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Ivan Schuller

University of California, San Diego, Physics, Faculty Member

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Rafael Abargues

Universitat de València

Diamond Light Source

Ulyanovsk State University

Taras Shevchenko National University of Kyiv

MUSTAFA GOKTEPE

Balikesir University

University of Illinois at Urbana-Champaign

VIT University

University of Cambridge

Muhammad Shahid Arshad

Jožef Stefan Institute

University of York

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Papers by Ivan Schuller

Manipulation of competing ferromagnetic and antiferromagnetic domains in exchange-biased nanostructures

Physical Review B, Nov 20, 2015

Using photoemission electron microscopy combined with X-ray magnetic circular dichroism we show t... more Using photoemission electron microscopy combined with X-ray magnetic circular dichroism we show that a progressive spatial confinement of a ferromagnet (FM), either through thickness variation or laterally via patterning, actively controls the domains of uncompensated spins in the antiferromagnet (AF) in exchange biased systems. Direct observations of the spin structure in both sides of the FM/AF interface in a model system, Ni/FeF 2 , show that the spin structure is determined by the balance between the competing FM and AF magnetic energies. Coexistence of exchange bias domains, with opposite directions, can be established in Ni/FeF 2 bilayers for Ni thicknesses below 10 nm. Patterning the Ni/FeF 2 heterostructures with antidots destabilizes the FM state, enhancing the formation of opposite exchange bias domains below a critical antidot separation of the order of a few FeF 2 crystal domains. The results suggest that dimensional confinement of the FM may be used to manipulate the AF spin structure in spintronic devices and ultra-high density information storage media. The underlying mechanism of the uncompensated AF domain formation in Ni/FeF 2 may be generic to other magnetic systems with complex non-collinear FM/AF spin structures.

The fabrication of ordered arrays of exchange biased Ni/FeF2nanostructures

Nanotechnology, 2010

The fabrication of ordered arrays of exchange biased Ni/FeF 2 nanostructures by focused ion beam ... more The fabrication of ordered arrays of exchange biased Ni/FeF 2 nanostructures by focused ion beam lithography is reported. High quality nano-elements, with controlled removal depth and no significant re-deposition, were carved using small ion beam currents (30 pA), moderate dwell times (1 µs) and repeated passages over the same area. Two types of nanostructures were fabricated: square arrays of circular dots with diameters from 125 ± 8 to 500 ± 12 nm and periodicities ranging from 200 ± 8 to 1000 ± 12 nm, and square arrays of square antidots (207 ± 8 nm in edge length) with periodicities ranging from 300 ± 8 to 1200 ± 12 nm. The arrays were characterized using scanning ion and electron microscopy, and atomic force microscopy. The effect of the patterning on the exchange bias field (i.e., the shift in the hysteresis loop of ferromagnetic Ni due to proximity to antiferromagnetic FeF 2 ) was studied using magneto-transport measurements. These high quality nanostructures offer a unique method to address some of the open questions regarding the microscopic origin of exchange bias. This is not only of major relevance in the fabrication and miniaturization of magnetic devices but it is also one of the important proximity phenomena in nanoscience and materials science.

Three-dimensional spin structure in exchange-biased antiferromagnetic/ferromagnetic thin films

Applied Physics Letters, 2009

A coexistence of lateral and in-depth domain walls in antiferromagnet/ferromagnet (AF/FM) thin fi... more A coexistence of lateral and in-depth domain walls in antiferromagnet/ferromagnet (AF/FM) thin films exhibiting double hysteresis loops (DHLs) is demonstrated. Comparison of single and DHLs together with local and global measurements confirms the formation of two oppositely oriented domains in the AF that imprint a lateral domain structure into the FM layer. Most significantly, the magnetization reversal mechanism within each opposite domain takes place by incoherent rotation of spring-like domain walls extending through the Ni thickness. Therefore, complex three-dimensional domain walls are created perpendicular and parallel to the AF/FM interface in exchange biased systems.

