Publications


Anisotropic giant magnetoresistance in granular CoAg films

J MAGN MAGN MATER 165 (1997) 323-325

M Gester, K Ounadjela, JF Gregg, SM Thompson

We present magnetisation and giant magnetoresistance (GMR) measurements for MBE grown CoAg granular films with the external field applied parallel and perpendicular to the film plane. Both the magnetisation and the GMR show uniaxial anisotropy with the easy axis of magnetisation out-of-plane for Co concentrations of 30 at% and less and in the film plane for higher Co concentrations. We attribute this behaviour to the shape anisotropy of the magnetic particles in the films.


Magnetic anisotropy in giant magnetoresistive CoAg granular films

JOURNAL OF APPLIED PHYSICS 81 (1997) 5207-5207

M Gester, Y Henry, SM Thompson, JF Gregg, K Ounadjela


The ferromagnetic domain wall as a GMR trilayer

J MAGN MAGN MATER 165 (1997) 121-124

W Allen, JF Gregg, K Ounadjela, M Viret, M Hehn, SM Thompson, JMD Coey

By exploiting the simplicity of a novel transport measurement on a ferromagnetic striped domain structure in a thin film of cobalt, we report the first direct observation of ferromagnetic domain wall scattering and what we believe to be the first clear indication of giant magnetoresistive effects (GMR) in a homogeneous magnetic system. A new model is proposed to describe these observations which highlights the crucial role played by electron spin precession in determining the electrical transport properties of magnetic interfaces.


Correlation between the structural and transport properties of as-grown epitaxial phase-separated Co-Ag thin films

Physical Review B - Condensed Matter and Materials Physics 54 (1996) 12252-12261

K Ounadjela, S Thompson

Giant magnetoresistive Co-Ag granular films are prepared by molecular beam epitaxy at three different temperatures with cobalt concentrations ranging from 30 to 80 at. %. The structural properties of these films are studied using a variety of diffraction techniques and real space probes. Magnetization and magnetotransport measurements are carried out at room temperature. Correlating the structural and magnetotransport properties reveals that the spin diffusion length strongly affects the amplitude of the giant magnetoresistance, which is otherwise dominated by the surface-to-volume ratio of the segregated magnetic precipitates. © 1996 The American Physical Society.


Spin scattering in ferromagnetic thin films

PHYSICAL REVIEW B 53 (1996) 8464-8468

M Viret, D Vignoles, D Cole, JMD Coey, W Allen, DS Daniel, JF Gregg


Giant magnetoresistive effects in a single element magnetic thin film

PHYSICAL REVIEW LETTERS 77 (1996) 1580-1583

JF Gregg, W Allen, K Ounadjela, M Viret, M Hehn, SM Thompson, JMD Coey


Structural characterization of epitaxial Co-Ag

J APPL PHYS 79 (1996) 6247-6249

A Azizi, L ElChahal, K Ounadjela, JP Deville, SM Thompson, JF Gregg

Co-Ag granular films are prepared by molecular-beam epitaxy to enable a detailed structural study to be conducted in conjunction with magnetotransport properties. It is demonstrated that the shape, size, location, and segregation of the magnetic Co grains and the morphology of the surface depend on the growth temperature. Detailed transmission electron microscopy reveals inherent differences between samples prepared at room temperature and those prepared at high temperature. Small clusters of only a few atoms are found to be present in the matrix of the low-temperature samples significantly affecting the spin diffusion length. Well-crystallized regions of both Co and Ag are found in the high-temperature samples, however, the Co grains remain relatively small in size even at the highest growth temperatures. (C) 1996 American Institute of Physics.


Jitterbug spin channel mixing in heterogeneous giant magnetoresistive material

J APPL PHYS 79 (1996) 5593-5595

JF Gregg, W Allen, SM Thompson, ML Watson, GA Gehring

A mechanism is described which considers the effect of small magnetic particles on the spin diffusion length in a granular giant magnetoresistive material. Spin depolarization occurs by precession of the spin orientation of the carrier due to the s-d exchange interaction within a magnetic particle. Numerical simulation of this jitterbug effect is found to generate a temperature and field dependence of the distance a carrier may travel within the sample without losing its spin memory. (C) 1996 American Institute of Physics.


Electron spin depolarisation in granular magnetic systems

J MAGN MAGN MATER 140 (1995) 501-502

GA GEHRING, JF GREGG, SM THOMPSON, ML WATSON

A new mechanism for spin channel mixing is discussed which contributes to the effective spin diffusion length in granular giant magnetoresistive materials. Two limiting cases are considered in which magnetic clusters act either to filter out the down component of the spin channel or cause the quantisation axis to precess.


