Stuart Parkin - Academia.edu (original) (raw)

Papers by Stuart Parkin

Physical Review Applied

The properties and characteristics of thin-film materials strongly depend on their textures and o... more The properties and characteristics of thin-film materials strongly depend on their textures and orientations. However, the attainable film morphologies are severely limited by substrates and growth thermodynamics. Metastable films that otherwise cannot be grown by conventional growth methods may overcome these limitations, thus allowing a dramatic expansion of the spectrum of film textures and orientations. Here we present a means to grow metastable platinum layers that are deposited from platinum alloyed with bismuth surfactant material. This is distinct from conventional surfactant-aided growth of films in which surfactants are typically deposited onto the substrate before film deposition, altering the film growth mode but not the film morphology by tuning the surface energy. Surprisingly, we find that almost no bismuth is incorporated into the platinum layer, but rather the structural morphology of this layer is significantly altered. When this metastable platinum layer is applied to spin-orbit-torque technology, a huge increase in the current-driven velocity of chiral domain walls in perpendicularly magnetized wire on top of the metastable platinum layer is observed for otherwise the same current density, while the platinum resistivity is found to barely increase. Our findings show that the metastable film grown from material alloyed with surfactant is promising for the development of devices in various fields, such as spintronics, semiconductors, and quantum materials.

Ir 2.1 Epitaxial growth, structural characterization, and exchange bias of noncollinear antiferro... more Ir 2.1 Epitaxial growth, structural characterization, and exchange bias of noncollinear antiferromagnetic Mn 3 Ir thin films. .. . i 2.2 Magnetic and electrical transport signatures of uncompensated moments in epitaxial thin films of the noncollinear antiferromagnet Mn 3 Ir .

Nature Reviews Physics, 2021

Very recently, it has been discovered that chiral molecules can play a significant role in spintr... more Very recently, it has been discovered that chiral molecules can play a significant role in spintronics and they can act as very efficient spin injectors and detectors. This effect is commonly dubbed as CISS or "Chirality induced spin selectivity" and has generated significant interest in recent years. Our group has also made significant contributions in this area. The purpose of this project is to explore this area further using various experimental techniques.

APL Materials, 2019

Tetragonal Heusler compounds that exhibit large perpendicular magnetic anisotropy are promising m... more Tetragonal Heusler compounds that exhibit large perpendicular magnetic anisotropy are promising materials for advanced spintronic devices. A prerequisite are thin films whose tetragonal axis is oriented perpendicular to the plane of the films. Here we show that highly textured, (001) oriented, tetragonal Mn 3 Sn layers can be prepared using metallic zinc-blende (ZB) MnN as underlayers. Moreover, we show that these layers can be deposited on amorphous substrates using reactive magnetron sputtering. The ferrimagnetic Mn 3 Sn layers exhibit perpendicularly magnetized hysteresis loops with coercive fields of ∼2 T. Stoichiometric ZB-MnN underlayers share an "equivalent" Mn-Mn layer at the interface with Mn 3 Sn, thus promoting their oriented growth. Other nitride underlayers are not effective due to their rock-salt (RS) crystal structure and the absence of Mn. Density functional theory calculations confirm that tetragonal Mn 3 Sn Heusler films are energetically stable when interfaced with ZB-MnN underlayers and not with any of the other RS nitride underlayers considered here. Such Heusler compounds have much promise as electrodes for magnetic tunnel junction memory elements for deeply scaled magnetic random access memories.

Annual Review of Condensed Matter Physics, 2010

Spintronics encompasses the ever-evolving field of magnetic electronics. It is an applied discipl... more Spintronics encompasses the ever-evolving field of magnetic electronics. It is an applied discipline that is so forward-looking that much of the research that supports it is at the center of basic condensed matter physics. This review provides a perspective on recent developments in switching magnetic moments by spin-polarized currents, electric fields, and photonic fields. Developments in the field continue to be strongly dependent on the exploration and discovery of novel material systems. An array of novel transport and thermoelectric effects dependent on the interplay between spin and charge currents have been explored theoretically and experimentally in recent years. The review highlights select areas that hold promise for future investigation and attempts to unify and further inform the field.