Mirror symmetry in magnetization reversal and coexistence of positive and negative exchange bias in Ni/FeF2

Applied Physics Letters, 2011

Positively and negatively exchange biased (PEB and NEB) magnetoresistance (MR) loops in Ni/FeF2 f... more Positively and negatively exchange biased (PEB and NEB) magnetoresistance (MR) loops in Ni/FeF2 ferromagnetic/antiferromagnetic (AF) heterostructures proceed through the same reversal mechanisms. The MR curves exhibit mirror symmetry: the increasing (decreasing) field branch of the PEB (NEB) loop is identical to the decreasing (increasing) branch of the NEB (PEB) loop, suggesting that the interfacial areal density of pinned uncompensated AF spins responsible for PEB and NEB is similar. Micromagnetic simulations are in agreement with experimental results and imply the coexistence of EB domains of opposite sign for all cooling fields, which results in a reversal mechanism not previously reported.

Antiferromagnetic/ferromagnetic nanostructures for multidigit storage units

Applied Physics Letters, 2014

The pursuit of higher densities in binary storage media is facing serious operating limitations. ... more The pursuit of higher densities in binary storage media is facing serious operating limitations. In order to overcome these constraints, several multistate techniques have been investigated as alternatives. Here, we report on an approach to define multistate switching memory units based on magnetic nanostructures exhibiting exchange bias. Writing and reading conditions were studied in patterned antiferromagnetic/ferromagnetic thin films. We establish the necessary and sufficient requirements for this multidigit memory concept that might open up new possibilities for the exploration and design of suitable room temperature spintronic devices. V

Copper Oxidation States, Vacancy Ordering and Their Effect on High Temperature Superconductivity

Novel Superconductivity, 1987

ABSTRACT

Coercivity enhancement above the Néel temperature of an antiferromagnet/ferromagnet bilayer

Journal of Applied Physics, Jul 18, 2002

Single-crystal thin films of the antiferromagnet FeF 2 have been used to exchange bias overlayers... more Single-crystal thin films of the antiferromagnet FeF 2 have been used to exchange bias overlayers of Fe. An unexpected coercivity enhancement is observed at temperatures above the Néel temperature of the FeF 2. This coercivity reaches a peak value of over 600 Oe close to the Néel temperature and persists to above 300 K. The coercivity is correlated with the growth of an anisotropy in the ferromagnet, the increase of the antiferromagnetic susceptibility and the increase of the ferromagnetic resonance linewidth. We argue that the growth of spin fluctuations in the antiferromagnet leads to an enhanced ferromagnetic anisotropy, and therefore coercivity, above the Néel temperature.

The spatial profile of unpinned and pinned uncompensated spins in the antiferromagnetic layer of an exchange bias heterostructure determined with polarized neutron reflectometry

Bulletin of the American Physical Society, Mar 23, 2005

Diego-The spin structure of an antiferromagnet plays a critically important role as a means to es... more Diego-The spin structure of an antiferromagnet plays a critically important role as a means to establish a reference state in magnetic devices; however, the spin structure at the surface and interior of an antiferromagnetic thin film remains unknown. We have used the unique spatial sensitivity of polarized neutron reflectometry to measure the depth dependence of magnetization across the interface between a ferromagnet and an antiferromagnet. The net uncompensated magnetization near the interface responds to applied field, while uncompensated spins in the antiferromagnet bulk are pinned. A new model is proposed for exchange bias.

Two-Stage Magnetization Reversal in Exchange Biased Bilayers

Physical Review Letters, May 7, 2001

MnF 2 ͞Fe bilayers exhibit asymmetric magnetization reversal that occurs by coherent rotation on ... more MnF 2 ͞Fe bilayers exhibit asymmetric magnetization reversal that occurs by coherent rotation on one side of the loop and by nucleation and propagation of domain walls on the other side of the loop. Here, we show by polarized neutron reflectometry, magnetization, and magnetotransport measurements that for samples with good crystalline "quality" the rotation is a two-stage process, due to coherent rotation to a stable state perpendicular to the cooling field direction. The result is remarkably asymmetrically shaped hysteresis loops.

Asymmetric Magnetization Reversal in Exchange-Biased Hysteresis Loops

Physical Review Letters, Apr 24, 2000

Polarized neutron reflectometry is used to probe the in-plane projection of the net-magnetization... more Polarized neutron reflectometry is used to probe the in-plane projection of the net-magnetization vector M of polycrystalline Fe films exchange coupled to twinned (110) MnF 2 or FeF 2 antiferromagnetic (AF) layers. The magnetization reversal mechanism depends upon the orientation of the cooling field with respect to the twinned microstructure of the AF, and whether the applied field is increased to (or decreased from) a positive saturating field; i.e., the magnetization reversal is asymmetric. The reversal of the sample magnetization from one saturated state to the other occurs via either domain wall motion or magnetization rotation on opposite sides of the same hysteresis loop.

