An interface between 3d transition metal and 5d(4d) heavy metal (HM) is a host of intriguing spin... more An interface between 3d transition metal and 5d(4d) heavy metal (HM) is a host of intriguing spin-related effects desirable for spin-orbitronic applications [1].
We report on the growth of Co -doped ZnO nanowires (NWs) on Si substrate using a self-catalytic v... more We report on the growth of Co -doped ZnO nanowires (NWs) on Si substrate using a self-catalytic vapor deposition method from a Co -doped ZnO nanopowder source and study its structural, optical and magnetic properties for the as-grown and rapid thermal annealed samples. Co (5%)-doped ZnO ( ZnCoO ) nanoparticles (NPs) are used as source material for the growth process. Electron microscopy imaging clearly reveals the formation of long ZnO NWs with uniform diameter. X-ray diffraction analysis confirms the single crystalline hexagonal structure of Co -doped ZnO NWs without impurities of metallic cobalt or other phases. Micro-Raman studies of doped samples show doping/disorder induced additional modes as compared to the undoped ZnO . Room temperature photoluminescence spectra of the doped ZnO NWs show strong emission band at ~380 nm and no significant emission was observed in the visible region indicating low defect content in the NWs. The field dependent magnetization (M–H curve) measure...
We report on the occurrence of high temperature ferromagnetism ͑FM͒ in ZnO nanoparticles ͑NPs͒ do... more We report on the occurrence of high temperature ferromagnetism ͑FM͒ in ZnO nanoparticles ͑NPs͒ doped with Co-atoms. ZnO NPs of two different initial sizes are doped with 3% and 5% Co using ball milling and FM is studied at room temperature and above. X-ray diffraction and high-resolution transmission electron microscopy analysis confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO. UV-visible absorption studies show change in band structure and photoluminescence studies show green emission band at 520 nm indicating incorporation of Co-atoms and presence of oxygen vacancy defects, respectively in ZnO lattice. Micro-Raman studies of doped samples shows defect related additional bands at 547 and 574 cm −1. XRD and Raman spectra provide clear evidence for strain in the doped ZnO NPs. The field dependence of magnetization ͑M-H curve͒ measured at room temperature exhibits the clear FM with saturation magnetization ͑M s ͒ and coercive field ͑H c ͒ of the order of 3-7 emu/g and 260 Oe, respectively. Temperature dependence of magnetization ͑M-T͒ measurement shows sharp ferromagnetic to paramagnetic transition with a high Curie temperature ͑T c ͒ of ϳ800 K for 3% Co doped ZnO NPs. It is found that doping at 5% and higher concentration does not exhibit a proper magnetic transition. We attempt to fit the observed FM data with the bound magnetic polaron ͑BMP͒ model involving localized carriers and magnetic cations. However, calculated concentration of the BMPs is well below the typical percolation threshold in ZnO. We believe that observed high temperature FM is primarily mediated by defects in the strained NPs. ZnO NPs of lower initial size show enhanced FM that may be attributed to size dependent doping effect.
The quantum interference assisted enhanced optical activity due to the emergence of a steady-stat... more The quantum interference assisted enhanced optical activity due to the emergence of a steady-state atomic polarization is investigated. The Rubidium atoms in an antirelaxation coated cell provide a suitable platform to address the phenomena at multiple Larmor's frequencies. It interacts with a narrow bandwidth frequency comb generated by the frequency modulation of the light field. The Lindblad master equation with a trichromatic field provides a microscopic picture of the atomic response to the narrow bandwidth frequency comb. The directive of the relative phase between the light fields, in the detuning dependence of the magnetic resonances, is conclusively captured with the trichromatic field model. The measured absorption, nonlinear magneto-optic rotation, and their dependencies on various experimental parameters are analysed. The ellipticity of the light field controls the extent of several physical processes at multiple Larmor's frequencies. The investigation provides an approach to address the Zeeman coherence in the interaction of a narrow bandwidth frequency comb with an atomic ensemble and will have applications in various quantum devices.
