Publications | Rajnandini Sharma

Thesis

Preprints

Element-specific probe of quantum criticality in $CeCoIn_5$

A Khansili, R. Sharma, R Hissariya, I Baev, J Schwarz, F Kielgast, M Nissen, M Martins, M-J Huang, M Hoesch, and others

arXiv preprint, Preprints

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Employing the elemental sensitivity of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), we study the valence and magnetic order in the heavy fermion superconductor CeCoIn. We probe spin population of the f-electrons in Ce and d-electrons in Co as a function of temperature (down to 0.1 K) and magnetic field (up to 6 T). From the XAS we find a pronounced contribution of Ce component at low temperature and a clear temperature dependence of the Ce valence below 5 K, suggesting enhanced valence fluctuations, an indication for the presence of a nearby quantum critical point (QCP). We observe no significant corresponding change with magnetic field. The XMCD displays a weak signal for Ce becoming clear only at 6 T. This splitting of the Kramers doublet ground state of Ce is significantly smaller than expected for independent but screened ions, indicating strong antiferromagnetic pair interactions. The unconventional character of superconductivity in CeCoIn is evident in the extremely large specific heat step at the superconducting transition.
@article{khansili2022element, title = {Element-specific probe of quantum criticality in $CeCoIn_5$}, author = {Khansili, A and Sharma, R. and Hissariya, R and Baev, I and Schwarz, J and Kielgast, F and Nissen, M and Martins, M and Huang, M-J and Hoesch, M and others}, journal = {arXiv preprint}, year = {Preprints}, google_scholar_id = {ohlvGxYAAAAJ&hl}, }
All solution grown epitaxial magnonic crystal of thulium iron garnet thin film

R. Sharma, Pawan Kumar Ojha, Simran Sahoo, Rijul Roychowdhury, and Shrawan Kumar Mishra

arXiv preprint, Preprints

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Magnonics has shown the immense potential of compatibility with CMOS devices and the ability to be utilized in futuristic quantum computing. Therefore, the magnonic crystals, both metallic and insulating, are under extensive exploration. The presence of high spin-orbit interaction induced by the presence of rare-earth elements in thulium iron garnet (TmIG) increases its potential in magnonic applications. Previously, TmIG thin films were grown using ultra-high vacuum-based techniques. Here, we present a cost-effective solution-based approach that enables the excellent quality interface and surface roughness of the epitaxial TmIG/GGG. The deposited TmIG (12.2 nm) thin film's physical and spin dynamic properties are investigated in detail. The confirmation of the epitaxy using X-ray diffraction in -scan geometry along with the X-ray reflectivity and atomic force for the thickness and roughness analysis and topography, respectively. The epitaxial TmIG/GGG have confirmed the perpendicular magnetic anisotropy utilizing the polar-magneto-optic Kerr effect. Analyzing the ferromagnetic resonance study of TmIG/GGG thin films provides the anisotropy constant K = 20.610 0.210 N/m and the Gilbert damping parameter $\alpha$ = 0.0216 0.0028. The experimental findings suggest that the solution-processed TmIG/GGG thin films have the potential to be utilized in device applications.
@article{sharma2023all, title = {All solution grown epitaxial magnonic crystal of thulium iron garnet thin film}, author = {Sharma, R. and Ojha, Pawan Kumar and Sahoo, Simran and Roychowdhury, Rijul and Mishra, Shrawan Kumar}, journal = {arXiv preprint}, year = {Preprints}, google_scholar_id = {ohlvGxYAAAAJ&hl}, }

2024

Dynamics of phonons, charge-regulated itinerant ${VO}_2$ states, and their impacts on the memristor properties of thin ${VO}_2$ films

