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Papers by Sam Foster

Research paper thumbnail of Resonant magnetic x-ray and neutron diffuse studies of transition metal multilayers

Journal of Applied Physics, 2003

Electron scattering mechanisms within metallic multilayers are affected by both structural and ma... more Electron scattering mechanisms within metallic multilayers are affected by both structural and magnetic disorders. Off-specular x-ray scattering has long been used to probe the structural interfaces, and it is only recently that it has been applied to the study of magnetic disorder. We compare the resonant magnetic x-ray scattering with off-specular neutron studies from magnetron-sputtered Co/Cu and Co/Ru multilayers grown at the second antiferromagnetic coupling peak. Both techniques yield similar results for the Cu system, and a simple domain model can be applied to extract the magnetic interface morphological parameters. For the Cu system, the in-plane correlation length is field dependent and is 880+/-20 Å after saturation along the hard axis, but increases to 7000+/-100 Å after saturation along the orthogonal easy axis. Both systems show strong out-of-plane correlations in both the structural and magnetic disorders. In all cases, the out-of-plane correlation length for the structural interfaces is 200-250 Å, but the ratio of the magnetic to structural correlations length is dependent on the magnitude of the exchange coupling and ranges from 0.4 to 1.4.

Research paper thumbnail of Propagation of magnetic and superconducting order in Gd/La superlattices

Journal of Physics-condensed Matter, 2005

Single-crystal Gd/La superlattices have been grown by molecular beam epitaxy. The magnetic orderi... more Single-crystal Gd/La superlattices have been grown by molecular beam epitaxy. The magnetic ordering has been determined using neutron scattering and the superconducting transitions have been identified using SQUID magnetometry. The Gd layers order ferromagnetically below T~280 K and the interlayer coupling across the La layers is antiferromagnetic. Surprisingly, when cooled in zero field below T~5 K the Gd/La superlattices exhibit

Research paper thumbnail of Advancing and exploring off-specular scattering” meets in Abingdon

Neutron News, 2000

ABSTRACT

Research paper thumbnail of Exchange Bias in Spin-Engineered Double Superlattices

Physical Review Letters, 2002

Exchange bias has been observed in sputtered magnetic double superlattices which consist of a fer... more Exchange bias has been observed in sputtered magnetic double superlattices which consist of a ferromagnetically coupled superlattice grown on an antiferromagnetically (AF) coupled superlattice. This system exhibits a parallel domain wall, a spin flop transition, and exchange bias when the anisotropy is large in the AF block. This work shows that neither the domain wall nor the spin flop are directly related to exchange bias but that the anisotropy is essential.

Research paper thumbnail of Interplay between superconductivity and magnetism in Gd/La superlattices

Journal of Magnetism and Magnetic Materials, 2002

High-pressure magnetization, structural and 57 Fe Mössbauer studies were performed on superconduc... more High-pressure magnetization, structural and 57 Fe Mössbauer studies were performed on superconducting Rb 0.8 Fe 1.6 Se 2.0 with T c = 32.4 K. The superconducting transition temperature gradually decreases on increasing pressure up to 5.0 GPa followed by a marked step-like suppression of superconductivity near 6 GPa. No structural phase transition in the Fe vacancy-ordered superstructure is observed in synchrotron XRD studies up to 15.6 GPa, while the Mössbauer spectra above 5 GPa reveal the appearance of a new paramagnetic phase and significant changes in the magnetic and electronic properties of the dominant antiferromagnetic phase, coinciding with the disappearance of superconductivity. These findings underline the strong correlation between antiferromagnetic order and superconductivity in phase-separated A x Fe 2-x/2 Se 2 (A = K, Rb, Cs) superconductors. PACS number(s): 74.70.Xa, 74.62.Fj, 75.30.-m * s.medvedev@mpic.de

Research paper thumbnail of Mapping domain disorder in exchange-biased magnetic multilayers

Physical Review B, 2002

Exchange anisotropy occurs at the interface between an antiferromagnetic (AF) layer and a ferroma... more Exchange anisotropy occurs at the interface between an antiferromagnetic (AF) layer and a ferromagnetic layer, and results in a ferromagnet hysteresis loop displaced along the field axis. We have performed off-specular neutron reflectometry in order to characterize the domain structure in Co layers that are exchange biased by FeMn. This allows us to determine the domain direction distributions and lateral

