We measure the magnetoresistance of Ta/NiFe/FeMn/NiFe/Co(t Co )/Cu/Co(t Co )/NiFe spin-valves as ... more We measure the magnetoresistance of Ta/NiFe/FeMn/NiFe/Co(t Co )/Cu/Co(t Co )/NiFe spin-valves as a function of field strength and direction. The exchange bias H EB and the giant magnetoresistance depend on the Co thickness t Co ; and the resistance shows rotational hysteresis when the external field is close to compensating H EB : A single-domain model reproduces the angular dependence of the magnetoresistance in regions without hysteresis, yields H EB values in agreement with MOKE, and allows one to precisely determine the interlayer coupling and uniaxial anisotropies although they are much weaker than H EB : r
ABSTRACT The novel BioRef reflectometer currently under construction in neutron guidehall-I of th... more ABSTRACT The novel BioRef reflectometer currently under construction in neutron guidehall-I of the BER-2 reactor at the Helmholtz Centre Berlin is designed for applications in soft matter science at solid-liquid interfaces especially under dynamic conditions. In addition to a time-of-flight (TOF) approach to be realised with three choppers in order to adapt resolutions and wavelength bands to the needs of specific applications, the instrument will be equipped with an in-situ infrared (IR) spectrometer operating in its surface sensitive ATR-geometry for simultaneous spectroscopic and conformational studies. The instrument will become operational in 2010.
Vortex domain walls poses chirality or 'handedness' which can be exploited to act as memo... more Vortex domain walls poses chirality or 'handedness' which can be exploited to act as memory units by changing their polarity with electric field or driving/manupulating the vortex itself by electric currents in multiferroics. Recently, domain walls formed by one dimensional array of vortex-like structures have been theoretically predicted to exist in disordered rare-earth helical magnets with topological defects. Here, in this report, we have used a combination of two rare-earth metals, e.g. superlattice that leads to long range magnetic order despite their competing anisotropies along the out-of-plane (Er) and in-plane (Tb) directions. Probing the vertically correlated magnetic structures by off-specular polarized neutron scattering we confirm the existence of such magnetic vortex-like domains associated with magnetic helical ordering within the Er layers. The vortex-like structures are predicted to have opposite chirality, side-by-side, and are fairly unaffected by the int...
In optoelectronic devices based on quantum dot arrays, thin nanolayers of gold are preferred as s... more In optoelectronic devices based on quantum dot arrays, thin nanolayers of gold are preferred as stable metal contacts and for connecting recombination centers. The optimal morphology requirements are uniform arrays with precisely controlled positions and sizes over a large area with long range ordering since this strongly affects device performance. To understand the development of gold layer nanomorphology, the detailed mechanism of structure formation are probed with time-resolved grazing incidence small-angle X-ray scattering (GISAXS) during gold sputter deposition. Gold is sputtered on a CdSe quantum dot array with a characteristic quantum dot spacing of ≈7 nm. In the initial stages of gold nanostructure growth, a preferential deposition of gold on top of quantum dots occurs. Thus, the quantum dots act as nucleation sites for gold growth. In later stages, the gold nanoparticles surrounding the quantum dots undergo a coarsening to form a complete layer comprised of gold-dot clust...
... a 12.0 0.5 150 0.4 b 12.0 0.5 150 0.4 c 12.0 0.5 150 0.4 d 12.0 0.5 150 0.4 AMITESH PAUL AND ... more ... a 12.0 0.5 150 0.4 b 12.0 0.5 150 0.4 c 12.0 0.5 150 0.4 d 12.0 0.5 150 0.4 AMITESH PAUL AND GS LODHA PHYSICAL REVIEW B 65 245416 245416-6 Page 7. crease in the lateral correlation lengths with increasing number of bilayers in Si/Mo multilayers systems. ...
Industrial Applications of the Mössbauer Effect, 2002
Fe/Tb multilayers have been prepared which exhibit significant perpendicular magnetic anisotropy ... more Fe/Tb multilayers have been prepared which exhibit significant perpendicular magnetic anisotropy (PMA) even at RT. The effect of systematic variation in the interface roughness on PMA has been studied in these multilayers which were deposited simultaneously on a set of float glass substrates prepared with varying root mean square surface roughness. The amount of intermixing at the interfaces and uncorrelated part of the interface roughness of different multilayers remain similar, thus allowing one to separate out the effect of the correlated interface roughness only. X-ray reflectivity and conversion electron Mössbauer spectroscopy and SQUID magnetometry was used to characterise the systems. PMA was found to depend weakly on the correlated interface roughness.