To stabilize nontrivial spin textures, e.g., skyrmions or chiral domain walls in ultrathin magnet... more To stabilize nontrivial spin textures, e.g., skyrmions or chiral domain walls in ultrathin magnetic films, an additional degree of freedom, such as the interfacial Dzyaloshinskii-Moriya interaction (IDMI), must be induced by the strong spin-orbit coupling (SOC) of a stacked heavy metal layer. However, advanced approaches to simultaneously control the IDMI and perpendicular magnetic anisotropy (PMA) are needed for future spin-orbitronic device implementations. Here, we show the effect of atomic-scale surface modulation on the magnetic properties and IDMI in ultrathin films composed of 5d heavy metal/ferromagnet/4d(5d) heavy metal or oxide interfaces, such as Pt/CoFeSiB/Ru, Pt/CoFeSiB/ Ta, and Pt/CoFeSiB/MgO. The maximum IDMI value corresponds to the correlated roughness of the bottom and top interfaces of the ferromagnetic layer. The proposed approach for significant enhancement of PMA and the IDMI through interface roughness engineering at the atomic scale offers a powerful tool for the development of spinorbitronic devices with precise and reliable controllability of their functionality.
Zn 1Àx Co x O (x = 0, 0.05, and 0.07) nanorods (NRs) exhibiting ferromagnetism above room tempera... more Zn 1Àx Co x O (x = 0, 0.05, and 0.07) nanorods (NRs) exhibiting ferromagnetism above room temperature and with high magnetic moment have been synthesized by a solvothermal route. XRD, FESEM, TEM, EDS and XPS measurements reveal the growth of single phase wurtzite structure Zn 1Àx Co x O NRs with the successful incorporation of Co ions inside the ZnO matrix. TEM micrograph reveals clearly the formation of long ZnO NRs with diameter of 50-90 nm and length of 0.3-0.6 lm. High resolution TEM lattice images and the electron diffraction patterns show that all the NRs are single crystalline. Room temperature magnetic measurements exhibit ferromagnetic behavior with high magnetic moment of 1.83 emu/g for 2 T field, coercivity of 53 G. Temperature dependent magnetization measurement shows a Curie temperature of the order of 398 K. Photoluminescence (PL) spectra exhibit near band edge UV emission and defect related visible emission, which is expected to play a significant role in the FM ordering. PL and UV-VIS spectra reveal slight modification of band edge due to doping effect. Systemic structural, magnetic, and optical studies reveal that both the nature of the defects as well as Co 2+ ions are significant ingredients to attain high moment as well as high ordering temperature in the 1-dimensional ZnO NRs. Magnetic interaction is quantitatively analyzed and explained using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Co ions and V Zn , O i related defects. These findings provides a better insight into the underlying mechanisms of high temperature ferromagnetism in Zn 1Àx Co x O NRs.
ABSTRACT We report on the room temperature ferromagnetism (RT FM) in the Zn1−xNixO (x = 0, 0.03, ... more ABSTRACT We report on the room temperature ferromagnetism (RT FM) in the Zn1−xNixO (x = 0, 0.03, and 0.05) nanoparticles (NPs) synthesized by a ball milling technique. X-ray diffraction analysis confirms the single crystalline ZnO wurtzite structure with presence of small intensity secondary phase related peak which disappear with increasing milling time for Ni doped samples. HRTEM lattice images show that the doped NPs are single crystalline with a dspacing of 2.44 Å. Energy-dispersive X-ray spectroscopy analysis confirms the presence of Ni ions in the ZnO matrix. Magnetic measurement (RT) exhibits the hysteresis loop with saturation magnetization (Ms) of 1.6–2.56 (emu/g) and coercive field (Hc) of 296–322 Oe. M-T measurement shows a Curie temperature of the order of 325°C for 3% Ni doped sample. Micro -Raman studies show doping/disorder induced additional modes at ∼510, 547, 572 cm−1 in addition to 437 cm−1 peak of pure ZnO. UV-Vis absorption spectra illustrate band gap shift due to doping. Alteration of Ms value with the variation of doping concentration and milling time has been studied and discussed.
