martin asmat | Colorado State University (original) (raw)
Papers by martin asmat
Journal of Magnetism and Magnetic Materials, 2008
Pure and Zn 1-x Co x O nanoparticles have been synthesized by a simple sol-gel method at low temp... more Pure and Zn 1-x Co x O nanoparticles have been synthesized by a simple sol-gel method at low temperature where neither a chelating agent nor subsequent annealing was required. The effect of Cobalt atomic fraction, 'x' ≤ 0.0625, on the structural and magnetic properties of the doped ZnO powders was evaluated. X-ray diffraction and Fourier-transform infrared spectroscopy analyses evidenced the exclusive formation of the ZnO-wurtzite structure; no isolated Co-phases were detected. The linear dependence of cell parameters a and c with 'x', suggested the actual replacement of Zn by Co ions in the oxide lattice. Micro Raman spectroscopy measurements showed a band centered at 534cm-1 , which can be assigned to a local vibrational mode related to Co species, in addition to the normal modes associated with wurtzite. The intensity and broadening of this band at 534 cm-1 were enhanced by increasing 'x'. In turn, the other bands corresponding to A 1 (E 2 , E 1) and E 2 High modes were red shifted at higher Co contents. Room-temperature magnetization measurements revealed the paramagnetic behavior of the Co-doped ZnO nanoparticles.
Applied Physics A: Materials Science & Processing, 2003
ABSTRACT Owing to its parallel image acquisition, photoemission electron microscopy is well suite... more ABSTRACT Owing to its parallel image acquisition, photoemission electron microscopy is well suited for real-time observation of fast processes on surfaces. Pulsed excitation sources like synchrotron radiation or lasers, fast electric pulsers for the study of magnetic switching, and/or time-resolved detection can be utilised. A standard approach also being used in light optical imaging is stroboscopic illumination of a periodic (or quasi-periodic) process. Using this technique, the time dependence of the magnetic field in a pulsed microstrip line has been imaged in real time exploiting Lorentz-type contrast. Similarly, the corresponding field-induced changes in the magnetisation of cobalt microstructures deposited on the microstrip line have been observed exploiting magnetic X-ray circular dichroism as a contrast mechanism. The experiment has been performed at the UE 56/1-PGM at BESSY II (Berlin) in the single-bunch mode.
Bulletin of the American Physical Society, Oct 19, 2013
magnetic antivortex (AV) state is a topological configuration that is expected to exhibit interes... more magnetic antivortex (AV) state is a topological configuration that is expected to exhibit interesting physical behavior and it may also be useful for applications. Recent work showed that magnetic antivortices can be created in pound-key-like nanostructures via a two-step magnetic field procedure [1]. In this procedure, magnetic hysteresis measurements are important to predict the field values at which the AV's will form. The Magneto-Optical Kerr Effect (MOKE) is widely used to make magnetic hysteresis measurements, especially for magnetic nanostructures since measurements can be made on individual structures or small arrays. MOKE measurements were made on a series of micron-sized pound-key-like structures made of Permalloy to examine how the reversal process and critical fields depend on the details of the structure shape and size. Hysteresis loops were obtained with a high signal-noise ratio even though the amount of magnetic material and consequently the Kerr rotation angle were small (less than 0.02 mrad). Subsequent magnetic force microscopy imaging of the structures showed successful AV formation at the fields predicted from the MOKE measurements.
Magnetic vortices are fundamental magnetic structures that form in patterned ferromagnetic materi... more Magnetic vortices are fundamental magnetic structures that form in patterned ferromagnetic materials. The study of magnetic vortices is an active field of research at present, in part because of the potential for new technologies. In addition to vortices, so called antivortex states have been found in some particular geometries such as four connected rings and cross-like nanomagnets. Antivortices may be useful for nonvolatile data storage applications, and they are also expected to show unusual transport properties in an applied magnetic field, for example, a ``topological'' Hall effect. In order to make use of magnetic antivortices, it is important to first understand how to stabilize systems that contain only a single antivortex. Micromagnetic calculations have been performed with OOMMF and LLG software to explore how the geometry of the structure affects the formation and stability of the antivortex state and whether the field history can be used to reliably select the st...
Journal of Applied Physics, 2015
In this work, we assess the effects of field history and structure shape on the formation of magn... more In this work, we assess the effects of field history and structure shape on the formation of magnetic antivortices. The magnetic reversal process was investigated for a series of patterned micron-sized permalloy pound-key structures with varying degrees of asymmetry using magneto-optical Kerr effect hysteresis measurements combined with magnetic force microscopy. The largest number of antivortices was observed in the structures with the highest level of structure asymmetry, which also show an intermediate state in the hysteresis loop. A significant enhancement of the antivortex formation rate—from 5% to almost 80%—was achieved by adjusting the structure dimensions. Images of the magnetic states obtained at various points in the hysteresis loop show that the highest rate of antivortex formation occurs near the coercive field, also the nucleation field, and that the antivortex formation is also sensitive to the angle of the applied field, where the highest antivortex formation rate is...
