Jingsheng Chen | National University of Singapore (original) (raw)
Papers by Jingsheng Chen
Proceedings of SPIE - The International Society for Optical Engineering
ABSTRACT
The evolution of the microstructure and magnetic properties of FePt films with different thicknes... more The evolution of the microstructure and magnetic properties of FePt films with different thicknesses fabricated by dc magnetron sputtering on TiN intermediate layer were investigated. MFM results showed that the magnetization reversal mechanism changed from Stoner-Wohlfarth rotation to domain wall motion with increasing film thickness. SEM and TEM images revealed that the morphologies of the films evolved from island particle to maze-like grains and then continuous films with the increase of the FePt film thickness. The critical thickness for FePt/TiN epitaxial growth was estimated to be 10 nm by lattice constant simulation and TEM measurement. The lattice relaxation in the FePt film of 40 nm was determined by high-resolution TEM images.
The effects of a TiN intermediate layer on the microstructure and magnetic properties of the FePt... more The effects of a TiN intermediate layer on the microstructure and magnetic properties of the FePt films were investigated. With introducing TiN intermediate layer the chemical ordering and magnetic properties of FePt films significantly improved. The FePt film with 5nm TiN exhibited a large perpendicular anisotropy and smaller opening-up of in-plane hysteresis loop with out-of-plane coercivity of 13.7 kOe and in-plane coercivity of 0.24 kOe. Moreover, with doping C and SiOx, FePt film with coercivity around 20 kOe, a single layer structure, and small FePt grain size of 5.6 nm in average diameter was obtained.
Journal of Physics D Applied Physics
We studied L10 ordered Fe50Pt50−x Ndx alloy films, which showed a large enhancement (~18.4% at ro... more We studied L10 ordered Fe50Pt50−x Ndx alloy films, which showed a large enhancement (~18.4% at room temperature and ~11.7% at 10 K) of magnetic moment with 6 atomic % of Nd. Analysis of the x-ray magnetic circular dichroism spectra at the Fe L 3,2 edges and Nd M 5,4 edges in Fe50Pt44Nd6 films indicated a significant contribution of the Nd orbital moment. The origin of the large enhancement of magnetic moment was attributed to the effect of ferromagnetic coupling of the total magnetic moments between Fe and Nd. Density functional theory based first principles calculations supported the experimental observations of increasing moment due to Nd substitution of Pt.
Applied Physics Letters
In order to increase the signal-to-noise ratio of heat assisted magnetic recording, it is desirab... more In order to increase the signal-to-noise ratio of heat assisted magnetic recording, it is desirable to fabricate high magnetic anisotropy FePt media with small grain size and high aspect ratio (grain height to size ratio). In the present paper, we report that FePt media with small grain size and high aspect ratio were achieved by doping ZrO2 into FePt film grown on TiON intermediate layer. The grain size was around 5.6 nm, and the aspect ratio was as high as 2.6. It is believed that this originated from the ZrO2 (002) tetragonal crystalline phase epitaxially grown on TiON intermediate layer. With a 5 vol. % carbon doping into FePt-ZrO2 films, the perpendicular anisotropy was improved and the out-of-plane coercivity was around 23.2 kOe. Doping of crystalline phase material with certain required crystal structure may offer a method for fabrication of nanostructured thin films with high aspect ratio grains at high processing temperature.
Journal of Applied Physics
A trilayer micromagnetic model based on the Landau-Lifshitz-Bloch equation of motion is utilized ... more A trilayer micromagnetic model based on the Landau-Lifshitz-Bloch equation of motion is utilized to study the properties of L10-FePt/TiN/L10-FePt pseudo spin valves (PSVs) in direct comparison with experiment. Theoretical studies give an insight on the crystallographic texture, magnetic properties, reversal behavior, interlayer coupling effects, and magneto-transport properties of the PSVs, in particular, with varying thickness of the top L10-FePt and TiN spacer. We show that morphological changes in the FePt layers, induced by varying the FePt layer thickness, lead to different hysteresis behaviors of the samples, caused by changes in the interlayer and intralayer exchange couplings. Such effects are important for the optimization of the PSVs due to the relationship between the magnetic properties, domain structures, and the magnetoresistance of the device.