Polarized neutron reflectivity (PNR) studies have been performed on granular multilayers [Co80Fe2... more Polarized neutron reflectivity (PNR) studies have been performed on granular multilayers [Co80Fe20(tn)/Al2O3 (3nm)]m. For a non-percolating superferromagnetic (SFM) sample (tn=1.3nm) no spin-flip (SF) scattering was observed, measured at various points of the hysteresis, confirming that the magnetization reversal is achieved merely by domain nucleation and growth. For a percolated sample (tn=1.6nm) an oscillating magnetization depth profile from FeCo layer to FeCo layer along the multilayer stack was observed around the coercive field. The competition between long-range dipolar, short-range Néel and external field interactions apparently give rise to such a modulated magnetization depth profile in these magnetic multilayers.
We report on a polarized neutron reflectivity investigation of the magnetization in Ge-based dilu... more We report on a polarized neutron reflectivity investigation of the magnetization in Ge-based dilute magnetic semiconductors. We could observe a net magnetization from the splitting of the non-spin flip reflectivity patterns, which measure the magnetic moment parallel and antiparallel to the applied field. This contrast is visible at 50 K, at remanence and it is pronounced at higher fields even at 250 K for an inhomogeneous specimen. For a homogeneous sample the magnetic variation is visible only at 50 K and above 1.0 kOe. Thus, polarized neutron reflectivity can be a useful tool for investigating the magnetism in homogeneous and inhomogeneous thin film magnetic semiconductors. r
We have investigated the impact of out-of-plane ferromagnetic (FM) anisotropy (which can be coinc... more We have investigated the impact of out-of-plane ferromagnetic (FM) anisotropy (which can be coincident with the direction of unidirectional anisotropy), where antiferromagnetic (AF) anisotropy is along the film plane. This provides a platform for non-collinear exchange coupling in an archetypal exchange coupled system in an unconventional way. We probe the in-plane magnetization by the depth-sensitive vector magnetometry technique. The experimental findings reveal a magnetization reversal (i) that is symmetric for both the branches of the hysteresis loop, (ii) that is characterized by vertically correlated domains associated with a strong transverse component of magnetization and (iii) that remains untrained (suppression of trained state) with field cycling. This scenario has been compared with in-plane magnetization reversal for a conventional in-plane unidirectional anisotropic case in the same system that shows usual asymmetric reversal and training for vertically uncorrelated domains. We explain the above observations for the out-of-plane case in terms of inhomogeneous magnetic states due to competing perpendicular anisotropies that result in non-collinear FM-AF coupling. This study provides direct evidence for the vertical correlation of domains mediated by out-of-plane exchange coupling.
The adsorption behavior of neurotransmitter biomolecule, glutamate, on terminal poly-(allylamine)... more The adsorption behavior of neurotransmitter biomolecule, glutamate, on terminal poly-(allylamine)hydrochloride (PAH) polyelectrolyte multilayer is compared with its adsorption on a terminal poly(styrenesulfonate) (PSS) polyelectrolyte multilayer. Using X-ray and neutron reflectivity experiments, the internal structure of such a supramolecular film has been revealed with high resolution and the volume fraction of the adsorbed glutamate is determined. It has been shown that the glutamate binds only to the terminal PAH multilayer. Multiple attenuated total reflection infrared spectroscopy indicates that glutamate is electrostatically physisorbed on PAH surface in the zwitterionic form. Index matching neutron experiments have been done where the scattering length density of the solvent is varied, by changing the ratio of heavy water and light water, until it completely matches with that of the polyelectrolyte layer. The resulting absorption of the glutamic acid leads to changes in scattering profile which are analyzed and it is seen that the adsorption is restricted only to the surface layers. On the other hand, terminal poly(styrenesulfonate) multilayers show resistance toward glutamate. Such repulsion and adsorption between the neurotransmitter and polyelectrolytes could be potentially used in a variety of medicinal applications. Figure 1. Structures of the chemicals used at pH 7 (a) poly-(allylamine)hydrochloride (PAH), (b) poly(styrenesulfonate) (PSS), and (c) glutamate. Article pubs.acs.org/JPCB
Depth-sensitive polarized neutron scattering in specular and off-specular mode has recently revea... more Depth-sensitive polarized neutron scattering in specular and off-specular mode has recently revealed that perpendicular field cycling brings about a modification in the interfacial magnetization of a trained exchange coupled interface. We show here by various model fits to our neutron reflectivity data that a restoration of the untrained state is not possible in the case of our polycrystalline multilayer specimen. This is due to the magnetic moment at the interface induced only after perpendicular field cycling, changing the initial field-cooled state.
