arXiv subjects

Partha Mitra

Publications and source records attributed to Partha Mitra.

At least 19 recordsLinked to original sources

arXiv2026

Temperature-driven enhancement and sign reversal of field-like torque in Py/FePS$_3$ bilayers

Electrical manipulation of magnetization via current-induced spin orbit torques offers a promising route toward nonvolatile and energy efficient spintronic devices. In this work, we present a comprehensive investigation of SOTs in Py/FePS$_3$ bilayer devices, where Py/FePS$_3$ is a layered van der Waals antiferromagnetic insulator. Using low frequency harmonic Hall measurements, we quantify both field like and damping like torque components and examine their dependence on temperature. We find that interfacing Py with Py/FePS$_3$ leads to a pronounced enhancement of the field-like torque efficiency compared to Py reference devices, while the damping-like torque remains largely unaffected. Strikingly, the field like torque efficiency exhibits a strong temperature dependence, including a clear sign reversal upon cooling. This behavior occurs despite negligible charge current flow through the Py/FePS$_3$ layer, indicating that the observed torque modulation arises from interfacial effects rather than bulk transport. The close correlation between the temperature evolution of the field like torque and the antiferromagnetic ordering of Py/FePS$_3$ highlights the active role of antiferromagnetic insulators in controlling spin orbit torque symmetry and efficiency, and suggests new pathways for torque engineering in magnetic heterostructures.

cond-mat.mes-hall↗

arXiv2025

Interface tuned Enhanced and Low Temperature Quenching of Orbital Hall Currents Induce Torque and magnetoresistance in Light Metal/Nickel Bilayers

We investigate orbital current induced effects arising from the orbital Hall effect in light-metal/ferromagnet bilayers. Thin films of Ti in ohmic contact with Ni were studied using second-harmonic longitudinal and transverse voltage measurements under an applied a.c. current. From these signals, we extract the orbital Hall torque (OHT) efficiency and the unidirectional orbital magnetoresistance (UOMR). Insertion of a Cu interlayer between the Ni/Ti interface leads to an enhancement of both OHT efficiency and UOMR compared to both Ni/Ti and Ni/Cu bilayers. Furthermore, systematic variation of Ti thickness reveals that both OHT efficiency and UOMR increase with increasing Ti thickness, indicating that the observed phenomena predominantly originate from the bulk orbital Hall effect rather than purely from interfacial mechanisms and Lowering the temperature leads to a clear reduction in both the orbital Hall torque (OHT) efficiency and the unidirectional orbital magnetoresistance (UOMR). The nearly linear and correlated temperature dependence of both parameters suggests a common underlying mechanism, namely, the orbital Hall effect in the light-metal layer, which governs both the generation of orbital current and its subsequent influence on the ferromagnet through orbital torque and orbital magnetoresistance.

cond-mat.mes-hall↗

arXiv2025

Mechanism of Conductivity Enhancement of Polymers Employing Microbubble Lithography

The pursuit of green methodologies for fabricating optoelectronic devices necessitates the adoption of self-assembly-based strategies to engineer efficient and sustainable platforms. Microbubble lithography (MBL) stands out as a directed self-assembly technique, enabling real-time micropatterning of conductive structures. Notably, this approach achieves significant enhancements in the conductivity of patterned polymers without requiring external dopants. However, the underlying mechanisms driving this enhancement remain poorly understood. In this study, we address this knowledge gap through a combined theoretical and experimental investigation of a binary polymer system. Molecular dynamics simulations and percolation theory reveal structural transformations that underpin improved charge transport. Furthermore, we demonstrate that phase separation at the interfaces of interacting polymers plays a pivotal role in enhancing conductivity. This separation optimizes the conformational states of the polymers, facilitating more efficient charge carrier transport and ultimately leading to higher conductivity. Our findings establish MBL-induced self-assembly as a robust and sustainable technique for fabricating conductive patterns, paving the way for its integration into next-generation optoelectronic devices.

physics.app-ph↗

arXiv2025

Decoupling of Spin-Orbit Torque Components in Py/W Bilayers unveiled through variation of W-resistivity

