Aquil Ahmad | National Changhua University of Education (original) (raw)

Papers by Aquil Ahmad

Journal of Physics: Condensed Matter, 2020

Density functional theory calculations within the generalized gradient approximation are employed... more Density functional theory calculations within the generalized gradient approximation are employed to study the ground state of Co2FeAl. Various magnetic configurations are considered to find out its most stable phase. The ferromagnetic ground state of the Co2FeAl is energetically observed with an optimized lattice constant of 5.70 Å. Thereafter, the system was subjected under uniform and non-uniform strains to see their effects on spin polarization (P) and half-metallicity. The effect of spin orbit coupling is considered in the present study. Half-metallicity (and 100 % P) is only retained under uniform strains started from 0 to +4%, and dropped rapidly from 90% to 16% for the negative strains started from-1% to-6%. We find that the present system is much sensitive under tetragonal distortions as half-metallicity (and 100% P) is preserved only for the cubic case. The main reason for the loss of half-metallicity is due to the shift of the bands with respect to the Fermi level. We also discuss the influence of these results on spintronics devices.

The European Physical Journal B, 2020

MATERIALS SCIENCE, ENERGY TECHNOLOGY AND POWER ENGINEERING III (MEP 2019), 2019

Journal of Magnetism and Magnetic Materials, 2019

The generalized gradient approximation (GGA) scheme in the first-principles calculations are used... more The generalized gradient approximation (GGA) scheme in the first-principles calculations are used to study the effect of L21 and XA ordering on the phase stability, halfmetallicity and magnetism of Co2FeAl (CFA) Heusler alloy. Various possible hypothetical structures: L21-I, L21-II, XA-I, and XA-II were prepared under the conventional L21 and inverse XA phases by altering the atomic occupancies at their Wyckoff sites. It is found that the XA-II phase of CFA is the most stable phase energetically among all the structures. The electronic structure calculations without U show the presence of half-metallic (HM) ground state only in L21-1 structure and the other structures are found to be metallic. However, the electronic structures of CFA are significantly modified in the presence of U, although the total magnetic moments per cell remained the same and consistent with the Slater-Pauling (SP) rule. The metallic ground states of CFA in L21-II and XA-II structures are converted into the half-metallic ground states in presence of U but remained the same (metallic) in XA-I structure. The results indicate that the electronic structures are not only dependent on the L21 and XA ordering of the atoms but also depend on the choice of U values. So experiments may only verify the superiority of GGA+U to GGA.

Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanopar... more Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanoparticles (NPs). The NPs of size 23 nm are prepared using the co-precipitation method. X-ray and selected area electron diffraction patterns have confirmed the cubic Heusler phase of the NPs with the A2-disorder. These NPs are soft ferromagnetic, and exhibit a high saturation magnetization (Ms) along with a very high Curie temperature (Tc) of 1060 K. The observed Tc value matches closely with the theoretically calculated one following a model provided by Wurmehl et al. [1]. The high Ms and Tc make the present system a potential candidate for magnetically activated nano-devices working at high temperatures. The near-integral value 5.9 mehoB/f.u. of Ms at low temperatures indicates that the half-metallic ferromagnetism is preserved even in the particles even on the 20 nm length scale. Additionally, we have facilitated the existing HA-NP preparation method, which can be used in synthesizing ot...

We have synthesized A2-disordered Co2FeAl Heusler nanoalloy and observed giant magnetocaloric eff... more We have synthesized A2-disordered Co2FeAl Heusler nanoalloy and observed giant magnetocaloric effect (GMCE) during the magnetic field induced magneto-structural phase transition (MST). The magnetic entropy change shows a positive anomaly at 1252 K. The entropy change is increased linearly with the magnetic field and a large value of 15 J/Kg is detected under a moderate field of 1.4 T near the magneto-structural phase transition temperature (Tt). It leads to a net relative cooling power of 89 J/Kg for the change of the magnetic field of 1.4 T. The Arrott plot explores the essential information of the nature of MST. This low-cost GMCE material is promising for high-temperature magnetic refrigeration in aircraft and military systems for localized cooling and suitable for shape memory application.

arXiv: Materials Science, 2020

Experimental and computational studies were performed on Co2FeGe Heusler alloy. It was found that... more Experimental and computational studies were performed on Co2FeGe Heusler alloy. It was found that the alloy has very high experimental magnetic moment of 6.1 muB/f.u., curie temperature of 1073K and very high spin-wave stiffness constant of 10.4 nm2-meV, which indicates that the magnetic moment is very high and do not vary with change in temperature in the range 0-300K. The alloy strictly follows Slater-Pauling (SP) rule and the minor experimental deviation from its SP value is justified by doing full-potential density functional calculations which gives more accurate result when electron-electron correlation parameter (U) is taken into account with conventional GGA method. Effect of lattice strain and electron correlation on individual atomic moments, total magnetic moment and spin-polarization is studied in detail and can be concluded that they have a role in the deviation of the experimental results from the expected theoretical values.

