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Tufan Roy

Publications and source records attributed to Tufan Roy.

At least 19 recordsLinked to original sources
arXiv2025

Magnetically Enhanced Thermoelectric Effect Driven by Martensitic Transformation in the Weak Itinerant Ferromagnet Co$_2$NbSn

We investigated the magnetic and thermoelectric properties of the full Heusler alloy Co$_2$NbSn, which exhibits a martensitic transformation at 240 K. Magnetization measurements reveal weak itinerant ferromagnetism in the martensitic phase, which is well described by Takahashi's spin fluctuation theory. The characteristic spin fluctuation parameters were estimated to be T_0 = 1.0$\times$10^3 K and T_A = 7.2$\times$10^3 K. Seebeck coefficient measurements under magnetic fields up to 9 T show complex temperature and field dependence, which we decomposed into electron diffusion, spin fluctuation drag, and magnon drag components. A significant magnon-drag contribution was identified in both austenite and martensitic phases. Remarkably, this contribution is strongly enhanced in the martensitic phase compared to the austenite phase, despite a smaller magnetic moment. These findings provide evidence for robust low-energy spin excitations and highlight the potential of martensitic transformation in enhancing the thermoelectric performance of itinerant ferromagnetic alloys.

cond-mat.mtrl-sci↗
arXiv2025

CoRuTiGe: A Possible Spin Gapless Semiconductor

We report experimental and theoretical investigations on the quaternary Heusler alloy CoRuTiGe, synthesized using the arc melting technique. Crystal structure analysis reveals a tetragonal structure at room temperature. Magnetization measurements as a function of temperature and magnetic field indicate ferromagnetic nature with a saturation magnetization of 0.681 mB/f.u. at 5 K. The temperature dependence of electrical resistivity shows a nearly linear decrease in the high-temperature range, indicating the spin gapless semiconductor like behavior of the material. This SGS nature is further supported by the temperature-independent carrier concentration and mobility. Hall effect analysis reveals that the anomalous Hall effect in CoRuTiGe arises from both intrinsic and extrinsic mechanisms. Additionally, a well-defined symmetric negative magnetoresistance is observed at low temperatures. These findings suggest that CoRuTiGe holds significant promise for spintronic applications.

cond-mat.mtrl-sci↗
arXiv2025

Low-Moment Semiconducting Properties of Quaternary Heusler Alloy CoRuTiSn

We investigate structural, magnetic and transport properties of CoRuTiSn equiatomic quaternary Heusler alloy. CoRuTiSn was synthesized by arc-melt technique. The room temperature powder XRD pattern was analyzed, and it was found that CoRuTiSn has a tetragonal crystal structure. Magnetic measurements show non-zero but small hysteresis indicating CoRuTiSn as a soft ferromagnetic with a Curie temperature of ~200 K. The magnetic moment determined from magnetization data is found to be 0.84 {\mu}B/f.u. at 4 K, which is close to the value, calculated using first principles calculations. Electrical resistivity decreases with temperatures, indicating semiconducting nature of CoRuTiSn. Hall effect measurements show anomalous behavior, consistent with the ferromagnetic nature of the sample. The low moment ferromagnetic semiconducting nature of CoRuTiSn could make this material promising for semiconducting spintronics.

cond-mat.mtrl-sci↗
arXiv2023

Ballistic spin-transport properties of magnetic tunnel junctions with MnCr-based ferrimagnetic quaternary Heusler alloys

We investigate the suitability of nearly half-metallic ferrimagnetic quaternary Heusler alloys, CoCrMnZ (Z=Al, Ga, Si, Ge) to assess the feasibility as electrode materials of MgO-based magnetic tunnel junctions (MTJ). Low magnetic moments of these alloys originated from the anti-ferromagnetic coupling between Mn and Cr spins ensure a negligible stray field in spintronics devices as well as a lower switching current required to flip their spin direction. We confirmed mechanical stability of these materials from the evaluated values of elastic constants, and the absence of any imaginary frequency in their phonon dispersion curves. The influence of swapping disorders on the electronic structures and their relative stability are also discussed. A high spin polarization of the conduction electrons are observed in case of CoCrMnZ/MgO hetrojunctions, independent of terminations at the interface. Based on our ballistic transport calculations, a large coherent tunnelling of the majority-spin $s$-like $Δ_1$ states can be expected through MgO-barrier. The calculated tunnelling magnetoresistance (TMR) ratios are in the order of 1000\%. A very high Curie temperatures specifically for CoCrMnAl and CoCrMnGa, which are comparable to $bcc$ Co, could also yield a weaker temperature dependece of TMR ratios for CoCrMnAl/MgO/CoCrMnAl (001) and CoCrMnGa/MgO/CoCrMnGa (001) MTJ.

