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Cliff Tanaka

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Papers by Cliff Tanaka

Dielectric tensor for magneto-optic NiMnSb

Physical Review B, 1999

The diagonal component xx and off-diagonal component xy of the complex dielectric tensor for the ... more The diagonal component xx and off-diagonal component xy of the complex dielectric tensor for the ferromagnetic compound NiMnSb are determined using ex situ spectroscopic ellipsometry and magneto-optic analysis over the spectral range from 0.7 to 6.2 eV. The effects of the overcoat on the raw data are removed by the analysis. First, the complex xx of thin-film NiMnSb were determined by ex situ spectroscopic ellipsometry; then xy was determined by analyzing Kerr rotation and ellipticity data using the determined xy data. Lorentz oscillators were used to model peaks seen in the xx spectra. The diagonal dielectric component xx is dominated by free-carrier effects below 1.15 eV, and dominated by interband transitions above 2.0 eV. The center energies of the Lorentz oscillators are consistent with the calculated band structure and minority-spin optical conductivity of NiMnSb. Joint density of states and optical conductivity calculated from xx (2) data with free-carrier effects removed shows onset energies at ϳ0.6 and ϳ0.2 eV, respectively. From a study of the xx and xy spectra, the Kerr rotation peak at lower energy is determined to be due to combined contributions from: ͑1͒ a crossover between the free-carrier effect and interband transitions, and ͑2͒ transitions involving spin-orbit coupling. The high-Kerr rotation peaks at higher energies result exclusively from transitions involving spin-orbit coupling. ͓S0163-1829͑99͒13615-4͔

Spin-polarized tunneling in a half-metallic ferromagnet

Journal of applied physics, Dec 1, 1999

Direct measurement of the conduction electron spin polarization (P) in epitaxial NiMnSb was perfo... more Direct measurement of the conduction electron spin polarization (P) in epitaxial NiMnSb was performed to test the prediction of half metallicity in this material. Spin-polarized tunneling in NiMnSb/Al 2 O 3/Al junctions showed P of 28%, contrary to the predicted value of 100%. Magnetoresistance measurements in NiMnSb/Al 2 O 3/Ni 80 Fe 20 junctions concurred with this result. The discrepancy between theory and experiment is discussed. Also, the latter junctions show four nonvolatile remanent states due to the ...

Calculation of magnetic moments in Ho2C3 nanocrystals

Journal of Applied Physics, 1994

A general approach to the computation of effective magnetic moments in rare-earth carbides is des... more A general approach to the computation of effective magnetic moments in rare-earth carbides is described, and details of this calculation for Ho(3+) ions in Ho2C3 are presented. This calculation is designed to explain the reduced magnetic moments, relative to free ion values, measured by dc SQUID magnetometry for Ho2C3 nanocrystals. Crystal-field splittings of the rare-earth ion in a particular symmetry

<title>Chemical vapor composite silicon carbide for space telescopes</title>

Space Telescopes and Instrumentation I: Optical, Infrared, and Millimeter, 2006

ABSTRACT Components for space telescopes using high quality silicon carbide (SiC) produced via th... more ABSTRACT Components for space telescopes using high quality silicon carbide (SiC) produced via the chemical vapor composite (CVC) process are currently under development. This CVC process is a modification of chemical vapor deposition (CVD) and results in a dramatic reduction in residual stress of the SiC deposit. The resultant CVC SiC material has high modulus, high thermal conductivity and can be polished to better than 1nm RMS surface roughness, making it ideal for space telescopes requiring lightweight, stiff and thermally stable components. Moreover, due to its lower intrinsic stress, CVC SiC is much more readily scaled to large sizes and manufactured into the complex geometries needed for the telescope assemblies. Results are presented on the optical figure for a lightweight 15cm CVC SiC mirror demonstrating low wavefront error (

Spin-polarized tunneling, magnetoresistance and interfacial effects in ferromagnetic junctions