Spherical nanoparticles (NPs) made of intermetallic Fe2CoAl (FCA) Heusler alloy are synthesized v... more Spherical nanoparticles (NPs) made of intermetallic Fe2CoAl (FCA) Heusler alloy are synthesized via co-precipitation method and its structural, magnetic and magnetocaloric properties are explored, for the first time. The basic structural characterizations have revealed A2-disordered cubic Heusler structure. HRTEM with the SAED pattern analysis revealed crystalline nature of the FCA-NPs with a mean diameter of around 14 nm. Field and temperature dependent magnetization (M) study shows that the NPs are soft ferromagnetic with a high saturation magnetization (Ms) and Curie temperature (Tc). We also observed that FCA-NPs do not follow the Slater Pauling (SP) rule possibly because of the disorder present in this system. We further investigate its phase transition and magnetocaloric properties. The peak value of -∆SM vs T curve at a magnetic field change of 20 kOe corresponds to about 2.65 J/Kg-K, and the observed value of refrigeration capacity (RCP) was as large as 44 J/Kg, suggesting a...

Journal of physics. Condensed matter : an Institute of Physics journal, 2021

Tunability of structural, magnetic and electronic properties of Co2FeGe Heusler alloy is experime... more Tunability of structural, magnetic and electronic properties of Co2FeGe Heusler alloy is experimentally demonstrated by doping Ti in the Fe site (i.e. Co2Fe1-xTixGe), followed by in-depth first principle calculations. Co2FeGe in its pure phase shows very high saturation magnetization, Curie temperature and spin-wave stiffness constant which were reported in our earlier work. With gradual increase in Ti doping concentration (x= 0.5 and 0.75), the experimental saturation magnetization is found to be decreased to 4.3 µB/f.u and 3.1 µB/f.u respectively as compared to the parent alloy (x=0) having the saturation magnetization of 6.1µB/f.u. Variation of spinwave stiffness constant is also studied for different x and found to be decreasing from peak value of 10.4 nm2- meV (for x=0) to the least value of 2.56 nm2-meV for x=0.5. Justification of the experimental results is given with first principle calculations. Computational phase diagram of the alloys is found in terms of formation energy...

Journal of Magnetism and Magnetic Materials

Spherical nanoparticles (NPs) of size 14±7 nm, made of intermetallic Fe2CoAl (FCA) Heusler alloy,... more Spherical nanoparticles (NPs) of size 14±7 nm, made of intermetallic Fe2CoAl (FCA) Heusler alloy, are synthesized via the co-precipitation and thermal deoxidization method. X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns confirm that the present nanoalloy is crystalized in A2-disordered cubic Heusler structure. Magnetic field (H) and temperature (T) dependent magnetization (M) results reveal that the NPs are soft ferromagnetic (FM) with high saturation magnetization (Ms) and Curie temperature (Tc). Fe2CoAl nanoalloy do not follow the Slater Pauling (SP) rule, possibly because of the disorder present in the system. We also investigate its magnetic phase transition (MPT) and magnetocaloric (MC) properties. The peak value of −∆SM (entropy change) vs T curve at a magnetic field change of 20 kOe corresponds to about 2.65 J/kg-K, and the observed value of refrigeration capacity (RCP) is as large as 44 J/kg, suggesting a large heat conversion in magnetic refrigeration cycle. The Arrott plot and the nature of the universal curve accomplish that the FM to paramagnetic (PM) phase transition in Fe2CoAl nanoalloy is of second-order. The present study suggests that the Fe2CoAl nanoscale system is proficient, useful

arXiv: Strongly Correlated Electrons, 2019

We have synthesized A2 disordered Heusler nanoalloy and observed giant magnetocaloric effect duri... more We have synthesized A2 disordered Heusler nanoalloy and observed giant magnetocaloric effect during the magnetic field induced magneto structural phase transition (MST). The magnetic entropy change shows a positive anomaly at 1252 K. The entropy change is increased linearly with magnetic field and a large value of ~15 J/Kg is detected under a moderate field of 1.4 T near the magneto-structural phase transition temperature (Tt). It leads to a net relative cooling power of 89 J/Kg for the change of magnetic field of 1.4 T. The Arrott plot explores the essential information of the nature of MST. This giant magnetocaloric effect is anticipated to be incorporated in magnetic refrigeration that will not only be an alternative of energy saving but also eco-friendly.