Research paper thumbnail of Resonant magnetic x-ray and neutron diffuse studies of transition metal multilayers

Journal of Applied Physics, 2003

Electron scattering mechanisms within metallic multilayers are affected by both structural and ma... more Electron scattering mechanisms within metallic multilayers are affected by both structural and magnetic disorders. Off-specular x-ray scattering has long been used to probe the structural interfaces, and it is only recently that it has been applied to the study of magnetic disorder. We compare the resonant magnetic x-ray scattering with off-specular neutron studies from magnetron-sputtered Co/Cu and Co/Ru multilayers grown at the second antiferromagnetic coupling peak. Both techniques yield similar results for the Cu system, and a simple domain model can be applied to extract the magnetic interface morphological parameters. For the Cu system, the in-plane correlation length is field dependent and is 880+/-20 Å after saturation along the hard axis, but increases to 7000+/-100 Å after saturation along the orthogonal easy axis. Both systems show strong out-of-plane correlations in both the structural and magnetic disorders. In all cases, the out-of-plane correlation length for the structural interfaces is 200-250 Å, but the ratio of the magnetic to structural correlations length is dependent on the magnitude of the exchange coupling and ranges from 0.4 to 1.4.

Research paper thumbnail of Propagation of magnetic and superconducting order in Gd/La superlattices

Journal of Physics-condensed Matter, 2005

Single-crystal Gd/La superlattices have been grown by molecular beam epitaxy. The magnetic orderi... more Single-crystal Gd/La superlattices have been grown by molecular beam epitaxy. The magnetic ordering has been determined using neutron scattering and the superconducting transitions have been identified using SQUID magnetometry. The Gd layers order ferromagnetically below T~280 K and the interlayer coupling across the La layers is antiferromagnetic. Surprisingly, when cooled in zero field below T~5 K the Gd/La superlattices exhibit

Research paper thumbnail of Advancing and exploring off-specular scattering” meets in Abingdon

Neutron News, 2000

ABSTRACT

Research paper thumbnail of Exchange Bias in Spin-Engineered Double Superlattices

Physical Review Letters, 2002

Exchange bias has been observed in sputtered magnetic double superlattices which consist of a fer... more Exchange bias has been observed in sputtered magnetic double superlattices which consist of a ferromagnetically coupled superlattice grown on an antiferromagnetically (AF) coupled superlattice. This system exhibits a parallel domain wall, a spin flop transition, and exchange bias when the anisotropy is large in the AF block. This work shows that neither the domain wall nor the spin flop are directly related to exchange bias but that the anisotropy is essential.

Research paper thumbnail of Interplay between superconductivity and magnetism in Gd/La superlattices

Journal of Magnetism and Magnetic Materials, 2002

High-pressure magnetization, structural and 57 Fe Mössbauer studies were performed on superconduc... more High-pressure magnetization, structural and 57 Fe Mössbauer studies were performed on superconducting Rb 0.8 Fe 1.6 Se 2.0 with T c = 32.4 K. The superconducting transition temperature gradually decreases on increasing pressure up to 5.0 GPa followed by a marked step-like suppression of superconductivity near 6 GPa. No structural phase transition in the Fe vacancy-ordered superstructure is observed in synchrotron XRD studies up to 15.6 GPa, while the Mössbauer spectra above 5 GPa reveal the appearance of a new paramagnetic phase and significant changes in the magnetic and electronic properties of the dominant antiferromagnetic phase, coinciding with the disappearance of superconductivity. These findings underline the strong correlation between antiferromagnetic order and superconductivity in phase-separated A x Fe 2-x/2 Se 2 (A = K, Rb, Cs) superconductors. PACS number(s): 74.70.Xa, 74.62.Fj, 75.30.-m * s.medvedev@mpic.de

Research paper thumbnail of Mapping domain disorder in exchange-biased magnetic multilayers

Physical Review B, 2002

Exchange anisotropy occurs at the interface between an antiferromagnetic (AF) layer and a ferroma... more Exchange anisotropy occurs at the interface between an antiferromagnetic (AF) layer and a ferromagnetic layer, and results in a ferromagnet hysteresis loop displaced along the field axis. We have performed off-specular neutron reflectometry in order to characterize the domain structure in Co layers that are exchange biased by FeMn. This allows us to determine the domain direction distributions and lateral