16 research outputs found
Defect-induced formation and frustration driven multiple magnetic transitions in Gd2Co0.90Si2.90
No full textA new ternary compound Gd2Co0.90Si2.90 been synthesized in chemically single phase by deliberately introducing lattice vaccancies in the Co and Si sites. The system is characterized through dc magnetization, heat capacity, resistivity along with density functional theory (DFT) calculations. A detailed experimental study reveals that the system exibits an antiferromagneic transition below TN ~ 11 K followed by spin freezing behaviour below TB ~ 5 K. An additional high-temperature magnetic transition could also be detected at TH ~150 K, which is short range in character and is associated with defect-induced polarization of the conduction electrons. The spin-glass-like state formation in the system is further supported by the observation of magnetic relaxation and associated aging phenomena as well as magnetic memory effect. On the basis of non-equilibrium dynamical behaviour, we argue that the glassy state in this compound favours the hierarchical model over the droplet model. DFT calculations as well as experimental outcomes establish that the ground state is magnetically frustrated due to small energy difference between distinct spin configurations having closely spaced ground-state energies.This preprint is available as Kundu, Mily and Pakhira, Santanu and Gupta, Shuvankar and Choudhary, Renu and Lakshminarasimhan, N. and Ranganathan, R. and Mandal, Kalyan and Johnson, Duane D. and Mazumdar, Chandan, Defect-Induced Formation and Frustration Driven Multiple Magnetic Transitions in Gd2co0.90si2.90. doi: http://dx.doi.org/10.2139/ssrn.4631937.
Published as Kundu, Mily, Santanu Pakhira, Shuvankar Gupta, Renu Choudhary, Sourav Sarkar, N. Lakshminarasimhan, R. Ranganathan, Kalyan Mandal, Duane D. Johnson, and Chandan Mazumdar. "Defect-induced formation and frustration-driven multiple magnetic transitions in Gd 2 Co 0.90 Si 2.90." Journal of Materials Chemistry C 12, no. 32 (2024): 12292-12303.
doi: https://doi.org/10.1039/D4TC01798F
Ground-state degeneracy and complex magnetism of geometrically frustrated Gd 2 Ir 0.97 Si 2.97
No full textA new triangular-lattice intermetallic compound Gd2Ir0.97Si2.97 was successfully synthesized as single phase
by deliberately introducing vacancies. Theoretical analysis suggests that the ground state is competing with
several low-energy spin configurations due to magnetic frustration on a nearly ideal triangular lattice. Despite
a number of competing magnetic states, the compound exhibits long-range antiferromagnetic order at 16 K, a
long-range ferrimagnetic transition at 6.5 K, and a reentrant cluster-glass transition below Tf ∼ 3 K. The complex
magnetism in the compound could be correlated with competing antiferromagnetic and ferrimagnetic structures
predicted theoretically.This article is published as Chakraborty, Sudip, Shuvankar Gupta, Santanu Pakhira, Renu Choudhary, Anis Biswas, Yaroslav Mudryk, Vitalij K. Pecharsky, Duane D. Johnson, and Chandan Mazumdar. "Ground-state degeneracy and complex magnetism of geometrically frustrated Gd 2 Ir 0.97 Si 2.97." Physical Review B 106, no. 22 (2022): 224427.
DOI: 10.1103/PhysRevB.106.224427.
Copyright 2022 American Physical Society.
Posted with permission.
DOE Contract Number(s): AC02-07CH1135
Heat transfer enhancement in hybrid metal-polymer heat exchangers using extended surfaces
No full textSubmission published under a 24 month embargo labeled 'U of I Access', the embargo will last until 2023-12-01The student, Shuvankar Goswami, accepted the attached license on 2021-12-10 at 16:05.The student, Shuvankar Goswami, submitted this Thesis for approval on 2021-12-10 at 16:08.This Thesis was approved for publication on 2021-12-10 at 16:37.DSpace SAF Submission Ingestion Package generated from Vireo submission #17445 on 2022-04-06 at 17:18:08Made available in DSpace on 2022-04-29T21:47:49Z (GMT). No. of bitstreams: 2