Journal of Physics D: Applied Physics

A giant magnetocaloric effect across the ferromagnetic (FM) to paramagnetic (PM) phase transition... more A giant magnetocaloric effect across the ferromagnetic (FM) to paramagnetic (PM) phase transition was observed in chemically synthesized Co2FeAl Heusler alloy nanoparticles with a mean diameter of 16 nm. In our previous report, we have observed a significant enhancement in its saturation magnetization (Ms) and Curie temperature (Tc) as compared with the bulk counterpart. Motivated from those results, here, we aim to explore its magnetocaloric properties near the Tc. The magnetic entropy change (−∆) shows a positive anomaly at 1252 K. −∆ increases linearly with the magnetic field, and a large value of ~15 J/Kg-K is detected under a moderate field of 14 kOe. It leads to a net relative cooling power of 89 J/Kg for the magnetic field change of 14 kOe. To confirm the nature of magnetic phase transition, a detailed study of its magnetization is performed. The Arrott plot and nature of universal curve conclude that FM to PM phase transition in the present system is of second-order.

Journal of Alloys and Compounds, 2021

Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanopar... more Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanoparticles (NPs). The NPs of size 23±10 nm are prepared using the co-precipitation method. X-ray and selected area electron diffraction patterns have confirmed the cubic Heusler phase of the NPs with the A2-disorder. These NPs are soft ferromagnetic, and exhibit a high saturation magnetization (Ms) along with a very high Curie temperature (Tc) of 1060 K. The observed Tc value matches closely with the theoretically calculated one following a model provided by Wurmehl et al. (2005). The high Ms and Tc make the present system a potential candidate for magnetically activated nano-devices working at high temperatures. The near-integral value 5.9 μB/f.u. of Ms at low temperatures indicates that the half-metallic ferromagnetism is preserved even in the particles even on the 20 nm length scale. Additionally, we have facilitated the existing HA-NP preparation method, which can be used in synthesizing other HA-NPs. The first-principles density functional theory computations complement the experimental results.

THE EUROPEAN PHYSICAL JOURNAL B, 2020

The physical properties of Fe2CoAl (FCA) Heusler alloy are systematically investigated using the ... more The physical properties of Fe2CoAl (FCA) Heusler alloy are systematically investigated using the Q1 first-principles calculations within generalized gradient approximation (GGA) and GGA+U. The influence of atomic ordering with respect to the Wyckoff sites on the phase stability, magnetism and half metallicity in both the conventional L21 and XA phases of FCA is focused in this study. Various possible hypothetical structures viz., L21, XA-I, and XA-II are prepared by altering atomic occupancies at their Wyckoff sites. At first, we have determined the stable phase of FCA considering various non-magnetic (or paramagnetic), ferromagnetic (FM) and antiferromagnetic (AFM) configurations. Out of these, the ferromagnetic (FM) XA-I structure is found to be energetically most stable. The total magnetic moments per cell are not in agreement with the Slater-Pauling (SP) rule in any phase; therefore, the half-metallicity is not observed in any configurations. However, FM ordered XA-I type FCA shows 78% spin polarization at EF. Interestingly, the results of XA-I type FCA are closely matched with the experimental results.

Journal of Physics: Condensed Matter, 2020

Density functional theory calculations within the generalized gradient approximation (GGA) are em... more Density functional theory calculations within the generalized gradient approximation (GGA) are employed to study the ground state of Co2FeAl. Various magnetic configurations are considered to find out its most stable phase. The ferromagnetic ground state of the Co2FeAl is energetically observed with an optimized lattice constant of 5.70 Å. After that, the system was subjected under uniform and non-uniform strains, to see their effects on spin polarization (P) and half-metallicity. The effect of spin-orbit coupling is considered in the present study. Half-metallicity (and 100 % P) is retained only under uniform strains started from 0 to +4%, and dropped rapidly from 90% to 16% for the negative strains started from-1% to-6%. We find that the present system is much sensitive under tetragonal distortions as half-metallicity (and 100% P) is preserved only for the cubic case. The main reason for the loss of half-metallicity is due to the shift of the bands with respect to the Fermi level (EF). We also discuss the influence of these results on spintronics devices.