Harmonic Hall measurements were performed on a series of ferromagnetic metal/heavy metal (FM/HM) bilayers consisting of Permalloy (Py) as the FM and beta-Tungsten (W) as the HM, and the efficiencies of the two orthogonal components of the spin-orbit torque (SOT) were extracted. Two sets of Hall bar-shaped devices, differing in the aspect ratio of the voltage pickup line width and the current channel width, were studied. Within each set, the resistivity of the W layer was systematically varied over a wide range (approximately 150-1000 micro-Ohm-cm). To account for geometry-induced variations in current distribution, numerical simulations were performed, and a correction protocol was developed to normalize the torque efficiencies obtained from the conventional analysis. After applying the correction, the Slonczewski-like (anti-damping, in-plane) torque efficiency exhibited a consistent dependence on W resistivity across both device sets. In contrast, the field-like (out-of-plane) torque efficiency remained largely independent of W resistivity, reinforcing its interfacial character.

cond-mat.mes-hall↗

arXiv2024

Evidence of orbital Hall current induced correlation in second harmonic response of longitudinal and transverse voltage in light metal-ferromagnet bilayers

We investigate the effect of orbital current arising from orbital Hall effect in thin films of Nb and Ti in ohmic contact with ferromagnetic Ni in the second harmonic longitudinal and transverse voltages in response to an a.c. current applied to the bilayer structures. Our experiments were analogous to those on Heavy Metal-Ferromagnet bilayers and we extract the Orbital Hall Torque efficiency and unidirectional magnetoresistance (UMR). Through second-harmonic measurements, we investigate orbital Hall torque and UMR in bilayer devices composed of ferromagnetic materials (FM), such as Ni and NiFe, paired with light metals (LM), such as Ti and Nb. Our results demonstrate that LM/Ni bilayers exhibit enhanced damping-like torque and unidirectional magnetoresistance (UMR) compared to LM/NiFe bilayers. This enhancement suggests that angular momentum is generated via the orbital Hall effect within the light metal, where it undergoes orbital-to-spin conversion within the Ni ferromagnet, ultimately transferring to the magnetization of the ferromagnetic layer. Torque and UMR are also absent in single-layer devices, highlighting the necessity of the bilayer structure for orbital current generation.

cond-mat.mes-hall↗

arXiv2024

Anomalous Inverse Spin Hall Effect (AISHE) due to Unconventional Spin Currents in Ferromagnetic Films with Tailored Interfacial Magnetic Anisotropy

A single layer ferromagnetic film magnetized in the plane of an ac current flow, exhibits a characteristic Hall voltage with harmonic and second harmonic components, which is attributed to the presence of spin currents with polarization non-collinear with the magnetization. A set of 30 nm thick permalloy (Py) films used in this study are deposited at an oblique angle with respect to the substrate plane which induces an in-plane easy axis in the magnetization of the initial nucleating layers of the films which is distinct from the overall bulk magnetic properties of the film. This unusual magnetic texture provides a platform for the direct detection of inverse spin Hall effect in Hall bar shaped macroscopic devices at room temperatures which we denote as Anomalous Inverse Spin Hall Effect (AISHE). Control samples fabricated by normal deposition of permalloy with slow rotation of substrate shows significant reduction of the harmonic Hall signal that further substantiates the model. The analysis of the second harmonic Hall signal corroborates the presence of spin-orbit torque arising from the unconventional spin-currents in the single-layer ferromagnets.

cond-mat.mes-hall↗

arXiv2024

Direct Electrical Detection of Spin Chemical Potential Due to Spin Hall Effect in $\beta$-Tungsten and Platinum Using a Pair of Ferromagnetic and Normal Metal Voltage Probes

The phenomenon of Spin Hall Effect (SHE) generates a pure spin current transverse to an applied current in materials with strong spin-orbit coupling, although not detectable through conventional electrical measurement. An intuitive Hall effect like measurement configuration is implemented to directly measure pure spin chemical potential of the accumulated spins at the edges of heavy metal (HM) channels that generates large SHE. A pair of transverse linearly aligned voltage probes in placed in ohmic contact with the top surface of HM , one being a ferromagnetic metal (FM) with non-zero spin polarization and other is the reference metal (RM) with zero polarization of carriers. This combination of FM/RM electrodes is shown to induce an additional voltage proportional to a spin accumulation potential, which is anti symmetric with respect to opposite orientations of FM controlled by a 2D vector magnet. Proof of concept of the measurement scheme is verified by comparing the signs of voltages for HM channels of Tungsten (W) and Platinum (Pt) which are known to generate opposite spin accumulation under similar conditions of applied current. The same devices are also able to detect the reciprocal effect, inverse spin Hall effect (ISHE) by swapping the current and voltage leads and the results are consistent with reciprocity principle. Further, exploiting a characteristic feature of W thin film deposition, a series of devices were fabricated with W resistivity varying over a wide range of 10 - 750 $\mu \Omega$-cm and the calculated spin Hall resistivity exhibits a pronounced power law dependence on resistivity. Our measurement scheme combined with almost two decades of HM resistivity variation provides the ideal platform required to test the underlying microscopic mechanism responsible for SHE/ISHE.