Scientific reports, 2015
The quest for solid state non-volatility memory devices on silicon with high storage density, hig... more The quest for solid state non-volatility memory devices on silicon with high storage density, high speed, low power consumption has attracted intense research on new materials and novel device architectures. Although flash memory dominates in the non-volatile memory market currently, it has drawbacks, such as low operation speed, and limited cycle endurance, which prevents it from becoming the "universal memory". In this report, we demonstrate ferroelectric tunnel junctions (Pt/BaTiO3/La0.67Sr0.33MnO3) epitaxially grown on silicon substrates. X-ray diffraction spectra and high resolution transmission electron microscope images prove the high epitaxial quality of the single crystal perovskite films grown on silicon. Furthermore, the write speed, data retention and fatigue properties of the device compare favorably with flash memories. The results prove that the silicon-based ferroelectric tunnel junction is a very promising candidate for application in future non-volatile m...
Journal of Applied Physics, 2014
ABSTRACT In structurally ordered magnetic thin films, the Curie temperature (T-C) of ferromagneti... more ABSTRACT In structurally ordered magnetic thin films, the Curie temperature (T-C) of ferromagnetic films depends on the exchange integral of the short range ordered neighboring atoms. The exchange integral may be adjusted by controlling the elemental substitutional concentration at the lattice site of interest. We show how to control the T-C in high anisotropy L1(0) Fe50Pt50 magnetic thin films by substituting Rh into the Pt site. Rh substitution in L1(0) FePt modified the local atomic environment and the corresponding electronic properties, while retaining the ordered L1(0) phase. The analysis of extended x-ray Absorption Fine Structure spectra shows that Rh uniformly substitutes for Pt in L1(0) FePt. A model of antiferromagnetic defects caused by controlled Rh substitution of the Pt site, reducing the T-C, is proposed to interpret this phenomenon and its validity is further examined by ab initio density functional calculations. (C) 2014 AIP Publishing LLC.
Scientific reports, 2015
Strain engineering is an effective way to modify functional properties of thin films. Recently, t... more Strain engineering is an effective way to modify functional properties of thin films. Recently, the importance of octahedral rotations in pervoskite films has been recognized in discovering and designing new functional phases. Octahedral behavior of SrRuO3 film as a popular electrode in heterostructured devices is of particular interest for its probable interfacial coupling of octahedra with the functional overlayers. Here we report the strain engineering of octahedral rotations and physical properties that has been achieved in SrRuO3 films in response to the substrate-induced misfit strains of almost the same amplitude but of opposite signs. It shows that the compressively strained film on NdGaO3 substrate displays a rotation pattern of a tetragonal phase whilst the tensilely strained film on KTaO3 substrate has the rotation pattern of the bulk orthorhombic SrRuO3 phase. In addition, the compressively strained film displays a perpendicular magnetic anisotropy while the tensilely st...
Materials Perspective, 2011
It was learned in the previous chapters that the media trilemma will pose limitations on the area... more It was learned in the previous chapters that the media trilemma will pose limitations on the areal density achieved using the existing granular perpendicular media technology. Although technologies such as exchange-coupled composite (ECC) media may extend the areal density beyond 1 Tb/in. 2 in perpendicular media, significant progress in research over the last several years has brought the introduction of alternative technologies such as heat-assisted magnetic recording (HAMR). Chapter 10 introduced the concept of HAMR at a ...
Scientific reports, 2014
The effects of TiN-ZrO₂ intermediate layer on the microstructures and magnetic properties of FePt... more The effects of TiN-ZrO₂ intermediate layer on the microstructures and magnetic properties of FePt films were investigated. The TiN-ZrO2 intermediate layer was granular consisting of grains of solid solution of Ti(Zr)ON segregated by amorphous ZrO₂. By doping ZrO₂ into TiN intermediate layer, the FePt grains became better isolated from each other and the FePt grain size was reduced. For 20 vol. % ZrO₂ doping into TiN, the grain size decreased dramatically from 11. 2 nm to 6. 4 nm, and good perpendicular anisotropy was achieved simultaneously. For the FePt 4nm-SiO₂ 35 vol. % -C 20 vol. % films grown on top of the TiN-ZrO₂ 20 vol. % intermediate layer, well isolated FePt (001) granular films with coercivity higher than 18. 1 kOe and an average size as small as 6. 4 nm were achieved.