Nature communications, Jan 28, 2017

Rabi oscillations describe the process whereby electromagnetic radiation interacts coherently wit... more Rabi oscillations describe the process whereby electromagnetic radiation interacts coherently with spin states in a non-equilibrium interaction. To date, Rabi oscillations have not been studied in one of the most common spin ensembles in nature: spins in ferromagnets. Here, using a combination of femtosecond laser pulses and microwave excitations, we report the classical analogue of Rabi oscillations in ensemble-averaged spins of a ferromagnet. The microwave stimuli are shown to extend the coherence-time resulting in resonant spin amplification. The results we present in a dense magnetic system are qualitatively similar to those reported previously in semiconductors which have five orders of magnitude fewer spins and which require resonant optical excitations to spin-polarize the ensemble. Our study is a step towards connecting concepts used in quantum processing with spin-transport effects in ferromagnets. For example, coherent control may become possible without the complications ...

Nature Communications, 2017

Ferromagnets are commonly magnetized by either external magnetic fields or spin polarized current... more Ferromagnets are commonly magnetized by either external magnetic fields or spin polarized currents. The manipulation of magnetization by spin-current occurs through the spin-transfer-torque effect, which is applied, for example, in modern magnetoresistive random access memory. However, the current density required for the spin-transfer torque is of the order of 1 × 106 A·cm−2, or about 1 × 1025 electrons s−1 cm−2. This relatively high current density significantly affects the devices’ structure and performance. Here we demonstrate magnetization switching of ferromagnetic thin layers that is induced solely by adsorption of chiral molecules. In this case, about 1013 electrons per cm2 are sufficient to induce magnetization reversal. The direction of the magnetization depends on the handedness of the adsorbed chiral molecules. Local magnetization switching is achieved by adsorbing a chiral self-assembled molecular monolayer on a gold-coated ferromagnetic layer with perpendicular magneti...

Applied Physics Express, 2015

ABSTRACT We studied the intrinsic and extrinsic Gilbert damping constants measured with an all-op... more ABSTRACT We studied the intrinsic and extrinsic Gilbert damping constants measured with an all-optical method in an exchange-biased IrMn/Cu/CoFe trilayer system with various Cu-spacer layer thicknesses. To understand the effect of the exchange bias on damping, the strength of the exchange bias was controlled by the Cu-spacer layer. When the external magnetic field is high enough compared with the exchange fields, the intrinsic Gilbert damping constant is independent of the exchange bias field. On the other hand, under a low external magnetic field, the extrinsic Gilbert damping is dominant because of the magnetic inhomogeneities, and is strongly correlated with the exchange bias field.

Proceedings of the National Academy of Sciences, 2015

Significance One of the most promising emerging memory technologies is magnetic random access mem... more Significance One of the most promising emerging memory technologies is magnetic random access memory (MRAM), which promises to be a high-performance, nonvolatile memory. The essential feature of MRAM is the efficient switching of the magnetic tunnel junction (MTJ) memory cell between two distinct resistance states associated with the relative magnetic orientation of the ferromagnetic electrodes which sandwich the tunnel barrier by using magnetic fields, charge currents, or pure spin currents. In this paper, a temperature difference of just a few kelvin across an ultrathin (∼1 nm) MgO tunnel barrier is found to generate giant spin currents sufficient to significantly influence the switching of the MTJ. These spin currents get created only in those devices that show a large asymmetry in their tunneling conductance across zero bias voltage.

Nature Nanotechnology, 2015

The operation of racetrack memories is based on the motion of domain walls in atomically thin, pe... more The operation of racetrack memories is based on the motion of domain walls in atomically thin, perpendicularly magnetized nanowires, which are interfaced with adjacent metal layers with high spin-orbit coupling. Such domain walls have a chiral Néel structure and can be moved efficiently by electrical currents. High-capacity racetrack memory requires closely packed domain walls, but their density is limited by dipolar coupling from their fringing magnetic fields. These fields can be eliminated using a synthetic antiferromagnetic structure composed of two magnetic sub-layers, exchange-coupled via an ultrathin antiferromagnetic-coupling spacer layer. Here, we show that nanosecond-long current pulses can move domain walls in synthetic antiferromagnetic racetracks that have almost zero net magnetization. The domain walls can be moved even more efficiently and at much higher speeds (up to ∼750 m s(-1)) compared with similar racetracks in which the sub-layers are coupled ferromagnetically. This is due to a stabilization of the Néel domain wall structure, and an exchange coupling torque that is directly proportional to the strength of the antiferromagnetic exchange coupling between the two sub-layers. Moreover, the dependence of the wall velocity on the magnetic field applied along the nanowire is distinct from that of the single-layer racetrack due to the exchange coupling torque. The high domain wall velocities in racetracks that have no net magnetization allow for densely packed yet highly efficient domain-wall-based spintronics.