The use of the anisotropic magnetoresistance to study exchange coupling

IEEE International Digest of Technical Papers on Magnetics Conference, Jun 25, 2003

Summary form only given. A microscopic understanding of the ferromagnetic/antiferromagnetic (F/AF... more Summary form only given. A microscopic understanding of the ferromagnetic/antiferromagnetic (F/AF) direct exchange coupling remains elusive. An experimental technique, based on the anisotropic magnetoresistance (AMR), to determine both the magnitude of small reversible rotations of the magnetization and the overall orientation of the magnetization has been developed. Three extensions of the original AMR technique include a separation of the exchange anisotropy into rotatable and rigid components, measurements of the time dependence of the exchange anisotropy and its relation to coercivity, and the symmetry of the interfacial exchange coupling-induced anisotropy and its effect on magnetization reversal. A discussion of the techniques and a summary of the results are presented.

In-Plane Magnetic Structure of Exchange Biased Co/FeF2

Bulletin of the American Physical Society, Mar 15, 2010

Diego-We report on measurements of magnetic X-Ray specular and diffuse scattering on an exchange ... more Diego-We report on measurements of magnetic X-Ray specular and diffuse scattering on an exchange biased Co/FeF2 bilayer. The data has been analyzed using the full theory of resonant x-ray scattering in the distorted wave born approximation, and various structural and magnetic roughness parameters have been extracted. We find that the length associated with magnetic corrections in the antiferromagnet increases as the temperature decreases, but remains unchanged for the ferromagnet. The diffuse scattering together with the specular reflectivity provide a more complete picture of the magnetic interfacial phenomena that give rise to exchange bias.

Quantitative x-ray photoelectron spectroscopy study of Al/AlOx bilayers

Journal of Applied Physics, May 30, 2002

An x-ray photoelectron spectroscopy ͑XPS͒ analysis of Nb/Al wedge bilayers, oxidized by both plas... more An x-ray photoelectron spectroscopy ͑XPS͒ analysis of Nb/Al wedge bilayers, oxidized by both plasma and natural oxidation, is reported. The main goal is to show that the oxidation state-i.e., O:͑oxidize͒Al ratio-, structure and thickness of the surface oxide layer, as well as the thickness of the metallic Al leftover, as functions of the oxidation procedure, can be quantitatively evaluated from the XPS spectra. This is relevant to the detailed characterization of the insulating barriers in ͑magnetic͒ tunnel junctions.

Temperature dependence of in-plane correlation lengths in exchange biased Co/FeF2

We have measured resonant soft x-ray diffuse magnetic scattering as a function of temperature in ... more We have measured resonant soft x-ray diffuse magnetic scattering as a function of temperature in a positively exchange biased Co/FeF 2 bilayer and analyzed the data in the distorted wave Born approximation to obtain in-plane charge and magnetic correlation lengths associated with the Co and FeF 2 layers and estimate interfacial roughness. Tuning to the Fe and Co L 3 edges reveals significantly different temperature trends in these quantities in the antiferromagnetic and ferromagnetic layers, respectively. While the magnetic correlation length of the uncompensated interfacial spins in FeF 2 layer increase as temperature decreases, these quantities remain unchanged in the Co layer. Our results indicate that uncompensated Fe spins order within a range of few hundred nanometers in otherwise randomly distributed uncompensated magnetic moments, giving rise to spin clusters in the antiferromagnet whose size increase as the temperature decrease.

Influence of interfacial disorder and temperature on magnetization reversal in exchange-coupled bilayers

Physical Review B, 2001

Polarized neutron reflectometry is used to measure the thermal response of the net-magnetization ... more Polarized neutron reflectometry is used to measure the thermal response of the net-magnetization vector of polycrystalline ferromagnetic ͑F͒ Fe films exchange coupled to twinned (110) MnF 2 antiferromagnetic ͑AF͒ layers. We observe a strong correlation between the temperature dependencies of the net sample magnetization perpendicular to the applied field at coercivity and exchange bias. For cooling field and measurement conditions involving magnetization reversal via rotation, we find a range of temperature dependencies. For the smoothest F-AF interface, the temperature dependence of exchange bias compares well to a Sϭ 5 2 Brillouin function-an observation predicted by some theoretical models. This temperature dependence is expected for the sublattice magnetization and the square root of the anisotropy constant ͱK 1 of bulk MnF 2. In contrast, for a rough F-AF interface the magnetization reversal process ͑and exchange bias͒ showed little temperature dependence up to temperatures approaching the AF Néel point-a clear consequence of increasing interfacial disorder in a F-AF epitaxial system.