International Journal of Modern Physics B, 2002
ABSTRACT Ta-C and iron containing amorphous carbon (a-C:Fe) films were deposited by filtered cath... more ABSTRACT Ta-C and iron containing amorphous carbon (a-C:Fe) films were deposited by filtered cathodic vacuum arc technique. The influences of Fe contents on the wettability of the films were investigated in terms of surface energy. The surface energy of a-C:Fe films was determined by the contact angle measurement. Atomic force microscopy (AFM), Raman spectroscopy and X-ray induced photoelectron spectroscopy (XPS) were employed to analyze the origin of the variation of surface energy with various Fe content. It is found that the contact angle for water increases significantly after incorporating Fe into the films and the films become hydrophobic. The roughness of these films has a little effect on the contact angle. The surface energy is reduced after incorporating Fe into the a-C film which is due to the reduction of both dispersive and polar component. The reduction in dispersive component is ascribed to the decrease of atomic density of the a-C:Fe films due to the increase in sp2 bonded carbon. The absorption of oxygen on the surface play an important role in the reduction of polar component for the a-C:Fe films. It is proposed that such network as (Ca-O-Fe)-O-(Fe-O-Ca) may be formed and responsible for the reduction of polar component.
ABSTRACT We report the spatio-temporal separation of electron and phonon thermal transports in na... more ABSTRACT We report the spatio-temporal separation of electron and phonon thermal transports in nanostructured magnetic L10 FePt films at the nanometer length scale and the time domain of tens of picosecond, when heated with a pulsed laser. We demonstrate that lattice dynamics measured using the picosecond time-resolved laser pump/X-ray probe method on the FePt (002) and Ag (002) Bragg reflections from different layers provided the information of nanoscale thermal transport between the layers. We also describe how the electron and phonon thermal transports in nanostructured magnetic thin films were separated.
Advanced Materials Interfaces, 2014
Advanced Materials Interfaces, 2014
Strongly correlated oxides are full of fascinating phenomena owing to their interacting lattice, ... more Strongly correlated oxides are full of fascinating phenomena owing to their interacting lattice, charge, spin and orbital degrees of freedom . Bandgap, a critical parameter for an oxide insulator, is well determined by those degrees of freedom and in turn directly affects electronic, magnetic and optical properties of the material. Typically, tunability of the bandgap in an oxide insulator can be achieved through chemical doping , which is important for electronic and photonic device applications. Here we report large bandgap enhancement in SrTiO 3 (STO) thin films, which can be up to 20% greater than the bulk value, depending on the deposition temperature. There is no significant change in density and cationic ratio of the oxide so the effect is attributed to Sr/Ti antisite defects, an attribution supported by density functional theory calculations. It was found that the bandgap enhancement significantly changes the electronic and magnetic phases in the oxygen-vacancy-induced two-dimensional electron gas at the interface between amorphous LaAlO 3 (LAO) and STO. This opens an attractive path to tailor electronic, magnetic and optical properties of STO-based oxide interface systems under intensive focus in the electronics community. Meanwhile, our study provides key insight into the origin of the fundamental issue that STO thin films are difficult to convert into metals by oxygen vacancy doping.
Physical Review X, 2013
The relative importance of atomic defects and electron transfer in explaining conductivity at the... more The relative importance of atomic defects and electron transfer in explaining conductivity at the crystalline LaAlO 3 =SrTiO 3 interface has been a topic of debate. Metallic interfaces with similar electronic properties produced by amorphous oxide overlayers on SrTiO 3 [Y. Chen et al., Nano Lett. 11, 3774 (2011); S. W. Lee et al., Nano Lett. 12, 4775 (2012)] have called in question the original polarization catastrophe model [N. Nakagawa et al., Nature Mater. 5, 204 (2006)]. We resolve the issue by a comprehensive comparison of (100)-oriented SrTiO 3 substrates with crystalline and amorphous overlayers of LaAlO 3 of different thicknesses prepared under different oxygen pressures. For both types of overlayers, there is a critical thickness for the appearance of conductivity, but its value is always 4 unit cells (around 1.6 nm) for the oxygen-annealed crystalline case, whereas in the amorphous case, the critical thickness could be varied in the range 0.5 to 6 nm according to the deposition conditions. Subsequent ion milling of the overlayer restores the insulating state for the oxygen-annealed crystalline heterostructures but not for the amorphous ones. Oxygen post-annealing removes the oxygen vacancies, and the interfaces become insulating in the amorphous case. However, the interfaces with a crystalline overlayer remain conducting with reduced carrier density. These results demonstrate that oxygen vacancies are the dominant source of mobile carriers when the LaAlO 3 overlayer is amorphous, while both oxygen vacancies and polarization catastrophe contribute to the interface conductivity in unannealed crystalline LaAlO 3 =SrTiO 3 heterostructures, and the polarization catastrophe alone accounts for the conductivity in oxygen-annealed crystalline LaAlO 3 =SrTiO 3 heterostructures. Furthermore, we find that the crystallinity of the LaAlO 3 layer is crucial for the polarization catastrophe mechanism in the case of crystalline LaAlO 3 overlayers.