The pioneering studies of spin-polarized tunnelling by Meservey and Tedrow in the early 1970s sho... more The pioneering studies of spin-polarized tunnelling by Meservey and Tedrow in the early 1970s showed that the conduction electrons in ferromagnetic (FM) metals are spin polarized and that the spin is conserved in the tunnelling process. Only recently (1995) improved material fabrication techniques have permitted realization of the Jullie Á re quantitative model, showing that tunnelling in ferromagnet/insulator/ferromagnet (FM/I/FM) junctions should lead to a large junction magnetoresistance (JMR); JMR values greater than 30% have been achieved at room temperature. This recent success has led to several fundamental questions regarding the phenomenon of spin tunnelling and also the development of JMR devices. In this paper, experimental results, such as the dependence on bias, temperature and barrier characteristics of FM/I/FM tunnelling are reviewed brie¯y. The in¯uence of inelastic tunnelling processes, metal at the interface and material properties on the JMR is discussed. The future direction from both the physics and the applications viewpoints, is also covered. } 1. INTRODUCTION Spin-polarized tunnelling (SPT), discovered by Meservey et al. (1970) and Merservey and Tedrow (1971, 1994), laid the foundation to a new ® eld of research. Meservey and Tedrow measured the conduction-electron spin polarization P in magnetic metals and compounds using the Zeeman split quasiparticle density of states in a superconductor as the spin detector. Tunnelling from a ferromagnetic (FM) ® lm, with its uneven spin distribution at the Fermi level E F , into such a spinsplit superconducting Al ® lm re¯ects the spin polarization of the tunnelling electrons coming from the ferromagnet. Values, of P recently measured are higher owing to improved junction preparation conditions including samples grown by molecularbeam epitaxy. Highly polarized tunnelling electrons can also be obtained through a phenomenon called the spin-® lter e ect using magnetic semiconductors such as EuS and EuSe as tunnel barriers (Moodera et al. 1988, 1990, 1993). Jullie Á re (1975) made the ® rst reported magnetoresistance measurement on a ferromagnet/insulator/ferromagnet (FM/I/FM) trilayer junction and interpreted it by stating that the tunnelling current should depend on the relative orientation of the magnetizations of the electrodes. The tunnel junction magnetoresistance (JMR) is

Low cost phase shifters and integrated phased antenna arrays designs based on the ferroelectric materials technology

A novel, high performance, and low cost phase shifter design is proposed and its performance has ... more A novel, high performance, and low cost phase shifter design is proposed and its performance has been simulated and numerically evaluated. The proposed design is based on a multi-dielectric material CPW design and the utilization of ferroelectric material for achieving the phase shifting. It is shown that, by including a low loss thin dielectric layer between the coplanar conductors and the ferroelectric materials, significant improvement in the insertion loss can be achieved (∼5 dB) and as high as three fold increase in the figure of merit (°/dB) is possible. A similar multilayer approach was implemented on a phased antenna array using continuous transverse stubs (CTS) technology to achieve low cost and high performance, including beam steering. Initial results are encouraging, particularly regarding the reduction in insertion loss, increasing the input impedance to values of practical interest, and also achieving beam steering.

High Performance Low Cost Ferroelectric Phase Shifters Designed for Simple Biasing

A novel approach in ferroelectric phase shifters design using Ba x Sr 1-x TiO 3 (BSTO) films in a... more A novel approach in ferroelectric phase shifters design using Ba x Sr 1-x TiO 3 (BSTO) films in a multilayer dielectric coplanar waveguide structure is described. By including a low loss dielectric layer (SiO 2) between the coplanar waveguide conductors and the ferroelectric material in conjunction with a via to allow the signal conductors to contact the ferroelectric layer to employ biasing, significant reduction in insertion loss can be achieved in conjunction with a three fold increase in figure of merit (o /dB) compared to the case with direct metallization on the ferroelectric layer.