Journal of Magnetism and Magnetic Materials, 2019

The generalized gradient approximation (GGA) scheme in the first -principles calculations is used... more The generalized gradient approximation (GGA) scheme in the first -principles calculations is used to study the effect of L21 and XA ordering on the phase stability, half-metallicity and magnetism of Co2FeAl (CFA) Heusler alloy. Various possible hypothetical structures: L21-I, L21-II, XA-I, and XA-II are prepared under the conventional L21 and inverse XA phases by altering the atomic occupancies at their Wyckoff sites. It is found that the XA-II phase of CFA is the most stable phase energetically among all the structures. The electronic structure calculations without U show the presence of half-metallic (HM) ground state only in L21-I structure and the other structures are found to be metallic. However, the electronic structures of CFA are significantly modified in the presence of U, although the total magnetic moments per cell remained the same and consistent with the Slater-Pauling (SP) rule. The metallic ground states of CFA in L21-II and XA-II structures are converted into the half-metallic ground states in presence of U, but remained the same (metallic) in XA-I structure. The results indicate that the electronic structures are not only dependent on the L21 and XA ordering of the atoms but also depend on the choice of U values. So experiments may only verify the superiority of GGA+U to GGA.

Co2FeAl (CFA) nanoparticles (NPs) of different sizes were synthesized by chemical route. The effe... more Co2FeAl (CFA) nanoparticles (NPs) of different sizes were synthesized by chemical route. The effect of the size of NPs upon the structure and magnetization compared to its bulk counterpart was investigated. The structure and composition were determined from X-ray diffraction (XRD) and electron microscopy. XRD analysis shows that the samples are having single (A2-type) disordered phase. Magnetization measurements suggest that the samples are soft ferromagnetic in nature with very low coercivity. Enhanced magnetic properties like saturation magnetization, coercive force, retentivity, and Curie-temperature are observed with a decrease in particle size. The effect of particle size on hysteresis losses is also discussed. The smallest particles of size 16 nm exhibited the highest saturation magnetization and transition temperature of 180.73 emu/g and 1261 K, respectively. The origin of enhancement in magnetization of Co2FeAl nano-alloy is attributed to the strong Co-Co exchange interaction due to disorder present in the systems.

Conference Presentations by Aquil Ahmad

AIP Conference Proceedings, 2019

In this report, Fe2CoAl Heusler alloy is selected as a target to study site-preferences of atoms ... more In this report, Fe2CoAl Heusler alloy is selected as a target to study site-preferences of atoms by first-principles calculations using WIEN2k code. It has been observed that Fe2CoAl (FCA) Alloy tend to form XA-I type structure (Hg2CuTi-type) which is energetically more favorable as compared to those of the others XA-II and XA-III structures. Further, we observed that the total magnetic moment per cell shows a strong dependence on-site preferences of Co in all available Wyckoff sites. However, this compound does not show a half-metallic character but a metallic behavior was observed for both spin direction at Fermi level (EF), still, it can be used as the spin-polarized material for spintronics application

DAE SOLID STATE PHYSICS SYMPOSIUM 2018, 2019

In this report, Fe 2 CoAl Heusler alloy is selected as a target to study site-preferences of atom... more In this report, Fe 2 CoAl Heusler alloy is selected as a target to study site-preferences of atoms by first-principles calculations using WIEN2k code. It has been observed that Fe 2 CoAl (FCA) Alloy tend to form XA-I type structure (Hg 2 CuTi-type) which is more energetically favorable and also much stable compared to XA-II and XA-III type structures. Further, we observed that the total magnetic moment M t (μB/f.u.) shows a strong dependence on site preferences of Co in all available Wyckoff sites. However, this compound does not show a half-metallic character but a metallic behavior has been observed for both spin direction at Fermi level E F, still, it can be used as the spin-polarized material for spintronics application.