Structural, optical and magnetic studies have been carried out for the Co-doped ZnO nanoparticles... more Structural, optical and magnetic studies have been carried out for the Co-doped ZnO nanoparticles (NPs). ZnO NPs are doped with 3% and 5% Co using ball milling and ferromagnetism (FM) is studied at room temperature and above. A high Curie temperature (T c has been observed from the Co doped ZnO NPs. X-ray diffraction and high resolution transmission electron microscopy analysis confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO. UV-visible absorption and photoluminescence studies on the doped samples show change in band structure and oxygen vacancy defects, respectively. Micro-Raman studies of doped samples shows defect related additional strong bands at 547 and 574 cm −1 confirming the presence of oxygen vacancy defects in ZnO lattice. The field dependence of magnetization (M-H curve) measured at room temperature exhibits the clear M-H loop with saturation magnetization and coercive field of the order of 4-6 emu/g and 260 G, respectively. Temperature dependence of magnetization measurement shows sharp ferromagnetic to paramagnetic transition with a high T C = 791 K for 3% Co doped ZnO NPs. Ferromagnetic ordering is interpreted in terms of overlapping of polarons mediated through oxygen vacancy defects based on the bound magnetic polaron (BMP) model. We show that the observed FM data fits well with the BMP model involving localised carriers and magnetic cations.
Enhanced upconversion fluorescence of Er3+:ZrO2 nanocrystals induced by phase transformation AIP ... more Enhanced upconversion fluorescence of Er3+:ZrO2 nanocrystals induced by phase transformation AIP Advances 2, 012102 The indirect to direct band gap transition in multilayered MoS2 as predicted by screened hybrid density functional theory Appl. Phys. Lett. 99, 261908 (2011) Robust thermal performance of Sr2Si5N8:Eu2+: An efficient red emitting phosphor for light emitting diode based white lighting Appl. Phys. Lett. 99, 241106 (2011) Downconversion from visible to near infrared through multi-wavelength excitation in Er3+/Yb3+ co-doped NaYF4 nanocrystals
We report on the occurrence of high temperature ferromagnetism ͑FM͒ in ZnO nanoparticles ͑NPs͒ do... more We report on the occurrence of high temperature ferromagnetism ͑FM͒ in ZnO nanoparticles ͑NPs͒ doped with Co-atoms. ZnO NPs of two different initial sizes are doped with 3% and 5% Co using ball milling and FM is studied at room temperature and above. X-ray diffraction and high-resolution transmission electron microscopy analysis confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO. UV-visible absorption studies show change in band structure and photoluminescence studies show green emission band at 520 nm indicating incorporation of Co-atoms and presence of oxygen vacancy defects, respectively in ZnO lattice. Micro-Raman studies of doped samples shows defect related additional bands at 547 and 574 cm −1 . XRD and Raman spectra provide clear evidence for strain in the doped ZnO NPs. The field dependence of magnetization ͑M-H curve͒ measured at room temperature exhibits the clear FM with saturation magnetization ͑M s ͒ and coercive field ͑H c ͒ of the order of 3-7 emu/g and 260 Oe, respectively. Temperature dependence of magnetization ͑M-T͒ measurement shows sharp ferromagnetic to paramagnetic transition with a high Curie temperature ͑T c ͒ of ϳ800 K for 3% Co doped ZnO NPs. It is found that doping at 5% and higher concentration does not exhibit a proper magnetic transition. We attempt to fit the observed FM data with the bound magnetic polaron ͑BMP͒ model involving localized carriers and magnetic cations. However, calculated concentration of the BMPs is well below the typical percolation threshold in ZnO. We believe that observed high temperature FM is primarily mediated by defects in the strained NPs. ZnO NPs of lower initial size show enhanced FM that may be attributed to size dependent doping effect. Downloaded 24 Dec 2010 to 210.212.8.62. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions 084322-2 B. Pal and P. K. Giri J. Appl. Phys. 108, 084322 ͑2010͒ Downloaded 24 Dec 2010 to 210.212.8.62. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions 084322-4 B. Pal and P. K. Giri J. Appl. Phys. 108, 084322 ͑2010͒ Downloaded 24 Dec 2010 to 210.212.8.62. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions 084322-6 B. Pal and P. K. Giri J. Appl. Phys. 108, 084322 ͑2010͒ Downloaded 24 Dec 2010 to 210.212.8.62. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions 084322-8 B. Pal and P. K. Giri J. Appl. Phys. 108, 084322 ͑2010͒
The room-temperature ferromagnetism properties of Co-doped zinc oxide are reported in this paper.... more The room-temperature ferromagnetism properties of Co-doped zinc oxide are reported in this paper. The Zn 1−x Co x O (0.02 < x < 0.10) specimens were synthesized by the solid state reaction method with the mixture of Co 2 O 3 and ZnO powders. The precursor Co 2 O 3 and ZnO powders were mixed and milled for 10 h. The mixed powders were calcined at temperature of 600 • C and 1000 • C, respectively for 12 h, and then were cooled at the ambient temperature. Next, the powders were milled, made block and sintered for 12 h. The specimens were then hydrogenated at 600 • C for 1 h and 3 h, respectively. The room-temperature ferromagnetism (RTFM) was found in the hydrogenated samples, and the hysteresis loops of the hydrogenated Co-doped ZnO samples were measured at 10 K and 300 K by using the superconducting quantum interference device (SQUID) magnetometer (Quantum Design). The coercive field was about 310 Oe for the room-temperature measurement, and the saturation magnetization of the hydrogenated sample was about 0.056µ B /Co. The samples were further measured by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The results suggested the observed RTFM in the Co-doped ZnO was partly due to the existence of the cobalt nanoparticles in the hydrogenated samples.
