arXiv subjects

Pintu Das

Publications and source records attributed to Pintu Das.

17 recordsLinked to original sources

arXiv2025

Intralayer antiferromagnetism in two-dimensional van der Waals magnet Fe$_3$GeTe$_2$

For the van der Waals magnet Fe$_3$GeTe$_2$, although a ferromagnetic ground state has been reported, there are also reports of complex magnetic behavior suggesting coexistence of ferromagnetism and antiferromagnetism due to the intricate interaction between Fe$^{+3}$ and Fe$^{+2}$ ions in this system. The exact nature of the interactions and the origin of antiferromagnetism are still under debate. Here, we report the observation of signature of ferromagnetic and antiferromagnetic couplings between different Fe-ions in the anomalous Hall effect measured for devices of mechanically exfoliated Fe$_3$GeTe$_2$ nano-flakes of thicknesses ranging from\,$\sim$\,15-20 layers. The temperature-dependent anomalous Hall effect data reveal two sharp step-like switchings at low temperature ($T\lesssim150\,$K). Our detailed analyses suggest the step-like sharp switchings in anomalous Hall resistance are due to the magnetization reversal behavior of different Fe-ions in individual layers of Fe$_3$GeTe$_2$. The experimental results can be explained by considering an intra-layer antiferromagnetic coupling between Fe$^{+3}$ and Fe$^{+3}$ ions, whereas intra-layer ferromagnetic coupling between Fe$^{+3}$ and Fe$^{+2}$ in the system. Our experimental results and the analyses are supported by the first-principles calculations for energetics and intralayer as well as interlayer exchange coupling constants.

cond-mat.mtrl-sci↗

arXiv2025

Optical contrast-based determination of number of layers for two-dimensional van der Waals magnet Fe$_3$GeTe$_2$

Recent advances in revealing intrinsic magnetism in two-dimensional (2D) materials have highlighted their potential for future spintronic applications, driven by their novel physical properties, promising for future spintronic devices. In order to explore layer dependent magnetic behavior, in general, mechanically exfoliated flakes from high-quality single crystals are used. It is crucial to determine the number of layers of these materials accurately. In the absence of an efficient and quick method, researchers often rely on atomic force microscopy (AFM) imaging to identify their number of layers. In this work, we report an optical contrast study as a quick and cost-effective technique to determine the number of layers of Fe$_3$GeTe$_2$ (FGT). Here, we observed a linear relationship between the optical contrast (derived from optical microscopic images) observed for mechanically exfoliated FGT nano-flakes and their thickness, as measured by the AFM imaging method. This technique requires no additional equipment; it relies solely on a conventional optical microscope. Additionally, our results reveal a thickness-dependent evolution of the intensity; in contrast, the Raman frequency demonstrates no significant dependence on layer thickness. Also, our studies reveal two additional Raman modes of FGT, at the frequency of 129\,cm$^-1$ \& 190\,cm$^-1$. Both modes show the intensity dependence on the thickness of FGT, same as out-of-plane (A$_{1g}$) Raman modes.

cond-mat.mtrl-sci↗

arXiv2025

Layer breathing Raman mode in two-dimensional van der Waals material $\mathrm{Cr_2Ge_2Te_6}$

Two-dimensional (2D) van der Waals (vdW) magnetic materials have emerged as key materials for next-generation magneto-electric and spintronic devices, where understanding the relationship between layer number, lattice dynamics, and magnetic interactions is very important. In this work, we report the observation of the layer breathing mode (LBM) in few-layer $\mathrm{Cr_2Ge_2Te_6}$, a ferromagnetic semiconductor with thickness dependent electronic, magnetic and optical properties, using Raman spectroscopy, which serves as a direct fingerprint of interlayer coupling and lattice symmetry. Group-theoretical symmetry analysis confirms that the CGT falls under the non-polar category of layered material. The evolution of the LBM-frequency with increasing layer number (N) reveals a distinct softening trend, characteristic of weakening restoring forces in thicker flakes. By fitting the experimental Raman data using the Linear Chain Model (LCM), we quantitatively extract the interlayer force constant ($\mathrm{K_c}$), providing a measure of the vdW coupling strength between layers.

