Sam Ladak | Cardiff University (original) (raw)
Papers by Sam Ladak
Applied Physics Letters, 2005
Hot electron transport has been studied in three terminal Ta/ TaO x / Co/ AlO x /Ni 81 Fe 19 stru... more Hot electron transport has been studied in three terminal Ta/ TaO x / Co/ AlO x /Ni 81 Fe 19 structures fabricated by magnetron sputtering through shadow masks. With the Co base and Ta collector connected together via a small resistor, the collector current contains contributions first from hot electrons injected from the Ni 81 Fe 19 emitter, and second from a geometrical artifact that leads to tunneling from the Fermi level in the base. Both sources of collector current lead to a room temperature magnetocurrent effect. The hot electron contribution begins to dominate as the emitter-base voltage −V eb exceeds 0.3 V.
Journal of Applied Physics, 2005
Double-barrier magnetic tunnel junctions (DBMTJs) of composition Co/AlOx/Co/AlOx/Ni81Fe19 have be... more Double-barrier magnetic tunnel junctions (DBMTJs) of composition Co/AlOx/Co/AlOx/Ni81Fe19 have been fabricated by magnetron sputtering through shadow masks. Two terminal measurements made upon the individual tunnel barriers revealed nonlinear I-V curves and significant room-temperature tunnel magnetoresistance (TMR) in all cases. Measurements were also performed with connections made to all three electrodes. The TMR of a particular tunnel barrier within the DBMTJ can be strongly modified by applying a bias voltage to the other barrier, while the TMR measured across the two barriers in series decreases more slowly with increasing bias voltage than for a single barrier. With zero bias applied between the central Co base electrode and the Co collector electrode, the collector current was measured as electrons were injected from the Ni81Fe19 electrode. For structures grown on Si/SiO2 substrates, the collector current showed a nonmonotonic dependence upon the emitter-base bias voltage, and collector magnetocurrent values in excess of 100% were observed at nonzero emitter-base bias values. For structures grown on quartz the collector current increased while the magnetocurrent decreased with increasing emitter-base voltage. We suggest that the enhanced TMR and magnetocurrent effects can be explained by substrate leakage and geometrical artifacts rather than by transport of spin-polarized hot electrons across the base layer.
Journal of Applied Physics, 2006
Soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co L2,3 edge have been ... more Soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co L2,3 edge have been applied to explore the near-interfacial magnetism of Co electrodes in Co/alumina based magnetic tunnel junctions. By taking into account the formation of CoO at the ferromagnetic (FM)/barrier interface, the change in the total magnetic moment on metallic Co atoms as a function of barrier oxidation has been determined. The results demonstrate a strong correlation between the Co moments and measured TMR values, and an enhancement in the Co moments for moderate oxidation times.
Journal of Physics D-applied Physics, 2003
We have used a magneto-optical pump-probe technique to stimulate and characterize small amplitude... more We have used a magneto-optical pump-probe technique to stimulate and characterize small amplitude precession of the magnetization in a variety of thin film magnetic structures. The sample is stimulated by an optically triggered magnetic field pulse and its response probed by means of a stroboscopic measurement of the magneto-optical Kerr effect. We demonstrate that the experimental technique is sensitive to small amplitude deflections of the magnetization and present formulae that describe the frequency of precession in continuous films containing either one or two magnetic layers. We show how the dependence of the precession frequency upon the orientation and magnitude of the static field may be used to characterize a 100 Å Co thin film, a Ni 81 Fe 19 (50 Å)/Cu(20 Å)/ Co(50 Å)/IrMn(100 Å) spin-valve structure, and a square Ni 81 Fe 19 element of 10 µm side and 150 nm thickness. Information may be obtained about the g factor, demagnetizing field, exchange bias effect and magnetic anisotropy within an individual ferromagnetic layer, and about the coupling between layers. We deduce the presence of a four-fold anisotropy in the square element that is associated with non-uniformity of the magnetization. Time resolved images show that the dynamic magnetization is generally non-uniform. A stripe pattern is observed when the static field is significantly larger than the anisotropy field. Measurements at low field show that the dynamic magnetization initially has a regular pattern associated with the underlying domain structure, but then evolves into a highly disordered state. Finally we discuss the outlook for further development and application of the experimental technique.
Applied Physics Letters, 2004
The electronic structure and polarization in magnetic tunnel junctions prepared with varying degr... more The electronic structure and polarization in magnetic tunnel junctions prepared with varying degrees of barrier-layer oxidation have been studied using x-ray absorption spectroscopy across the Co L2,3 absorption edges. It was found that the Co electronic structure near the Co/alumina interface tended to that of cobalt oxide as the barrier oxidation time was increased. However, the net Co 3d spin polarization, determined from x-ray magnetic circular dichroism, increased for moderate oxidation times compared to that obtained for an under-oxidized Co/Al interface. It is proposed that the expected dilution of the measured polarization due to the formation of (room temperature) paramagnetic cobalt oxide, is offset by an increase in the Co 3d spin-polarization of the interface layer as the interface bonding changes from Co-Al to Co-O with increasing oxidation times.
Journal of Applied Physics, 2005
It is shown that a coplanar stripline structure containing indium tin oxide windows can be used t... more It is shown that a coplanar stripline structure containing indium tin oxide windows can be used to perform optical ferromagnetic resonance measurements on a sample grown on an opaque substrate, using a pulsed magnetic field of any desired orientation. The technique is demonstrated by applying it to a thin film of permalloy grown on a Si substrate. The measured precession frequency was found to be in good agreement with macrospin simulations. The phase of the oscillatory Kerr response was observed to vary as the probe spot was scanned across the coplanar stripline structure, confirming that the orientation of the pulsed field varied from parallel to perpendicular relative to the plane of the sample.