Applied Physics Letters, 2014

We report on the experimental demonstration of a magnetic domain wall logic device—a 2 bit demult... more We report on the experimental demonstration of a magnetic domain wall logic device—a 2 bit demultiplexer or a biplexer—that sorts vortex domain walls, created in the input branch, into one of the two output branches of a Y-shaped magnetic nanostructure based on their chiralities. We show that this sorting behavior is insensitive to the angle that the two output branches of the Y-shaped nanostructure subtend with each other, which we attribute to a topological protection stemming from the constituent fractional topological defects of the domain wall.

Physical Review Letters, 2001

The magnetic anisotropy in antiferromagnetic 500 Å thick NiO films, before and after the establis... more The magnetic anisotropy in antiferromagnetic 500 Å thick NiO films, before and after the establishment of an exchange bias field with Co 84 Fe 16 ferromagnetic layers, was measured using magnetic linear dichroism in soft x-ray absorption. Both ͗111͘ textured NiO and untextured NiO films show exchange-bias induced in-plane magnetic anisotropy of nearly equal magnitude and with the Ni moment axis being nearly parallel to the exchange bias field direction. These results represent the first observation of the key step in the exchange biasing process, namely, repopulation of the antiferromagnetic domains whose magnetization axis is closest to the exchange bias field direction.

Physical Review B, 2011

Energy-filtered transmission electron microscopy and electron holography were used to study chang... more Energy-filtered transmission electron microscopy and electron holography were used to study changes in the MgO tunnel barrier of CoFe/MgO/CoFe magnetic tunnel junctions as a function of annealing and in-situ applied electrical bias. Annealing was found to increase the homogeneity and crystallinity of the MgO tunnel barrier. Cobalt, oxygen and trace amounts of iron diffused into the MgO upon annealing. Annealing also resulted in a reduction of the tunneling barrier height, and decreased the resistance of the annealed MTJ relative to that of the asgrown sample. In-situ off-axis electron holography was employed to image the barrier potential profile of an MTJ directly, with the specimen under electrical bias. Varying the bias voltage from −1.5 V to +1.5 V was found to change the asymmetry of the barrier potential and decrease the effective barrier width as a result of charge accumulation at the MgO-CoFe interface. Introduction Metal-oxide interfaces are the subject of extensive experimental and theoretical research for next generation nano-scale spintronic devices that exploit spin as a degree of freedom for charged electrons. 1 They play a key role in metal-oxide based science and engineering, with applications including magnetic tunnel junctions (MTJs) 2 and other heterogeneous structures such as resistance switching oxides 3 with uses or potentials uses in low-power non-volatile memories. In its simplest form, the MTJ is a trilayer structure consisting of two ferromagnetic (FM) electrode layers separated by an ultra-thin dielectric layer. The electrical resistance across the insulating tunnel barrier is dependent upon the relative orientation of the magnetizations of the two ferromagnetic electrodes. In most cases, the electrical resistance is lower when the magnetization of the two ferromagnetic layers is

Physical Review B, 2011

Spin-dependent tunneling across a highly textured MgO insulating barrier has received much attent... more Spin-dependent tunneling across a highly textured MgO insulating barrier has received much attention due to its potential applications in various spintronic devices. However, the interfacial magnetic and electronic structure of a prototypical realization of this in Fe/MgO/Fe and the effective band gap of the MgO layer are still under debate. In order to resolve these issues, we have employed standing-wave excited core and valence photoemission, as well as core-level magnetic circular dichroism (MCD) in photoemission, to study the Fe/MgO interface with subnanometer depth resolution. For our synthetic procedure, we show that the Fe/MgO interface is linearly intermixed in composition over a length of ∼8Å (∼4 monolayers) and that there is a magnetic dead layer ∼2-3Å thick. The unambiguous extraction of depth-resolved density of states (DOS) reveals that the interfacial layer composition is mostly metallic and nonmagnetic FeO x , with x ∼ = 1, which accounts for a smaller magnetoresistance compared to theoretical predictions. The formation of the magnetic dead layer (FeO) at the interface should also reduce the tunneling spin polarization. The analysis of our data also shows a clear valence band edge of ultrathin MgO layer at ∼3.5 eV below the Fermi level (E F) that is very close to that of single crystal bulk MgO. An analysis that does not consider the interdiffused region separately exhibits the valence band edge for MgO layer ∼1.3 eV below E F , which is significantly closer to the MgO barrier height estimated from magnetotransport measurements and further suggests that the Fe/MgO interdiffusion effectively reduces the MgO band gap.

