arXiv subjects

Dipanjan Mazumdar

Publications and source records attributed to Dipanjan Mazumdar.

10 recordsLinked to original sources

arXiv2026

Measurement of Kerr rotation using a variable-angle polarizer method

The magneto-optical Kerr effect (MOKE) occurs when polarized light reflects from a magnetized surface, causing a small change in the polarization angle and state. Measurement of the rotation in the angle (Kerr rotation) is well established and typically performed close to the null configuration in a polarizer-analyzer geometry. However, accurate measurement always remains a challenge and as the effect depends intricately on several optical parameters. Here we performed a series of longitudinal magneto-optical Kerr effect (MOKE) measurements on $p$ and $s$ polarized laser light at various polarizer angles on a Cobalt thin film and measured the orthogonal components of the reflected polarized light using a Wollaston prism. Analytical expressions for the orthogonal light components were fitted to the average intensity and the MOKE signal measured at different polarizer angles to obtain the Kerr roations. Our analysis yielded a $p (s)$-Kerr rotation of 0.46 (0.65) milliradians for a 633~nm laser at 45$^\circ$ angle of incidence, which agrees very well with our estimated value for Co using available literature data. Apart from being very accurate, the advantage of the process is that it eliminates the inherent uncertainties in single-point measurements of the Kerr rotation.

cond-mat.mtrl-sci↗

arXiv2026

Effect of annealing temperature on the structure and properties of co-sputtered Fe-Mn-Sn films near 2:1:1 ratio

Research in recent years has focused on the thin-film synthesis of high-quality ternary alloys, identified for their tunable properties and potential in spintronics (e.g., Heusler alloys, Kagome magnets). In a previous study, we identified the conditions for stabilizing Fe$_2$MnSn, a Kagome magnet with a high Curie temperature and magnetic anisotropy. However, ternary phases such as Fe$_2$MnSn are challenging to synthesize and stabilize within a narrow temperature window, as binary and elemental phases can also form during the growth process. To highlight these observations, we investigated the thin film phases in the Fe-Mn-Sn system near the 2:1:1 ratio as a function of annealing temperature, ranging from 400 to 700\degree C. The elemental Fe, Mn, and Sn targets were pre-calibrated to a close to 2:1:1 ratio and co-sputtered at room temperature, followed by annealing. Two binary hexagonal structures, Fe$_3$Sn$_2$ and Fe$_5$Sn$_3$, along with the elemental Fe phase, are stabilized between 400-550\degree C, but disappear at 580\degree C, where Fe$_2$MnSn is the only stable phase. Elemental Mn phase starts to appear starting from 600\degree C, and becomes dominant by 750\degree C. Electrical, magnetic and magneto-optical properties are observed to correlate with the structural findings and the best properties are observed in the temperature range where Fe$_2$MnSn is the dominant phase. In general, our study highlights the difficulty in growing phase-pure ternary alloys such as Fe$_2$MnSn, which is very strongly based on precise temperature conditions. We also observed significant disordered growth below 100 nm for Fe$_2$MnSn, implying poor thickness scaling behavior.

cond-mat.mtrl-sci↗

arXiv2025

Spectroscopic Characterization of Metallocene Single Crystals Grown by Physical Vapor Transport Method

High-quality metallocene single crystals with a low density of impurities and high homogeneity were prepared using the physical vapor transport method. These crystals were then characterized using various spectroscopic tools and X-ray diffraction. Laser-induced breakdown spectroscopy confirmed the presence of metal ions in each freshly grown sample despite all these crystals undergoing physical deformation with different lifetimes. X-ray diffraction analysis confirmed that all our metallocene single crystals retained a monoclinic structure at room temperature. The vibrational properties of our metallocene crystals were examined using Raman and Fourier-transform infrared spectroscopy. The inter- and intra-ring vibrational modes, along with additional modes associated with the crystalline form, were identified as inherent vibrational properties of our metallocene single crystals. Given the increasingly important role of metallocene in organic solar cells, organic light-emitting displays and molecular quantum systems, this research will enhance our understanding of the intrinsic physical properties of cleaner, more crystalline metallocene single crystals.

cond-mat.mtrl-sci↗

arXiv2021

Topological properties of multilayer magnon insulators

Two-dimensional magnetic insulators can be promising hosts for topological magnons. In this study, we show that ABC-stacked honeycomb lattice multilayers with alternating Dzyaloshinskii-Moriya interaction (DMI) reveal a rich topological magnon phase diagram. Based on our bandstructure and Berry curvature calculations, we demonstrate jumps in the thermal Hall behavior that corroborate with topological phase transitions triggered by adjusting the DMI and interlayer coupling. We connect the phase diagram of generic multilayers to a bilayer and a trilayer system. We find an even-odd effect amongst the multilayers where the even layers show no jump in thermal Hall conductivity, but the odd layers do. We also observe the presence of topological proximity effect in our trilayer. Our results offer new schemes to manipulate Chern numbers and their measurable effects in topological magnonic systems.

cond-mat.mes-hall↗

arXiv2018

Bulk transport properties of Bismuth selenide thin films approaching the two-dimensional limit

We have investigated the transport properties of topological insulator Bi2Se3 thin films grown using magnetron sputtering with an emphasis on understanding the behavior as a function of thickness. We show that thickness has a strong influence on all aspects of transport as the two-dimensional limit is approached. Bulk resistivity and Hall mobility show disproportionately large changes below 6 quintuple layer which we directly correlate to an increase in the bulk band gap of few-layer Bi2Se3, an effect that is concomitant with surface gap opening. A tendency to crossover from a metallic to an insulating behavior in temperature-dependent resistivity measurements in ultra-thin Bi2Se3 is also consistent with an increase in the bulk band gap along with enhanced disorder at the film-substrate interface. Our work highlights that the properties of few-layer Bi2Se3 are tunable that may be attractive for a variety of device applications in areas such as optoelectronics, nanoelectronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2017