Journal of Physics D: Applied Physics, 2014
The magnetic properties of L1 0 -FePt/Fe exchange-coupled composite (ECC) nanodots were investiga... more The magnetic properties of L1 0 -FePt/Fe exchange-coupled composite (ECC) nanodots were investigated by Hall effect measurement due to its high sensitivity. The FePt/Fe nanodots showed coercivity changing irregularly with respect to the Fe thickness, which deviated from the simulated results, where the coercivity of the ECC dots reduced with increasing Fe soft layer thickness. It was found that the edge damage induced by ion milling affected the coercivity of the nanodots significantly and the magnetization reversal mechanism. Domain wall nucleation and propagation was revealed in the FePt single domain dots. In the FePt/Fe ECC nanodots, the magnetization gradually reversed from the out-of-plane direction towards the in-plane direction before switching to the reverse out-of-plane direction. The critical size of single magnetic domains in the FePt/Fe ECC nanodots increased with increasing Fe soft layer thickness.
Thin Solid Films, 2008
Multi-layer nanocomposite structures of Ta/Ru/CoCr 1 /FeCoTaCr(soft magnetic layer)/CoCr 2 /CoCrP... more Multi-layer nanocomposite structures of Ta/Ru/CoCr 1 /FeCoTaCr(soft magnetic layer)/CoCr 2 /CoCrPt-SiO 2 (hard magnetic layer or recording layer)/C and Ta/Ru/CoCr 1 /CoCrPt-SiO 2 /CoCr 2 /FeCoTaCr/C were proposed. This exchange coupled composite (ECC) media consisting of hard/ soft stacked magnetic layers were promising in improving the writability of perpendicular magnetic recording media. A small CoCrPt c-axis orientation dispersion of about 3°was achieved with the optimized sputter conditions. The CoCrPt-SiO 2 grains were well segregated by SiO 2 at grain boundaries. The macro-magnetic properties showed that the stacked magnetic grains switched in a coherent mode and that switching field decreased with increasing the thickness of the soft magnetic layer.
Surface Review and Letters, 2008
ABSTRACT Co-ferrite films were prepared using pulsed laser deposition with both post-annealing an... more ABSTRACT Co-ferrite films were prepared using pulsed laser deposition with both post-annealing and in situ heating processes. Magnetic properties of these films were studied in the function of temperature, film thickness, and substrate. The films using post-annealing processes exhibited isotropic microstructure, and the coercivity showed no obvious magnetic anisotropy and no strong dependence on film thickness. Co-ferrite films using in situ heating exhibited (111) highly textured structure and possessed perpendicular anisotropy as well as large coercivity. The preferential texture and magnetic anisotropy were closely associated with substrate temperature and thickness. Perpendicular Hc over 12.5 kOe was obtained in the 33 nm Co-ferrite film deposited on single crystal quartz substrate at 550°C. The high coercivity and perpendicular coercivity may be attributed to the nanocrystalline grain, textured orientation, and large residual strain in these films since large residual strain may induce strong stress anisotropy.
Surface Review and Letters, 2001
The unique electron field emission properties of carbon nanotube films are promising for flat pan... more The unique electron field emission properties of carbon nanotube films are promising for flat panel display application as the cold cathode. In this work, the multiwall carbon nanotube (1010 2 nm in diameter, and 10 3 10 4 nm in length) film arrays (the pixel size: 50 μm ...