Design, development and experimental verification of voltage tunable ferroelectric coplanar phase shifters

2010 IEEE Antennas and Propagation Society International Symposium, 2010

Page 1. Design, Development and Experimental Verification of Voltage Tunable Ferroelectric Coplan... more

<title>Recent achievements using chemical vapor composite silicon carbide (CVC SiC)</title>

Optical Materials and Structures Technologies IV, 2009

ABSTRACT This annual review documents our progress towards inexpensive mass production of silicon... more ABSTRACT This annual review documents our progress towards inexpensive mass production of silicon carbide mirrors and optical structures. Results are provided for a NASA Small Business Technology Transfer (STTR) X-Ray Mirror project. Trex partnered with the University of Alabama-Huntsville Center for Advanced Optics (UAH-CAO) to develop fabrication methods for polished cylindrical and conical chemical vapor composite (CVCTM) SiC mandrels. These mandrels are envisioned as pre-forms for the replication of fused silica x-ray optics to be eventually used in the International X-Ray Observatory (IXO). CVC SiCTM offers superior high temperature stability, thermal and mechanical performance and polishability required for this precision replication process. In this program, Trex fabricated prototype mandrels with design diameters of 10.5cm, 20cm and 45cm. UAH-CAO was Trex&#39;s university partner in this effort and worked on polishing and metrology of the unusual x-ray mandrel geometries. UAH-CAO successfully developed an innovative interferometric method for measuring the CVC SiCTM x-ray mandrels based on a precision cylindrical lens system. UAH-CAO also developed finishing and polishing methods for CVC SiCTM that utilized a Zeeko IRP200 computer controlled polishing tool. The three technologies key technologies demonstrated in this program (near net shape forming of CVC SiCTM mandrels, the x-ray mandrel metrology and free-form polishing capability on CVC SiCTM) could enable cost-effective manufacture of the x-ray mandrels required for the International X-Ray Observatory (IXO).

Spin-resolved density of states at the surface of NiMnSb

Physical Review B, 2001

Spin-resolved photoemission measurements from polycrystalline NiMnSb films reveal spin-resolved d... more

Cluster-based Monte Carlo simulation of ferrofluids

Physical Review E, 1999

We demonstrate a Monte Carlo algorithm for efficiently simulating ferrofluids. By identifying par... more We demonstrate a Monte Carlo algorithm for efficiently simulating ferrofluids. By identifying particle clusters and evolving them as single units, we reduce correlation times by more than two orders of magnitude. This method enables accurate calculations of ferrofluid thermodynamics in the limit of strong magnetic coupling that would be impossible by conventional means. We apply the method to study magnetic anisotropy in dilute thin films. ͓S1063-651X͑99͒05702-5͔

Formation of doped Al2O3 tunnel barriers by plasma oxidation of δ-doped Al

Surface Science, 2000

The formation of doped Al 2 O 3 tunnel barriers, by plasma oxidation of doped Al, has been studie... more The formation of doped Al 2 O 3 tunnel barriers, by plasma oxidation of doped Al, has been studied by in situ Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Ultrathin layers (<1 ML) of Ni or Si dopants were incorporated into an Al film at different depths, and subsequently plasma oxidized. Comparison of AES peak heights reveals two different types of oxidation behavior of the dopants. In the case where Ni or Si are initially present on top of the Al, they completely disappear from the immediate surface region after oxidation. Thus, Al from underneath the dopants goes through the dopant layer during oxidation, thereby burying the dopants in the Al-oxide. In contrast, for dopants sandwiched between two 7 Å Al films no significant vertical redistribution occurs, while XPS spectra show that dopants are completely oxidized. The data is used to support previous magnetoresistance measurements on magnetic tunnel junctions with doped barriers.

Calculation of magnetic moments in Ho2C3 nanocrystals

Journal of Applied Physics, 1994

A general approach to the computation of effective magnetic moments in rare-earth carbides is des... more A general approach to the computation of effective magnetic moments in rare-earth carbides is described, and details of this calculation for Ho3+ ions in Ho,C, are presented. This calculation is designed to explain the reduced magnetic moments, relative to free ion values, measured by dc SQUID magnetometry for Ho& nanocrystals. Crystal-field splittings of the rare-earth ion in a particular symmetry site are determined by the operator equivalent method. Using the eigenvalues and eigenfunctions of the crystal-field Hamiltonian, the effective magnetic moment is then determined. For Ho3+ ions in Ho&&, this method predicts a reduced magnetic moment, but the degree of reduction depends on the energy-level splittings and, therefore, the temperature. This magnetic moment is compared with previous experimental results, and the implications of the formal carbon charge, screening, and temperature are discussed.