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Reason: Author requested U of Illinois access only (OA after 2yrs) in Vireo ETD systemAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD systemU of I OnlyUp to half of the total energy input into the industrial sector of the United States can reasonably be lost through waste heat. Most of this waste heat exists in the low temperature regime (<450°F). Metal heat exchangers prevalent in waste heat recovery are vulnerable to corrosion and costly to install and maintain. Polymers, though inadequate for most heat transfer applications due to low thermal conductivities (~0.1 W/m.K), provide a means to mitigate these issues. A hybrid configuration of combined metal-polymer tapes provides a method for reaching a suitable conductivity for waste heat recovery usage. Heat exchangers comprised of helically wound copper-polymer tapes are manufactured using a roll-to-roll process, as established in previous research. This work investigates the capability of adapting the process to integrate external fins into the hybrid design. Surface area enhancement of the outermost metal layer is a proposed way of improving the heat transfer characteristics without affecting material or thermal parameters. We report findings from conducted experiments to analyze the effectiveness of the external fins. This achieved design can enable future advancements in waste heat recovery technology for eventual industrial usage
Experimental observation of spin glass state in the highly disordered quaternary Heusler alloy FeRuMnGa
No full textThe realization of the spin-glass (SG) state in Heusler alloys is very rare despite the presence of inherent structural and elemental disorder in those compounds. Although a few half- and full-Heusler alloys are known to exhibit the SG state, there is hardly any manifestation of the same in cases of quaternary Heusler compounds. Here we report the observation of a SG state in a highly disordered equiatomic quaternary Heusler compound: FeRuMnGa where the SG state is in between the canonical SG and the cluster glass. Different intricate features of the SG state including nonequilibrium magnetic dynamics at low temperatures in the compound are unveiled through our comprehensive magnetic, heat capacity, and neutron-diffraction studies. Further the structural disorder in the sample is neither conventional A2 - nor B2 -type whereas those two types are commonly observed for Heusler compounds. The presence of disorder also plays a significant role in electron transport properties of the alloy, which is reflected in its exhibition of semimetallic behavior and anomalous Hall effect at low temperatures.This article is published as Gupta, Shuvankar, Sudip Chakraborty, Santanu Pakhira, Anis Biswas, Yaroslav Mudryk, Amit Kumar, Bodhoday Mukherjee et al. "Experimental observation of spin glass state in the highly disordered quaternary Heusler alloy FeRuMnGa." Physical Review B 107, no. 18 (2023): 184408.
DOI: 10.1103/PhysRevB.107.184408.
Copyright 2023 American Physical Society.
Posted with permission.
DOE Contract Number(s): AC02-07CH1135
Magnetic frustration driven by conduction carrier blocking in Nd2Co0.85Si2.88
No full textThe intermetallic compound Nd2Co0.85Si2.88 having a triangular lattice could be synthesized in single-phase only with defect crystal structure. Investigation through different experimental techniques indicate the presence of two magnetic transitions in the system. As verified experimentally and theoretically, the high-temperature transition TH ~ 140 K is associated with the development of ferromagnetic interaction between itinerant Co moments, whereas the low-temperature transition at TL ~ 6.5 K is due to the coupling among Nd-4f and Co-3d moments, which is antiferromagnetic in nature. Detailed studies of temperature-dependent dc magnetic susceptibility, field dependence of isothermal magnetization, non-equilibrium dynamical behavior, viz. magnetic relaxation, aging effect, magnetic-memory effect, and temperature dependence of heat capacity, along with density functional theory (DFT) calculations, suggest that the ground state is magnetically frustrated spin-glass in nature, having competing magnetic interactions of equivalent energies. DFT results further reveal that the 3d/5d-conduction carriers are blocked in the system and act as a barrier for the 4f-4f RKKY interactions, resulting in spin-frustration. Presence of vacancy defects in the crystal are also conducive to the spin-frustration. This is an unique mechanism of magnetic frustration, not emphasized so far in any of the ternary R2TX3 (R=rare-earth, T=transition elements and X=Si, Ge, In) type compounds. Due to the competing character of the itinerant 3d and localized 4f moments, the compound exhibits anomalous field dependence of magnetic coercivity.This article is published as Kundu, Mily, Santanu Pakhira, Renu Choudhary, Shuvankar Gupta, Sudip Chakraborty, N. Lakshminarasimhan, R. Ranganathan, Duane D. Johnson, and Chandan Mazumdar. "Magnetic frustration driven by conduction carrier blocking in Nd2Co0.85Si2.88." Physical Review B 107, no. 9 (2023): 094421.
DOI: 10.1103/PhysRevB.107.094421.
Copyright 2023 American Physical Society.
Posted with permission.