Proceedings of SPIE - The International Society for Optical Engineering
ABSTRACT
The evolution of the microstructure and magnetic properties of FePt films with different thicknes... more The evolution of the microstructure and magnetic properties of FePt films with different thicknesses fabricated by dc magnetron sputtering on TiN intermediate layer were investigated. MFM results showed that the magnetization reversal mechanism changed from Stoner-Wohlfarth rotation to domain wall motion with increasing film thickness. SEM and TEM images revealed that the morphologies of the films evolved from island particle to maze-like grains and then continuous films with the increase of the FePt film thickness. The critical thickness for FePt/TiN epitaxial growth was estimated to be 10 nm by lattice constant simulation and TEM measurement. The lattice relaxation in the FePt film of 40 nm was determined by high-resolution TEM images.
The effects of a TiN intermediate layer on the microstructure and magnetic properties of the FePt... more The effects of a TiN intermediate layer on the microstructure and magnetic properties of the FePt films were investigated. With introducing TiN intermediate layer the chemical ordering and magnetic properties of FePt films significantly improved. The FePt film with 5nm TiN exhibited a large perpendicular anisotropy and smaller opening-up of in-plane hysteresis loop with out-of-plane coercivity of 13.7 kOe and in-plane coercivity of 0.24 kOe. Moreover, with doping C and SiOx, FePt film with coercivity around 20 kOe, a single layer structure, and small FePt grain size of 5.6 nm in average diameter was obtained.
Journal of Physics D Applied Physics
We studied L10 ordered Fe50Pt50−x Ndx alloy films, which showed a large enhancement (~18.4% at ro... more We studied L10 ordered Fe50Pt50−x Ndx alloy films, which showed a large enhancement (~18.4% at room temperature and ~11.7% at 10 K) of magnetic moment with 6 atomic % of Nd. Analysis of the x-ray magnetic circular dichroism spectra at the Fe L 3,2 edges and Nd M 5,4 edges in Fe50Pt44Nd6 films indicated a significant contribution of the Nd orbital moment. The origin of the large enhancement of magnetic moment was attributed to the effect of ferromagnetic coupling of the total magnetic moments between Fe and Nd. Density functional theory based first principles calculations supported the experimental observations of increasing moment due to Nd substitution of Pt.
Applied Physics Letters
In order to increase the signal-to-noise ratio of heat assisted magnetic recording, it is desirab... more In order to increase the signal-to-noise ratio of heat assisted magnetic recording, it is desirable to fabricate high magnetic anisotropy FePt media with small grain size and high aspect ratio (grain height to size ratio). In the present paper, we report that FePt media with small grain size and high aspect ratio were achieved by doping ZrO2 into FePt film grown on TiON intermediate layer. The grain size was around 5.6 nm, and the aspect ratio was as high as 2.6. It is believed that this originated from the ZrO2 (002) tetragonal crystalline phase epitaxially grown on TiON intermediate layer. With a 5 vol. % carbon doping into FePt-ZrO2 films, the perpendicular anisotropy was improved and the out-of-plane coercivity was around 23.2 kOe. Doping of crystalline phase material with certain required crystal structure may offer a method for fabrication of nanostructured thin films with high aspect ratio grains at high processing temperature.
Journal of Applied Physics
A trilayer micromagnetic model based on the Landau-Lifshitz-Bloch equation of motion is utilized ... more A trilayer micromagnetic model based on the Landau-Lifshitz-Bloch equation of motion is utilized to study the properties of L10-FePt/TiN/L10-FePt pseudo spin valves (PSVs) in direct comparison with experiment. Theoretical studies give an insight on the crystallographic texture, magnetic properties, reversal behavior, interlayer coupling effects, and magneto-transport properties of the PSVs, in particular, with varying thickness of the top L10-FePt and TiN spacer. We show that morphological changes in the FePt layers, induced by varying the FePt layer thickness, lead to different hysteresis behaviors of the samples, caused by changes in the interlayer and intralayer exchange couplings. Such effects are important for the optimization of the PSVs due to the relationship between the magnetic properties, domain structures, and the magnetoresistance of the device.