Polarized neutron reflectivity ͑PNR͒ and magnetometry studies have been performed on the granular... more Polarized neutron reflectivity ͑PNR͒ and magnetometry studies have been performed on the granular multilayer ͓Co 80 Fe 20 ͑1.3 nm͒ /Al 2 O 3 ͑3 nm͔͒ 10 . Due to strong interparticle interactions, a collective superferromagnetic state is encountered. Cole-Cole plots drawn from the complex ac susceptibility are measured as functions of frequency, temperature, and field amplitudes that hint at the relaxation, creep, sliding, and switching regimes of pinned domain walls that are in close agreement with results obtained from simulations. Very slow switching with exponential relaxation under near-coercive fields is confirmed by PNR measurements. The complete absence of spin-flip scattering confirms that the magnetization reversal is achieved merely by domain nucleation and growth.
We show that the extent of training in exchange-biased systems can be strongly influenced by the ... more We show that the extent of training in exchange-biased systems can be strongly influenced by the field cooling procedure. This phenomenon is revealed by comparing the behavior of the system in two different magnetic configurations: cooling the system in a remanent state without an external magnetic field ͑state I͒ results in a suppression of the training effect, whereas the same sample being field cooled ͑state II͒ exhibits a clear training response. Interestingly, by cooling the sample in a critical field H FC close to the coercive field H C of the ferromagnet, we find a peculiar situation with a coexistence of states I and II. By using a depth-sensitive polarized neutron reflectivity technique, we can establish a clear correlation of the reversal mechanism with either the untrained or trained state.
... Amitesh Paul,* Emmanuel Kentzinger, Ulrich Rücker, Daniel E. Bürgler, and Thomas Brückel Inst... more ... Amitesh Paul,* Emmanuel Kentzinger, Ulrich Rücker, Daniel E. Bürgler, and Thomas Brückel Institut für Festkörperforschung, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany Received 8 September 2005; revised manuscript ... The latter is proportional to sin ; see Fig. ...
We measure the magnetoresistance of Ta/NiFe/FeMn/NiFe/Co(t Co )/Cu/Co(t Co )/NiFe spin-valves as ... more We measure the magnetoresistance of Ta/NiFe/FeMn/NiFe/Co(t Co )/Cu/Co(t Co )/NiFe spin-valves as a function of field strength and direction. The exchange bias H EB and the giant magnetoresistance depend on the Co thickness t Co ; and the resistance shows rotational hysteresis when the external field is close to compensating H EB : A single-domain model reproduces the angular dependence of the magnetoresistance in regions without hysteresis, yields H EB values in agreement with MOKE, and allows one to precisely determine the interlayer coupling and uniaxial anisotropies although they are much weaker than H EB : r
ABSTRACT The novel BioRef reflectometer currently under construction in neutron guidehall-I of th... more ABSTRACT The novel BioRef reflectometer currently under construction in neutron guidehall-I of the BER-2 reactor at the Helmholtz Centre Berlin is designed for applications in soft matter science at solid-liquid interfaces especially under dynamic conditions. In addition to a time-of-flight (TOF) approach to be realised with three choppers in order to adapt resolutions and wavelength bands to the needs of specific applications, the instrument will be equipped with an in-situ infrared (IR) spectrometer operating in its surface sensitive ATR-geometry for simultaneous spectroscopic and conformational studies. The instrument will become operational in 2010.
Vortex domain walls poses chirality or 'handedness' which can be exploited to act as memo... more Vortex domain walls poses chirality or 'handedness' which can be exploited to act as memory units by changing their polarity with electric field or driving/manupulating the vortex itself by electric currents in multiferroics. Recently, domain walls formed by one dimensional array of vortex-like structures have been theoretically predicted to exist in disordered rare-earth helical magnets with topological defects. Here, in this report, we have used a combination of two rare-earth metals, e.g. superlattice that leads to long range magnetic order despite their competing anisotropies along the out-of-plane (Er) and in-plane (Tb) directions. Probing the vertically correlated magnetic structures by off-specular polarized neutron scattering we confirm the existence of such magnetic vortex-like domains associated with magnetic helical ordering within the Er layers. The vortex-like structures are predicted to have opposite chirality, side-by-side, and are fairly unaffected by the int...