Pawan Kumar Ojha, R. Sharma, VG Sathe, S Ram, and SK Mishra

Surfaces and Interfaces, 2024

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A molecular VO2 semiconductor (SC) reveals exotic properties of electronic switches, memristors, neuromorphic computing, and other devices at a nonlinear charge order at coupled ‘e-e’ and ‘e-p’ interactions (e: electron and p: phonon). Temperature (and/or other means) induced electron-hole ‘e−−h+’ pairs order at successive VO2 polymorphic phases, with a congruent SC to metallic phase transition at Tc ∼ 345 K. To develop these strategies, here we study the phonons (Raman bands, 100–800 cm−1), minor I-V (current-voltage) loops at small ramps of 1 to 30 V fields, and impedance resistance (at 102 to 106 Hz frequencies) at thin VO2 films (≤ 100 nm thickness). The films contain epitaxial (011) VO2 nanocrystals (NCs). The temperature is varied from 295 K to 385 K in which the VO2 phase orders led over thermal-induced charge carriers. A model VO68− molecular VO2 unit is used to analyse the phonon bands that soften and lose intensities (vanish at the Tc) at the metallicity rises up towards the Tc point. It describes the total 18 phonons (with three ones at a shorter V4+−O − V4+ dimer bond) that are sorted out at varied temperatures in a thermal cycle. Due transitions M1 → M2/R1/R2/R-VO2 are resolved (at a monoclinic M1-VO2 phase) at the specific conditions. An applied frequency is shown to promote the distinct VO2 phases at a nonlinear charge order mediated with the phonons states. An inbuilt strain induces a phonon hardening what is it releases at softer metallic VO2 states. The minor I-V loops swipe oppositely over the applied fields at the two major types of VO2 electronic bands. The results are described in correlation to the VO2 network structure in small NCs in the films of a memristor.
@article{ojha2024dynamics, title = {Dynamics of phonons, charge-regulated itinerant ${VO}_2$ states, and their impacts on the memristor properties of thin ${VO}_2$ films}, author = {Ojha, Pawan Kumar and Sharma, R. and Sathe, VG and Ram, S and Mishra, SK}, journal = {Surfaces and Interfaces}, volume = {46}, pages = {104029}, year = {2024}, google_scholar_id = {ohlvGxYAAAAJ&hl}, doi = {https://doi.org/10.1016/j.surfin.2024.104029}, publisher = {Elsevier} }

2023

Antisites disorder mediated magnetization relaxation and polydispersity in ${La_2NiMnO}_6$ crystallites

R. Hissariya, R. Sharma, and SK Mishra

Journal of Physics and Chemistry of Solids, 2023

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Antisite disorders play a critical role in determining the magnetic ground state of the La2NiMnO6 double perovskite compounds that show multifunctional behaviours and spin-glass phases at lower temperatures. The spin-glass (SG) state and magnetization relaxation in these compounds have been studied extensively; however, the polydispersity of the SG state remains elusive. Employing ac-magnetic susceptibility and temperature-dependent electron spin resonance (ESR) spectroscopy, herein, we probe the polydispersity of the SG phase in La2NiMnO6 crystallites. We employed X-ray photoelectron spectroscopy (XPS) to -investigate the crystallite size-dependent variable fractions of Mn3+/Mn4+ (Ni2+/Ni3+) cations and their influence on completing exchange interactions. Mn-2p core-level XPS spectra suggest a dominant Mn4+ + Ni2+→Mn3+ + Ni3+ charge transfer peak that appeared as a satellite peak at lower binding energy than the Mn core-level. The observed differences in saturation magnetization (MS) as a function of crystallite size indicate the variation in the degree of antisites disorder. Competing magnetic interactions driven by mixed valence and disorder facilitate a spin-glass (SG) phase at lower temperatures. Utilizing ESR measurement across paramagnetic → competing ferro/antiferromagnetic phase, a modified g-factor that ranges from 2.050-2.037 is observed in the paramagnetic region. The line width of the ESR signals is found to be increased across the transition suggesting spin-freezing characteristics. Cole-Cole plots obtained from data indicate collective spin relaxation dynamics in the proximity of the freezing temperature. Our findings suggest the importance of the size effect and its relation with the degree of antisites disorders in these crystallites. These experimental results enable the La2NiMnO6 compound to find its possible applications in new areas of material research.
@article{hissariya2023antisites, title = {Antisites disorder mediated magnetization relaxation and polydispersity in ${La_2NiMnO}_6$ crystallites}, author = {Hissariya, R. and Sharma, R. and Mishra, SK}, journal = {Journal of Physics and Chemistry of Solids}, volume = {181}, pages = {111549}, year = {2023}, google_scholar_id = {ohlvGxYAAAAJ&hl}, doi = {https://doi.org/10.1016/j.jpcs.2023.111549}, publisher = {Elsevier} }
Magnetic ordering in sol-gel-based $Tm_3Fe_5O_{12}$ thin films