We report on the growth of Co-doped ZnO nanowires (NWs) on Si substrate using a self-catalytic va... more We report on the growth of Co-doped ZnO nanowires (NWs) on Si substrate using a self-catalytic vapor deposition method from a Co-doped ZnO nanopowder source and study its structural, optical and magnetic properties for the as-grown and rapid thermal annealed samples. Co (5%)-doped ZnO (ZnCoO) nanoparticles (NPs) are used as source material for the growth process. Electron microscopy imaging clearly reveals the formation of long ZnO NWs with uniform diameter. X-ray di®raction analysis con¯rms the single crystalline hexagonal structure of Co-doped ZnO NWs without impurities of metallic cobalt or other phases. Micro-Raman studies of doped samples show doping/disorder induced additional modes as compared to the undoped ZnO. Room temperature photoluminescence spectra of the doped ZnO NWs show strong emission band at $380 nm and no signi¯cant emission was observed in the visible region indicating low defect content in the NWs. Thē eld dependent magnetization (MÀH curve) measured at room temperature exhibits paramagnetic nature for the NWs with the magnetic moment in the range 2À3.7 milli-emu/cm 2 for the applied eld of 2 Tesla, while the source ZnCoO NPs exhibit room temperature ferromagnetism with saturation magnetization $6 emu/g. Possible mechanism of alteration in magnetic behavior in doped NWs are discussed based on the growth conditions and role of defects.
An interface between 3d transition metal and 5d(4d) heavy metal (HM) is a host of intriguing spin... more An interface between 3d transition metal and 5d(4d) heavy metal (HM) is a host of intriguing spin-related effects desirable for spin-orbitronic applications [1].
We report on the growth of Co -doped ZnO nanowires (NWs) on Si substrate using a self-catalytic v... more We report on the growth of Co -doped ZnO nanowires (NWs) on Si substrate using a self-catalytic vapor deposition method from a Co -doped ZnO nanopowder source and study its structural, optical and magnetic properties for the as-grown and rapid thermal annealed samples. Co (5%)-doped ZnO ( ZnCoO ) nanoparticles (NPs) are used as source material for the growth process. Electron microscopy imaging clearly reveals the formation of long ZnO NWs with uniform diameter. X-ray diffraction analysis confirms the single crystalline hexagonal structure of Co -doped ZnO NWs without impurities of metallic cobalt or other phases. Micro-Raman studies of doped samples show doping/disorder induced additional modes as compared to the undoped ZnO . Room temperature photoluminescence spectra of the doped ZnO NWs show strong emission band at ~380 nm and no significant emission was observed in the visible region indicating low defect content in the NWs. The field dependent magnetization (M–H curve) measure...
We report on the occurrence of high temperature ferromagnetism ͑FM͒ in ZnO nanoparticles ͑NPs͒ do... more We report on the occurrence of high temperature ferromagnetism ͑FM͒ in ZnO nanoparticles ͑NPs͒ doped with Co-atoms. ZnO NPs of two different initial sizes are doped with 3% and 5% Co using ball milling and FM is studied at room temperature and above. X-ray diffraction and high-resolution transmission electron microscopy analysis confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO. UV-visible absorption studies show change in band structure and photoluminescence studies show green emission band at 520 nm indicating incorporation of Co-atoms and presence of oxygen vacancy defects, respectively in ZnO lattice. Micro-Raman studies of doped samples shows defect related additional bands at 547 and 574 cm −1. XRD and Raman spectra provide clear evidence for strain in the doped ZnO NPs. The field dependence of magnetization ͑M-H curve͒ measured at room temperature exhibits the clear FM with saturation magnetization ͑M s ͒ and coercive field ͑H c ͒ of the order of 3-7 emu/g and 260 Oe, respectively. Temperature dependence of magnetization ͑M-T͒ measurement shows sharp ferromagnetic to paramagnetic transition with a high Curie temperature ͑T c ͒ of ϳ800 K for 3% Co doped ZnO NPs. It is found that doping at 5% and higher concentration does not exhibit a proper magnetic transition. We attempt to fit the observed FM data with the bound magnetic polaron ͑BMP͒ model involving localized carriers and magnetic cations. However, calculated concentration of the BMPs is well below the typical percolation threshold in ZnO. We believe that observed high temperature FM is primarily mediated by defects in the strained NPs. ZnO NPs of lower initial size show enhanced FM that may be attributed to size dependent doping effect.