In optoelectronic devices based on quantum dot arrays, thin nanolayers of gold are preferred as s... more In optoelectronic devices based on quantum dot arrays, thin nanolayers of gold are preferred as stable metal contacts and for connecting recombination centers. The optimal morphology requirements are uniform arrays with precisely controlled positions and sizes over a large area with long range ordering since this strongly affects device performance. To understand the development of gold layer nanomorphology, the detailed mechanism of structure formation are probed with time-resolved grazing incidence small-angle X-ray scattering (GISAXS) during gold sputter deposition. Gold is sputtered on a CdSe quantum dot array with a characteristic quantum dot spacing of ≈7 nm. In the initial stages of gold nanostructure growth, a preferential deposition of gold on top of quantum dots occurs. Thus, the quantum dots act as nucleation sites for gold growth. In later stages, the gold nanoparticles surrounding the quantum dots undergo a coarsening to form a complete layer comprised of gold-dot clust...
... a 12.0 0.5 150 0.4 b 12.0 0.5 150 0.4 c 12.0 0.5 150 0.4 d 12.0 0.5 150 0.4 AMITESH PAUL AND ... more ... a 12.0 0.5 150 0.4 b 12.0 0.5 150 0.4 c 12.0 0.5 150 0.4 d 12.0 0.5 150 0.4 AMITESH PAUL AND GS LODHA PHYSICAL REVIEW B 65 245416 245416-6 Page 7. crease in the lateral correlation lengths with increasing number of bilayers in Si/Mo multilayers systems. ...
Industrial Applications of the Mössbauer Effect, 2002
Fe/Tb multilayers have been prepared which exhibit significant perpendicular magnetic anisotropy ... more Fe/Tb multilayers have been prepared which exhibit significant perpendicular magnetic anisotropy (PMA) even at RT. The effect of systematic variation in the interface roughness on PMA has been studied in these multilayers which were deposited simultaneously on a set of float glass substrates prepared with varying root mean square surface roughness. The amount of intermixing at the interfaces and uncorrelated part of the interface roughness of different multilayers remain similar, thus allowing one to separate out the effect of the correlated interface roughness only. X-ray reflectivity and conversion electron Mössbauer spectroscopy and SQUID magnetometry was used to characterise the systems. PMA was found to depend weakly on the correlated interface roughness.
Polarized neutron reflectivity (PNR) studies have been performed on granular multilayers [Co80Fe2... more Polarized neutron reflectivity (PNR) studies have been performed on granular multilayers [Co80Fe20(tn)/Al2O3 (3nm)]m. For a non-percolating superferromagnetic (SFM) sample (tn=1.3nm) no spin-flip (SF) scattering was observed, measured at various points of the hysteresis, confirming that the magnetization reversal is achieved merely by domain nucleation and growth. For a percolated sample (tn=1.6nm) an oscillating magnetization depth profile from FeCo layer to FeCo layer along the multilayer stack was observed around the coercive field. The competition between long-range dipolar, short-range Néel and external field interactions apparently give rise to such a modulated magnetization depth profile in these magnetic multilayers.
We report on a polarized neutron reflectivity investigation of the magnetization in Ge-based dilu... more We report on a polarized neutron reflectivity investigation of the magnetization in Ge-based dilute magnetic semiconductors. We could observe a net magnetization from the splitting of the non-spin flip reflectivity patterns, which measure the magnetic moment parallel and antiparallel to the applied field. This contrast is visible at 50 K, at remanence and it is pronounced at higher fields even at 250 K for an inhomogeneous specimen. For a homogeneous sample the magnetic variation is visible only at 50 K and above 1.0 kOe. Thus, polarized neutron reflectivity can be a useful tool for investigating the magnetism in homogeneous and inhomogeneous thin film magnetic semiconductors. r
We have investigated the impact of out-of-plane ferromagnetic (FM) anisotropy (which can be coinc... more We have investigated the impact of out-of-plane ferromagnetic (FM) anisotropy (which can be coincident with the direction of unidirectional anisotropy), where antiferromagnetic (AF) anisotropy is along the film plane. This provides a platform for non-collinear exchange coupling in an archetypal exchange coupled system in an unconventional way. We probe the in-plane magnetization by the depth-sensitive vector magnetometry technique. The experimental findings reveal a magnetization reversal (i) that is symmetric for both the branches of the hysteresis loop, (ii) that is characterized by vertically correlated domains associated with a strong transverse component of magnetization and (iii) that remains untrained (suppression of trained state) with field cycling. This scenario has been compared with in-plane magnetization reversal for a conventional in-plane unidirectional anisotropic case in the same system that shows usual asymmetric reversal and training for vertically uncorrelated domains. We explain the above observations for the out-of-plane case in terms of inhomogeneous magnetic states due to competing perpendicular anisotropies that result in non-collinear FM-AF coupling. This study provides direct evidence for the vertical correlation of domains mediated by out-of-plane exchange coupling.