High-performance low-cost phase-shifter design based on ferroelectric materials technology

Electronics Letters, 2004

ABSTRACT A novel approach to ferroelectric phase-shifter design using BaxSr1-xTiO3 (BSTO) films i... more ABSTRACT A novel approach to ferroelectric phase-shifter design using BaxSr1-xTiO3 (BSTO) films in a multilayer dielectric coplanar waveguide structure is described. By including a low-loss dielectric layer (SiO2) between the coplanar waveguide conductors and a layer of ferroelectric materials, significant reduction in insertion loss can be achieved in conjunction with a threefold increase in figure of merit (°/dB).

Spin-polarized tunnelling, magnetoresistance and interfacial effects in ferromagnetic junctions

The pioneering studies of spin-polarized tunnelling by Meservey and Tedrow in the early 1970s sho... more The pioneering studies of spin-polarized tunnelling by Meservey and Tedrow in the early 1970s showed that the conduction electrons in ferromagnetic (FM) metals are spin polarized and that the spin is conserved in the tunnelling process. Only recently (1995) improved material fabrication techniaues have permitted realization of the Julliere quantitative model, showing that tunnelling in ferromagnet/insulator/ferromagnet (FM/I/FM) junctions should lead to a large junction magnetoresistance (JMR); JMR values greater than 30% have been achieved at room temperature. This recent success has led to several fundamental auestions regarding the phenomenon of spin tunnelling and also the development of JMR devices. In this paper, experimental results, such as the dependence on bias, temperature and barrier characteristics of FM/I/FM tunnelling are reviewed briefly. The influence of inelastic tunnelling processes, metal at the interface and material properties on the JMR is discussed. The futur...

Dielectric tensor for magneto-optic NiMnSb

Physical Review B, 1999

The diagonal component xx and off-diagonal component xy of the complex dielectric tensor for the ... more The diagonal component xx and off-diagonal component xy of the complex dielectric tensor for the ferromagnetic compound NiMnSb are determined using ex situ spectroscopic ellipsometry and magneto-optic analysis over the spectral range from 0.7 to 6.2 eV. The effects of the overcoat on the raw data are removed by the analysis. First, the complex xx of thin-film NiMnSb were determined by ex situ spectroscopic ellipsometry; then xy was determined by analyzing Kerr rotation and ellipticity data using the determined xy data. Lorentz oscillators were used to model peaks seen in the xx spectra. The diagonal dielectric component xx is dominated by free-carrier effects below 1.15 eV, and dominated by interband transitions above 2.0 eV. The center energies of the Lorentz oscillators are consistent with the calculated band structure and minority-spin optical conductivity of NiMnSb. Joint density of states and optical conductivity calculated from xx (2) data with free-carrier effects removed shows onset energies at ϳ0.6 and ϳ0.2 eV, respectively. From a study of the xx and xy spectra, the Kerr rotation peak at lower energy is determined to be due to combined contributions from: ͑1͒ a crossover between the free-carrier effect and interband transitions, and ͑2͒ transitions involving spin-orbit coupling. The high-Kerr rotation peaks at higher energies result exclusively from transitions involving spin-orbit coupling. ͓S0163-1829͑99͒13615-4͔

Spin-polarized tunneling in a half-metallic ferromagnet

Journal of applied physics, Dec 1, 1999

Direct measurement of the conduction electron spin polarization (P) in epitaxial NiMnSb was perfo... more Direct measurement of the conduction electron spin polarization (P) in epitaxial NiMnSb was performed to test the prediction of half metallicity in this material. Spin-polarized tunneling in NiMnSb/Al 2 O 3/Al junctions showed P of 28%, contrary to the predicted value of 100%. Magnetoresistance measurements in NiMnSb/Al 2 O 3/Ni 80 Fe 20 junctions concurred with this result. The discrepancy between theory and experiment is discussed. Also, the latter junctions show four nonvolatile remanent states due to the ...