Relation between exchange anisotropy and magnetization reversal asymmetry in Fe/MnF2 bilayers

Physical Review B, 2002

The angular dependence of the magnetic anisotropy of exchange biased Fe/MnF 2 bilayers was measur... more The angular dependence of the magnetic anisotropy of exchange biased Fe/MnF 2 bilayers was measured. Below the Néel temperature of the antiferromagnetic MnF 2 layer, an exchange anisotropy is observed which consists of unidirectional, uniaxial, threefold and fourfold symmetry components. The threefold exchange anisotropy term is responsible for the asymmetric magnetization reversal process recently observed in this system.

Influence of in-plane crystalline quality of an antiferromagnet on perpendicular exchange coupling and exchange bias

Physical Review B, 2002

We have undertaken a systematic study of the influence of in-plane crystalline quality of the ant... more We have undertaken a systematic study of the influence of in-plane crystalline quality of the antiferromagnet on exchange bias. Polarized neutron reflectometry and magnetometry were used to determine the anisotropies of polycrystalline ferromagnetic ͑F͒ Fe thin films exchange coupled to antiferromagnetic ͑AF͒ untwinned single crystal ͑110͒ FeF 2 , twinned single crystal ͑110͒ FeF 2 thin films and ͑110͒ textured polycrystalline FeF 2 thin films. A correlation between the anisotropies of the AF and F thin films with exchange bias was identified. Specifically, when exchange coupling across the F-AF interface introduces an additional anisotropy axis in the F thin film-one perpendicular to the cooling field, the magnetization reversal mechanism is affected ͑as observed with neutron scattering͒ and exchange bias is significantly enhanced.

Induced anisotropy and positive exchange bias: A temperature, angular, and cooling field study by ferromagnetic resonance

Physical Review B, 2002

Exchange-biased MnF 2 /Fe bilayers, examined by variable angle and temperature ferromagnetic reso... more Exchange-biased MnF 2 /Fe bilayers, examined by variable angle and temperature ferromagnetic resonance ͑FMR͒, exhibit a sudden onset of a unidirectional and fourfold anisotropy below the MnF 2 Néel temperature. This unexpected fourfold symmetry arises from frustrated perpendicular coupling between the MnF 2 and the Fe overlayer in the presence of twinning in the antiferromagnet layer. These data are consistent with earlier polarized-neutron-reflectometry results. The FMR data show a clear reversal in the direction of the unidirectional anisotropy as a function of cooling field, switching sign at H FC ϭ13 kOe, which is consistent with the onset of positive exchange bias observed in conventional magnetometry experiments. The low-temperature FMR linewidth reflects the in-plane symmetry of the resonance itself, exhibiting surprising divergence in the hard directions. Temperature-dependent FMR measurements reveal a sharp reduction in the resonance field below the Néel point due to the ferromagnetic/antiferromagnetic coupling.

Effect of anisotropy and exchange bias on reversal of sub-100 nm magnetic dots

INTERMAG 2006 - IEEE International Magnetics Conference, 2006

Detailed knowledge of magnetization reversal and possible magnetic configurations in magnetic nan... more Detailed knowledge of magnetization reversal and possible magnetic configurations in magnetic nanostructures is essential for high-density magnetic memory. Magnetism of nanostructured magnets, of size comparable to or smaller than ferromagnetic domain size, offers a great potential for new physics. We present results of a systematic study of magnetic reversal in sub-100 nm nanodots, and how this reversal can be affected

Coercivity of a percolative magnetic system

Physical Review B, 2000

A model that describes the behavior of the coercivity H c , of coalescing Ni films, as a percolat... more A model that describes the behavior of the coercivity H c , of coalescing Ni films, as a percolation phenomenon is presented. We show that there is a nonmonotonic dependence, of the coercive field as a function of the occupation probability p. This behavior is a function of cluster size and topology, and is independent of the growth direction of the thin film. Based on these ideas we predict theoretically, and confirm experimentally, the dependence of H c as a function of external magnetic field orientation. For a ͓111͔ oriented fcc film, if p is less than the percolation threshold p cr , a sixfold symmetry is observed. However, if pϷp cr the percolation is isotropic.