cond-mat.mes-hall↗

arXiv2024

Anisotropic magneto-photothermal voltage in Sb2Te3 topological insulator thin films

We studied longitudinal and Hall photothermal voltages under a planar magnetic field scan in epitaxial thin films of the Topological Insulator (TI) Sb2Te3, grown using pulsed laser deposition (PLD). Unlike prior research that utilised polarised light-induced photocurrent to investigate the TI, our study introduces advancements based on unpolarized light-induced local heating. This method yields a thermoelectric response exhibiting a direct signature of strong spin-orbit coupling. Our analysis reveals three distinct contributions when fitting the photothermal voltage data to the angular dependence of the planar magnetic field. The interaction between the applied magnetic field and the thermal gradient on the bulk band orbitals enables the differentiation between the ordinary Nernst effect from the out-of-plane thermal gradient and an extraordinary magneto-thermal contribution from the planar thermal gradient. The fitting of our data to theoretical models indicates that these effects primarily arise from the bulk states of the TI rather than the surface states. These findings highlight PLD-grown epitaxial topological insulator thin films as promising candidates for optoelectronic devices, including sensors and actuators. Such devices offer controllable responses through position-dependent, non-invasive local heating via focused incident light and variations in the applied magnetic field direction.

cond-mat.mes-hall↗

arXiv2023

Modulating Spin Current Induced Effective Damping in $β-W/Py$ Heterostructures by a Systematic Variation in Resistivity of the Sputtered Deposited $β-W$ films

Utilizing the spin-induced pumping from a ferromagnet (FM) into a heavy metal (HM) under the ferromagnetic resonance (FMR) condition, we report an enhancement in effective damping in $β$- W/Py bilayers by systematically varying resistivity ($ρ_{W}$) of $β$-W films. Different resistivity ranging from 100 $μΩ$-cm to 1400 $μΩ$-cm with a thickness of 8 nm can be achieved by varying the argon pressure ($P_{Ar}$) during the growth by the method of sputtering. The coefficient of effective damping $α_{eff}$ is observed to increase from 0.010 to 0.025 with $ρ_{W}$, which can be modulated by $P_{Ar}$. We observe a modest dependence of $α_{eff}$ on the sputtering power ($p_{S}$) while keeping the $P_{Ar}$ constant. $α_{eff}$ dependence on both $P_{Ar}$ and $p_{S}$ suggests that there exists a strong correlation between $α_{eff}$ and $ρ_{W}$. It is thus possible to utilize $ρ_{W}$ as a tuning parameter to regulate the $α_{eff}$, which can be advantageous for faster magnetization dynamics switching. The thickness dependence study of Py in the aforementioned bilayers manifests a higher spin mixing conductance ($g^{\uparrow\downarrow}_{eff}$) which suggests a strong spin pumping from Py into the $β$-W layer. The effective spin current ($J_{S(eff)}$) is also evaluated by considering the spin-back flow in this process. Intrinsic spin mixing conductance ($g^{\uparrow\downarrow}_{W}$) and spin diffusion length ($λ_{SD}$) of $β$-W are additionally investigated using thickness variations in $β$-W. Furthermore, the low-temperature study in $β$-W/Py reveals an intriguing temperature dependence in $α_{eff}$ which is quite different from $α_{b}$ of single Py layer and the enhancement in $α_{eff}$ at low temperature can be attributed to the spin-induced pumping from Py layer into $β$-W.

cond-mat.mes-hall↗

arXiv2022

Role of electron and hole doping in NdNi1_xVxO3 Nanostructure

Neodymium nickelate, NdNiO3 attracts attraction due to the simultaneous occurrence of several phase transitions around the same temperature. The electronic properties of NdNiO3 are extremely complex as structural distortion, electron correlation, charge ordering, and orbital overlapping play significant roles in the transitions. We report the effects of electron and hole injection via doping a single 3d metal, V, in the NdNiO3 nanostructures to understand the variations in the electronic properties without any structural distortion. A reversible resistivity modulation more than five orders of magnitude via hole doping and complete suppression of metal to insulator transition via electron doping is observed along with the switching of major charge carriers. The modulation of electronic properties without any structural distortion and external strain opens up new directions to consider the NdNi1_xVxO3 nanostructures applicable as emerging electronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Controlling Fano resonance using the Geometrical Phase of light in spatially tailored waveguided plasmonic crystals