The magnetic vortex state has received increasing attention during recent years however its topol... more The magnetic vortex state has received increasing attention during recent years however its topological counterpart, the magnetic antivortex (AV), has not been explored with the same intensity. The antivortex spin configuration may have some advantages over vortices, especially for channeling spin waves emitted from the dynamic core reversal. In order to study the properties of antivortices it is necessary to have geometries and procedures that reliable stabilize antivortices, however this task is more challenging than forming a vortex. In this work, we use micromagnetic simulations to show that pound-key-like structures can be used to form stable AV's and to explore the role of shape anisotropy in the AV formation. Our results are compared to Magneto Optical Kerr Effect (MOKE) hysteresis measurements and magnetic force microscopy images. The simulations show that the AV nucleation field depends on the sample global and relative dimensions and these results are in good agreement with the experiments. Acknowledgement: This work was supported by the NSF.
magnetic antivortex (AV) state is a topological configuration that is expected to exhibit interes... more magnetic antivortex (AV) state is a topological configuration that is expected to exhibit interesting physical behavior and it may also be useful for applications. Recent work showed that magnetic antivortices can be created in pound-key-like nanostructures via a two-step magnetic field procedure [1]. In this procedure, magnetic hysteresis measurements are important to predict the field values at which the AV's will form. The Magneto-Optical Kerr Effect (MOKE) is widely used to make magnetic hysteresis measurements, especially for magnetic nanostructures since measurements can be made on individual structures or small arrays. MOKE measurements were made on a series of micron-sized pound-key-like structures made of Permalloy to examine how the reversal process and critical fields depend on the details of the structure shape and size. Hysteresis loops were obtained with a high signal-noise ratio even though the amount of magnetic material and consequently the Kerr rotation angle were small (less than 0.02 mrad). Subsequent magnetic force microscopy imaging of the structures showed successful AV formation at the fields predicted from the MOKE measurements.
Journal of Applied Physics, 2015
Magnetostatic interactions between vortices in closely spaced planar structures are important for... more Magnetostatic interactions between vortices in closely spaced planar structures are important for applications including vortex-based magnonic crystals and spin torque oscillator networks. Analytical theories that include magnetostatic interaction effects have been proposed but have not yet been rigorously tested. Here we compare micromagnetic simulations of the dynamics of magnetic vortices confined in three disks in an equilateral triangle configuration to analytical theories that include coupling. Micromagnetic simulations show that the magnetostatic coupling between the disks leads to splitting of the gyrotropic resonance into three modes and that the frequency splitting increases with decreasing separation. The temporal profiles of the magnetization depend on the vortex polarities and chiralities, however, the frequencies depend only on the polarity combinations and will fall into one of two categories: all polarities equal or one polarity opposite to the others, where the latter leads to a larger frequency splitting. Although the magnitude of the splitting observed in the simulations is larger than what is expected based on purely dipolar interactions, a simple analytical model that assumes dipole-dipole coupling captures the functional form of the frequency splitting and the motion patterns just as well as more complex models.
This work presents a comparative study on optical and electrical properties of CuAlO2 thin films ... more This work presents a comparative study on optical and electrical properties of CuAlO2 thin films on sapphire (0001) substrates deposited with two different growth conditions using reactive RF-magnetron sputtering technique from metallic Cu and Al targets. CuAlO2 is a very promising material for transparent electronic applications, it is intended that comparison of results obtained from both approaches, could lead to optimization and control of the physical properties of this material, namely its electrical conductivity and optical transmittance. All samples were heat treated at 1100°C using rapid thermal annealing with varying time and rate of cooling. The effect of sputtering conditions and different annealing time on phase formation and evolution is studied with X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that for most of the samples CuAlO2 phase is formed after 60 min of annealing time, but secondary phases were also present that depend on the deposition conditions. However, pure CuAlO2 phase was obtained for annealed CuO on sapphire films with annealing time of 60 min. The optical properties obtained from UV-Visible spectroscopic measurement reveals indirect and direct optical band gaps for CuAlO2 films and were found to be 2.58 and 3.72 eV respectively. The films show a transmittance of about 60% in the visible range. Hall effect measurements indicate p-type conductivity. Van der Pauw technique was used to measure resistivity of the samples. The highest electrical conductivity and charge carrier concentration obtained were of 1.01x10-1S.cm -1 and 3.63 x1018 cm-3 respectively.