Manipulation of competing ferromagnetic and antiferromagnetic domains in exchange-biased nanostructures

Physical Review B, Nov 20, 2015

Using photoemission electron microscopy combined with X-ray magnetic circular dichroism we show t... more Using photoemission electron microscopy combined with X-ray magnetic circular dichroism we show that a progressive spatial confinement of a ferromagnet (FM), either through thickness variation or laterally via patterning, actively controls the domains of uncompensated spins in the antiferromagnet (AF) in exchange biased systems. Direct observations of the spin structure in both sides of the FM/AF interface in a model system, Ni/FeF 2 , show that the spin structure is determined by the balance between the competing FM and AF magnetic energies. Coexistence of exchange bias domains, with opposite directions, can be established in Ni/FeF 2 bilayers for Ni thicknesses below 10 nm. Patterning the Ni/FeF 2 heterostructures with antidots destabilizes the FM state, enhancing the formation of opposite exchange bias domains below a critical antidot separation of the order of a few FeF 2 crystal domains. The results suggest that dimensional confinement of the FM may be used to manipulate the AF spin structure in spintronic devices and ultra-high density information storage media. The underlying mechanism of the uncompensated AF domain formation in Ni/FeF 2 may be generic to other magnetic systems with complex non-collinear FM/AF spin structures.

The fabrication of ordered arrays of exchange biased Ni/FeF2nanostructures

Nanotechnology, 2010

The fabrication of ordered arrays of exchange biased Ni/FeF 2 nanostructures by focused ion beam ... more The fabrication of ordered arrays of exchange biased Ni/FeF 2 nanostructures by focused ion beam lithography is reported. High quality nano-elements, with controlled removal depth and no significant re-deposition, were carved using small ion beam currents (30 pA), moderate dwell times (1 µs) and repeated passages over the same area. Two types of nanostructures were fabricated: square arrays of circular dots with diameters from 125 ± 8 to 500 ± 12 nm and periodicities ranging from 200 ± 8 to 1000 ± 12 nm, and square arrays of square antidots (207 ± 8 nm in edge length) with periodicities ranging from 300 ± 8 to 1200 ± 12 nm. The arrays were characterized using scanning ion and electron microscopy, and atomic force microscopy. The effect of the patterning on the exchange bias field (i.e., the shift in the hysteresis loop of ferromagnetic Ni due to proximity to antiferromagnetic FeF 2 ) was studied using magneto-transport measurements. These high quality nanostructures offer a unique method to address some of the open questions regarding the microscopic origin of exchange bias. This is not only of major relevance in the fabrication and miniaturization of magnetic devices but it is also one of the important proximity phenomena in nanoscience and materials science.

Three-dimensional spin structure in exchange-biased antiferromagnetic/ferromagnetic thin films

Applied Physics Letters, 2009

A coexistence of lateral and in-depth domain walls in antiferromagnet/ferromagnet (AF/FM) thin fi... more A coexistence of lateral and in-depth domain walls in antiferromagnet/ferromagnet (AF/FM) thin films exhibiting double hysteresis loops (DHLs) is demonstrated. Comparison of single and DHLs together with local and global measurements confirms the formation of two oppositely oriented domains in the AF that imprint a lateral domain structure into the FM layer. Most significantly, the magnetization reversal mechanism within each opposite domain takes place by incoherent rotation of spring-like domain walls extending through the Ni thickness. Therefore, complex three-dimensional domain walls are created perpendicular and parallel to the AF/FM interface in exchange biased systems.

Mirror symmetry in magnetization reversal and coexistence of positive and negative exchange bias in Ni/FeF2

Applied Physics Letters, 2011

Positively and negatively exchange biased (PEB and NEB) magnetoresistance (MR) loops in Ni/FeF2 f... more Positively and negatively exchange biased (PEB and NEB) magnetoresistance (MR) loops in Ni/FeF2 ferromagnetic/antiferromagnetic (AF) heterostructures proceed through the same reversal mechanisms. The MR curves exhibit mirror symmetry: the increasing (decreasing) field branch of the PEB (NEB) loop is identical to the decreasing (increasing) branch of the NEB (PEB) loop, suggesting that the interfacial areal density of pinned uncompensated AF spins responsible for PEB and NEB is similar. Micromagnetic simulations are in agreement with experimental results and imply the coexistence of EB domains of opposite sign for all cooling fields, which results in a reversal mechanism not previously reported.