cond-mat.mtrl-sci↗
arXiv2023

Electronic and magnetic properties of the topological semimetal SmMg$_2$Bi$_2$

Dirac semimetals show nontrivial physical properties and can host exotic quantum states like Weyl semimetals and topological insulators under suitable external conditions. Here, by combining angle-resolved photoemission spectroscopy measurements (ARPES) and first-principle calculations, we demonstrate that Zintl-phase compound SmMg$_2$Bi$_2$ belongs to the close proximity to a topological Dirac semimetallic state. ARPES results show a Dirac-like band crossing at the zone-center near the Fermi level ($E_\mathrm {F}$) which is further confirmed by first-principle calculations. Theoretical studies also reveal that SmMg$_2$Bi$_2$ belongs to a $Z_2$ topological class and hosts spin-polarized states around the $E_\mathrm {F}$. Zintl's theory predicts that the valence state of Sm in this material should be Sm$^{2+}$, however we detect many Sm-4$f$ multiplet states (flat-bands) whose energy positions suggest the presence of both Sm$^{2+}$ and Sm$^{3+}$. It is also evident that these flat-bands and other dispersive states are strongly hybridized when they cross each other. Due to the presence of Sm$^{3+}$ ions, the temperature dependence of magnetic susceptibility $χ(T)$ shows Curie-Weiss-like contribution in the low temperature region, in addition to the Van Vleck-like behaviour expected for the Sm$^{2+}$ ions. The present study will help in better understanding of the electronic structure, magnetism and transport properties of related materials.

cond-mat.str-el↗
arXiv2023

Anisotropic magnetism and electronic structure of trigonal EuAl$_2$Ge$_2$ single crystals

The magnetic and electronic properties of the layered Zintl-phase compound EuAl$_2$Ge$_2$ crystallizing in the trigonal CaAl$_2$Si$_2$-type structure are reported. Our neutron-diffraction measurements show that EuAl$_2$Ge$_2$ undergoes A-type antiferromagnetic (AFM) ordering below $T_{\rm N} = 27.5(5)$~K, with the Eu moments (Eu$^{2+}$, $S = 7/2$) aligned ferromagnetically in the $ab$ plane. The $H = 0$ magnetic structure consists of trigonal AFM domains associated with $ab$-plane magnetic anisotropy and a field-induced reorientation of the Eu spins in the domains is evident at $T = 2$~K below the critical field $H_{c1} = 2.5(1)$ kOe. Electrical resistivity and ARPES measurements show that EuAl$_2$Ge$_2$ is metallic both above and below $T_{\rm N}$. In the AFM phase, we directly observe folded bands in ARPES due to the doubling of the magnetic unit cell along the $c$ axis with an enhancement of quasiparticle weight due to the complex change in the coupling between the magnetic moments and itinerant electrons on cooling below $T_{\rm N}$. The observed electronic structure is well reproduced by first-principle calculations, which also predict the presence of nontrivial electronic states near the Fermi level in the AFM phase with $Z_2$ topological numbers 1;(000).

cond-mat.str-el↗
arXiv2022

Topological electronic structure of YbMg$_2$Bi$_2$ and CaMg$_2$Bi$_2$

Zintl compounds have been extensively studied for their outstanding thermoelectric properties, but their electronic structure remains largely unexplored. Here, we present a detailed investigation of the electronic structure of the isostructural thermopower materials YbMg$_2$Bi$_2$ and CaMg$_2$Bi$_2$ using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). The ARPES results show a significantly smaller Fermi surface and Fermi velocity in CaMg$_2$Bi$_2$ than in YbMg$_2$Bi$_2$. Our ARPES results also reveal that in the case of YbMg$_2$Bi$_2$, Yb-4$f$ states reside well below the Fermi level and likely have a negligible impact on transport properties. To properly model the position of 4$f$-states, as well as the overall electronic structure, a Hubbard $U$ at the Yb sites and spin-orbit coupling (SOC) have to be included in the DFT calculations. Interestingly, the theoretical results reveal that both materials belong to a $Z_2$ topological class and host robust topological surface states around $E_\mathrm {F}$. Due to the intrinsic hole doping, the topological states reside above the Fermi level, inaccessible by ARPES. Our results also suggest that in addition to SOC, vacancies and the resulting hole doping play an important role in the transport properties of these materials.

cond-mat.mtrl-sci↗
arXiv2021

IrCrMnZ (Z=Al, Ga, Si, Ge) Heusler alloys as electrode materials for MgO-based magnetic tunneling junctions: A first-principles study