Journal of Magnetism and Magnetic Materials, 2008
Pure and Zn 1-x Co x O nanoparticles have been synthesized by a simple sol-gel method at low temp... more Pure and Zn 1-x Co x O nanoparticles have been synthesized by a simple sol-gel method at low temperature where neither a chelating agent nor subsequent annealing was required. The effect of Cobalt atomic fraction, 'x' ≤ 0.0625, on the structural and magnetic properties of the doped ZnO powders was evaluated. X-ray diffraction and Fourier-transform infrared spectroscopy analyses evidenced the exclusive formation of the ZnO-wurtzite structure; no isolated Co-phases were detected. The linear dependence of cell parameters a and c with 'x', suggested the actual replacement of Zn by Co ions in the oxide lattice. Micro Raman spectroscopy measurements showed a band centered at 534cm-1 , which can be assigned to a local vibrational mode related to Co species, in addition to the normal modes associated with wurtzite. The intensity and broadening of this band at 534 cm-1 were enhanced by increasing 'x'. In turn, the other bands corresponding to A 1 (E 2 , E 1) and E 2 High modes were red shifted at higher Co contents. Room-temperature magnetization measurements revealed the paramagnetic behavior of the Co-doped ZnO nanoparticles.
Applied Physics A: Materials Science & Processing, 2003
ABSTRACT Owing to its parallel image acquisition, photoemission electron microscopy is well suite... more ABSTRACT Owing to its parallel image acquisition, photoemission electron microscopy is well suited for real-time observation of fast processes on surfaces. Pulsed excitation sources like synchrotron radiation or lasers, fast electric pulsers for the study of magnetic switching, and/or time-resolved detection can be utilised. A standard approach also being used in light optical imaging is stroboscopic illumination of a periodic (or quasi-periodic) process. Using this technique, the time dependence of the magnetic field in a pulsed microstrip line has been imaged in real time exploiting Lorentz-type contrast. Similarly, the corresponding field-induced changes in the magnetisation of cobalt microstructures deposited on the microstrip line have been observed exploiting magnetic X-ray circular dichroism as a contrast mechanism. The experiment has been performed at the UE 56/1-PGM at BESSY II (Berlin) in the single-bunch mode.
Bulletin of the American Physical Society, Oct 19, 2013
magnetic antivortex (AV) state is a topological configuration that is expected to exhibit interes... more magnetic antivortex (AV) state is a topological configuration that is expected to exhibit interesting physical behavior and it may also be useful for applications. Recent work showed that magnetic antivortices can be created in pound-key-like nanostructures via a two-step magnetic field procedure [1]. In this procedure, magnetic hysteresis measurements are important to predict the field values at which the AV's will form. The Magneto-Optical Kerr Effect (MOKE) is widely used to make magnetic hysteresis measurements, especially for magnetic nanostructures since measurements can be made on individual structures or small arrays. MOKE measurements were made on a series of micron-sized pound-key-like structures made of Permalloy to examine how the reversal process and critical fields depend on the details of the structure shape and size. Hysteresis loops were obtained with a high signal-noise ratio even though the amount of magnetic material and consequently the Kerr rotation angle were small (less than 0.02 mrad). Subsequent magnetic force microscopy imaging of the structures showed successful AV formation at the fields predicted from the MOKE measurements.
Magnetic vortices are fundamental magnetic structures that form in patterned ferromagnetic materi... more Magnetic vortices are fundamental magnetic structures that form in patterned ferromagnetic materials. The study of magnetic vortices is an active field of research at present, in part because of the potential for new technologies. In addition to vortices, so called antivortex states have been found in some particular geometries such as four connected rings and cross-like nanomagnets. Antivortices may be useful for nonvolatile data storage applications, and they are also expected to show unusual transport properties in an applied magnetic field, for example, a ``topological'' Hall effect. In order to make use of magnetic antivortices, it is important to first understand how to stabilize systems that contain only a single antivortex. Micromagnetic calculations have been performed with OOMMF and LLG software to explore how the geometry of the structure affects the formation and stability of the antivortex state and whether the field history can be used to reliably select the st...
Journal of Applied Physics, 2015
In this work, we assess the effects of field history and structure shape on the formation of magn... more In this work, we assess the effects of field history and structure shape on the formation of magnetic antivortices. The magnetic reversal process was investigated for a series of patterned micron-sized permalloy pound-key structures with varying degrees of asymmetry using magneto-optical Kerr effect hysteresis measurements combined with magnetic force microscopy. The largest number of antivortices was observed in the structures with the highest level of structure asymmetry, which also show an intermediate state in the hysteresis loop. A significant enhancement of the antivortex formation rate—from 5% to almost 80%—was achieved by adjusting the structure dimensions. Images of the magnetic states obtained at various points in the hysteresis loop show that the highest rate of antivortex formation occurs near the coercive field, also the nucleation field, and that the antivortex formation is also sensitive to the angle of the applied field, where the highest antivortex formation rate is...