cond-mat.mtrl-sci↗

arXiv2025

Thermal gradient-driven skyrmion dynamics with near-zero skyrmion Hall angle

Thermal gradient driven skyrmion dynamics offers a promising route toward green spintronics, enabling the utilization of waste heat for information transport and processing. Using micromagnetic simulations, we investigate Neel skyrmions in a Co-Pt bilayer nanoracetrack and demonstrate that stochastic torques induced by a thermal gradient drive skyrmion motion toward the hotter region with a nearly vanishing Hall angle. The dynamics depends sensitively on intrinsic material parameters - the skyrmion velocity decreases with increasing damping constant, increases with stronger thermal gradients, and varies systematically with saturation magnetization, interfacial DMI strength, and uniaxial out of plane anisotropy. Importantly, we identify a specific range of material parameters within which the skyrmion velocity changes sharply while the Hall angle remains strongly suppressed, saturating near zero. This comprehensive parameter-dependent study establishes a universal design framework for minimizing the Hall effect in thermal gradient driven spintronic systems.

cond-mat.mes-hall↗

arXiv2025

New Methods for Critical Analysis: Revealing the Simultaneous Existence of Universality Classes in Nontrivial Magnetic Systems

In magnetic systems, the microscopic constituents exhibit power law behavior near the paramagnetic transition temperature, $T_C$. The critical exponents (CEs) associated with the physical quantities that demonstrate singular behavior at $T_C$ illustrate the critical behavior, specifically the range and type of exchange interactions emerging in magnetic systems. However, it is realized that the developed methodologies may not yield accurate values of CEs, especially for magnetic systems with competing interactions, referred to as nontrivial magnetic systems. Currently, no comprehensive method effectively addresses the competing effects of the range of magnetic interactions among the constituent entities emerging in such systems. Additionally, there is no definitive explanation for CE values that do not belong to any single universality class. Here, we present new methodologies for critical analysis aimed at determining both the range of exchange interaction(s) and appropriate values of CEs. Using computational and experimental investigations, we analyze the magnetic behavior of trivial Ni and nontrivial Gd. Our findings demonstrate that (i) the critical behavior remains the same on either side of $T_C$, (ii) the critical behavior associated with local electron moments remains unaffected by the magnetic field, and (iii) in Gd, the critical role of competing interactions becomes evident: local electron moments follow a three-dimensional Ising-type short-range interaction, while itinerant electron moments exhibit a mean-field-type long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, which weakens under an external magnetic field due to the localization effect on itinerant electrons.

cond-mat.str-el↗

arXiv2023

Spin wave behavior of a novel hopfion-like chiral state in Co/Pt nanodiscs

This work discusses the rich phase diagram of non-trivial chiral spin textures in confined ferromagnetic/heavy-metal (FM/HM) bilayer nanomagnets of circular cross-section. These spin textures are realized as a minimum-energy ground state during an external bias field sweep for a range of nanomagnet's diameter (d). Our study, based on micromagnetic simulations, has revealed a novel Hopfion-like state which can be stabilized for a wide range of diameters and external magnetic fields. We explored the dynamical characteristics of this novel Hopfion-like state under a transient magnetic field applied along the plane's perpendicular direction. Simulation results have demonstrated the excitation of nonreciprocal spin wave (SW) modes for this novel chiral state, in contrast to other stabilized chiral states. These modes are characterized as breathing and quantized radial modes, which also exhibit hybridization with azimuthal modes. The resonant SW modes have been used to demonstrate the switching from a Hopfion-like state to a skyrmion within a few nanoseconds of SW excitation. Furthermore, we establish a correlation between the behavior of excited SW modes as a function of external magnetic field strength and underlying chiral spin texture states.

cond-mat.mes-hall↗

arXiv2022

Spin wave excitation and directional propagation in presence of magnetic charges in square artificial spin ice

Artificial spin ice is a special class of engineered lattice of highly shape anisotropic single domain magnetic nanostructures which is used as one of the model systems to study the spin ice behavior observed in pyrochlore oxides. The nanomagnets interact via dipolar interaction which results in correlated magnetization dynamics exhibiting macroscopic spin configuration states. Here, we exploit the interplay of underlying magnetic state and external bias field orientation to study controlled spin wave propagation in square Artificial Spin Ice (sASI) by performing detailed micromagnetic simulations. We report that careful selection of vertices with local magnetic charges can effectively direct the anisotropic spin wave in presence of an external field. Further, we explore the influence of local charges due to the excited state in even-coordinated vertices as well as uncompensated charges due to odd-coordinated vertices on spin wave behavior. Our studies suggest that there is no perceptible difference on spin wave dynamical behavior due to the origin of local magnetic charge in sASI. Our results of controlled and directional spin wave propagation in sASI system may be useful for low-power consumption based all magnonic on-chip devices.