Scientific reports, 2015
The quest for solid state non-volatility memory devices on silicon with high storage density, hig... more The quest for solid state non-volatility memory devices on silicon with high storage density, high speed, low power consumption has attracted intense research on new materials and novel device architectures. Although flash memory dominates in the non-volatile memory market currently, it has drawbacks, such as low operation speed, and limited cycle endurance, which prevents it from becoming the "universal memory". In this report, we demonstrate ferroelectric tunnel junctions (Pt/BaTiO3/La0.67Sr0.33MnO3) epitaxially grown on silicon substrates. X-ray diffraction spectra and high resolution transmission electron microscope images prove the high epitaxial quality of the single crystal perovskite films grown on silicon. Furthermore, the write speed, data retention and fatigue properties of the device compare favorably with flash memories. The results prove that the silicon-based ferroelectric tunnel junction is a very promising candidate for application in future non-volatile m...
Journal of Applied Physics, 2014
ABSTRACT In structurally ordered magnetic thin films, the Curie temperature (T-C) of ferromagneti... more ABSTRACT In structurally ordered magnetic thin films, the Curie temperature (T-C) of ferromagnetic films depends on the exchange integral of the short range ordered neighboring atoms. The exchange integral may be adjusted by controlling the elemental substitutional concentration at the lattice site of interest. We show how to control the T-C in high anisotropy L1(0) Fe50Pt50 magnetic thin films by substituting Rh into the Pt site. Rh substitution in L1(0) FePt modified the local atomic environment and the corresponding electronic properties, while retaining the ordered L1(0) phase. The analysis of extended x-ray Absorption Fine Structure spectra shows that Rh uniformly substitutes for Pt in L1(0) FePt. A model of antiferromagnetic defects caused by controlled Rh substitution of the Pt site, reducing the T-C, is proposed to interpret this phenomenon and its validity is further examined by ab initio density functional calculations. (C) 2014 AIP Publishing LLC.
Scientific reports, 2015
Strain engineering is an effective way to modify functional properties of thin films. Recently, t... more Strain engineering is an effective way to modify functional properties of thin films. Recently, the importance of octahedral rotations in pervoskite films has been recognized in discovering and designing new functional phases. Octahedral behavior of SrRuO3 film as a popular electrode in heterostructured devices is of particular interest for its probable interfacial coupling of octahedra with the functional overlayers. Here we report the strain engineering of octahedral rotations and physical properties that has been achieved in SrRuO3 films in response to the substrate-induced misfit strains of almost the same amplitude but of opposite signs. It shows that the compressively strained film on NdGaO3 substrate displays a rotation pattern of a tetragonal phase whilst the tensilely strained film on KTaO3 substrate has the rotation pattern of the bulk orthorhombic SrRuO3 phase. In addition, the compressively strained film displays a perpendicular magnetic anisotropy while the tensilely st...
Materials Perspective, 2011
It was learned in the previous chapters that the media trilemma will pose limitations on the area... more It was learned in the previous chapters that the media trilemma will pose limitations on the areal density achieved using the existing granular perpendicular media technology. Although technologies such as exchange-coupled composite (ECC) media may extend the areal density beyond 1 Tb/in. 2 in perpendicular media, significant progress in research over the last several years has brought the introduction of alternative technologies such as heat-assisted magnetic recording (HAMR). Chapter 10 introduced the concept of HAMR at a ...
Scientific reports, 2014
The effects of TiN-ZrO₂ intermediate layer on the microstructures and magnetic properties of FePt... more The effects of TiN-ZrO₂ intermediate layer on the microstructures and magnetic properties of FePt films were investigated. The TiN-ZrO2 intermediate layer was granular consisting of grains of solid solution of Ti(Zr)ON segregated by amorphous ZrO₂. By doping ZrO₂ into TiN intermediate layer, the FePt grains became better isolated from each other and the FePt grain size was reduced. For 20 vol. % ZrO₂ doping into TiN, the grain size decreased dramatically from 11. 2 nm to 6. 4 nm, and good perpendicular anisotropy was achieved simultaneously. For the FePt 4nm-SiO₂ 35 vol. % -C 20 vol. % films grown on top of the TiN-ZrO₂ 20 vol. % intermediate layer, well isolated FePt (001) granular films with coercivity higher than 18. 1 kOe and an average size as small as 6. 4 nm were achieved.