R. Sharma, PK Ojha, S Choudhary, and SK Mishra

Materials Letters, 2023

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In the quest to utilize rare-earth garnets (ReIG) with perpendicular magnetic anisotropy (PMA) in futuristic spintronics, the attention turned towards iron garnet thin films. Among this family, the one candidate is Tm3Fe5O12 (TmIG) that manifests PMA with excellent Gilbert damping, making this material a potential candidate in magnonics devices. Here, we report a cost-effective deposition approach of the TmIG thin films deposited on a thermally oxidized Si(100) substrate. The homogeneous TmIG (22 nm) thin film shows a roughness of 1.5 nm. The magnetic properties of thin films indicating the compensation temperature 15 K. Experimental findings present a method for the deposition of TmIG thin films, which is a potential candidate for fabricating futuristic compact devices.
@article{sharma2023magnetic, title = {Magnetic ordering in sol-gel-based $Tm_3Fe_5O_{12}$ thin films}, author = {Sharma, R. and Ojha, PK and Choudhary, S and Mishra, SK}, journal = {Materials Letters}, volume = {352}, pages = {135154}, year = {2023}, google_scholar_id = {ohlvGxYAAAAJ&hl}, doi = {https://doi.org/10.1016/j.matlet.2023.135154}, publisher = {Elsevier} }
Charge ordering at a dielectric gate in itinerant metallic states with low-field memristor properties in ${VO}_2$ thin films

PK Ojha, R. Sharma, SK Mishra, and S Ram

Surfaces and Interfaces, 2023

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Vanadium dioxide (VO2) − a non-stoichiometric oxide semiconductor (SC) offers exotic properties at a self-confined structure of correlated 3d1-electrons (spins) useful for non-volatile memory devices, smart switches, and human brain-inspired neuromorphic devices. Poor chemical stability and fragile nature limit its technologies of thin films. In view of resolving some of these issues, we developed polymer stabilized VO2 films (thickness t ≤ 100 nm), using VO2 nanocolloids in poly(vinylpyrrolidone) (PVP) (as a VO2 dispersoid, a molecular template, and a film former) in water, at a (100) Si(p++) substrate. Using a nano SiO2/TiO2 gate (t ≤ 10 nm), VO2 is grown (011) preferentially in a confined shape of nanoplates (nanocrystals) along the films, mostly of 15 to 40 nm widths at 20–30 nm crystallite size. The results are described with X-ray diffraction, surface topologies, lattice images, and X-ray photoelectron spectroscopy (XPS) of films in the variable charges 2V4+ → V3+ + V5+ order at the itinerant metallic states. A significant V5+-3d0, ≤ 33 at%, is shown in the XPS bands, which induces metallic states at conducting ‘V4+ → V5+ + e−’ channels. So, a charge-regulated SC → metal transition incurs via an induced M1 → R-VO2 metallic state near room temperature. A memristor VO2@TiO2/Si so made renders a wide current-voltage (I-V) loop at room temperature, with a leakage current that is well controlled at a high-k TiO2 gate. It exhibits a reversible switching at a duly small threshold field, Vt ≤ 0.2 V. This is the smallest Vt tuned so far beneficial for the low field, ≤ 1.0 V, devices. The charge models corroborate the effect of field-induced charge order at the interfaces of ‘conducting through channels’, regulating a reversible I-V hysteresis in an on-off cycle.
@article{ojha2023charge, title = {Charge ordering at a dielectric gate in itinerant metallic states with low-field memristor properties in ${VO}_2$ thin films}, author = {Ojha, PK and Sharma, R. and Mishra, SK and Ram, S}, journal = {Surfaces and Interfaces}, volume = {42}, pages = {103445}, year = {2023}, google_scholar_id = {ohlvGxYAAAAJ&hl}, doi = {https://doi.org/10.1016/j.surfin.2023.103445}, publisher = {Elsevier} }
Magnetic energy dissipative factors of spin-coated $Y_3Fe_5O_{12}$ thin films