Antiferromagnetic/ferromagnetic nanostructures for multidigit storage units

Applied Physics Letters, 2014

The pursuit of higher densities in binary storage media is facing serious operating limitations. ... more The pursuit of higher densities in binary storage media is facing serious operating limitations. In order to overcome these constraints, several multistate techniques have been investigated as alternatives. Here, we report on an approach to define multistate switching memory units based on magnetic nanostructures exhibiting exchange bias. Writing and reading conditions were studied in patterned antiferromagnetic/ferromagnetic thin films. We establish the necessary and sufficient requirements for this multidigit memory concept that might open up new possibilities for the exploration and design of suitable room temperature spintronic devices. V

Copper Oxidation States, Vacancy Ordering and Their Effect on High Temperature Superconductivity

Novel Superconductivity, 1987

ABSTRACT

Coercivity enhancement above the Néel temperature of an antiferromagnet/ferromagnet bilayer

Journal of Applied Physics, Jul 18, 2002

Single-crystal thin films of the antiferromagnet FeF 2 have been used to exchange bias overlayers... more Single-crystal thin films of the antiferromagnet FeF 2 have been used to exchange bias overlayers of Fe. An unexpected coercivity enhancement is observed at temperatures above the Néel temperature of the FeF 2. This coercivity reaches a peak value of over 600 Oe close to the Néel temperature and persists to above 300 K. The coercivity is correlated with the growth of an anisotropy in the ferromagnet, the increase of the antiferromagnetic susceptibility and the increase of the ferromagnetic resonance linewidth. We argue that the growth of spin fluctuations in the antiferromagnet leads to an enhanced ferromagnetic anisotropy, and therefore coercivity, above the Néel temperature.

The spatial profile of unpinned and pinned uncompensated spins in the antiferromagnetic layer of an exchange bias heterostructure determined with polarized neutron reflectometry

Bulletin of the American Physical Society, Mar 23, 2005

Diego-The spin structure of an antiferromagnet plays a critically important role as a means to es... more Diego-The spin structure of an antiferromagnet plays a critically important role as a means to establish a reference state in magnetic devices; however, the spin structure at the surface and interior of an antiferromagnetic thin film remains unknown. We have used the unique spatial sensitivity of polarized neutron reflectometry to measure the depth dependence of magnetization across the interface between a ferromagnet and an antiferromagnet. The net uncompensated magnetization near the interface responds to applied field, while uncompensated spins in the antiferromagnet bulk are pinned. A new model is proposed for exchange bias.

Two-Stage Magnetization Reversal in Exchange Biased Bilayers

Physical Review Letters, May 7, 2001

MnF 2 ͞Fe bilayers exhibit asymmetric magnetization reversal that occurs by coherent rotation on ... more MnF 2 ͞Fe bilayers exhibit asymmetric magnetization reversal that occurs by coherent rotation on one side of the loop and by nucleation and propagation of domain walls on the other side of the loop. Here, we show by polarized neutron reflectometry, magnetization, and magnetotransport measurements that for samples with good crystalline "quality" the rotation is a two-stage process, due to coherent rotation to a stable state perpendicular to the cooling field direction. The result is remarkably asymmetrically shaped hysteresis loops.

Asymmetric Magnetization Reversal in Exchange-Biased Hysteresis Loops

Physical Review Letters, Apr 24, 2000

Polarized neutron reflectometry is used to probe the in-plane projection of the net-magnetization... more Polarized neutron reflectometry is used to probe the in-plane projection of the net-magnetization vector M of polycrystalline Fe films exchange coupled to twinned (110) MnF 2 or FeF 2 antiferromagnetic (AF) layers. The magnetization reversal mechanism depends upon the orientation of the cooling field with respect to the twinned microstructure of the AF, and whether the applied field is increased to (or decreased from) a positive saturating field; i.e., the magnetization reversal is asymmetric. The reversal of the sample magnetization from one saturated state to the other occurs via either domain wall motion or magnetization rotation on opposite sides of the same hysteresis loop.