Calculation of magnetic moments in Ho2C3 nanocrystals

Journal of Applied Physics, 1994

A general approach to the computation of effective magnetic moments in rare-earth carbides is des... more A general approach to the computation of effective magnetic moments in rare-earth carbides is described, and details of this calculation for Ho(3+) ions in Ho2C3 are presented. This calculation is designed to explain the reduced magnetic moments, relative to free ion values, measured by dc SQUID magnetometry for Ho2C3 nanocrystals. Crystal-field splittings of the rare-earth ion in a particular symmetry

<title>Chemical vapor composite silicon carbide for space telescopes</title>

Space Telescopes and Instrumentation I: Optical, Infrared, and Millimeter, 2006

ABSTRACT Components for space telescopes using high quality silicon carbide (SiC) produced via th... more ABSTRACT Components for space telescopes using high quality silicon carbide (SiC) produced via the chemical vapor composite (CVC) process are currently under development. This CVC process is a modification of chemical vapor deposition (CVD) and results in a dramatic reduction in residual stress of the SiC deposit. The resultant CVC SiC material has high modulus, high thermal conductivity and can be polished to better than 1nm RMS surface roughness, making it ideal for space telescopes requiring lightweight, stiff and thermally stable components. Moreover, due to its lower intrinsic stress, CVC SiC is much more readily scaled to large sizes and manufactured into the complex geometries needed for the telescope assemblies. Results are presented on the optical figure for a lightweight 15cm CVC SiC mirror demonstrating low wavefront error (

Spin-polarized tunneling, magnetoresistance and interfacial effects in ferromagnetic junctions

The pioneering studies of spin-polarized tunnelling by Meservey and Tedrow in the early 1970s sho... more The pioneering studies of spin-polarized tunnelling by Meservey and Tedrow in the early 1970s showed that the conduction electrons in ferromagnetic (FM) metals are spin polarized and that the spin is conserved in the tunnelling process. Only recently (1995) improved material fabrication techniques have permitted realization of the Jullie Á re quantitative model, showing that tunnelling in ferromagnet/insulator/ferromagnet (FM/I/FM) junctions should lead to a large junction magnetoresistance (JMR); JMR values greater than 30% have been achieved at room temperature. This recent success has led to several fundamental questions regarding the phenomenon of spin tunnelling and also the development of JMR devices. In this paper, experimental results, such as the dependence on bias, temperature and barrier characteristics of FM/I/FM tunnelling are reviewed brie¯y. The in¯uence of inelastic tunnelling processes, metal at the interface and material properties on the JMR is discussed. The future direction from both the physics and the applications viewpoints, is also covered. } 1. INTRODUCTION Spin-polarized tunnelling (SPT), discovered by Meservey et al. (1970) and Merservey and Tedrow (1971, 1994), laid the foundation to a new ® eld of research. Meservey and Tedrow measured the conduction-electron spin polarization P in magnetic metals and compounds using the Zeeman split quasiparticle density of states in a superconductor as the spin detector. Tunnelling from a ferromagnetic (FM) ® lm, with its uneven spin distribution at the Fermi level E F , into such a spinsplit superconducting Al ® lm re¯ects the spin polarization of the tunnelling electrons coming from the ferromagnet. Values, of P recently measured are higher owing to improved junction preparation conditions including samples grown by molecularbeam epitaxy. Highly polarized tunnelling electrons can also be obtained through a phenomenon called the spin-® lter e ect using magnetic semiconductors such as EuS and EuSe as tunnel barriers (Moodera et al. 1988, 1990, 1993). Jullie Á re (1975) made the ® rst reported magnetoresistance measurement on a ferromagnet/insulator/ferromagnet (FM/I/FM) trilayer junction and interpreted it by stating that the tunnelling current should depend on the relative orientation of the magnetizations of the electrodes. The tunnel junction magnetoresistance (JMR) is