R. Sharma, PK Ojha, and SK Mishra

Thin Solid Films, 2023

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YFeO12 (YIG) is a ferrimagnetic insulator that shows significant potential in various applications such as spin pumping and optical devices. In the case of YIG, the uncompensated antiferromagnetic coupled Fe3＋ ions originate the ferrimagnetic ordering. A homogeneous YIG thin film with lower Gilbert damping constant is essential for its industrial applications. Therefore, the YIG’s intrinsic and extrinsic magnetic energy dissipative factors are crucial to explore. Here, we report the growth of uniform polycrystalline YIG thin films on thermally oxidized Si (100) substrates. X-ray diffraction confirms the single-phase formation of YIG. Surface morphology and thickness on thin films are studied using scanning electron microscopy. X-ray photoemission is used to probe the valence state of constituent elements of the ferrimagnetic thin films with saturation magnetization 3.11 /f.u. The lowest Gilbert damping constant $\alpha$ = 4.754 × 10−3 with an inhomogeneous contribution to the linewidth of 5.04 mT is observed due to extrinsic inhomogeneous growth of the deposited films.
@article{sharma2023magnetic1, title = {Magnetic energy dissipative factors of spin-coated $Y_3Fe_5O_{12}$ thin films}, author = {Sharma, R. and Ojha, PK and Mishra, SK}, journal = {Thin Solid Films}, volume = {764}, pages = {139625}, year = {2023}, google_scholar_id = {ohlvGxYAAAAJ&hl}, doi = {https://doi.org/10.1016/j.tsf.2022.139625}, publisher = {Elsevier} }
The micromagnetic study of stabilizing parameters for the interfacial skyrmions

R. Sharma, and Shrawan Kumar Mishra

Materials Today: Proceedings, 2023

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Skyrmions are localized topologically protected spin arrangements that can be mapped to a unit sphere. Various energy factors such as exchange energy, Dzyaloshinskii- Moriya (DM) interaction, uniaxial magnetocrystalline anisotropy, and Zeeman energy play a significant role in the stabilization of the skyrmions. Using micromagnetics as a tool, here we simulate these fundamental parameters required for stabilizing magnetic skyrmions. Our work comprehensively predicts how magnetic texture modification can be expected with variable values of the exchange constant, uniaxial magnetocrystalline anisotropy constant, DM constant, and saturation magnetization. Our results indicate that higher saturation magnetization dissolves the skyrmions. Among all these factors, DM interaction plays a crucial role, as the higher value of the DM constant helps in the evolution and creation of skyrmions. An optimum value of exchange energy is favorable for skyrmion stability. Similarly, a higher anisotropy constant further reduces the skyrmions size.
@article{sharma2023micromagnetic, title = {The micromagnetic study of stabilizing parameters for the interfacial skyrmions}, author = {Sharma, R. and Mishra, Shrawan Kumar}, journal = {Materials Today: Proceedings}, volume = {80}, pages = {1205--1208}, year = {2023}, google_scholar_id = {ohlvGxYAAAAJ&hl}, doi = {https://doi.org/10.1016/j.matpr.2022.12.199}, publisher = {Elsevier} }

2022

Interfacial skyrmion in magnetic thin films and its applications

R. Sharma, and Shrawan Kumar Mishra

Journal of Magnetism and Magnetic Materials, 2022

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Topologically protected spin configurations are predicted to be the future of information storage and other computational applications. Through various investigations it has been proven that magnetic skyrmions consume a low driving current compare to magnetic domain wall, which can reduce the Joule heating significantly in memory storage devices and facilitate energy efficient, faster and miniaturized memory storage. Recently, numerous host materials are studied in condensed matter for exploiting the skyrmions. Experimentally magnetic multilayers consisting of repeated ferromagnetic (FM)/ heavy metal (HM) can stabilize interfacial skyrmions at near room temperature. In this seminal review, a brief introduction of the microscopic origin of the magnetic skyrmions with the theoretical models that have been used to simulate the interfacial skyrmions along with the important experimental techniques, which have been utilized to detect the interfacial skyrmions is presented. We also highlighted the progress on the potential applications of skyrmions in racetrack memory, logic gates, and neuromorphic devices.
@article{sharma2022interfacial, title = {Interfacial skyrmion in magnetic thin films and its applications}, author = {Sharma, R. and Mishra, Shrawan Kumar}, journal = {Journal of Magnetism and Magnetic Materials}, volume = {551}, pages = {169107}, year = {2022}, google_scholar_id = {ohlvGxYAAAAJ&hl}, publisher = {Elsevier}, doi = {https://doi.org/10.1016/j.jmmm.2022.169107} }
Observation of V--V dimers softening and distinct length scales in nanostructured ${VO}_2$ thin films