ADVANCES IN BASIC SCIENCE (ICABS 2019), 2019

Journal of Physics: Condensed Matter, 2020

Density functional theory calculations within the generalized gradient approximation are employed... more Density functional theory calculations within the generalized gradient approximation are employed to study the ground state of Co2FeAl. Various magnetic configurations are considered to find out its most stable phase. The ferromagnetic ground state of the Co2FeAl is energetically observed with an optimized lattice constant of 5.70 Å. Thereafter, the system was subjected under uniform and non-uniform strains to see their effects on spin polarization (P) and half-metallicity. The effect of spin orbit coupling is considered in the present study. Half-metallicity (and 100 % P) is only retained under uniform strains started from 0 to +4%, and dropped rapidly from 90% to 16% for the negative strains started from-1% to-6%. We find that the present system is much sensitive under tetragonal distortions as half-metallicity (and 100% P) is preserved only for the cubic case. The main reason for the loss of half-metallicity is due to the shift of the bands with respect to the Fermi level. We also discuss the influence of these results on spintronics devices.

The European Physical Journal B, 2020

MATERIALS SCIENCE, ENERGY TECHNOLOGY AND POWER ENGINEERING III (MEP 2019), 2019

Journal of Magnetism and Magnetic Materials, 2019

The generalized gradient approximation (GGA) scheme in the first-principles calculations are used... more The generalized gradient approximation (GGA) scheme in the first-principles calculations are used to study the effect of L21 and XA ordering on the phase stability, halfmetallicity and magnetism of Co2FeAl (CFA) Heusler alloy. Various possible hypothetical structures: L21-I, L21-II, XA-I, and XA-II were prepared under the conventional L21 and inverse XA phases by altering the atomic occupancies at their Wyckoff sites. It is found that the XA-II phase of CFA is the most stable phase energetically among all the structures. The electronic structure calculations without U show the presence of half-metallic (HM) ground state only in L21-1 structure and the other structures are found to be metallic. However, the electronic structures of CFA are significantly modified in the presence of U, although the total magnetic moments per cell remained the same and consistent with the Slater-Pauling (SP) rule. The metallic ground states of CFA in L21-II and XA-II structures are converted into the half-metallic ground states in presence of U but remained the same (metallic) in XA-I structure. The results indicate that the electronic structures are not only dependent on the L21 and XA ordering of the atoms but also depend on the choice of U values. So experiments may only verify the superiority of GGA+U to GGA.

Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanopar... more Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanoparticles (NPs). The NPs of size 23 nm are prepared using the co-precipitation method. X-ray and selected area electron diffraction patterns have confirmed the cubic Heusler phase of the NPs with the A2-disorder. These NPs are soft ferromagnetic, and exhibit a high saturation magnetization (Ms) along with a very high Curie temperature (Tc) of 1060 K. The observed Tc value matches closely with the theoretically calculated one following a model provided by Wurmehl et al. [1]. The high Ms and Tc make the present system a potential candidate for magnetically activated nano-devices working at high temperatures. The near-integral value 5.9 mehoB/f.u. of Ms at low temperatures indicates that the half-metallic ferromagnetism is preserved even in the particles even on the 20 nm length scale. Additionally, we have facilitated the existing HA-NP preparation method, which can be used in synthesizing ot...

We have synthesized A2-disordered Co2FeAl Heusler nanoalloy and observed giant magnetocaloric eff... more We have synthesized A2-disordered Co2FeAl Heusler nanoalloy and observed giant magnetocaloric effect (GMCE) during the magnetic field induced magneto-structural phase transition (MST). The magnetic entropy change shows a positive anomaly at 1252 K. The entropy change is increased linearly with the magnetic field and a large value of 15 J/Kg is detected under a moderate field of 1.4 T near the magneto-structural phase transition temperature (Tt). It leads to a net relative cooling power of 89 J/Kg for the change of the magnetic field of 1.4 T. The Arrott plot explores the essential information of the nature of MST. This low-cost GMCE material is promising for high-temperature magnetic refrigeration in aircraft and military systems for localized cooling and suitable for shape memory application.

arXiv: Materials Science, 2020

Experimental and computational studies were performed on Co2FeGe Heusler alloy. It was found that... more Experimental and computational studies were performed on Co2FeGe Heusler alloy. It was found that the alloy has very high experimental magnetic moment of 6.1 muB/f.u., curie temperature of 1073K and very high spin-wave stiffness constant of 10.4 nm2-meV, which indicates that the magnetic moment is very high and do not vary with change in temperature in the range 0-300K. The alloy strictly follows Slater-Pauling (SP) rule and the minor experimental deviation from its SP value is justified by doing full-potential density functional calculations which gives more accurate result when electron-electron correlation parameter (U) is taken into account with conventional GGA method. Effect of lattice strain and electron correlation on individual atomic moments, total magnetic moment and spin-polarization is studied in detail and can be concluded that they have a role in the deviation of the experimental results from the expected theoretical values.