We study IrCrMnZ (Z=Al, Ga, Si, Ge) systems using first-principles calculations from the perspective of their application as the electrode materials of MgO-based MTJs. These materials have highly spin-polarized conduction electrons with partially occupied $Δ_1$ band, which is important for coherent tunneling in parallel magnetization configuration. The Curie temperatures of IrCrMnAl and IrCrMnGa are very high (above 1300 K) as predicted from mean-field-approximation. The stability of ordered phase against various antisite disorders has been investigated. We discuss here the effect of "spin-orbit-coupling" on the electronic structure around Fermi level. Further, we investigate the electronic structure of IrCrMnZ/MgO heterojunction along (001) direction. IrCrMnAl/MgO and IrCrMnGa/MgO maintain half-metallicity even at the MgO interface, with no interfacial states at/around Fermi level in the minority-spin channel. Large majority-spin conductance of IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa is reported from the calculation of ballistic spin-transport property for parallel magnetization configuration. We propose IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa as promising MTJs with a weaker temperature dependence of tunneling magnetoresistance ratio, owing to their very high Curie temperatures.

cond-mat.mtrl-sci↗
arXiv2020

Ab-initio study of electronic and magnetic properties of Mn$_2$RuZ/MgO (001) heterojunctions (Z= Al, Ga, Si, Ge)

Using first-principles calculations, we studied Mn$_2$RuZ (Z=Al, Ga, Si, Ge) and their heterojunctions with MgO along (001) direction. All these alloys possess Hg$_2$CuTi-type inverse Heusler alloy structure and ferrimagnetic ground state. Our study reveals the half-metallic electronic structure with highly spin-polarized $Δ_1$ band, which is robust against atomic disorder. Next we studied the electronic structure of Mn$_2$RuAl/MgO and Mn$_2$RuGe/MgO heterojunctions. We found that the MnAl- or MnGe-terminated interface is energetically more favorable compared to the MnRu-terminated interface. Interfacial states appear at the Fermi level in the minority-spin gap for the Mn$_2$RuGe/MgO junction. We discuss the origin of these interfacial states in terms of local environment around each constituent atom. On the other hand, in the Mn$_2$RuAl/MgO junction, high spin polarization of bulk Mn$_2$RuAl is preserved independent of its termination.

cond-mat.mtrl-sci↗
arXiv2020

Structural and magnetic properties of epitaxial films of CoIrMnAl equiatomic quaternary Heusler alloy designed from first-principles calculation

MgO-barrier magnetic tunnel junctions with half-metallic Heusler alloy electrodes attracted much attentions for spintronics applications. However, a couples of issues related to materials still remain to be resolved for practical uses. Recently, quarterly equiatomic Heusler alloys attracted attentions as advanced Heusler alloys. CoIrMnZ (Z = Al, Si, Ga, and Ge) half-metallic Heusler alloys were designed and predicted to have moderate Curie temperatures and to be a lattice-matched with the MgO barrier, being advantageous to traditional Co2 Heusler alloys [T. Roy et al., J. Magn. Magn. Mater. 498, 166092 (2020)]. Here we experimentally investigated structure and magnetic properties for thin films of one of those alloys, CoIrMnAl with a sputtering deposition. We successfully obtained the films with the B2 chemical ordering even with no post-annealing process. The lattice constant for the films annealed at 500-600$^\circ$C approximates the predicted values. The magnetization at 10 K was near 500 kA/m and the Curie temperature was approximately 400 K were observed, which were about 70% of the values predicted for the fully ordered structure. The magnetic properties observed in those B2 ordered films were well explained by ferrimagnetism appeared in B2 ordered CoIrMnAl with full-swap disorders of Co-Ir and Mn-Al and almost full-swap disorder of Co-Mn, predicted from the first-principles calculations.

cond-mat.mtrl-sci↗
arXiv2020

Lattice Softening in Metastable bcc CoxMn100-x(001) Ferromagnetic Layers for a Strain-Less Magnetic Tunnel Junction