GIANT MAGNETORESISTANCE IN METASTABLE ALLOYS PREPARED BY PLASMAS SPUTTERING AND MECHANICAL ALLOYING

FIZIKA METALLOV I METALLOVEDENIE 79 (1995) 88-97

SM THOMPSON, JF GREGG, D DANIEL, K UNADIELA


Thermal conductivity of a giant magnetoresistive mechanical alloy

J MAGN MAGN MATER 140 (1995) 493-494

D DANIEL, JF GREGG, SM THOMPSON, JMD COEY, A FAGAN, K OUNADJELA, C FERMON, G SAUX

We report correlated measurements of magnetothermal and magnetoelectric conductivities in a cobalt-silver mechanical alloy. Their variation with magnetisation indicates that their respective relaxation times have similar origins. At low temperatures the thermal and electric conductivities are similar; however at higher temperatures an increasing phonon contribution reduces the magnetothermal effect.


MAGNETIC-INTERACTIONS IN GIANT MAGNETORESISTIVE CO-AG HETEROGENEOUS FILMS

IEEE T MAGN 30 (1994) 726-728

SM THOMPSON, JF GREGG, SJ DAWSON, CR STADDON, K OUNADJELA, J HAMMANN, C FERMON


Effect of magnetic interactions and multiple magnetic phases on the giant magnetoresistance of heterogeneous cobalt-silver thin films

Physical Review B 49 (1994) 1064-1072

JF Gregg, SM Thompson, SJ Dawson, K Ounadjela, CR Staddon, J Hamman, C Fermon, G Saux, K O'Grady

We have observed magnetic and electrical transport properties in giant magnetoresistive inhomogeneous cobalt-silver films. The material consists of two distinct magnetic phases: large clusters which dominate the magnetization and magnetoresistive processes at room temperature, between which cooperative behavior is observed; and small clusters which dominate the magnetization below 10 K, but make only a minor contribution to the magnetoresistance. Both the variation of the magnetoresistance with magnetization in the film and the difference in the magnetoresistance between the zero-field-cooled and field-cycled state are interpreted by invoking interactions between the active magnetic regions in the sample. © 1994 The American Physical Society.


Effects of annealing on microstructure of Co-Ag granular alloy films

(1994) 1171-1172

AK PETFORDLONG, RC DOOLE, JF GREGG, CR STADDON


GIANT MAGNETORESISTANCE AND INDUCED EXCHANGE-ANISOTROPY IN MECHANICALLY ALLOYED CO30AG70

J APPL PHYS 75 (1994) 6921-6923

K OUNADJELA, A HERR, R POINSOT, JMD COEY, A FAGAN, CR STADDON, D DANIEL, JF GREGG, SM THOMPSON, K OGRADY, S GRIEVES


GIANT MAGNETORESISTANCE OF COBALT SILVER METASTABLE ALLOYS PREPARED BY SPUTTERING AND MECHANICAL ALLOYING

PHILOSOPHICAL MAGAZINE B-PHYSICS OF CONDENSED MATTER STATISTICAL MECHANICS ELECTRONIC OPTICAL AND MAGNETIC PROPERTIES 68 (1993) 923-937

SM THOMPSON, JF GREGG, CR STADDON, D DANIEL, SJ DAWSON, K OUNADJELA, J HAMMANN, C FERMON, G SAUX, K OGRADY, SJ GRIEVES, JMD COEY, A FAGAN


ONION SKIN DOMAINS IN A RELAXING METASTABLE ANTIFERROMAGNET

J MAGN MAGN MATER 104 (1992) 373-374

SJ DAWSON, JF GREGG, JS LORD, MR WELLS, WP WOLF


MICROWAVE ULTRASONIC MEASUREMENT OF NUCLEAR-SPIN DIFFUSION

J MAGN MAGN MATER 104 (1992) 957-958

JF GREGG, JS LORD


RELAXATION DYNAMICS OF METASTABLE ANTIFERROMAGNETIC STATES

J APPL PHYS 67 (1990) 5430-5432

JF GREGG, W KOB, JS LORD, ID MORRIS, JZ PFEFFER, R SCHILLING, MR WELLS, WP WOLF


RELAXATION DYNAMICS OF METASTABLE ANTIFERROMAGNETIC STATES

J MAGN MAGN MATER 90-1 (1990) 184-186

SJ DAWSON, JF GREGG, JS LORD, ID MORRIS, JZ PFEFFER, MR WELLS, WP WOLF

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