International Journal of Modern Physics B, 2002
ABSTRACT Ta-C and iron containing amorphous carbon (a-C:Fe) films were deposited by filtered cath... more ABSTRACT Ta-C and iron containing amorphous carbon (a-C:Fe) films were deposited by filtered cathodic vacuum arc technique. The influences of Fe contents on the wettability of the films were investigated in terms of surface energy. The surface energy of a-C:Fe films was determined by the contact angle measurement. Atomic force microscopy (AFM), Raman spectroscopy and X-ray induced photoelectron spectroscopy (XPS) were employed to analyze the origin of the variation of surface energy with various Fe content. It is found that the contact angle for water increases significantly after incorporating Fe into the films and the films become hydrophobic. The roughness of these films has a little effect on the contact angle. The surface energy is reduced after incorporating Fe into the a-C film which is due to the reduction of both dispersive and polar component. The reduction in dispersive component is ascribed to the decrease of atomic density of the a-C:Fe films due to the increase in sp2 bonded carbon. The absorption of oxygen on the surface play an important role in the reduction of polar component for the a-C:Fe films. It is proposed that such network as (Ca-O-Fe)-O-(Fe-O-Ca) may be formed and responsible for the reduction of polar component.
ABSTRACT We report the spatio-temporal separation of electron and phonon thermal transports in na... more ABSTRACT We report the spatio-temporal separation of electron and phonon thermal transports in nanostructured magnetic L10 FePt films at the nanometer length scale and the time domain of tens of picosecond, when heated with a pulsed laser. We demonstrate that lattice dynamics measured using the picosecond time-resolved laser pump/X-ray probe method on the FePt (002) and Ag (002) Bragg reflections from different layers provided the information of nanoscale thermal transport between the layers. We also describe how the electron and phonon thermal transports in nanostructured magnetic thin films were separated.
Advanced Materials Interfaces, 2014
Advanced Materials Interfaces, 2014
Strongly correlated oxides are full of fascinating phenomena owing to their interacting lattice, ... more Strongly correlated oxides are full of fascinating phenomena owing to their interacting lattice, charge, spin and orbital degrees of freedom . Bandgap, a critical parameter for an oxide insulator, is well determined by those degrees of freedom and in turn directly affects electronic, magnetic and optical properties of the material. Typically, tunability of the bandgap in an oxide insulator can be achieved through chemical doping , which is important for electronic and photonic device applications. Here we report large bandgap enhancement in SrTiO 3 (STO) thin films, which can be up to 20% greater than the bulk value, depending on the deposition temperature. There is no significant change in density and cationic ratio of the oxide so the effect is attributed to Sr/Ti antisite defects, an attribution supported by density functional theory calculations. It was found that the bandgap enhancement significantly changes the electronic and magnetic phases in the oxygen-vacancy-induced two-dimensional electron gas at the interface between amorphous LaAlO 3 (LAO) and STO. This opens an attractive path to tailor electronic, magnetic and optical properties of STO-based oxide interface systems under intensive focus in the electronics community. Meanwhile, our study provides key insight into the origin of the fundamental issue that STO thin films are difficult to convert into metals by oxygen vacancy doping.
Physical Review X, 2013
The relative importance of atomic defects and electron transfer in explaining conductivity at the... more The relative importance of atomic defects and electron transfer in explaining conductivity at the crystalline LaAlO 3 =SrTiO 3 interface has been a topic of debate. Metallic interfaces with similar electronic properties produced by amorphous oxide overlayers on SrTiO 3 [Y. Chen et al., Nano Lett. 11, 3774 (2011); S. W. Lee et al., Nano Lett. 12, 4775 (2012)] have called in question the original polarization catastrophe model [N. Nakagawa et al., Nature Mater. 5, 204 (2006)]. We resolve the issue by a comprehensive comparison of (100)-oriented SrTiO 3 substrates with crystalline and amorphous overlayers of LaAlO 3 of different thicknesses prepared under different oxygen pressures. For both types of overlayers, there is a critical thickness for the appearance of conductivity, but its value is always 4 unit cells (around 1.6 nm) for the oxygen-annealed crystalline case, whereas in the amorphous case, the critical thickness could be varied in the range 0.5 to 6 nm according to the deposition conditions. Subsequent ion milling of the overlayer restores the insulating state for the oxygen-annealed crystalline heterostructures but not for the amorphous ones. Oxygen post-annealing removes the oxygen vacancies, and the interfaces become insulating in the amorphous case. However, the interfaces with a crystalline overlayer remain conducting with reduced carrier density. These results demonstrate that oxygen vacancies are the dominant source of mobile carriers when the LaAlO 3 overlayer is amorphous, while both oxygen vacancies and polarization catastrophe contribute to the interface conductivity in unannealed crystalline LaAlO 3 =SrTiO 3 heterostructures, and the polarization catastrophe alone accounts for the conductivity in oxygen-annealed crystalline LaAlO 3 =SrTiO 3 heterostructures. Furthermore, we find that the crystallinity of the LaAlO 3 layer is crucial for the polarization catastrophe mechanism in the case of crystalline LaAlO 3 overlayers.