In spintronics, one of the long standing questions is why the MgO-based magnetic tunnel junction (MTJ) is almost the only option to achieve a large tunnelling magnetoresistance (TMR) ratio at room temperature (RT) but not as large as the theoretical prediction. This study focuses on the development of an almost strain-free MTJ using metastable bcc CoxMn100-x ferromagnetic films. We have investigated the degree of crystallisation in MTJ consisting of CoxMn100-x/MgO/CoxMn100-x (x = 66, 75, 83 and 86) in relation to their TMR ratios. Cross-sectional high resolution transmission electron microscopy (HRTEM) reveals that almost consistent lattice constants of these layers for 66 < x < 83 with maintaining large TMR ratios of 229% at RT, confirming the soft nature of the CoxMn100-x layer with some dislocations at the MgO/Co75Mn25 interfaces. For x = 86, on the other hand, the TMR ratio is found to be reduced to 142% at RT, which is partially attributed to the increased number of the dislocations at the MgO/Co86Mn14 interfaces and amorphous grains identified in the MgO barrier. Ab-initio calculations confirm the crystalline deformation stability across a broad compositional range in CoMn, proving the advantage of a strain-free interface for much larger TMR ratios.

cond-mat.mtrl-sci↗
arXiv2020

High tunnel magnetoresistance and magnetism in metastable bcc Co$_{1-x}$Mn$_x$-based magnetic tunnel junctions

Co-rich Co$_{1-x}$Mn$_x$ alloys have hcp or fcc disordered phases and those ferromagnetic orderings are significantly deteriorated with increasing Mn concentration $x$ in bulk. On the other hand, those metastable bcc phases show properties attractive to spintronics, e.g., high tunnel magnetoresistance (TMR) ratio of more than 200% (600%) at 300 K (10 K) in magnetic tunnel junctions (MTJs) with the $x$ = 0.25 bcc alloy electrodes [Kunimatsu et al., Appl. Phys. Express 13, 083007 (2020)]. Here, we report systematic study of structure and magnetism for epitaxial thin films as well as the TMR effect in MgO(001)-barrier MTJs with electrodes comprising those bcc films. The single phase bcc Co$_{1-x}$Mn$_x$(001) films were pseudomorphically grown on Cr(001) for 0.14 < $x$ < 0.50 with a sputtering technique. The magnetization was larger than that of pure Co for $x$ = 0.14-0.25 and deceased with further increasing $x$. This behavior mainly stemmed from the composition dependence of magnetic moment of Mn that exceeded 2 $μ_B$ at the maximum, unveiled by X-ray magnetic circular dichroism. Correspondingly, within the range of 0.25 < $x$ < 0.37, the TMR ratio decreased from 620% (229%) to 450% (194%) at 10 K (300 K) as $x$ increased. We discussed the relationship between the magnetism and high TMR ratio with different $x$ with the aid of the ab-initio band structure calculations.

cond-mat.mtrl-sci↗
arXiv2019

Electronic structure of Au-Sn compounds grown on Au(111)

The electronic structure of Au-Sn intermetallic layers of different compositions grown on Au(111) to the thickness of several nanometers has been studied in this work. The layer, interface and the substrate related components in the Au 4$f$ and Sn 4$d$ core-level spectra obtained using x-ray photoelectron spectroscopy (XPS) vary with deposition parameters to reveal the details of the Au-Sn formation. While AuSn is grown by deposition at room temperature, Au rich compounds form as a result of heat treatment through inter diffusion of Au and Sn. Deposition at high temperature forms more Au rich compositions compared to post annealing at the same temperature due to the kinetic energy of the impinging Sn atoms in the former case. Post annealing, on the other hand, stabilizes the bulk phases such as AuSn and Au$_5$Sn and exhibits an activated behavior for transition from the former to the latter with increasing temperature. The XPS valence band spectra of AuSn and Au$_{5}$Sn layers show good agreement with the density functional theory calculation, indicating that these have the bulk structure reported in literature. However, the influence of anti-site defects is observed in Au$_5$Sn. Low energy electron diffraction study reveals that although the AuSn layer is ordered, its top surface is disordered at room temperature. Surface order is obtained by annealing or deposition at elevated temperatures and dispersing bands are observed by angle resolved photoemission spectroscopy. Both electron-like and hole-like bands are evident for the ($\sqrt{3}$$\times$$\sqrt{3}$)R30$^{\circ}$ phase, while a nearly free electron-like parabolic surface state is observed for the p(3$\times$3)R15$^{\circ}$ phase.