The quantum interference assisted enhanced optical activity due to the emergence of a steady-stat... more The quantum interference assisted enhanced optical activity due to the emergence of a steady-state atomic polarization is investigated. The Rubidium atoms in an antirelaxation coated cell provide a suitable platform to address the phenomena at multiple Larmor's frequencies. It interacts with a narrow bandwidth frequency comb generated by the frequency modulation of the light field. The Lindblad master equation with a trichromatic field provides a microscopic picture of the atomic response to the narrow bandwidth frequency comb. The directive of the relative phase between the light fields, in the detuning dependence of the magnetic resonances, is conclusively captured with the trichromatic field model. The measured absorption, nonlinear magneto-optic rotation, and their dependencies on various experimental parameters are analysed. The ellipticity of the light field controls the extent of several physical processes at multiple Larmor's frequencies. The investigation provides an approach to address the Zeeman coherence in the interaction of a narrow bandwidth frequency comb with an atomic ensemble and will have applications in various quantum devices.
To stabilize nontrivial spin textures, e.g., skyrmions or chiral domain walls in ultrathin magnet... more To stabilize nontrivial spin textures, e.g., skyrmions or chiral domain walls in ultrathin magnetic films, an additional degree of freedom, such as the interfacial Dzyaloshinskii-Moriya interaction (IDMI), must be induced by the strong spin-orbit coupling (SOC) of a stacked heavy metal layer. However, advanced approaches to simultaneously control the IDMI and perpendicular magnetic anisotropy (PMA) are needed for future spin-orbitronic device implementations. Here, we show the effect of atomic-scale surface modulation on the magnetic properties and IDMI in ultrathin films composed of 5d heavy metal/ferromagnet/4d(5d) heavy metal or oxide interfaces, such as Pt/CoFeSiB/Ru, Pt/CoFeSiB/ Ta, and Pt/CoFeSiB/MgO. The maximum IDMI value corresponds to the correlated roughness of the bottom and top interfaces of the ferromagnetic layer. The proposed approach for significant enhancement of PMA and the IDMI through interface roughness engineering at the atomic scale offers a powerful tool for the development of spinorbitronic devices with precise and reliable controllability of their functionality.
Zn 1Àx Co x O (x = 0, 0.05, and 0.07) nanorods (NRs) exhibiting ferromagnetism above room tempera... more Zn 1Àx Co x O (x = 0, 0.05, and 0.07) nanorods (NRs) exhibiting ferromagnetism above room temperature and with high magnetic moment have been synthesized by a solvothermal route. XRD, FESEM, TEM, EDS and XPS measurements reveal the growth of single phase wurtzite structure Zn 1Àx Co x O NRs with the successful incorporation of Co ions inside the ZnO matrix. TEM micrograph reveals clearly the formation of long ZnO NRs with diameter of 50-90 nm and length of 0.3-0.6 lm. High resolution TEM lattice images and the electron diffraction patterns show that all the NRs are single crystalline. Room temperature magnetic measurements exhibit ferromagnetic behavior with high magnetic moment of 1.83 emu/g for 2 T field, coercivity of 53 G. Temperature dependent magnetization measurement shows a Curie temperature of the order of 398 K. Photoluminescence (PL) spectra exhibit near band edge UV emission and defect related visible emission, which is expected to play a significant role in the FM ordering. PL and UV-VIS spectra reveal slight modification of band edge due to doping effect. Systemic structural, magnetic, and optical studies reveal that both the nature of the defects as well as Co 2+ ions are significant ingredients to attain high moment as well as high ordering temperature in the 1-dimensional ZnO NRs. Magnetic interaction is quantitatively analyzed and explained using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Co ions and V Zn , O i related defects. These findings provides a better insight into the underlying mechanisms of high temperature ferromagnetism in Zn 1Àx Co x O NRs.