Fano resonance exhibiting an asymmetric spectral line shape is a universal phenomenon observed in diverse physical systems. Here we experimentally establish a direct link between the spectral asymmetry parameter and a physically realizable phase factor of interference between a continuum and a discrete mode that leads to Fano resonance. Using a specially designed metamaterial, namely waveguided plasmonic crystal with a spatially varying orientation axis of plasmonic grating, we demonstrate control on the spectral asymmetry of the Fano resonance through changes in the geometric phase of polarized light. In this scenario, the changes in the geometric phase for input left, and right circular polarized light arises due to varying orientation angle of the grating axis. The systematic changes in the geometric phase and the resulting q-parameter of Fano resonance is interpreted by an appropriate theoretical model connecting the two physical entities. The demonstrated control over the spectral line shape of Fano resonance achieved by tailoring geometric phase may open up novel routes for polarization-based photonic applications.

physics.optics↗

arXiv2019

On Fundamental Limitations of Dynamic Feedback Control in Regular Large-Scale Networks

We study fundamental performance limitations of distributed feedback control in large-scale networked dynamical systems. Specifically, we address the question of whether dynamic feedback controllers perform better than static (memoryless) ones when subject to locality constraints. We consider distributed linear consensus and vehicular formation control problems modeled over toric lattice networks. For the resulting spatially invariant systems we study the large-scale asymptotics (in network size) of global performance metrics that quantify the level of network coherence. With static feedback from relative state measurements, such metrics are known to scale unfavorably in lattices of low spatial dimensions, preventing, for example, a 1-dimensional string of vehicles to move like a rigid object. We show that the same limitations in general apply also to dynamic feedback control that is locally of first order. This means that the addition of one local state to the controller gives a similar asymptotic performance to the memoryless case. This holds unless the controller can access noiseless measurements of its local state with respect to an absolute reference frame, in which case the addition of controller memory may fundamentally improve performance. In simulations of platoons with 20-200 vehicles we show that the performance limitations we derive manifest as unwanted accordion-like motions. Similar behaviors are to be expected in any network that is embeddable in a low-dimensional toric lattice, and the same fundamental limitations would apply. To derive our results, we present a general technical framework for the analysis of stability and performance of spatially invariant systems in the limit of large networks.

math.OC↗

arXiv2018

Overfitting or perfect fitting? Risk bounds for classification and regression rules that interpolate

Many modern machine learning models are trained to achieve zero or near-zero training error in order to obtain near-optimal (but non-zero) test error. This phenomenon of strong generalization performance for "overfitted" / interpolated classifiers appears to be ubiquitous in high-dimensional data, having been observed in deep networks, kernel machines, boosting and random forests. Their performance is consistently robust even when the data contain large amounts of label noise. Very little theory is available to explain these observations. The vast majority of theoretical analyses of generalization allows for interpolation only when there is little or no label noise. This paper takes a step toward a theoretical foundation for interpolated classifiers by analyzing local interpolating schemes, including geometric simplicial interpolation algorithm and singularly weighted $k$-nearest neighbor schemes. Consistency or near-consistency is proved for these schemes in classification and regression problems. Moreover, the nearest neighbor schemes exhibit optimal rates under some standard statistical assumptions. Finally, this paper suggests a way to explain the phenomenon of adversarial examples, which are seemingly ubiquitous in modern machine learning, and also discusses some connections to kernel machines and random forests in the interpolated regime.

stat.ML↗

arXiv2018

Estimating Diffeomorphic Mappings between Templates and Noisy Data: Variance Bounds on the Estimated Canonical Volume Form

Anatomy is undergoing a renaissance driven by availability of large digital data sets generated by light microscopy. A central computational task is to map individual data volumes to standardized templates. This is accomplished by regularized estimation of a diffeomorphic transformation between the coordinate systems of the individual data and the template, building the transformation incrementally by integrating a smooth flow field. The canonical volume form of this transformation is used to quantify local growth, atrophy, or cell density. While multiple implementations exist for this estimation, less attention has been paid to the variance of the estimated diffeomorphism for noisy data. Notably, there is an infinite dimensional un-observable space defined by those diffeomorphisms which leave the template invariant. These form the stabilizer subgroup of the diffeomorphic group acting on the template. The corresponding flat directions in the energy landscape are expected to lead to increased estimation variance. Here we show that a least-action principle used to generate geodesics in the space of diffeomorphisms connecting the subject brain to the template removes the stabilizer. This provides reduced-variance estimates of the volume form. Using simulations we demonstrate that the asymmetric large deformation diffeomorphic mapping methods (LDDMM), which explicitly incorporate the asymmetry between idealized template images and noisy empirical images, provide lower variance estimators than their symmetrized counterparts (cf. ANTs). We derive Cramer-Rao bounds for the variances in the limit of small deformations. Analytical results are shown for the Jacobian in terms of perturbations of the vector fields and divergence of the vector field.