Applied Physics Letters, 2005
Hot electron transport has been studied in three terminal Ta/ TaO x / Co/ AlO x /Ni 81 Fe 19 stru... more Hot electron transport has been studied in three terminal Ta/ TaO x / Co/ AlO x /Ni 81 Fe 19 structures fabricated by magnetron sputtering through shadow masks. With the Co base and Ta collector connected together via a small resistor, the collector current contains contributions first from hot electrons injected from the Ni 81 Fe 19 emitter, and second from a geometrical artifact that leads to tunneling from the Fermi level in the base. Both sources of collector current lead to a room temperature magnetocurrent effect. The hot electron contribution begins to dominate as the emitter-base voltage −V eb exceeds 0.3 V.
Journal of Applied Physics, 2005
Double-barrier magnetic tunnel junctions (DBMTJs) of composition Co/AlOx/Co/AlOx/Ni81Fe19 have be... more Double-barrier magnetic tunnel junctions (DBMTJs) of composition Co/AlOx/Co/AlOx/Ni81Fe19 have been fabricated by magnetron sputtering through shadow masks. Two terminal measurements made upon the individual tunnel barriers revealed nonlinear I-V curves and significant room-temperature tunnel magnetoresistance (TMR) in all cases. Measurements were also performed with connections made to all three electrodes. The TMR of a particular tunnel barrier within the DBMTJ can be strongly modified by applying a bias voltage to the other barrier, while the TMR measured across the two barriers in series decreases more slowly with increasing bias voltage than for a single barrier. With zero bias applied between the central Co base electrode and the Co collector electrode, the collector current was measured as electrons were injected from the Ni81Fe19 electrode. For structures grown on Si/SiO2 substrates, the collector current showed a nonmonotonic dependence upon the emitter-base bias voltage, and collector magnetocurrent values in excess of 100% were observed at nonzero emitter-base bias values. For structures grown on quartz the collector current increased while the magnetocurrent decreased with increasing emitter-base voltage. We suggest that the enhanced TMR and magnetocurrent effects can be explained by substrate leakage and geometrical artifacts rather than by transport of spin-polarized hot electrons across the base layer.
Journal of Applied Physics, 2006
Soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co L2,3 edge have been ... more Soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co L2,3 edge have been applied to explore the near-interfacial magnetism of Co electrodes in Co/alumina based magnetic tunnel junctions. By taking into account the formation of CoO at the ferromagnetic (FM)/barrier interface, the change in the total magnetic moment on metallic Co atoms as a function of barrier oxidation has been determined. The results demonstrate a strong correlation between the Co moments and measured TMR values, and an enhancement in the Co moments for moderate oxidation times.
Journal of Physics D-applied Physics, 2003
We have used a magneto-optical pump-probe technique to stimulate and characterize small amplitude... more We have used a magneto-optical pump-probe technique to stimulate and characterize small amplitude precession of the magnetization in a variety of thin film magnetic structures. The sample is stimulated by an optically triggered magnetic field pulse and its response probed by means of a stroboscopic measurement of the magneto-optical Kerr effect. We demonstrate that the experimental technique is sensitive to small amplitude deflections of the magnetization and present formulae that describe the frequency of precession in continuous films containing either one or two magnetic layers. We show how the dependence of the precession frequency upon the orientation and magnitude of the static field may be used to characterize a 100 Å Co thin film, a Ni 81 Fe 19 (50 Å)/Cu(20 Å)/ Co(50 Å)/IrMn(100 Å) spin-valve structure, and a square Ni 81 Fe 19 element of 10 µm side and 150 nm thickness. Information may be obtained about the g factor, demagnetizing field, exchange bias effect and magnetic anisotropy within an individual ferromagnetic layer, and about the coupling between layers. We deduce the presence of a four-fold anisotropy in the square element that is associated with non-uniformity of the magnetization. Time resolved images show that the dynamic magnetization is generally non-uniform. A stripe pattern is observed when the static field is significantly larger than the anisotropy field. Measurements at low field show that the dynamic magnetization initially has a regular pattern associated with the underlying domain structure, but then evolves into a highly disordered state. Finally we discuss the outlook for further development and application of the experimental technique.
Applied Physics Letters, 2004
The electronic structure and polarization in magnetic tunnel junctions prepared with varying degr... more The electronic structure and polarization in magnetic tunnel junctions prepared with varying degrees of barrier-layer oxidation have been studied using x-ray absorption spectroscopy across the Co L2,3 absorption edges. It was found that the Co electronic structure near the Co/alumina interface tended to that of cobalt oxide as the barrier oxidation time was increased. However, the net Co 3d spin polarization, determined from x-ray magnetic circular dichroism, increased for moderate oxidation times compared to that obtained for an under-oxidized Co/Al interface. It is proposed that the expected dilution of the measured polarization due to the formation of (room temperature) paramagnetic cobalt oxide, is offset by an increase in the Co 3d spin-polarization of the interface layer as the interface bonding changes from Co-Al to Co-O with increasing oxidation times.
Journal of Applied Physics, 2005
It is shown that a coplanar stripline structure containing indium tin oxide windows can be used t... more It is shown that a coplanar stripline structure containing indium tin oxide windows can be used to perform optical ferromagnetic resonance measurements on a sample grown on an opaque substrate, using a pulsed magnetic field of any desired orientation. The technique is demonstrated by applying it to a thin film of permalloy grown on a Si substrate. The measured precession frequency was found to be in good agreement with macrospin simulations. The phase of the oscillatory Kerr response was observed to vary as the probe spot was scanned across the coplanar stripline structure, confirming that the orientation of the pulsed field varied from parallel to perpendicular relative to the plane of the sample.