arXiv subjects

Suprotim Saha

Publications and source records attributed to Suprotim Saha.

3 recordsLinked to original sources

arXiv2025

Unravelling magnetic vortex-like excitations through rapid thermal quenching in low-carbon steel

Steel, traditionally valued for its structural strength, emerges in this study as a remarkable material for exploring novel magnetic phenomena. We investigate how common processing techniques-thermal treatments and mechanical strain-significantly affect the magnetic properties of low-carbon steels (0.05 percent by weight). Our findings show that slow annealing enlarges the grain size, enhancing magnetic susceptibility, while rapid quenching reduces grain size, resulting in a decreased magnetic response. Quenching low-carbon steel produces significant increase in the fraction of high-angle grain boundaries and a rapid spatial variation of local magnetic anisotropy between grains, a feature which is unachievable with mechanical straining even up to the material's ultimate tensile strength. Tensile-straining of low-carbon steel enhances magnetic susceptibility through altered magnetic anisotropy, contrary to the observed decrease of susceptibility in quenched low-carbon steel. Magnetic force microscopy and micromagnetic modelling of our data reveal that, the reduced magnetic susceptibility in quenched steel is a result of the presence of intriguing magnetic excitations akin to magnetic vortices. These localized structures act as strong magnetic domain wall pinning centres, causing the observed decrease in magnetic susceptibility in these quenched low-carbon steels. Beyond its established structural utility, low-carbon steel combines mechanical stability with favourable magnetic properties, positioning it as a strong platform for magnetic device applications.

cond-mat.mtrl-sci↗

arXiv2024

Dimension-Dependent Critical Scaling Analysis and Emergent Competing Interaction Scales in a 2D Van der Waals magnet Cr$_{2}$Ge$_{2}$Te$_{6}$

We investigate thickness-dependent transformation from a paramagnetic to ferromagnetic phase in Cr$_{2}$Ge$_{2}$Te$_{6}$ (CGT) in bulk and few-layer flake forms. 2D Ising-like critical transition in bulk CGT occurs at $T_{c}$ = 67 K with out-of-plane magnetic anisotropy. Few-layer CGT on hBN/SiO$_{2}$/Si substrate displays the same $T_{c}$ but also exhibits a new critical transition at $T^{\prime}_c$ = 14.2 K. Here, critical scaling analysis reveals the critical exponents differ significantly from those in bulk and do not align with the known universality classes. Our Density Functional Theory (DFT) and classical calculations indicate competition between magnetocrystalline and dipolar anisotropy emerges with reduced dimensions. The observed behavior is due to minor structural distortions in low dimensional CGT, which modify the balance between spin-orbit coupling, exchange interactions and dipolar anisotropy. This triggers a critical crossover at $T^{\prime}_c$. Our study shows the emergence of a complex interplay of short- and long-range interactions below $T^{\prime}_c$ as CGT approaches the 2D limit.

cond-mat.mtrl-sci↗

arXiv2023

Probing the strongly correlated magnetic state of Co$_2$C nanoparticles at low temperatures using $μ$SR

Co$_2$C nanoparticles (NPs) are amongst transition metal carbides whose magnetic properties have not been well explored. A recent study by Nirmal Roy et al. [1] showed that a collection of Co$_2$C NPs exhibit an exchange bias (EB) effect below T$_{EB}$ = 50 K and also a spin glass (SG) state below T$_{SG}$ = 5 K. We use magnetic, electrical transport, specific heat, and muon spin rotation ($μ$SR) measurements to explore further the magnetic properties of these NPs. We uncover the onset of Kondo localization at Kondo temperature T$_K$ (= 40.1 K), near the onset of EB effect. A crossover from the Kondo-screened scenario to an RKKY interaction-dominated regime is also observed for T < T$_K$. Specific heat measurements confirm Kondo localization and heavy fermionic nature in Co$_2$C at low T. At low T, zero field $μ$SR spectra reveal a dominant magnetically disordered fraction with slow relaxation and a smaller fraction with short-range order exhibiting fast relaxation, with no evidence of long-range magnetic order. We observe an increase in this fast relaxation rate between T$_{EB}$ and T$_{SG}$, suggesting a slowing down of the fluctuating local magnetic environment around muons. Transverse field $μ$SR spectra show the emergence of a stable, multi-peaked local magnetic field distribution below T$_{EB}$. Longitudinal field $μ$SR spectra shows distinct changes in the dynamics of fluctuations suggesting the presence of a frozen glassy like state below 6 K. Our results suggest that below T$_{EB}$, Co$_2$C NPs pellet develops a magnetic interface, separating disordered and short-range order fractions. The Exchange interaction that sets in below T$_{EB}$ at the interface couples them and suppresses the fluctuations. With the suppression of magnetic fluctuations below T$_{EB}$, strong correlation effects in the electronic state of Co$_2$C lead to Kondo localization.

cond-mat.str-el↗