arXiv subjects

Enamullah

Publications and source records attributed to Enamullah.

7 recordsLinked to original sources

arXiv2019

Cu-substituted Fe2P: An emerging candidates for magnetic RAM application

We propose that Cu-substituted Fe$_2$P, (Fe$_{1-x}$Cu$_x$)$_2$P ($x\sim 0.16$), to be an outstanding contender for the STT-MRAM application. Using first principles based calculations in the framework of density functional theory and through Monte Carlo simulations, we demonstrate that this material can be used as ferromagnetic electrode in the magnetic tunnel junction (MTJ) of STT-MRAM due to its moderate perpendicular magnetic anisotropy (PMA), large tunnel magneto-resistance (TMR), good thermal stability and high ferromagnetic transition temperature. We point out that the simplicity in the synthesis, huge abundance, and non-toxicity make this material a very good candidate to replace the current MTJ materials for STT-MRAM such as FePt,~FeCo or FeCoB.

cond-mat.mtrl-sci↗

arXiv2018

Spin Gapless Semiconducting Nature in Co-rich Co1+xFe1-xCrGa: Insight and Advancements

In this report, we present structural, electronic, magnetic and transport properties of Co-rich spin gapless semiconductor CoFeCrGa using both theoretical and experimental techniques. The key advantage of Co-rich samples $\mathrm{Co_{1+x}Fe_{1-x}CrGa}$ is the high Curie temperature (T$\mathrm{_C}$) and magnetization, without compromising the SGS nature (up to x = 0.4), and hence our choice. The quaternary Heusler alloys $\mathrm{Co_{1+x}Fe_{1-x}CrGa}$ (x = 0.1 to 0.5) are found to crystallize in LiMgPdSn-type structure having space group $F\bar{4}3m$ (\# 216). The measured Curie temperature increases from 690 K (x = 0) to 870 K (x = 0.5). Observed magnetization values follow the Slater-Pauling rule. Measured electrical resistivity, in the temperature range of 5-350 K, suggests that the alloys retain the SGS behavior up to x = 0.4, beyond which it reflects metallic character. Unlike conventional semiconductors, the conductivity value ($\mathrm{\sigma_{xx}}$) at 300 K lies in the range of 2289 S $\mathrm{cm^{-1}}$ to 3294 S $\mathrm{cm^{-1}}$, which is close to that of other reported SGS materials. The anomalous Hall effect is comparatively low. The intrinsic contribution to the anomalous Hall conductivity increase with x, which can be correlated with the enhancement in chemical order. The anomalous Hall coefficient is found to increase from 38 S/cm for x = 0.1 to 43 S/cm for 0.3. Seebeck coefficients turn out to be vanishingly small below 300 K, another signature for being SGS. All the alloys (for different x) are found to be both chemically and thermally stable. Simulated magnetization agrees fairly with the experiment. As such Co-rich CoFeCrGa is a promising candidate for room temperature spintronic applications, with enhanced T$\mathrm{_C}$, magnetic properties and SGS nature.

cond-mat.mtrl-sci↗

arXiv2017

Competing magnetic and spin gap-less semiconducting behaviour in fully compensated ferrimagnet CrVTiAl: Theory and Experiment

We report the structural, magnetic and transport properties of polycrystalline CrVTiAl alloy along with first principles calculations. It crystallizes in the LiMgPdSn type structure with lattice parameter 6.14 \AA\ at room temperature. Absence of (111) peak along with the presence of a weak (200) peak indicates the antisite disorder of Al with Cr and V atoms. The magnetization measurements reveal a ferrimagnetic transition near 710 K and a coercive field of 100 Oe at 3 K. Very low moment and coercive field indicate fully compensated ferrimagnetism in the alloy. Temperature coefficient of resistivity is found to be negative, indicating a characteristic of semiconducting nature. Absence of exponential dependence of resistivity on temperature indicates a gapless/spin-gapless semiconducting behaviour. Electronic and magnetic properties of CrVTiAl for three possible crystallograpic configurations are studied theoretically. All the three configurations are found to be different forms of semiconductors. Ground state configuration is a fully compensated ferrimagnet with band gaps 0.58 eV and 0.30 eV for up and down spin bands respectively. The next higher energy configuration is also ferrimagnetic, but has spin-gapless semiconducting nature. The highest energy configuration corresponds to a non-magnetic gapless semiconductor. The energy differences among these configurations are quite small ($<$ 1 $\mathrm{mRy/atom}$) which hints that at finite temperatures, the alloy exists in a disordered phase, which is a mixture of the three configurations. By taking into account the theoretical and the experimental findings, we conclude that CrVTiAl is a fully compensated ferrimagnet with predominantly spin gap-less semiconductor nature.

cond-mat.mtrl-sci↗

arXiv2017

Structural, electronic, magnetic and transport properties of equiatomic quaternary Heusler Alloy CoRhMnGe: Theory and Experiment