PK Ojha, R. Sharma, R Hissariya, S Babu, E Ketkar, S Singh, S Neema, A Rana, N Pal, VG Sathe, and others

Journal of Physics and Chemistry of Solids, 2022

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Employing temperature-controlled Raman spectroscopy, we demonstrate V–V dimers softening in nanostructured VO2 thin films. Temperature-dependent Raman band shifts and their spectral width (FWHM) suggest an intriguing phonon characteristic that evolves across the metal-insulator transition (MIT) during the heating/cooling thermal cycles. The V–V dimers start to collapse above a critical temperature (T > TMIT) and restoring their initial phase during the cooling process below TMIT. Raman bands suggest an abrupt reversible switching in the optical behaviour at a temperature (To = 332 K). Temperature-dependent sheet resistance variations indicate a smooth reversible resistive switching relatively at a lower temperature (Te = 340 K). The difference in transition temperatures has been discussed in the framework of light scattering cross-section from the metallic domains that become more extensive at the critical length scale required for percolation conduction in resistive switching. 2D-mean-field approximation associated with Mie scattering are modelled to explain the percolation process during phase transition and to estimate the metallic domain length scale evolved in the insulating matrix across the MIT. Our experimental findings explain the occurrence of optical and electrical transitions at the district temperature range, suggesting a weak coupling between lattice and optical switching.
@article{ojha2022observation, title = {Observation of V--V dimers softening and distinct length scales in nanostructured ${VO}_2$ thin films}, author = {Ojha, PK and Sharma, R. and Hissariya, R and Babu, S and Ketkar, E and Singh, S and Neema, S and Rana, A and Pal, N and Sathe, VG and others}, journal = {Journal of Physics and Chemistry of Solids}, volume = {163}, pages = {110564}, year = {2022}, google_scholar_id = {ohlvGxYAAAAJ&hl}, doi = {https://doi.org/10.1016/j.jpcs.2021.110564}, publisher = {Elsevier} }

2018

Effect of 120 MeV $^{28}Si^{9+}$ ion irradiation on structural and magnetic properties of $NiFe_2O_4$ and $Ni_{0.5}Zn_{0.5}Fe_2O_4$

R. Sharma, S Raghuvanshi, M Satalkar, SN Kane, TR Tatarchuk, and F Mazaleyrat

In AIP Conference Proceedings, 2018

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$NiFe_2O_4$, $Ni_{0.5}Zn_{0.5}Fe_2O_4$ samples were synthesized using sol-gel auto combustion method, and irradiated by using 120 MeV $^{28}Si^{9+}$ ion with ion fluence of 1×1012 ions/cm2. Characterization of pristine, irradiated samples were done using X-Ray Diffraction (XRD), Field Emission Scanning Microscopy (FE-SEM), Energy Dispersive X-ray Analysis (EDAX) and Vibrating Sample Magnetometer (VSM). XRD validates the single phase nature of pristine, irradiated Ni- Zn nano ferrite except for Ni ferrite (pristine, irradiated) where secondary phases of α−Fe2O3 and Ni is observed. FE- SEM images of pristine Ni, Ni-Zn ferrite show inhomogeneous nano-range particle size distribution. Presence of diamagnetic ion (Zn2+) in NiFe2O4 increases oxygen positional parameter (u 4̄3m ), experimental, theoretical saturation magnetization (Msexp., Msth.), while decreases the grain size (Ds) and coercivity (Hc). With irradiation Msexp., Msth. increases but not much change are observed in Hc. New antistructure modeling for the pristine, irradiated Ni and Ni-Zn ferrite samples was used for describing the surface active centers.
@inproceedings{sharma2018effect, title = {Effect of 120 MeV $^{28}Si^{9+}$ ion irradiation on structural and magnetic properties of $NiFe_2O_4$ and $Ni_{0.5}Zn_{0.5}Fe_2O_4$ }, author = {Sharma, R. and Raghuvanshi, S and Satalkar, M and Kane, SN and Tatarchuk, TR and Mazaleyrat, F}, booktitle = {AIP Conference Proceedings}, volume = {1953}, number = {1}, year = {2018}, google_scholar_id = {ohlvGxYAAAAJ&hl}, doi = {https://doi.org/10.1063/1.5032452}, organization = {AIP Publishing} }