Microscopy and Microanalysis, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia... more Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

MRS Bulletin, 2006

Spin-polarized currents can be generated by spin-dependent diffusive scattering in magnetic thin-... more Spin-polarized currents can be generated by spin-dependent diffusive scattering in magnetic thin-film structures or by spin-dependent tunneling across ultrathin dielectrics sandwiched between magnetic electrodes.By manipulating the magnetic moments of the magnetic components of these spintronic materials, their resistance can be significantly changed, allowing the development of highly sensitive magnetic-field detectors or advanced magnetic memory storage elements.Whereas the magneto-resistance of useful devices based on spin-dependent diffusive scattering has hardly changed since its discovery nearly two decades ago, in the past five years there has been a remarkably rapid development in both the basic understanding of spin-dependent tunneling and the magnitude of useful tunnel magnetoresistance values.In particular, it is now evident that the magnitude of the spin polarization of tunneling currents in magnetic tunnel junctions not only is related to the spin-dependent electronic s...

Journal of Magnetism and Magnetic Materials, 2013

We report an asymmetry of magnetic disorder in exchange-biased IrMn(t IrMn ¼ 5-20 nm)/CoFe(50 nm)... more We report an asymmetry of magnetic disorder in exchange-biased IrMn(t IrMn ¼ 5-20 nm)/CoFe(50 nm) films observed by means of a Kerr microscope, capable of direct domain observation. From the correlation between the magnetization half-reversal time and applied magnetic field, we find that the magnetization switching in all the films occurs via a thermally activated reversal mechanism for both branches of hysteresis loops. Surprisingly, in the forward branch reversal where the applied magnetic field is antiparallel to the direction of exchange-bias field, degree of magnetic disorder decreases as exchange-bias field increases, which is definitely contrasted with the case of backward branch reversal. This result is likely ascribed to the fact that the local values of exchange-bias field and coercive field are oppositely fluctuating with each other in the film.

Physical Review Applied

The properties and characteristics of thin-film materials strongly depend on their textures and o... more The properties and characteristics of thin-film materials strongly depend on their textures and orientations. However, the attainable film morphologies are severely limited by substrates and growth thermodynamics. Metastable films that otherwise cannot be grown by conventional growth methods may overcome these limitations, thus allowing a dramatic expansion of the spectrum of film textures and orientations. Here we present a means to grow metastable platinum layers that are deposited from platinum alloyed with bismuth surfactant material. This is distinct from conventional surfactant-aided growth of films in which surfactants are typically deposited onto the substrate before film deposition, altering the film growth mode but not the film morphology by tuning the surface energy. Surprisingly, we find that almost no bismuth is incorporated into the platinum layer, but rather the structural morphology of this layer is significantly altered. When this metastable platinum layer is applied to spin-orbit-torque technology, a huge increase in the current-driven velocity of chiral domain walls in perpendicularly magnetized wire on top of the metastable platinum layer is observed for otherwise the same current density, while the platinum resistivity is found to barely increase. Our findings show that the metastable film grown from material alloyed with surfactant is promising for the development of devices in various fields, such as spintronics, semiconductors, and quantum materials.

Ir 2.1 Epitaxial growth, structural characterization, and exchange bias of noncollinear antiferro... more Ir 2.1 Epitaxial growth, structural characterization, and exchange bias of noncollinear antiferromagnetic Mn 3 Ir thin films. .. . i 2.2 Magnetic and electrical transport signatures of uncompensated moments in epitaxial thin films of the noncollinear antiferromagnet Mn 3 Ir .

Nature Reviews Physics, 2021

Very recently, it has been discovered that chiral molecules can play a significant role in spintr... more Very recently, it has been discovered that chiral molecules can play a significant role in spintronics and they can act as very efficient spin injectors and detectors. This effect is commonly dubbed as CISS or "Chirality induced spin selectivity" and has generated significant interest in recent years. Our group has also made significant contributions in this area. The purpose of this project is to explore this area further using various experimental techniques.