ABSTRACT We report on the room temperature ferromagnetism (RT FM) in the Zn1−xNixO (x = 0, 0.03, ... more ABSTRACT We report on the room temperature ferromagnetism (RT FM) in the Zn1−xNixO (x = 0, 0.03, and 0.05) nanoparticles (NPs) synthesized by a ball milling technique. X-ray diffraction analysis confirms the single crystalline ZnO wurtzite structure with presence of small intensity secondary phase related peak which disappear with increasing milling time for Ni doped samples. HRTEM lattice images show that the doped NPs are single crystalline with a dspacing of 2.44 Å. Energy-dispersive X-ray spectroscopy analysis confirms the presence of Ni ions in the ZnO matrix. Magnetic measurement (RT) exhibits the hysteresis loop with saturation magnetization (Ms) of 1.6–2.56 (emu/g) and coercive field (Hc) of 296–322 Oe. M-T measurement shows a Curie temperature of the order of 325°C for 3% Ni doped sample. Micro -Raman studies show doping/disorder induced additional modes at ∼510, 547, 572 cm−1 in addition to 437 cm−1 peak of pure ZnO. UV-Vis absorption spectra illustrate band gap shift due to doping. Alteration of Ms value with the variation of doping concentration and milling time has been studied and discussed.
Structural, optical and magnetic studies have been carried out for the Co-doped ZnO nanoparticles... more Structural, optical and magnetic studies have been carried out for the Co-doped ZnO nanoparticles (NPs). ZnO NPs are doped with 3% and 5% Co using ball milling and ferromagnetism (FM) is studied at room temperature and above. A high Curie temperature (T c has been observed from the Co doped ZnO NPs. X-ray diffraction and high resolution transmission electron microscopy analysis confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO. UV-visible absorption and photoluminescence studies on the doped samples show change in band structure and oxygen vacancy defects, respectively. Micro-Raman studies of doped samples shows defect related additional strong bands at 547 and 574 cm −1 confirming the presence of oxygen vacancy defects in ZnO lattice. The field dependence of magnetization (M-H curve) measured at room temperature exhibits the clear M-H loop with saturation magnetization and coercive field of the order of 4-6 emu/g and 260 G, respectively. Temperature dependence of magnetization measurement shows sharp ferromagnetic to paramagnetic transition with a high T C = 791 K for 3% Co doped ZnO NPs. Ferromagnetic ordering is interpreted in terms of overlapping of polarons mediated through oxygen vacancy defects based on the bound magnetic polaron (BMP) model. We show that the observed FM data fits well with the BMP model involving localised carriers and magnetic cations.
Enhanced upconversion fluorescence of Er3+:ZrO2 nanocrystals induced by phase transformation AIP ... more Enhanced upconversion fluorescence of Er3+:ZrO2 nanocrystals induced by phase transformation AIP Advances 2, 012102 The indirect to direct band gap transition in multilayered MoS2 as predicted by screened hybrid density functional theory Appl. Phys. Lett. 99, 261908 (2011) Robust thermal performance of Sr2Si5N8:Eu2+: An efficient red emitting phosphor for light emitting diode based white lighting Appl. Phys. Lett. 99, 241106 (2011) Downconversion from visible to near infrared through multi-wavelength excitation in Er3+/Yb3+ co-doped NaYF4 nanocrystals
We report on the occurrence of high temperature ferromagnetism ͑FM͒ in ZnO nanoparticles ͑NPs͒ do... more We report on the occurrence of high temperature ferromagnetism ͑FM͒ in ZnO nanoparticles ͑NPs͒ doped with Co-atoms. ZnO NPs of two different initial sizes are doped with 3% and 5% Co using ball milling and FM is studied at room temperature and above. X-ray diffraction and high-resolution transmission electron microscopy analysis confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO. UV-visible absorption studies show change in band structure and photoluminescence studies show green emission band at 520 nm indicating incorporation of Co-atoms and presence of oxygen vacancy defects, respectively in ZnO lattice. Micro-Raman studies of doped samples shows defect related additional bands at 547 and 574 cm −1 . XRD and Raman spectra provide clear evidence for strain in the doped ZnO NPs. The field dependence of magnetization ͑M-H curve͒ measured at room temperature exhibits the clear FM with saturation