The use of the anisotropic magnetoresistance to study exchange coupling

IEEE International Digest of Technical Papers on Magnetics Conference, Jun 25, 2003

Summary form only given. A microscopic understanding of the ferromagnetic/antiferromagnetic (F/AF... more Summary form only given. A microscopic understanding of the ferromagnetic/antiferromagnetic (F/AF) direct exchange coupling remains elusive. An experimental technique, based on the anisotropic magnetoresistance (AMR), to determine both the magnitude of small reversible rotations of the magnetization and the overall orientation of the magnetization has been developed. Three extensions of the original AMR technique include a separation of the exchange anisotropy into rotatable and rigid components, measurements of the time dependence of the exchange anisotropy and its relation to coercivity, and the symmetry of the interfacial exchange coupling-induced anisotropy and its effect on magnetization reversal. A discussion of the techniques and a summary of the results are presented.

In-Plane Magnetic Structure of Exchange Biased Co/FeF2

Bulletin of the American Physical Society, Mar 15, 2010

Diego-We report on measurements of magnetic X-Ray specular and diffuse scattering on an exchange ... more Diego-We report on measurements of magnetic X-Ray specular and diffuse scattering on an exchange biased Co/FeF2 bilayer. The data has been analyzed using the full theory of resonant x-ray scattering in the distorted wave born approximation, and various structural and magnetic roughness parameters have been extracted. We find that the length associated with magnetic corrections in the antiferromagnet increases as the temperature decreases, but remains unchanged for the ferromagnet. The diffuse scattering together with the specular reflectivity provide a more complete picture of the magnetic interfacial phenomena that give rise to exchange bias.

Quantitative x-ray photoelectron spectroscopy study of Al/AlOx bilayers

Journal of Applied Physics, May 30, 2002

An x-ray photoelectron spectroscopy ͑XPS͒ analysis of Nb/Al wedge bilayers, oxidized by both plas... more An x-ray photoelectron spectroscopy ͑XPS͒ analysis of Nb/Al wedge bilayers, oxidized by both plasma and natural oxidation, is reported. The main goal is to show that the oxidation state-i.e., O:͑oxidize͒Al ratio-, structure and thickness of the surface oxide layer, as well as the thickness of the metallic Al leftover, as functions of the oxidation procedure, can be quantitatively evaluated from the XPS spectra. This is relevant to the detailed characterization of the insulating barriers in ͑magnetic͒ tunnel junctions.

Temperature dependence of in-plane correlation lengths in exchange biased Co/FeF2

We have measured resonant soft x-ray diffuse magnetic scattering as a function of temperature in ... more We have measured resonant soft x-ray diffuse magnetic scattering as a function of temperature in a positively exchange biased Co/FeF 2 bilayer and analyzed the data in the distorted wave Born approximation to obtain in-plane charge and magnetic correlation lengths associated with the Co and FeF 2 layers and estimate interfacial roughness. Tuning to the Fe and Co L 3 edges reveals significantly different temperature trends in these quantities in the antiferromagnetic and ferromagnetic layers, respectively. While the magnetic correlation length of the uncompensated interfacial spins in FeF 2 layer increase as temperature decreases, these quantities remain unchanged in the Co layer. Our results indicate that uncompensated Fe spins order within a range of few hundred nanometers in otherwise randomly distributed uncompensated magnetic moments, giving rise to spin clusters in the antiferromagnet whose size increase as the temperature decrease.

Influence of interfacial disorder and temperature on magnetization reversal in exchange-coupled bilayers

Physical Review B, 2001

Polarized neutron reflectometry is used to measure the thermal response of the net-magnetization ... more Polarized neutron reflectometry is used to measure the thermal response of the net-magnetization vector of polycrystalline ferromagnetic ͑F͒ Fe films exchange coupled to twinned (110) MnF 2 antiferromagnetic ͑AF͒ layers. We observe a strong correlation between the temperature dependencies of the net sample magnetization perpendicular to the applied field at coercivity and exchange bias. For cooling field and measurement conditions involving magnetization reversal via rotation, we find a range of temperature dependencies. For the smoothest F-AF interface, the temperature dependence of exchange bias compares well to a Sϭ 5 2 Brillouin function-an observation predicted by some theoretical models. This temperature dependence is expected for the sublattice magnetization and the square root of the anisotropy constant ͱK 1 of bulk MnF 2. In contrast, for a rough F-AF interface the magnetization reversal process ͑and exchange bias͒ showed little temperature dependence up to temperatures approaching the AF Néel point-a clear consequence of increasing interfacial disorder in a F-AF epitaxial system.