Spherical nanoparticles (NPs) made of intermetallic Fe2CoAl (FCA) Heusler alloy are synthesized v... more Spherical nanoparticles (NPs) made of intermetallic Fe2CoAl (FCA) Heusler alloy are synthesized via co-precipitation method and its structural, magnetic and magnetocaloric properties are explored, for the first time. The basic structural characterizations have revealed A2-disordered cubic Heusler structure. HRTEM with the SAED pattern analysis revealed crystalline nature of the FCA-NPs with a mean diameter of around 14 nm. Field and temperature dependent magnetization (M) study shows that the NPs are soft ferromagnetic with a high saturation magnetization (Ms) and Curie temperature (Tc). We also observed that FCA-NPs do not follow the Slater Pauling (SP) rule possibly because of the disorder present in this system. We further investigate its phase transition and magnetocaloric properties. The peak value of -∆SM vs T curve at a magnetic field change of 20 kOe corresponds to about 2.65 J/Kg-K, and the observed value of refrigeration capacity (RCP) was as large as 44 J/Kg, suggesting a...

Journal of physics. Condensed matter : an Institute of Physics journal, 2021

Tunability of structural, magnetic and electronic properties of Co2FeGe Heusler alloy is experime... more Tunability of structural, magnetic and electronic properties of Co2FeGe Heusler alloy is experimentally demonstrated by doping Ti in the Fe site (i.e. Co2Fe1-xTixGe), followed by in-depth first principle calculations. Co2FeGe in its pure phase shows very high saturation magnetization, Curie temperature and spin-wave stiffness constant which were reported in our earlier work. With gradual increase in Ti doping concentration (x= 0.5 and 0.75), the experimental saturation magnetization is found to be decreased to 4.3 µB/f.u and 3.1 µB/f.u respectively as compared to the parent alloy (x=0) having the saturation magnetization of 6.1µB/f.u. Variation of spinwave stiffness constant is also studied for different x and found to be decreasing from peak value of 10.4 nm2- meV (for x=0) to the least value of 2.56 nm2-meV for x=0.5. Justification of the experimental results is given with first principle calculations. Computational phase diagram of the alloys is found in terms of formation energy...

Journal of Magnetism and Magnetic Materials

Spherical nanoparticles (NPs) of size 14±7 nm, made of intermetallic Fe2CoAl (FCA) Heusler alloy,... more Spherical nanoparticles (NPs) of size 14±7 nm, made of intermetallic Fe2CoAl (FCA) Heusler alloy, are synthesized via the co-precipitation and thermal deoxidization method. X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns confirm that the present nanoalloy is crystalized in A2-disordered cubic Heusler structure. Magnetic field (H) and temperature (T) dependent magnetization (M) results reveal that the NPs are soft ferromagnetic (FM) with high saturation magnetization (Ms) and Curie temperature (Tc). Fe2CoAl nanoalloy do not follow the Slater Pauling (SP) rule, possibly because of the disorder present in the system. We also investigate its magnetic phase transition (MPT) and magnetocaloric (MC) properties. The peak value of −∆SM (entropy change) vs T curve at a magnetic field change of 20 kOe corresponds to about 2.65 J/kg-K, and the observed value of refrigeration capacity (RCP) is as large as 44 J/kg, suggesting a large heat conversion in magnetic refrigeration cycle. The Arrott plot and the nature of the universal curve accomplish that the FM to paramagnetic (PM) phase transition in Fe2CoAl nanoalloy is of second-order. The present study suggests that the Fe2CoAl nanoscale system is proficient, useful

arXiv: Strongly Correlated Electrons, 2019

We have synthesized A2 disordered Heusler nanoalloy and observed giant magnetocaloric effect duri... more We have synthesized A2 disordered Heusler nanoalloy and observed giant magnetocaloric effect during the magnetic field induced magneto structural phase transition (MST). The magnetic entropy change shows a positive anomaly at 1252 K. The entropy change is increased linearly with magnetic field and a large value of ~15 J/Kg is detected under a moderate field of 1.4 T near the magneto-structural phase transition temperature (Tt). It leads to a net relative cooling power of 89 J/Kg for the change of magnetic field of 1.4 T. The Arrott plot explores the essential information of the nature of MST. This giant magnetocaloric effect is anticipated to be incorporated in magnetic refrigeration that will not only be an alternative of energy saving but also eco-friendly.