cond-mat.mtrl-sci↗
arXiv2019

Magnetic tunnel junctions with a B2-ordered CoFeCrAl equiatomic Heusler alloy

The equiatomic quaternary Heusler alloy CoFeCrAl is a candidate material for spin-gapless semiconductors (SGSs). However, to date, there have been no experimental attempts at fabricating a junction device. This paper reports a fully epitaxial (001)-oriented MgO barrier magnetic tunnel junction (MTJ) with CoFeCrAl electrodes grown on a Cr buffer. X-ray and electron diffraction measurements show that the (001) CoFeCrAl electrode films with atomically flat surfaces have a $B2$-ordered phase. The saturation magnetization is 380 emu/cm$^3$, almost the same as the value given by the Slater--Pauling--like rule, and the maximum tunnel magnetoresistance ratios at 300 K and 10 K are 87% and 165%, respectively. Cross-sectional electron diffraction analysis shows that the MTJs have MgO interfaces with fewer dislocations. The temperature- and bias-voltage-dependence of the transport measurements indicates magnon-induced inelastic electron tunneling overlapping with the coherent electron tunneling. X-ray magnetic circular dichroism (XMCD) measurements show a ferromagnetic arrangement of the Co and Fe magnetic moments of $B2$-ordered CoFeCrAl, in contrast to the ferrimagnetic arrangement predicted for the $Y$-ordered state possessing SGS characteristics. Ab-initio calculations taking account of the Cr-Fe swap disorder qualitatively explain the XMCD results. Finally, the effect of the Cr-Fe swap disorder on the ability for electronic states to allow coherent electron tunneling is discussed.

cond-mat.mtrl-sci↗
arXiv2017

Co$_{2}$PtGa: A promising magnetic shape memory alloy with high martensite transition temperature

In the present work, a combined theoretical and experimental study on Co$_{2}$PtGa Heusler alloy shows that it exhibits a martensite transition around 1320 K with a small thermal hysteresis (10 K). Dynamical stability of Co$_2$PtGa in the tetragonal phase has been established by the theoretically calculated phonon dispersion curves. Magnetization measurements suggest that this alloy is a ferromagnetic material with a saturation magnetic moment of 2.83 $μ_B$/f.u. at 2 K, which is in excellent agreement with the value obtained from \textit{ab-initio} calculations (2.87 $μ_B$/f.u.). Our present study demonstrates that Co$_{2}$PtGa is a promising material for high temperature magnetic shape memory application.

cond-mat.mtrl-sci↗
arXiv2016

Ab initio Study of Effect of Co Substitution on the Magnetic Properties of Ni and Pt-based Heusler Alloys

Using density functional theory based calculations, we have carried out in-depth studies of effect of Co substitution on the magnetic properties of Ni and Pt-based shape memory alloys. We show the systematic variation of the total magnetic moment, as a function of Co doping. A detailed analysis of evolution of Heisenberg exchange coupling parameters as a function of Co doping has been presented here. The strength of RKKY type of exchange interaction is found to decay with the increase of Co doping.

cond-mat.mtrl-sci↗
arXiv2016

Ab initio Studies on Electronic and Magnetic Properties of X$_{2}$PtGa (X = Cr, Mn, Fe, Co) Heusler Alloys

Using first-principles calculations based on density functional theory, we probe the electronic and magnetic properties of X$_{2}$PtGa (X being Cr, Mn, Fe, Co) Heusler alloys. Our calculations predict that all these systems possess inverse Heusler alloy structure in their respective ground states. Application of tetragonal distortion leads to lowering of energy with respect to the cubic phase for all the materials. The equilibrium volumes of both the phases are nearly the same. These results of our calculations indicate that all these materials are prone to undergo martensite transition, as has been recently shown theoretically for Mn$_{2}$PtGa in the literature. Ground state with a tetragonal symmetry of these materials is supported by the observation of soft tetragonal shear constants in their cubic phase. By comparing the energies of various types of magnetic configurations of these alloys we predict that Cr$_{2}$PtGa and Mn$_{2}$PtGa possess ferrimagnetic configuration whereas Fe$_{2}$PtGa and Co$_{2}$PtGa possess ferromagneic configuration in their respective ground states.

cond-mat.mtrl-sci↗
arXiv2016

Magnetic Interactions and Electronic Structure of Pt$_{2}$Mn$_{1-x}$Y$_{x}$Ga (Y = Cr and Fe) system : An ab-initio calculation

First principles density functional theory based calculations have been carried out to predict the effects of Mn replacement by Fe and Cr on electronic as well as magnetic properties of Pt$_{2}$MnGa as well as Ni$_{2}$MnGa. All the materials studied here are predicted to have conventional Heusler alloy structure in their ground state and they are found to be electronically stable on the basis of their respective formation energy. The replacement of Mn by Fe leads to a ferromagnetic ground state whereas in case of Mn replacement by Cr an {\it intra-sublattice} anti-ferromagnetic configuration has been observed to have lower energy. We study the magnetic exchange interaction between the atoms for the materials with ferromagnetic and anti-ferromagnetic configurations to show the effects of Fe and Cr substitution at Mn site on the magnetic interactions of these systems. Detailed analysis of electronic structure in terms of density of states has been carried out to study the effect of substitution.

cond-mat.mtrl-sci↗