magnetization ͑M s ͒ and coercive field ͑H c ͒ of the order of 3-7 emu/g and 260 Oe, respectively. Temperature dependence of magnetization ͑M-T͒ measurement shows sharp ferromagnetic to paramagnetic transition with a high Curie temperature ͑T c ͒ of ϳ800 K for 3% Co doped ZnO NPs. It is found that doping at 5% and higher concentration does not exhibit a proper magnetic transition. We attempt to fit the observed FM data with the bound magnetic polaron ͑BMP͒ model involving localized carriers and magnetic cations. However, calculated concentration of the BMPs is well below the typical percolation threshold in ZnO. We believe that observed high temperature FM is primarily mediated by defects in the strained NPs. ZnO NPs of lower initial size show enhanced FM that may be attributed to size dependent doping effect. Downloaded 24 Dec 2010 to 210.212.8.62. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions 084322-2 B. Pal and P. K. Giri J. Appl. Phys. 108, 084322 ͑2010͒ Downloaded 24 Dec 2010 to 210.212.8.62. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions 084322-4 B. Pal and P. K. Giri J. Appl. Phys. 108, 084322 ͑2010͒ Downloaded 24 Dec 2010 to 210.212.8.62. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions 084322-6 B. Pal and P. K. Giri J. Appl. Phys. 108, 084322 ͑2010͒ Downloaded 24 Dec 2010 to 210.212.8.62. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions 084322-8 B. Pal and P. K. Giri J. Appl. Phys. 108, 084322 ͑2010͒
The room-temperature ferromagnetism properties of Co-doped zinc oxide are reported in this paper.... more The room-temperature ferromagnetism properties of Co-doped zinc oxide are reported in this paper. The Zn 1−x Co x O (0.02 < x < 0.10) specimens were synthesized by the solid state reaction method with the mixture of Co 2 O 3 and ZnO powders. The precursor Co 2 O 3 and ZnO powders were mixed and milled for 10 h. The mixed powders were calcined at temperature of 600 • C and 1000 • C, respectively for 12 h, and then were cooled at the ambient temperature. Next, the powders were milled, made block and sintered for 12 h. The specimens were then hydrogenated at 600 • C for 1 h and 3 h, respectively. The room-temperature ferromagnetism (RTFM) was found in the hydrogenated samples, and the hysteresis loops of the hydrogenated Co-doped ZnO samples were measured at 10 K and 300 K by using the superconducting quantum interference device (SQUID) magnetometer (Quantum Design). The coercive field was about 310 Oe for the room-temperature measurement, and the saturation magnetization of the hydrogenated sample was about 0.056µ B /Co. The samples were further measured by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The results suggested the observed RTFM in the Co-doped ZnO was partly due to the existence of the cobalt nanoparticles in the hydrogenated samples.
We report on the growth of Co-doped ZnO nanowires (NWs) on Si substrate using a self-catalytic va... more We report on the growth of Co-doped ZnO nanowires (NWs) on Si substrate using a self-catalytic vapor deposition method from a Co-doped ZnO nanopowder source and study its structural, optical and magnetic properties for the as-grown and rapid thermal annealed samples. Co (5%)-doped ZnO (ZnCoO) nanoparticles (NPs) are used as source material for the growth process. Electron microscopy imaging clearly reveals the formation of long ZnO NWs with uniform diameter. X-ray di®raction analysis con¯rms the single crystalline hexagonal structure of Co-doped ZnO NWs without impurities of metallic cobalt or other phases. Micro-Raman studies of doped samples show doping/disorder induced additional modes as compared to the undoped ZnO. Room temperature photoluminescence spectra of the doped ZnO NWs show strong emission band at $380 nm and no signi¯cant emission was observed in the visible region indicating low defect content in the NWs. Thē eld dependent magnetization (MÀH curve) measured at room temperature exhibits paramagnetic nature for the NWs with the magnetic moment in the range 2À3.7 milli-emu/cm 2 for the applied eld of 2 Tesla, while the source ZnCoO NPs exhibit room temperature ferromagnetism with saturation magnetization $6 emu/g. Possible mechanism of alteration in magnetic behavior in doped NWs are discussed based on the growth conditions and role of defects.
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Papers by Bappaditya Pal