In this work, we present structural, electronic, magnetic, mechanical and transport properties of equiatomic quaternary Heusler alloy, CoRhMnGe using both theoretical and experimental techniques. A detailed structural analysis is performed using X-ray diffraction(XRD) and extended X-ray absorption fine structure(EXAFS) spectroscopy. The alloy is found to crystallize in Y-type structure having space group $F\bar{4}3m$ (\# 216). The ab-initio simulation pedict half-metallic ferromagnetic characteristics leading to large spin polarization. The calculated magnetization is found to be in fair agreement with experiment as well as those predicted by the Slater-Pauling rule, which is a prerequisite for half-metallicity. The magnetic transition temperature($\mathrm{T_{C}}$) is found to be $\sim 760$ K. Measured electrical resistivity in the temperature range 2-400 K also gives an indication of half-metallic behavior. Simulated resistivity matches fairly well with those measured, with the temperature dependant carrier relaxation time lying in the range $1-2$ fs. Effect of hydrostatic pressure on electronic structure, magnetic and mechanical properties are investigated in detail. The alloy is found to preserve half-metallic characteristics upto 30.27 GPa beyond which it transit to metallic phase. No magnetic phase transition is found to occur in the whole range of pressure. The system also satisfies the Born-Huang criteria for mechanical stability upto a limited range of pressure. All these properties make CoRhMnGe alloy promising for spintronics devices.

cond-mat.mtrl-sci↗

arXiv2016

Bismuth based Half Heusler Alloys with giant thermoelectric figure of merit

Half Heusler (HH) thermoelectric alloys provide a wide platform to choose materials with non-toxic and earth abundant elements. This article presents an ab-initio theoretical evaluation of electrical and thermal transport properties of three Bismuth-based most promising thermoelectric alloys, selected out of 54 stable HH compounds. These are brand new compounds which are recently proposed to be stable (Nature Chem. 7, 308 (2015)) and may have interesting properties. The calculated band structure of the three compounds, namely HfRhBi, ZrIrBi and ZrRhBi, served as a hint for their promising thermoelectric properties. To gain confidence on the theoretical predictions of these unreported systems, we first checked our calculated results for a well studied similar compound, ZrNiSn, and showed reasonable agreement with the measured ones. HfRhBi and ZrIrBi turn out to be narrow band gap while ZrRhBi is a moderate band gap semiconductor. A detailed study of the carrier concentration and temperature dependance of the Seebeck coefficient (S), Power factor (S$^2 \sigma$), lattice ($\kappa_L$) and electronic ($\kappa_e$) thermal conductivity and hence the figure of merit (ZT) is carried out. In contrast to most promising known thermoelectric materials, we found high power factor for these materials (highest S$^2 \sigma\sim$17.36 mWm$^{-1}$K$^{-2}$ for p-type ZrIrBi). All the three systems (specially p-type) show high figure of merit, with ZT value as high as 0.45 for ideal crystal. Maximum ZT and the corresponding optimal n- and p-type doping concentrations ($n_c$) are calculated for all the three compounds, which shall certainly pave guidance to future experimental work.

cond-mat.mtrl-sci↗

arXiv2015

Half-metallic, Co-based quaternary Heuslers for spintronics: defect- and pressure-induced transitions and properties

Heusler compounds offer potential as spintronic devices due to their spin-polarization and half-metallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin-polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin-polarization. Using density-functional theory, we analyze the electronic and magnetic properties of equiatomic Heusler ($L$2$_{1}$) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (thermal antisites, binary swaps, and vacancies). Under pressure, CoMnCrAl undergoes a metallic transition, while half-metallicity in CoFeCrGe is retained for a limited range. Antisite disorder between Co-Al pairs in CoMnCrAl and Co-Ge pairs in CoFeCrGe is energetically the most favored, and retain half-metallic character in Co-excess samples. However, Co-deficient samples undergo a transition from half-metallic to metallic, with a discontinuity in the saturation magnetization. For binary swaps, configurations that compete with the ground state are identified and show no loss of half-metallicity; however, the minority-spin bandgap and magnetic moments vary depending on the atoms swapped. For single binary swaps, there is a significant energy cost in CoMnCrAl but with no loss of half metallicity. Although a few configurations in CoFeCrGe energetically compete with the ground statei, however the minority-spin bandgap and magnetic moments vary depending on the atoms swapped. These informations should help in controlling these potential spintronic materials.

cond-mat.mtrl-sci↗

arXiv2015

Electronic structure, magnetism and antisite disorder in CoFeCrGe and CoMnCrAl quaternary Heusler alloys

We present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrGe (CFCG) and CoMnCrAl (CMCA), promising candidates for spintronics applications. Magnetization measurement shows the saturation magnetization and transition temperature to be $3\; \mu_B$, $866$ K and $0.9 \; \mu_B$, $358$ K for CFCG and CMCA respectively. The magnetization values agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity. A striking difference between the two systems is their structure; CFCG crystallizes in fully ordered Y-type structure while CMCA has L2$_1$ disordered structure. The antisite disorder adds a somewhat unique property to the second compound, which arises due to the probabilistic mutual exchange of Al positions with Cr/Mn and such an effect is possibly expected due to comparable electronegativities of Al and Cr/Mn. {\it Ab-initio} simulation predicted a unique transition from half metallic ferromagnet to metallic antiferromagnet beyond a critical excess amount of Al in the alloy.

cond-mat.mtrl-sci↗