Relation between exchange anisotropy and magnetization reversal asymmetry in Fe/MnF2 bilayers

Physical Review B, 2002

The angular dependence of the magnetic anisotropy of exchange biased Fe/MnF 2 bilayers was measur... more The angular dependence of the magnetic anisotropy of exchange biased Fe/MnF 2 bilayers was measured. Below the Néel temperature of the antiferromagnetic MnF 2 layer, an exchange anisotropy is observed which consists of unidirectional, uniaxial, threefold and fourfold symmetry components. The threefold exchange anisotropy term is responsible for the asymmetric magnetization reversal process recently observed in this system.

Influence of in-plane crystalline quality of an antiferromagnet on perpendicular exchange coupling and exchange bias

Physical Review B, 2002

We have undertaken a systematic study of the influence of in-plane crystalline quality of the ant... more We have undertaken a systematic study of the influence of in-plane crystalline quality of the antiferromagnet on exchange bias. Polarized neutron reflectometry and magnetometry were used to determine the anisotropies of polycrystalline ferromagnetic ͑F͒ Fe thin films exchange coupled to antiferromagnetic ͑AF͒ untwinned single crystal ͑110͒ FeF 2 , twinned single crystal ͑110͒ FeF 2 thin films and ͑110͒ textured polycrystalline FeF 2 thin films. A correlation between the anisotropies of the AF and F thin films with exchange bias was identified. Specifically, when exchange coupling across the F-AF interface introduces an additional anisotropy axis in the F thin film-one perpendicular to the cooling field, the magnetization reversal mechanism is affected ͑as observed with neutron scattering͒ and exchange bias is significantly enhanced.

Induced anisotropy and positive exchange bias: A temperature, angular, and cooling field study by ferromagnetic resonance

Physical Review B, 2002

Exchange-biased MnF 2 /Fe bilayers, examined by variable angle and temperature ferromagnetic reso... more Exchange-biased MnF 2 /Fe bilayers, examined by variable angle and temperature ferromagnetic resonance ͑FMR͒, exhibit a sudden onset of a unidirectional and fourfold anisotropy below the MnF 2 Néel temperature. This unexpected fourfold symmetry arises from frustrated perpendicular coupling between the MnF 2 and the Fe overlayer in the presence of twinning in the antiferromagnet layer. These data are consistent with earlier polarized-neutron-reflectometry results. The FMR data show a clear reversal in the direction of the unidirectional anisotropy as a function of cooling field, switching sign at H FC ϭ13 kOe, which is consistent with the onset of positive exchange bias observed in conventional magnetometry experiments. The low-temperature FMR linewidth reflects the in-plane symmetry of the resonance itself, exhibiting surprising divergence in the hard directions. Temperature-dependent FMR measurements reveal a sharp reduction in the resonance field below the Néel point due to the ferromagnetic/antiferromagnetic coupling.

Effect of anisotropy and exchange bias on reversal of sub-100 nm magnetic dots

INTERMAG 2006 - IEEE International Magnetics Conference, 2006

Detailed knowledge of magnetization reversal and possible magnetic configurations in magnetic nan... more Detailed knowledge of magnetization reversal and possible magnetic configurations in magnetic nanostructures is essential for high-density magnetic memory. Magnetism of nanostructured magnets, of size comparable to or smaller than ferromagnetic domain size, offers a great potential for new physics. We present results of a systematic study of magnetic reversal in sub-100 nm nanodots, and how this reversal can be affected

Coercivity of a percolative magnetic system

Physical Review B, 2000

A model that describes the behavior of the coercivity H c , of coalescing Ni films, as a percolat... more A model that describes the behavior of the coercivity H c , of coalescing Ni films, as a percolation phenomenon is presented. We show that there is a nonmonotonic dependence, of the coercive field as a function of the occupation probability p. This behavior is a function of cluster size and topology, and is independent of the growth direction of the thin film. Based on these ideas we predict theoretically, and confirm experimentally, the dependence of H c as a function of external magnetic field orientation. For a ͓111͔ oriented fcc film, if p is less than the percolation threshold p cr , a sixfold symmetry is observed. However, if pϷp cr the percolation is isotropic.