The adsorption behavior of neurotransmitter biomolecule, glutamate, on terminal poly-(allylamine)... more The adsorption behavior of neurotransmitter biomolecule, glutamate, on terminal poly-(allylamine)hydrochloride (PAH) polyelectrolyte multilayer is compared with its adsorption on a terminal poly(styrenesulfonate) (PSS) polyelectrolyte multilayer. Using X-ray and neutron reflectivity experiments, the internal structure of such a supramolecular film has been revealed with high resolution and the volume fraction of the adsorbed glutamate is determined. It has been shown that the glutamate binds only to the terminal PAH multilayer. Multiple attenuated total reflection infrared spectroscopy indicates that glutamate is electrostatically physisorbed on PAH surface in the zwitterionic form. Index matching neutron experiments have been done where the scattering length density of the solvent is varied, by changing the ratio of heavy water and light water, until it completely matches with that of the polyelectrolyte layer. The resulting absorption of the glutamic acid leads to changes in scattering profile which are analyzed and it is seen that the adsorption is restricted only to the surface layers. On the other hand, terminal poly(styrenesulfonate) multilayers show resistance toward glutamate. Such repulsion and adsorption between the neurotransmitter and polyelectrolytes could be potentially used in a variety of medicinal applications. Figure 1. Structures of the chemicals used at pH 7 (a) poly-(allylamine)hydrochloride (PAH), (b) poly(styrenesulfonate) (PSS), and (c) glutamate. Article pubs.acs.org/JPCB
Depth-sensitive polarized neutron scattering in specular and off-specular mode has recently revea... more Depth-sensitive polarized neutron scattering in specular and off-specular mode has recently revealed that perpendicular field cycling brings about a modification in the interfacial magnetization of a trained exchange coupled interface. We show here by various model fits to our neutron reflectivity data that a restoration of the untrained state is not possible in the case of our polycrystalline multilayer specimen. This is due to the magnetic moment at the interface induced only after perpendicular field cycling, changing the initial field-cooled state.
Polarized neutron reflectivity ͑PNR͒ and magnetometry studies have been performed on the granular... more Polarized neutron reflectivity ͑PNR͒ and magnetometry studies have been performed on the granular multilayer ͓Co 80 Fe 20 ͑1.3 nm͒ /Al 2 O 3 ͑3 nm͔͒ 10 . Due to strong interparticle interactions, a collective superferromagnetic state is encountered. Cole-Cole plots drawn from the complex ac susceptibility are measured as functions of frequency, temperature, and field amplitudes that hint at the relaxation, creep, sliding, and switching regimes of pinned domain walls that are in close agreement with results obtained from simulations. Very slow switching with exponential relaxation under near-coercive fields is confirmed by PNR measurements. The complete absence of spin-flip scattering confirms that the magnetization reversal is achieved merely by domain nucleation and growth.
We show that the extent of training in exchange-biased systems can be strongly influenced by the ... more We show that the extent of training in exchange-biased systems can be strongly influenced by the field cooling procedure. This phenomenon is revealed by comparing the behavior of the system in two different magnetic configurations: cooling the system in a remanent state without an external magnetic field ͑state I͒ results in a suppression of the training effect, whereas the same sample being field cooled ͑state II͒ exhibits a clear training response. Interestingly, by cooling the sample in a critical field H FC close to the coercive field H C of the ferromagnet, we find a peculiar situation with a coexistence of states I and II. By using a depth-sensitive polarized neutron reflectivity technique, we can establish a clear correlation of the reversal mechanism with either the untrained or trained state.
... Amitesh Paul,* Emmanuel Kentzinger, Ulrich Rücker, Daniel E. Bürgler, and Thomas Brückel Inst... more ... Amitesh Paul,* Emmanuel Kentzinger, Ulrich Rücker, Daniel E. Bürgler, and Thomas Brückel Institut für Festkörperforschung, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany Received 8 September 2005; revised manuscript ... The latter is proportional to sin ; see Fig. ...
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