Computational Investigation of Inverse-Heusler compounds for Spintronics Applications

First-principles calculations of the electronic structure, magnetism and structural stability of inverse-Heusler compounds with the chemical formula \textit{X$_2$YZ} are presented and discussed with a goal of identifying compounds of interest for spintronics. Compounds for which the number of electrons per atom for \textit{Y} exceed that for \textit{X} and for which \textit{X} is one of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, or Cu; \textit{Y} is one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn; and \textit{Z} is one of Al, Ga, In, Si, Ge, Sn, P, As or Sb were considered. The formation energy per atom of each compound was calculated. By comparing our calculated formation energies to those calculated for phases in the Inorganic Crystal Structure Database (ICSD) of observed phases, we estimate that inverse-Heuslers with formation energies within 0.052 eV/atom of the calculated convex hull are reasonably likely to be synthesizable in equilibrium. The observed trends in the formation energy and relative structural stability as the \textit{X}, \textit{Y} and \textit{Z} elements vary are described. In addition to the Slater-Pauling gap after 12 states per formula unit in one of the spin channels, inverse-Heusler phases often have gaps after 9 states or 14 states. We describe the origin and occurrence of these gaps. We identify 14 inverse-Heusler semiconductors, 51 half-metals and 50 near half-metals with negative formation energy. In addition, our calculations predict 4 half-metals and 6 near half-metals to lie close to the respective convex hull of stable phases, and thus may be experimentally realized under suitable synthesis conditions, resulting in potential candidates for future spintronics applications.

cond-mat.mtrl-sci↗

arXiv2017

Optical evidence of blue shift in topological insulator bismuth selenide in the few-layer limit

Optical band gap properties of high-quality few-layer topological insulator Bi2Se3 thin films grown with magnetron sputtering are investigated using broadband absorption spectroscopy. We provide direct optical evidence of a rigid blue-shift to up to 0.5 eV in the band gap of Bi2Se3 as it approaches the two-dimensional limit. The onset of this behavior is most significant below six quintuple layers. The blue shift is very robust and is observed in both protected (capped) and exposed (uncapped) thin films. Our results are consistent with observations that finite-size effects have profound impact on the electronic character of topological insulators, particularly when the top and bottom surface states are coupled. Our result provides new insights, and the need for deeper investigations, into the scaling behavior of topological materials before they can have significant impact on electronic applications.

cond-mat.mtrl-sci↗

arXiv2016

Viable route towards large-area two dimensional MoS2 using magnetron sputtering

Structural, interfacial, optical, and transport properties of large-area MoS2 ultra-thin films on BN-buffered silicon substrates fabricated using magnetron sputtering are investigated. A relatively simple growth strategy is demonstrated here that simultaneously promotes superior interfacial and bulk MoS2 properties. Few layers of MoS2 are established using X-ray reflectivity, diffraction, ellipsometry, and Raman spectroscopy measurements. Layer-specific modeling of optical constants shows very good agreement with first-principles calculations. Conductivity measurements reveal that few-layer MoS2 films are more conducting than many-layer films. Photo-conductivity measurements reveal that the sputter deposited MoS2 films compare favorably with other large-area methods. Our work illustrates that sputtering is a viable route for large-area device applications using transition metal dichalcogenides.

cond-mat.mtrl-sci↗

arXiv2010

Ferroelectric domain scaling and electronic properties in ultrathin BiFeO3 films on vicinal substrates

We report electrically switchable polarization and ferroelectric domain scaling over a thickness range of 5-100 nm in BiFeO3 films deposited on [110] vicinal substrates. The BiFeO3 films of variable thickness were deposited with SrRuO3 bottom layer using pulsed laser deposition technique. These films have fractal domain patterns and the domain width scales closely with the square root of film thickness, in accordance with the Landau-Lifschitz-Kittel (LLK) law. The Switching Spectroscopy Piezo-response Force Microscopy provides clear evidence for the ferroelectric switching behavior in all the films. Using Quasi-particle Self-consistent GW (QPGW) approximation we have investigated physical parameters relevant for direct tunneling behavior, namely the effective mass and effective barrier height of electrons. For rhombohedral BFO, we report a large effective barrier height value of 3.6 eV, which is in reasonable agreement with nanoscale transport measurements. QPGW investigations into the tetragonal BFO structure with P4mm symmetry revealed a barrier height of 0.38 eV, significantly lower compared to its rhombohedral counterpart. This difference has very significant implications on the transport properties of nearly tetragonal BFO phase.

cond-mat.mtrl-sci↗

arXiv2010

Nanoscale switching characteristics of nearly tetragonal BiFeO3 thin films

We have investigated the nanoscale switching properties of strain-engineered BiFeO3 thin films deposited on LaAlO3 substrates using a combination of scanning probe techniques. Polarized Raman spectral analysis indicate that the nearly-tetragonal films have monoclinic (Cc) rather than P4mm tetragonal symmetry. Through local switching-spectroscopy measurements and piezoresponse force microscopy we provide clear evidence of ferroelectric switching of the tetragonal phase but the polarization direction, and therefore its switching, deviates strongly from the expected (001) tetragonal axis. We also demonstrate a large and reversible, electrically-driven structural phase transition from the tetragonal to the rhombohedral polymorph in this material which is promising for a plethora of applications.

cond-mat.mtrl-sci↗