APL Materials, 2019

Tetragonal Heusler compounds that exhibit large perpendicular magnetic anisotropy are promising m... more Tetragonal Heusler compounds that exhibit large perpendicular magnetic anisotropy are promising materials for advanced spintronic devices. A prerequisite are thin films whose tetragonal axis is oriented perpendicular to the plane of the films. Here we show that highly textured, (001) oriented, tetragonal Mn 3 Sn layers can be prepared using metallic zinc-blende (ZB) MnN as underlayers. Moreover, we show that these layers can be deposited on amorphous substrates using reactive magnetron sputtering. The ferrimagnetic Mn 3 Sn layers exhibit perpendicularly magnetized hysteresis loops with coercive fields of ∼2 T. Stoichiometric ZB-MnN underlayers share an "equivalent" Mn-Mn layer at the interface with Mn 3 Sn, thus promoting their oriented growth. Other nitride underlayers are not effective due to their rock-salt (RS) crystal structure and the absence of Mn. Density functional theory calculations confirm that tetragonal Mn 3 Sn Heusler films are energetically stable when interfaced with ZB-MnN underlayers and not with any of the other RS nitride underlayers considered here. Such Heusler compounds have much promise as electrodes for magnetic tunnel junction memory elements for deeply scaled magnetic random access memories.

Annual Review of Condensed Matter Physics, 2010

Spintronics encompasses the ever-evolving field of magnetic electronics. It is an applied discipl... more Spintronics encompasses the ever-evolving field of magnetic electronics. It is an applied discipline that is so forward-looking that much of the research that supports it is at the center of basic condensed matter physics. This review provides a perspective on recent developments in switching magnetic moments by spin-polarized currents, electric fields, and photonic fields. Developments in the field continue to be strongly dependent on the exploration and discovery of novel material systems. An array of novel transport and thermoelectric effects dependent on the interplay between spin and charge currents have been explored theoretically and experimentally in recent years. The review highlights select areas that hold promise for future investigation and attempts to unify and further inform the field.

Nature communications, Jan 28, 2017

Rabi oscillations describe the process whereby electromagnetic radiation interacts coherently wit... more Rabi oscillations describe the process whereby electromagnetic radiation interacts coherently with spin states in a non-equilibrium interaction. To date, Rabi oscillations have not been studied in one of the most common spin ensembles in nature: spins in ferromagnets. Here, using a combination of femtosecond laser pulses and microwave excitations, we report the classical analogue of Rabi oscillations in ensemble-averaged spins of a ferromagnet. The microwave stimuli are shown to extend the coherence-time resulting in resonant spin amplification. The results we present in a dense magnetic system are qualitatively similar to those reported previously in semiconductors which have five orders of magnitude fewer spins and which require resonant optical excitations to spin-polarize the ensemble. Our study is a step towards connecting concepts used in quantum processing with spin-transport effects in ferromagnets. For example, coherent control may become possible without the complications ...

Nature Communications, 2017

Ferromagnets are commonly magnetized by either external magnetic fields or spin polarized current... more Ferromagnets are commonly magnetized by either external magnetic fields or spin polarized currents. The manipulation of magnetization by spin-current occurs through the spin-transfer-torque effect, which is applied, for example, in modern magnetoresistive random access memory. However, the current density required for the spin-transfer torque is of the order of 1 × 106 A·cm−2, or about 1 × 1025 electrons s−1 cm−2. This relatively high current density significantly affects the devices’ structure and performance. Here we demonstrate magnetization switching of ferromagnetic thin layers that is induced solely by adsorption of chiral molecules. In this case, about 1013 electrons per cm2 are sufficient to induce magnetization reversal. The direction of the magnetization depends on the handedness of the adsorbed chiral molecules. Local magnetization switching is achieved by adsorbing a chiral self-assembled molecular monolayer on a gold-coated ferromagnetic layer with perpendicular magneti...

Applied Physics Express, 2015

ABSTRACT We studied the intrinsic and extrinsic Gilbert damping constants measured with an all-op... more ABSTRACT We studied the intrinsic and extrinsic Gilbert damping constants measured with an all-optical method in an exchange-biased IrMn/Cu/CoFe trilayer system with various Cu-spacer layer thicknesses. To understand the effect of the exchange bias on damping, the strength of the exchange bias was controlled by the Cu-spacer layer. When the external magnetic field is high enough compared with the exchange fields, the intrinsic Gilbert damping constant is independent of the exchange bias field. On the other hand, under a low external magnetic field, the extrinsic Gilbert damping is dominant because of the magnetic inhomogeneities, and is strongly correlated with the exchange bias field.

