by strong spin-lattice coupling across its spin reorientation transition (150-250 K). The role of two Debye temperatures pertaining to differently coordinated sites in the dielectric relaxations is established. The positive giant magneto-dielectricity is shown to be a direct consequence of the modulations in the lattice degrees of freedom through applied external field across the spin reorientation transition. Our study illustrates novel control of magneto-dielectricity by tuning the spin reorientation transition in a material that possess strong spin lattice coupling.

cond-mat.mtrl-sci↗

arXiv2018

Length-scale dependent average structures, piezoelectricity enhancement and depolarization mechanisms in a non-MPB high-performance piezoelectric alloy system PbTiO3-Bi(Zr1/2Ni1/2)O3

There is a general perception that large piezoelectric response in ferroelectric alloys requires tuning the system towards a morphotropic phase boundary (MPB), i.e., a composition driven inter-ferroelectric instability. Here we show that high piezoelectric response can be realized even in non-MPB alloy systems. This is demonstrated on (1-x)PbTiO3-(x)Bi(Zr0.5Ni0.5)O3 (PT-BNZ) by a comprehensive study involving electric-field and temperature dependent XRD, Raman spectroscopy, dielectric, piezoelectric and high field electrostrain measurements. We found that poling-field irreversibly suppresses the cubic-like phase at room temperature. Based on our results, we argue that that which appears as MPB, comprising of tetragonal and cubic-like phases on the global scale, is not so actually. The large piezoresponse is due to coexistence of tetragonal regions of long and short-range coherence. The PT-BNZ system is therefore qualitatively different from the conventional MPB systems such as PZT, PMN-PT, and PbTiO3-BiScO3, etc., which exhibits coexisting tetragonal and rhombohedral/monoclinic phases in thermodynamic equilibrium. In the absence of inter-ferroelectric instability as a phenomenon, field induced polarization-rotation and inter-ferroelectric transformation are no longer plausible mechanisms to explain the large piezoelectric response in PT-BNZ. The large piezoelectricity is primarily due to enhanced mobility of the tetragonal domain walls enabled by domain miniaturization. Our study proves that attainment of large piezoelectricity does not require inter-ferroelectric instability as a necessary criterion.

cond-mat.mtrl-sci↗