The magnetic vortex state has received increasing attention during recent years however its topol... more The magnetic vortex state has received increasing attention during recent years however its topological counterpart, the magnetic antivortex (AV), has not been explored with the same intensity. The antivortex spin configuration may have some advantages over vortices, especially for channeling spin waves emitted from the dynamic core reversal. In order to study the properties of antivortices it is necessary to have geometries and procedures that reliable stabilize antivortices, however this task is more challenging than forming a vortex. In this work, we use micromagnetic simulations to show that pound-key-like structures can be used to form stable AV's and to explore the role of shape anisotropy in the AV formation. Our results are compared to Magneto Optical Kerr Effect (MOKE) hysteresis measurements and magnetic force microscopy images. The simulations show that the AV nucleation field depends on the sample global and relative dimensions and these results are in good agreement with the experiments. Acknowledgement: This work was supported by the NSF.
magnetic antivortex (AV) state is a topological configuration that is expected to exhibit interes... more magnetic antivortex (AV) state is a topological configuration that is expected to exhibit interesting physical behavior and it may also be useful for applications. Recent work showed that magnetic antivortices can be created in pound-key-like nanostructures via a two-step magnetic field procedure [1]. In this procedure, magnetic hysteresis measurements are important to predict the field values at which the AV's will form. The Magneto-Optical Kerr Effect (MOKE) is widely used to make magnetic hysteresis measurements, especially for magnetic nanostructures since measurements can be made on individual structures or small arrays. MOKE measurements were made on a series of micron-sized pound-key-like structures made of Permalloy to examine how the reversal process and critical fields depend on the details of the structure shape and size. Hysteresis loops were obtained with a high signal-noise ratio even though the amount of magnetic material and consequently the Kerr rotation angle were small (less than 0.02 mrad). Subsequent magnetic force microscopy imaging of the structures showed successful AV formation at the fields predicted from the MOKE measurements.
Journal of Applied Physics, 2015
Magnetostatic interactions between vortices in closely spaced planar structures are important for... more Magnetostatic interactions between vortices in closely spaced planar structures are important for applications including vortex-based magnonic crystals and spin torque oscillator networks. Analytical theories that include magnetostatic interaction effects have been proposed but have not yet been rigorously tested. Here we compare micromagnetic simulations of the dynamics of magnetic vortices confined in three disks in an equilateral triangle configuration to analytical theories that include coupling. Micromagnetic simulations show that the magnetostatic coupling between the disks leads to splitting of the gyrotropic resonance into three modes and that the frequency splitting increases with decreasing separation. The temporal profiles of the magnetization depend on the vortex polarities and chiralities, however, the frequencies depend only on the polarity combinations and will fall into one of two categories: all polarities equal or one polarity opposite to the others, where the latter leads to a larger frequency splitting. Although the magnitude of the splitting observed in the simulations is larger than what is expected based on purely dipolar interactions, a simple analytical model that assumes dipole-dipole coupling captures the functional form of the frequency splitting and the motion patterns just as well as more complex models.
This work presents a comparative study on optical and electrical properties of CuAlO2 thin films ... more This work presents a comparative study on optical and electrical properties of CuAlO2 thin films on sapphire (0001) substrates deposited with two different growth conditions using reactive RF-magnetron sputtering technique from metallic Cu and Al targets. CuAlO2 is a very promising material for transparent electronic applications, it is intended that comparison of results obtained from both approaches, could lead to optimization and control of the physical properties of this material, namely its electrical conductivity and optical transmittance. All samples were heat treated at 1100°C using rapid thermal annealing with varying time and rate of cooling. The effect of sputtering conditions and different annealing time on phase formation and evolution is studied with X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that for most of the samples CuAlO2 phase is formed after 60 min of annealing time, but secondary phases were also present that depend on the deposition conditions. However, pure CuAlO2 phase was obtained for annealed CuO on sapphire films with annealing time of 60 min. The optical properties obtained from UV-Visible spectroscopic measurement reveals indirect and direct optical band gaps for CuAlO2 films and were found to be 2.58 and 3.72 eV respectively. The films show a transmittance of about 60% in the visible range. Hall effect measurements indicate p-type conductivity. Van der Pauw technique was used to measure resistivity of the samples. The highest electrical conductivity and charge carrier concentration obtained were of 1.01x10-1S.cm -1 and 3.63 x1018 cm-3 respectively.