Proceedings of the National Academy of Sciences, 2015

Significance One of the most promising emerging memory technologies is magnetic random access mem... more Significance One of the most promising emerging memory technologies is magnetic random access memory (MRAM), which promises to be a high-performance, nonvolatile memory. The essential feature of MRAM is the efficient switching of the magnetic tunnel junction (MTJ) memory cell between two distinct resistance states associated with the relative magnetic orientation of the ferromagnetic electrodes which sandwich the tunnel barrier by using magnetic fields, charge currents, or pure spin currents. In this paper, a temperature difference of just a few kelvin across an ultrathin (∼1 nm) MgO tunnel barrier is found to generate giant spin currents sufficient to significantly influence the switching of the MTJ. These spin currents get created only in those devices that show a large asymmetry in their tunneling conductance across zero bias voltage.

Nature Nanotechnology, 2015

The operation of racetrack memories is based on the motion of domain walls in atomically thin, pe... more The operation of racetrack memories is based on the motion of domain walls in atomically thin, perpendicularly magnetized nanowires, which are interfaced with adjacent metal layers with high spin-orbit coupling. Such domain walls have a chiral Néel structure and can be moved efficiently by electrical currents. High-capacity racetrack memory requires closely packed domain walls, but their density is limited by dipolar coupling from their fringing magnetic fields. These fields can be eliminated using a synthetic antiferromagnetic structure composed of two magnetic sub-layers, exchange-coupled via an ultrathin antiferromagnetic-coupling spacer layer. Here, we show that nanosecond-long current pulses can move domain walls in synthetic antiferromagnetic racetracks that have almost zero net magnetization. The domain walls can be moved even more efficiently and at much higher speeds (up to ∼750 m s(-1)) compared with similar racetracks in which the sub-layers are coupled ferromagnetically. This is due to a stabilization of the Néel domain wall structure, and an exchange coupling torque that is directly proportional to the strength of the antiferromagnetic exchange coupling between the two sub-layers. Moreover, the dependence of the wall velocity on the magnetic field applied along the nanowire is distinct from that of the single-layer racetrack due to the exchange coupling torque. The high domain wall velocities in racetracks that have no net magnetization allow for densely packed yet highly efficient domain-wall-based spintronics.

Applied Physics Letters, 2014

We report on the experimental demonstration of a magnetic domain wall logic device—a 2 bit demult... more We report on the experimental demonstration of a magnetic domain wall logic device—a 2 bit demultiplexer or a biplexer—that sorts vortex domain walls, created in the input branch, into one of the two output branches of a Y-shaped magnetic nanostructure based on their chiralities. We show that this sorting behavior is insensitive to the angle that the two output branches of the Y-shaped nanostructure subtend with each other, which we attribute to a topological protection stemming from the constituent fractional topological defects of the domain wall.

Physical Review Letters, 2001

The magnetic anisotropy in antiferromagnetic 500 Å thick NiO films, before and after the establis... more The magnetic anisotropy in antiferromagnetic 500 Å thick NiO films, before and after the establishment of an exchange bias field with Co 84 Fe 16 ferromagnetic layers, was measured using magnetic linear dichroism in soft x-ray absorption. Both ͗111͘ textured NiO and untextured NiO films show exchange-bias induced in-plane magnetic anisotropy of nearly equal magnitude and with the Ni moment axis being nearly parallel to the exchange bias field direction. These results represent the first observation of the key step in the exchange biasing process, namely, repopulation of the antiferromagnetic domains whose magnetization axis is closest to the exchange bias field direction.