Low cost phase shifters and integrated phased antenna arrays designs based on the ferroelectric materials technology

A novel, high performance, and low cost phase shifter design is proposed and its performance has ... more A novel, high performance, and low cost phase shifter design is proposed and its performance has been simulated and numerically evaluated. The proposed design is based on a multi-dielectric material CPW design and the utilization of ferroelectric material for achieving the phase shifting. It is shown that, by including a low loss thin dielectric layer between the coplanar conductors and the ferroelectric materials, significant improvement in the insertion loss can be achieved (∼5 dB) and as high as three fold increase in the figure of merit (°/dB) is possible. A similar multilayer approach was implemented on a phased antenna array using continuous transverse stubs (CTS) technology to achieve low cost and high performance, including beam steering. Initial results are encouraging, particularly regarding the reduction in insertion loss, increasing the input impedance to values of practical interest, and also achieving beam steering.

High Performance Low Cost Ferroelectric Phase Shifters Designed for Simple Biasing

A novel approach in ferroelectric phase shifters design using Ba x Sr 1-x TiO 3 (BSTO) films in a... more A novel approach in ferroelectric phase shifters design using Ba x Sr 1-x TiO 3 (BSTO) films in a multilayer dielectric coplanar waveguide structure is described. By including a low loss dielectric layer (SiO 2) between the coplanar waveguide conductors and the ferroelectric material in conjunction with a via to allow the signal conductors to contact the ferroelectric layer to employ biasing, significant reduction in insertion loss can be achieved in conjunction with a three fold increase in figure of merit (o /dB) compared to the case with direct metallization on the ferroelectric layer.

Design, development and experimental verification of voltage tunable ferroelectric coplanar phase shifters

2010 IEEE Antennas and Propagation Society International Symposium, 2010

Page 1. Design, Development and Experimental Verification of Voltage Tunable Ferroelectric Coplan... more

<title>Recent achievements using chemical vapor composite silicon carbide (CVC SiC)</title>

Optical Materials and Structures Technologies IV, 2009

ABSTRACT This annual review documents our progress towards inexpensive mass production of silicon... more ABSTRACT This annual review documents our progress towards inexpensive mass production of silicon carbide mirrors and optical structures. Results are provided for a NASA Small Business Technology Transfer (STTR) X-Ray Mirror project. Trex partnered with the University of Alabama-Huntsville Center for Advanced Optics (UAH-CAO) to develop fabrication methods for polished cylindrical and conical chemical vapor composite (CVCTM) SiC mandrels. These mandrels are envisioned as pre-forms for the replication of fused silica x-ray optics to be eventually used in the International X-Ray Observatory (IXO). CVC SiCTM offers superior high temperature stability, thermal and mechanical performance and polishability required for this precision replication process. In this program, Trex fabricated prototype mandrels with design diameters of 10.5cm, 20cm and 45cm. UAH-CAO was Trex&#39;s university partner in this effort and worked on polishing and metrology of the unusual x-ray mandrel geometries. UAH-CAO successfully developed an innovative interferometric method for measuring the CVC SiCTM x-ray mandrels based on a precision cylindrical lens system. UAH-CAO also developed finishing and polishing methods for CVC SiCTM that utilized a Zeeko IRP200 computer controlled polishing tool. The three technologies key technologies demonstrated in this program (near net shape forming of CVC SiCTM mandrels, the x-ray mandrel metrology and free-form polishing capability on CVC SiCTM) could enable cost-effective manufacture of the x-ray mandrels required for the International X-Ray Observatory (IXO).