cond-mat.mes-hall↗

arXiv2021

Fabrication and Characterization of InAs/AlSb based Magnetic Hall Sensors

Hall effect based magnetic field sensors are vector sensors which are sensitive to the perpendicular component of the magnetic field. Typically, Si-based Hall sensors are used for most usages requiring low field sensitivity. For higher sensitivity, III-V semiconductors such as GaAs, InSb, etc., provide better alternatives due to higher carrier mobilities at room temperature. In this work, Hall sensors based on two-dimensional electron gas (2DEG) at the heterostructure of InAs/AlSb are fabricated. The 2DEG is about 20 nm below the surface. Carrier density of 3.2x1016 /m2 and mobility of 1.8 m2/Vs were deduced via Hall measurements at RT. 1/f-noise analyses were carried out to calculate the magnetic field resolution of ~1 uT/Hz1/2.

physics.app-ph↗

arXiv2020

Spin wave spectral probing of possible microstates in building-block of macroscopically degenerate artificial spin ice

We have investigated the spin wave modes of strongly dipolar coupled, highly anisotropic nanoislands forming square artificial spin ice system using micromagnetic simulation in MUMAX3 in combination with Matlab coding. Artificial spin ice is considered to be formed by the four square ring-type structure of elliptical cross-section nanoislands. Our results state the direct relation between the spin wave modes generated and the micro-states formed in the system. We have shown that single ring type structure can alone be adequately used to understand the spin wave modes of square artificial spin ice.

cond-mat.mes-hall↗

arXiv2020

Accessing low-energy magnetic microstates in symmetry-broken isolated square artificial spin ice vertices with magnetic field

In artificial spin ice systems, an interplay of defects and dipolar interactions is expected to play important roles in stabilizing different collective magnetic states. In this work, we investigated the magnetization reversal of individual defective square artificial spin ice vertices where defects break four-fold rotational symmetry of the system. By varying the angle between the applied field and the geometrical axis of the vertices, we observe a change in energy landscape of the system resulting into the stabilization of collective low-energy magnetic states. We also observe that by changing the angle, it is possible to access different vertex configurations. Micromagnetic simulations are performed for varying angle as well as external field, the results of which are consistent with the experimental data.

cond-mat.mes-hall↗

arXiv2019

On the micromagnetic behavior of dipolar-coupled nanomagnets in defective square artificial spin ice systems

We report here the results of micromagnetic simulations of square artificial spin ice (ASI) systems with defects. The defects are introduced by misaligning of a nanomagnet at the vertex. In these defective systems, we are able to stabilize emergent monopole-like state by applying a small external field. We observe a systematic change of dipolar energies of the systems with varying misalignment angle. The fields at which the emergent monopoles are created vary linearly with the dipolar energies of the systems. Our results clearly show that the magnetization reversal of the ASI systems is intricately related to the interplay of defects and dipolar interactions.

cond-mat.mes-hall↗

arXiv2018

Magneto-optical imaging of stepwise magnetic domain disintegration at characteristic temperatures in EuB_6

Prior to the onset of the ferromagnetic transition in semimetallic EuB_6, unusual magnetic and electric behavior have been reported. Using a highly sensitive magneto-optical imaging (MOI) technique, we visualize the behavior of magnetic domains in a EuB_6 single crystal.The transformation from a paramagnetic to a ferromagnetic state is shown to be non-Curie Weiss like and proceeds via multiple breaks in the curvature of the temperature-dependent local magnetization. From our experiments, we identify three characteristic boundaries, T^*(H),T^*_c1(H) and T_c2(H),in a field - temperature magnetic phase diagram.Using scaling and Modified Arrott plot analysis of isothermal bulk magnetization data, we determine the critical exponents $β$ = 0.22 $\pm$ 0.01, $γ$ = 0.88 $\pm$ 0.05 and $δ$= 5.0 $\pm$ 0.1 and a critical transition temperature = 12.0 $\pm$ 0.2 K which is found to be equal to T_c2. The critical exponents are close to those associated with the universality class of tricritical mean field model. The absence of a model with which the exponents correspond with directly, suggests the presence of large critical fluctuations in this system. The critical fluctuations in this system are sensitive to the applied magnetic field, which leads to field dependence of boundaries in the magnetic phase diagram. With increasing T the magnetic domains disintegrate into finger-like patterns before fragmenting into disjoint magnetized puddles at T^*_c1 and ultimately disappearing at T*. At T^*_c1 we observe a significant increase in the spatial inhomogeneity of the local magnetic field distribution associated with the magnetic domain structure disintegrating into smaller magnetized structures. We explain our results via the formation of magnetic polaronic clusters and their coalescing into larger domains.