Physical Review B, 2011

Energy-filtered transmission electron microscopy and electron holography were used to study chang... more Energy-filtered transmission electron microscopy and electron holography were used to study changes in the MgO tunnel barrier of CoFe/MgO/CoFe magnetic tunnel junctions as a function of annealing and in-situ applied electrical bias. Annealing was found to increase the homogeneity and crystallinity of the MgO tunnel barrier. Cobalt, oxygen and trace amounts of iron diffused into the MgO upon annealing. Annealing also resulted in a reduction of the tunneling barrier height, and decreased the resistance of the annealed MTJ relative to that of the asgrown sample. In-situ off-axis electron holography was employed to image the barrier potential profile of an MTJ directly, with the specimen under electrical bias. Varying the bias voltage from −1.5 V to +1.5 V was found to change the asymmetry of the barrier potential and decrease the effective barrier width as a result of charge accumulation at the MgO-CoFe interface. Introduction Metal-oxide interfaces are the subject of extensive experimental and theoretical research for next generation nano-scale spintronic devices that exploit spin as a degree of freedom for charged electrons. 1 They play a key role in metal-oxide based science and engineering, with applications including magnetic tunnel junctions (MTJs) 2 and other heterogeneous structures such as resistance switching oxides 3 with uses or potentials uses in low-power non-volatile memories. In its simplest form, the MTJ is a trilayer structure consisting of two ferromagnetic (FM) electrode layers separated by an ultra-thin dielectric layer. The electrical resistance across the insulating tunnel barrier is dependent upon the relative orientation of the magnetizations of the two ferromagnetic electrodes. In most cases, the electrical resistance is lower when the magnetization of the two ferromagnetic layers is

Physical Review B, 2011

Spin-dependent tunneling across a highly textured MgO insulating barrier has received much attent... more Spin-dependent tunneling across a highly textured MgO insulating barrier has received much attention due to its potential applications in various spintronic devices. However, the interfacial magnetic and electronic structure of a prototypical realization of this in Fe/MgO/Fe and the effective band gap of the MgO layer are still under debate. In order to resolve these issues, we have employed standing-wave excited core and valence photoemission, as well as core-level magnetic circular dichroism (MCD) in photoemission, to study the Fe/MgO interface with subnanometer depth resolution. For our synthetic procedure, we show that the Fe/MgO interface is linearly intermixed in composition over a length of ∼8Å (∼4 monolayers) and that there is a magnetic dead layer ∼2-3Å thick. The unambiguous extraction of depth-resolved density of states (DOS) reveals that the interfacial layer composition is mostly metallic and nonmagnetic FeO x , with x ∼ = 1, which accounts for a smaller magnetoresistance compared to theoretical predictions. The formation of the magnetic dead layer (FeO) at the interface should also reduce the tunneling spin polarization. The analysis of our data also shows a clear valence band edge of ultrathin MgO layer at ∼3.5 eV below the Fermi level (E F) that is very close to that of single crystal bulk MgO. An analysis that does not consider the interdiffused region separately exhibits the valence band edge for MgO layer ∼1.3 eV below E F , which is significantly closer to the MgO barrier height estimated from magnetotransport measurements and further suggests that the Fe/MgO interdiffusion effectively reduces the MgO band gap.

Microscopy and Microanalysis, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia... more Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

MRS Bulletin, 2006

Spin-polarized currents can be generated by spin-dependent diffusive scattering in magnetic thin-... more Spin-polarized currents can be generated by spin-dependent diffusive scattering in magnetic thin-film structures or by spin-dependent tunneling across ultrathin dielectrics sandwiched between magnetic electrodes.By manipulating the magnetic moments of the magnetic components of these spintronic materials, their resistance can be significantly changed, allowing the development of highly sensitive magnetic-field detectors or advanced magnetic memory storage elements.Whereas the magneto-resistance of useful devices based on spin-dependent diffusive scattering has hardly changed since its discovery nearly two decades ago, in the past five years there has been a remarkably rapid development in both the basic understanding of spin-dependent tunneling and the magnitude of useful tunnel magnetoresistance values.In particular, it is now evident that the magnitude of the spin polarization of tunneling currents in magnetic tunnel junctions not only is related to the spin-dependent electronic s...

Journal of Magnetism and Magnetic Materials, 2013

We report an asymmetry of magnetic disorder in exchange-biased IrMn(t IrMn ¼ 5-20 nm)/CoFe(50 nm)... more We report an asymmetry of magnetic disorder in exchange-biased IrMn(t IrMn ¼ 5-20 nm)/CoFe(50 nm) films observed by means of a Kerr microscope, capable of direct domain observation. From the correlation between the magnetization half-reversal time and applied magnetic field, we find that the magnetization switching in all the films occurs via a thermally activated reversal mechanism for both branches of hysteresis loops. Surprisingly, in the forward branch reversal where the applied magnetic field is antiparallel to the direction of exchange-bias field, degree of magnetic disorder decreases as exchange-bias field increases, which is definitely contrasted with the case of backward branch reversal. This result is likely ascribed to the fact that the local values of exchange-bias field and coercive field are oppositely fluctuating with each other in the film.