Spin-resolved density of states at the surface of NiMnSb

Physical Review B, 2001

Spin-resolved photoemission measurements from polycrystalline NiMnSb films reveal spin-resolved d... more

Cluster-based Monte Carlo simulation of ferrofluids

Physical Review E, 1999

We demonstrate a Monte Carlo algorithm for efficiently simulating ferrofluids. By identifying par... more We demonstrate a Monte Carlo algorithm for efficiently simulating ferrofluids. By identifying particle clusters and evolving them as single units, we reduce correlation times by more than two orders of magnitude. This method enables accurate calculations of ferrofluid thermodynamics in the limit of strong magnetic coupling that would be impossible by conventional means. We apply the method to study magnetic anisotropy in dilute thin films. ͓S1063-651X͑99͒05702-5͔

Formation of doped Al2O3 tunnel barriers by plasma oxidation of δ-doped Al

Surface Science, 2000

The formation of doped Al 2 O 3 tunnel barriers, by plasma oxidation of doped Al, has been studie... more The formation of doped Al 2 O 3 tunnel barriers, by plasma oxidation of doped Al, has been studied by in situ Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Ultrathin layers (<1 ML) of Ni or Si dopants were incorporated into an Al film at different depths, and subsequently plasma oxidized. Comparison of AES peak heights reveals two different types of oxidation behavior of the dopants. In the case where Ni or Si are initially present on top of the Al, they completely disappear from the immediate surface region after oxidation. Thus, Al from underneath the dopants goes through the dopant layer during oxidation, thereby burying the dopants in the Al-oxide. In contrast, for dopants sandwiched between two 7 Å Al films no significant vertical redistribution occurs, while XPS spectra show that dopants are completely oxidized. The data is used to support previous magnetoresistance measurements on magnetic tunnel junctions with doped barriers.

Calculation of magnetic moments in Ho2C3 nanocrystals

Journal of Applied Physics, 1994

A general approach to the computation of effective magnetic moments in rare-earth carbides is des... more A general approach to the computation of effective magnetic moments in rare-earth carbides is described, and details of this calculation for Ho3+ ions in Ho,C, are presented. This calculation is designed to explain the reduced magnetic moments, relative to free ion values, measured by dc SQUID magnetometry for Ho& nanocrystals. Crystal-field splittings of the rare-earth ion in a particular symmetry site are determined by the operator equivalent method. Using the eigenvalues and eigenfunctions of the crystal-field Hamiltonian, the effective magnetic moment is then determined. For Ho3+ ions in Ho&&, this method predicts a reduced magnetic moment, but the degree of reduction depends on the energy-level splittings and, therefore, the temperature. This magnetic moment is compared with previous experimental results, and the implications of the formal carbon charge, screening, and temperature are discussed.

High-performance low-cost phase-shifter design based on ferroelectric materials technology

Electronics Letters, 2004

ABSTRACT A novel approach to ferroelectric phase-shifter design using BaxSr1-xTiO3 (BSTO) films i... more ABSTRACT A novel approach to ferroelectric phase-shifter design using BaxSr1-xTiO3 (BSTO) films in a multilayer dielectric coplanar waveguide structure is described. By including a low-loss dielectric layer (SiO2) between the coplanar waveguide conductors and a layer of ferroelectric materials, significant reduction in insertion loss can be achieved in conjunction with a threefold increase in figure of merit (°/dB).

Spin-polarized tunnelling, magnetoresistance and interfacial effects in ferromagnetic junctions

The pioneering studies of spin-polarized tunnelling by Meservey and Tedrow in the early 1970s sho... more The pioneering studies of spin-polarized tunnelling by Meservey and Tedrow in the early 1970s showed that the conduction electrons in ferromagnetic (FM) metals are spin polarized and that the spin is conserved in the tunnelling process. Only recently (1995) improved material fabrication techniaues have permitted realization of the Julliere quantitative model, showing that tunnelling in ferromagnet/insulator/ferromagnet (FM/I/FM) junctions should lead to a large junction magnetoresistance (JMR); JMR values greater than 30% have been achieved at room temperature. This recent success has led to several fundamental auestions regarding the phenomenon of spin tunnelling and also the development of JMR devices. In this paper, experimental results, such as the dependence on bias, temperature and barrier characteristics of FM/I/FM tunnelling are reviewed briefly. The influence of inelastic tunnelling processes, metal at the interface and material properties on the JMR is discussed. The futur...