cond-mat.str-el↗

arXiv2014

Lattice strain accompanying the colossal magnetoresistance effect in EuB$_6$

The coupling of magnetic and electronic degrees of freedom to the crystal lattice in the ferromagnetic semimetal EuB$_6$, which exhibits a complex ferromagnetic order and a colossal magnetoresistance (CMR) effect, %, very likely involving magnetic polarons, is studied by high-resolution thermal expansion and magnetostriction experiments. EuB$_6$ may be viewed as a model system, where pure magnetism-tuned transport and the response of the crystal lattice can be studied in a comparatively simple environment,i.e., not influenced by strong crystal-electric field effects and Jahn-Teller distortions. We find a very large lattice response, quantified by (i) the magnetic Grüneisen parameter, (ii) the spontaneous strain when entering the ferromagnetic region and (iii) the magnetostriction in the paramagnetic temperature regime. Our analysis reveals that a significant part of the lattice effects originates in the magnetically-driven delocalization of charge carriers, consistent with the scenario of percolating magnetic polarons. A strong effect of the formation and dynamics of local magnetic clusters on the lattice parameters is suggested to be a general feature of CMR materials.

cond-mat.str-el↗

arXiv2012

On the influence of nanometer-thin antiferromagnetic surface layer on ferromagnetic CrO$_2$

We present magnetic stray field measurements performed on a single micro-crystal of the half metallic ferromagnet CrO$_2$, covered by a naturally grown 2\,-\,5\,nm surface layer of antiferromagnetic (AFM) Cr$_2$O$_3$. The temperature variation of the stray field of the micro-crystal measured by micro-Hall magnetometry shows an anomalous increase below $\sim$\,60\,K. We find clear evidence that this behavior is due to the influence of the AFM surface layer, which could not be isolated in the corresponding bulk magnetization data measured using SQUID magnetometry. The distribution of pinning potentials, analyzed from Barkhausen jumps, exhibits a similar temperature dependence. Overall, the results indicate that the surface layer plays a role in defining the potential landscape seen by the domain configuration in the ferromagnetic grain.

cond-mat.mtrl-sci↗

arXiv2010

Magnetization dynamics of a CrO$_2$ grain studied by micro-Hall magnetometry

Micro-Hall magnetometry is employed to study the magnetization dynamics of a single, micron-size CrO$_2$ grain. With this technique we track the motion of a single domain wall, which allows us to probe the distribution of imperfections throughout the material. An external magnetic field along the grain's easy magnetization direction induces magnetization reversal, giving rise to a series of sharp jumps in magnetization. Supported by micromagnetic simulations, we identify the transition to a state with a single cross-tie domain wall, where pinning/depinning of the wall results in stochastic Barkhausen jumps.

cond-mat.str-el↗

arXiv2008

Ferromagnetism in transparent Mn(II)-doped indium tin oxide films prepared by sol-gel process

We observe remarkably strong room temperature ferromagnetism (~1.5 Bohr Magneton/Mn) in optically transparent Mn(II)-doped indium tin oxide (ITO) films. The nanocrystalline films with average grain size 10-22 nm and thickness 150-350 nm are prepared by sol-gel coating technique on sodalime silica glass substrate. The ferromagnetic property is, of course, weak for films deposited on pure silica glass substrate. The structural parameters of the films appear to be governing the magnetic property strongly which vary appreciably depending on the substrate. The observation of room temperature ferromagnetism in transparent conducting ITO films may find a plethora of applications in the area of magneto-optics.

cond-mat.mtrl-sci↗

arXiv2000

On the factors affecting the high temperature insulator-metal transition in rare-earth manganites

The measurement of resistivity across a wide temperature range - from 15 to 1473 K - in rare-earth manganite series of compounds reveals a very interesting feature : normally observed insulating pattern beyond Tc (Curie Point) gives way to a reentrant metallic pattern around a characteristic temperature T*. The transport activation barrier Ea collapses to zero around T*. T* is found to be dependent on the carrier concentration or the concentration of the Jahn-Teller-active Mn(3+) ions as well as on the average A-site radius for a fixed carrier concentration. These factors govern the effective lattice distortion and hence lead to the variation in the conduction bandwidth. Our data cover a wide range - from T*>>Tc for smaller bandwidth to T* tending towards Tc for larger bandwidth. These results seem to provide evidence of the onset of lattice distortion at high temperature (around T*) and its variation. Since lattice distortion governs the magnetic, transport and other important behaviors significantly, our data assume importance as they offer a new measure of the effective distortion and its tunability.

cond-mat.str-el↗