Journal of Physics D: Applied Physics

A giant magnetocaloric effect across the ferromagnetic (FM) to paramagnetic (PM) phase transition... more A giant magnetocaloric effect across the ferromagnetic (FM) to paramagnetic (PM) phase transition was observed in chemically synthesized Co2FeAl Heusler alloy nanoparticles with a mean diameter of 16 nm. In our previous report, we have observed a significant enhancement in its saturation magnetization (Ms) and Curie temperature (Tc) as compared with the bulk counterpart. Motivated from those results, here, we aim to explore its magnetocaloric properties near the Tc. The magnetic entropy change (−∆) shows a positive anomaly at 1252 K. −∆ increases linearly with the magnetic field, and a large value of ~15 J/Kg-K is detected under a moderate field of 14 kOe. It leads to a net relative cooling power of 89 J/Kg for the magnetic field change of 14 kOe. To confirm the nature of magnetic phase transition, a detailed study of its magnetization is performed. The Arrott plot and nature of universal curve conclude that FM to PM phase transition in the present system is of second-order.

Journal of Alloys and Compounds, 2021

Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanopar... more Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanoparticles (NPs). The NPs of size 23±10 nm are prepared using the co-precipitation method. X-ray and selected area electron diffraction patterns have confirmed the cubic Heusler phase of the NPs with the A2-disorder. These NPs are soft ferromagnetic, and exhibit a high saturation magnetization (Ms) along with a very high Curie temperature (Tc) of 1060 K. The observed Tc value matches closely with the theoretically calculated one following a model provided by Wurmehl et al. (2005). The high Ms and Tc make the present system a potential candidate for magnetically activated nano-devices working at high temperatures. The near-integral value 5.9 μB/f.u. of Ms at low temperatures indicates that the half-metallic ferromagnetism is preserved even in the particles even on the 20 nm length scale. Additionally, we have facilitated the existing HA-NP preparation method, which can be used in synthesizing other HA-NPs. The first-principles density functional theory computations complement the experimental results.

THE EUROPEAN PHYSICAL JOURNAL B, 2020

The physical properties of Fe2CoAl (FCA) Heusler alloy are systematically investigated using the ... more The physical properties of Fe2CoAl (FCA) Heusler alloy are systematically investigated using the Q1 first-principles calculations within generalized gradient approximation (GGA) and GGA+U. The influence of atomic ordering with respect to the Wyckoff sites on the phase stability, magnetism and half metallicity in both the conventional L21 and XA phases of FCA is focused in this study. Various possible hypothetical structures viz., L21, XA-I, and XA-II are prepared by altering atomic occupancies at their Wyckoff sites. At first, we have determined the stable phase of FCA considering various non-magnetic (or paramagnetic), ferromagnetic (FM) and antiferromagnetic (AFM) configurations. Out of these, the ferromagnetic (FM) XA-I structure is found to be energetically most stable. The total magnetic moments per cell are not in agreement with the Slater-Pauling (SP) rule in any phase; therefore, the half-metallicity is not observed in any configurations. However, FM ordered XA-I type FCA shows 78% spin polarization at EF. Interestingly, the results of XA-I type FCA are closely matched with the experimental results.

Journal of Physics: Condensed Matter, 2020

Density functional theory calculations within the generalized gradient approximation (GGA) are em... more Density functional theory calculations within the generalized gradient approximation (GGA) are employed to study the ground state of Co2FeAl. Various magnetic configurations are considered to find out its most stable phase. The ferromagnetic ground state of the Co2FeAl is energetically observed with an optimized lattice constant of 5.70 Å. After that, the system was subjected under uniform and non-uniform strains, to see their effects on spin polarization (P) and half-metallicity. The effect of spin-orbit coupling is considered in the present study. Half-metallicity (and 100 % P) is retained only under uniform strains started from 0 to +4%, and dropped rapidly from 90% to 16% for the negative strains started from-1% to-6%. We find that the present system is much sensitive under tetragonal distortions as half-metallicity (and 100% P) is preserved only for the cubic case. The main reason for the loss of half-metallicity is due to the shift of the bands with respect to the Fermi level (EF). We also discuss the influence of these results on spintronics devices.