DOE Contract Number(s): AC02-07CH1135
Comparative study of Kondo effect in Vanadium dichalcogenides VX (X=Se & Te)
No full textWe report on the electrical transport, magnetotransport, and magnetic
properties studies on the transition metal dichalcogenides VSe and VTe
and draw a comprehensive comparison between them. We observe Kondo effect in
both systems induced by the exchange interaction between localized moments and
conduction electrons at low temperature, resulting into resistance upturn at 6
K for VSe and 17 K for VTe. From the field dependent resistance
measurements we find that the data is fitted best with modified Hamann equation
corrected by the quantum Brillouin function for VSe, while the data is
fitted best with modified Hamann equation corrected by the classical Langevin
function for VTe. Interestingly, we observe a contrasting magnetoresistance
(MR) property between these systems across the Kondo temperature. That means,
negative MR is found in both systems in the Kondo state. In the normal state MR
is positive for VSe, while it is negligible for VTe. In addition, both
systems show weak ferromagnetism at low temperature due to intercalated V
atoms
Rare observation of spin-gapless semiconducting characteristics and related band topology of quaternary Heusler alloy CoFeMnSn
No full textIn this paper, we report the theoretical investigation and experimental
realization of a new spin-gapless semiconductor (SGSs) compound CoFeMnSn
belonging to the family of quaternary Heusler alloys. Through the use of
several ground-state energy calculations, the most stable structure has been
identified. Calculations of the spin-polarized band structure in optimized
structure's reveals the SGS nature of the compound. The compound form in an
ordered crystal structure and exhibit a high ferromagnetic transition
temperature (T = 560 K), making the material excellent for room
temperature applications. Adherence of saturation magnetization to the
Slater-Pauling rule, together with the nearly temperature-independent
resistivity, conductivity, and carrier concentration of the compound in the
temperature regime 5300 K along with the low value of anomalous Hall
conductivity (AHC) further confirms the SGS nature. Theoretical calculations
also reveal the robustness of the SGS state due to lattice contraction and one
can obtain a high value of intrinsic AHC using hole doping. Combined SGS and
topological properties of the compound make CoFeMnSn suitable for spintronics
and magneto-electronics devices
Does carrier localization affect the anomalous Hall effect?
No full textThe effect of carrier localization due to electron-electron interaction in
anomalous Hall effect is elusive and there are contradictory results in the
literature. To address the issue, we report here the detailed transport study
including the Hall measurements on -Mn type cubic compound
CoZnMn with chiral crystal structure, which lacks global mirror
symmetry. The alloy orders magnetically below = 204 K, and reported to
show spin glass state at low temperature. The longitudinal resistivity
() shows a pronounced upturn below = 75 K, which is found
to be associated with carrier localization due to quantum interference effect.
The upturn in shows a dependence and it is practically
insensitive to the externally applied magnetic field, which indicate that
electron-electron interaction is primarily responsible for the low- upturn.
The studied sample shows considerable value of anomalous Hall effect below
. We found that the localization effect is present in the ordinary Hall
coefficient (), but we failed to observe any signature of localization in
the anomalous Hall resistivity or conductivity. The absence of localization
effect in the anomalous Hall effect in CoZnMn may be due to large
carrier density, and it warrants further theoretical investigations,
particularly with systems having broken mirror symmetry.Comment: 9 pages, 5 figure
Giant Nernst Angle in Self-Intercalated van der Waals Magnet CrTe
No full textThe discovery of two-dimensional van der Waals (vdW) magnetic materials has propelled advancements in technological devices. The Nernst effect, which generates a transverse electric voltage in the presence of a longitudinal thermal gradient, shows great promise for thermoelectric applications. In this work, we report the electronic and thermoelectric transport properties of CrTe, a layered self-intercalated vdW material which exhibits an antiferromagnetic ordering at TN ~ 191 K followed by a ferromagnetic-like phase transition at TC ~171 K. We observe a prominent topological Hall effect and topological Nernst effect between TC and TN, which is ascribable to non-coplanar spin textures inducing a real-space Berry phase due to competing ferromagnetic and antiferromagnetic interactions. Furthermore, we show that CrTe exhibits a substantial anomalous Nernst effect, featuring a giant Nernst angle of ~37% near TC and a maximum Nernst thermoelectric coefficient of 0.52 uV/K. These results surpass those of conventional ferromagnets and other two-dimensional vdW materials, highlighting CrTe as a promising candidate for advanced thermoelectric devices based on the Nernst effect.Accepted in Materials Today Physic
Restructuring disorder: Transformation from the antiferromagnetic order in Fe2VSi to the ferromagnetic state in FeRuVSi by substitution of a non-magnetic element
No full textThe delicate nature of the half-metallic ferromagnetic (HMF) property in
Heusler alloys is often compromised by inherent structural disorder within the
systems. Fe2VSi is a prime example, where such disorder prevents the
realization of the theoretically proposed HMF state as the anti-site disorder
leads to the formation of two anti-parallel magnetic lattices resulting in
antiferromagnetic order. In this study, we propose an innovative and simple
strategy to prevent this atomic disorder by replacing 50% of the magnetic
element Fe by a large, isoelectronic, non-magnetic element, Ru. In this way,
one of the magnetic sublattices of the antiferromagnetic lattice ceases to
order while ferromagnetic order is restored, an essential criterion for
exhibiting HMF properties. Through various experimental measurements and
theoretical calculations, we have shown that such partial replacement of Fe by
Ru prevents the cross-site substitution of V/Si sites and the system regains
its ferromagnetic order. Our theoretical calculations suggest that a perfect
structural arrangement in Fe and Ru would have restored the HMF property in
FeRuVSi. However, the local atomic disorder of Fe and Ru was found to decrease
the spin polarization value. The present work sheds light on the complex
interplay between structural disorder and magnetic properties in Heusler alloys
and provides insights for future design strategies in the pursuit of robust
half-metallic ferromagnets