eess.IV↗

arXiv2018

Topological Skeletonization and Tree-Summarization of Neurons Using Discrete Morse Theory

Neuroscientific data analysis has classically involved methods for statistical signal and image processing, drawing on linear algebra and stochastic process theory. However, digitized neuroanatomical data sets containing labelled neurons, either individually or in groups labelled by tracer injections, do not fully fit into this classical framework. The tree-like shapes of neurons cannot mathematically be adequately described as points in a vector space. There is therefore a need for new approaches. Methods from computational topology and geometry are naturally suited to the analysis of neuronal shapes. Here we introduce methods from Discrete Morse Theory to extract tree-skeletons of individual neurons from volumetric brain image data, or to summarize collections of neurons labelled by localized anterograde tracer injections. Since individual neurons are topologically trees, it is sensible to summarize the collection of neurons labelled by a localized anterograde tracer injection using a consensus tree-shape. The algorithmic procedure includes an initial pre-processing step to extract a density field from the raw volumetric image data, followed by initial skeleton extraction from the density field using a discrete version of a 1-(un)stable manifold of the density field. Heuristically, if the density field is regarded as a mountainous landscape, then the 1-(un)stable manifold follows the "mountain ridges" connecting the maxima of the density field. We then simplify this skeleton-graph into a tree using a shortest-path approach and methods derived from persistent homology. The advantage of this approach is that it uses global information about the density field and is therefore robust to local fluctuations and non-uniformly distributed input signals. To be able to handle large data sets, we use a divide-and-conquer approach. The resulting software DiMorSC is available on Github.

cs.CG↗

arXiv2018

The Active Atlas: Combining 3D Anatomical Models with Texture Detectors

While modern imaging technologies such as fMRI have opened exciting new possibilities for studying the brain in vivo, histological sections remain the best way to study the anatomy of the brain at the level of single neurons. The histological atlas changed little since 1909 and localizing brain regions is a still a labor intensive process performed only by experienced neuro-anatomists. Existing digital atlases such as the Allen Brain atlas are limited to low resolution images which cannot identify the detailed structure of the neurons. We have developed a digital atlas methodology that combines information about the 3D organization of the brain and the detailed texture of neurons in different structures. Using the methodology we developed an atlas for the mouse brainstem and mid-brain, two regions for which there are currently no good atlases. Our atlas is "active" in that it can be used to automatically align a histological stack to the atlas, thus reducing the work of the neuroanatomist.

cs.CV↗

arXiv2016

Grand Challenges for Global Brain Sciences

The next grand challenges for society and science are in the brain sciences. A collection of 60+ scientists from around the world, together with 10+ observers from national, private, and foundations, spent two days together discussing the top challenges that we could solve as a global community in the next decade. We eventually settled on three challenges, spanning anatomy, physiology, and medicine. Addressing all three challenges requires novel computational infrastructure. The group proposed the advent of The International Brain Station (TIBS), to address these challenges, and launch brain sciences to the next level of understanding.

q-bio.NC↗

arXiv2014

Automated placement of stereotactic injections using a laser scan of the skull

Stereotactic targeting is a commonly used technique for performing injections in the brains of mice and other animals. The most common method for targeting stereoscopic injections uses the skull indentations bregma and lambda as reference points and is limited in its precision by factors such as skull curvature and individual variation, as well as an incomplete correspondence between skull landmarks and brain locations. In this software tool, a 3D laser scan of the mouse skull is taken in vitro and registered onto a reference skull using a point cloud matching algorithm, and the parameters of the transformation are used to position a glass pipette to place tracer injections. The software was capable of registering sample skulls with less than 100 micron error, and was able to target an injection in a mouse with error of roughly 500 microns. These results indicate that using skull scan registration has the potential to be widely applicable in automating stereotactic targeting of tracer injections.

q-bio.QM↗