Journal of Magnetism and Magnetic Materials, 2019

The generalized gradient approximation (GGA) scheme in the first -principles calculations is used... more The generalized gradient approximation (GGA) scheme in the first -principles calculations is used to study the effect of L21 and XA ordering on the phase stability, half-metallicity and magnetism of Co2FeAl (CFA) Heusler alloy. Various possible hypothetical structures: L21-I, L21-II, XA-I, and XA-II are prepared under the conventional L21 and inverse XA phases by altering the atomic occupancies at their Wyckoff sites. It is found that the XA-II phase of CFA is the most stable phase energetically among all the structures. The electronic structure calculations without U show the presence of half-metallic (HM) ground state only in L21-I structure and the other structures are found to be metallic. However, the electronic structures of CFA are significantly modified in the presence of U, although the total magnetic moments per cell remained the same and consistent with the Slater-Pauling (SP) rule. The metallic ground states of CFA in L21-II and XA-II structures are converted into the half-metallic ground states in presence of U, but remained the same (metallic) in XA-I structure. The results indicate that the electronic structures are not only dependent on the L21 and XA ordering of the atoms but also depend on the choice of U values. So experiments may only verify the superiority of GGA+U to GGA.

Co2FeAl (CFA) nanoparticles (NPs) of different sizes were synthesized by chemical route. The effe... more Co2FeAl (CFA) nanoparticles (NPs) of different sizes were synthesized by chemical route. The effect of the size of NPs upon the structure and magnetization compared to its bulk counterpart was investigated. The structure and composition were determined from X-ray diffraction (XRD) and electron microscopy. XRD analysis shows that the samples are having single (A2-type) disordered phase. Magnetization measurements suggest that the samples are soft ferromagnetic in nature with very low coercivity. Enhanced magnetic properties like saturation magnetization, coercive force, retentivity, and Curie-temperature are observed with a decrease in particle size. The effect of particle size on hysteresis losses is also discussed. The smallest particles of size 16 nm exhibited the highest saturation magnetization and transition temperature of 180.73 emu/g and 1261 K, respectively. The origin of enhancement in magnetization of Co2FeAl nano-alloy is attributed to the strong Co-Co exchange interaction due to disorder present in the systems.

AIP Conference Proceedings, 2019

In this report, Fe2CoAl Heusler alloy is selected as a target to study site-preferences of atoms ... more In this report, Fe2CoAl Heusler alloy is selected as a target to study site-preferences of atoms by first-principles calculations using WIEN2k code. It has been observed that Fe2CoAl (FCA) Alloy tend to form XA-I type structure (Hg2CuTi-type) which is energetically more favorable as compared to those of the others XA-II and XA-III structures. Further, we observed that the total magnetic moment per cell shows a strong dependence on-site preferences of Co in all available Wyckoff sites. However, this compound does not show a half-metallic character but a metallic behavior was observed for both spin direction at Fermi level (EF), still, it can be used as the spin-polarized material for spintronics application

DAE SOLID STATE PHYSICS SYMPOSIUM 2018, 2019

In this report, Fe 2 CoAl Heusler alloy is selected as a target to study site-preferences of atom... more In this report, Fe 2 CoAl Heusler alloy is selected as a target to study site-preferences of atoms by first-principles calculations using WIEN2k code. It has been observed that Fe 2 CoAl (FCA) Alloy tend to form XA-I type structure (Hg 2 CuTi-type) which is more energetically favorable and also much stable compared to XA-II and XA-III type structures. Further, we observed that the total magnetic moment M t (μB/f.u.) shows a strong dependence on site preferences of Co in all available Wyckoff sites. However, this compound does not show a half-metallic character but a metallic behavior has been observed for both spin direction at Fermi level E F, still, it can be used as the spin-polarized material for spintronics application.

ADVANCES IN BASIC SCIENCE (ICABS 2019), 2019

AIP Conference Proceedings , 2019

Co2FeAl Heusler alloy was selected as a target to study L21 and XA ordering of atoms by first-pri... more Co2FeAl Heusler alloy was selected as a target to study L21 and XA ordering of atoms by first-principles calculations using WIEN2k code. It has been observed that Co2FeAl (CFA) Alloy tend to form L21 type structure, which is more energetically favorable and much stable compared to XA type structure. Further, we observed that the total magnetic moment (Mt) shows a strong dependence on site preferences of Fe in all available Wyckoff sites. However, this compound does not show a half-metallic character in both types of ordered structures still, it can be used as the spin-polarized material for spintronics application. Our results are opposite to usual site preference rule and approving the validity of the Slater-Pauling rule.