Journal of Physics D: Applied Physics, 2014
The magnetic properties of L1 0 -FePt/Fe exchange-coupled composite (ECC) nanodots were investiga... more The magnetic properties of L1 0 -FePt/Fe exchange-coupled composite (ECC) nanodots were investigated by Hall effect measurement due to its high sensitivity. The FePt/Fe nanodots showed coercivity changing irregularly with respect to the Fe thickness, which deviated from the simulated results, where the coercivity of the ECC dots reduced with increasing Fe soft layer thickness. It was found that the edge damage induced by ion milling affected the coercivity of the nanodots significantly and the magnetization reversal mechanism. Domain wall nucleation and propagation was revealed in the FePt single domain dots. In the FePt/Fe ECC nanodots, the magnetization gradually reversed from the out-of-plane direction towards the in-plane direction before switching to the reverse out-of-plane direction. The critical size of single magnetic domains in the FePt/Fe ECC nanodots increased with increasing Fe soft layer thickness.
Thin Solid Films, 2008
Multi-layer nanocomposite structures of Ta/Ru/CoCr 1 /FeCoTaCr(soft magnetic layer)/CoCr 2 /CoCrP... more Multi-layer nanocomposite structures of Ta/Ru/CoCr 1 /FeCoTaCr(soft magnetic layer)/CoCr 2 /CoCrPt-SiO 2 (hard magnetic layer or recording layer)/C and Ta/Ru/CoCr 1 /CoCrPt-SiO 2 /CoCr 2 /FeCoTaCr/C were proposed. This exchange coupled composite (ECC) media consisting of hard/ soft stacked magnetic layers were promising in improving the writability of perpendicular magnetic recording media. A small CoCrPt c-axis orientation dispersion of about 3°was achieved with the optimized sputter conditions. The CoCrPt-SiO 2 grains were well segregated by SiO 2 at grain boundaries. The macro-magnetic properties showed that the stacked magnetic grains switched in a coherent mode and that switching field decreased with increasing the thickness of the soft magnetic layer.
Surface Review and Letters, 2008
ABSTRACT Co-ferrite films were prepared using pulsed laser deposition with both post-annealing an... more ABSTRACT Co-ferrite films were prepared using pulsed laser deposition with both post-annealing and in situ heating processes. Magnetic properties of these films were studied in the function of temperature, film thickness, and substrate. The films using post-annealing processes exhibited isotropic microstructure, and the coercivity showed no obvious magnetic anisotropy and no strong dependence on film thickness. Co-ferrite films using in situ heating exhibited (111) highly textured structure and possessed perpendicular anisotropy as well as large coercivity. The preferential texture and magnetic anisotropy were closely associated with substrate temperature and thickness. Perpendicular Hc over 12.5 kOe was obtained in the 33 nm Co-ferrite film deposited on single crystal quartz substrate at 550°C. The high coercivity and perpendicular coercivity may be attributed to the nanocrystalline grain, textured orientation, and large residual strain in these films since large residual strain may induce strong stress anisotropy.
Surface Review and Letters, 2001
The unique electron field emission properties of carbon nanotube films are promising for flat pan... more The unique electron field emission properties of carbon nanotube films are promising for flat panel display application as the cold cathode. In this work, the multiwall carbon nanotube (1010 2 nm in diameter, and 10 3 10 4 nm in length) film arrays (the pixel size: 50 μm ...