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ABSTRACT: Magnetic and magneto-optical properties of Co films are studied as a function of the morphology and the capping layer. We show that the nanoparticulate structure of the Co films has a clear influence on the magnetic and magneto-optical properties of the system. Kerr measurements combined with x-ray magnetic circular dichroism provide evidence of a strong correlation between the collective magnetic behavior of the system and the individual atomic magnetic response. The influence of the magnetic nature of the capping layer (Al, Au, and Pt) is also analyzed. Polarized capping layers, such as Pt, magnetically couple the nanostructures and not only increase the effective anisotropy of the system but also enhance the atomic magnetic moment of Co and the global magneto-optical activity.
Physical Review B 01/2008; 77(9). · 3.69 Impact Factor
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ABSTRACT: General rules are given for the branching ratio of x-ray absorption lines. The branching ratio between two spin-orbit split absorption edges of a deep core level will deviate from its statistical value only if the spin-orbit splitting in the valence band is large compared to the band width. The branching ratio obtains a maximum value for the Hund's rule ground state and decreases gradually for the other J values. This regular behaviour can be used to assess the spin-orbit splitting in the 4d and 5d band of transition metal compounds.
EPL (Europhysics Letters) 07/2007; 4(9):1083. · 2.17 Impact Factor
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ABSTRACT: We present experimental results on resonant inelastic x-ray scattering (RIXS) in perpendicular geometry excited with circularly polarized x rays at the Co L3,2 edges in Co ferrite (CoFe2O4) and Co metal and at the Fe L3,2 edges in Fe metal. In this geometry with magnetic samples the incident x-ray beam is perpendicular to the magnetization and the magnetic dichroism is only present in RIXS. We study the 3s-RIXS channel terminating in an inner shell 3s hole and extra 3d electron (2p63dn→2p53dn+1→2p63s13dn+1). The work is based on the measurement of the spectra vs incident photon energy and integrated over the scattered photon energy. We present data resolved in linear polarization of the scattered beam for scattering at 45° and data summed over the outgoing polarizations and taken vs the scattering angle φ. The results show common features in both metals and ionic system: the dichroism scales as sin 2φ while the spectral shape remains constant. The dichroism is maximum when the outgoing linear polarization is in the scattering plane and zero when the outgoing linear polarization is perpendicular to the scattering plane. The presence of the dichroism is due to the breaking of the mirror symmetry of the geometry consisting of the helicity vector of the incident radiation, the sample magnetization, and the linear polarization of the scattered beam. This is expected, in general, when the scattering occurs without rearrangement in the intermediate state. It is noteworthy that this occurs also in the metallic systems, where the scattering takes place with rearrangement due to a spin-dependent screening of the intermediate state core hole. This shows that this rearrangement does not require a more complex vector system in its description. The results show the relation between the selection of the scattering angle and the measurement of the polarization of the scattered radiation. We also present results from a geometry where the dichroism is absent in the emission integrated over outgoing polarization but where it is observed when this polarization is selected.
Phys. Rev. B. 05/2007; 75(18).
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ABSTRACT: The Fe 2p photoemission of ferromagnetic iron shows a dichroism which indicates a strong difference in hybridization for spin-up and spin-down electrons. In contrast, the dichroism of the localized Mn surface alloys reveals intense unscreened satellite structure. The spin polarization of the core hole is quantified using the dichroism in the Fe L3M2,3M2,3 Auger spectrum measured with the light helicity vector perpendicular to the magnetization direction. The observations in the 3d metals are in strong contradiction with the expectations from an independent-particle model.
EPL (Europhysics Letters) 01/2007; 40(2):171. · 2.17 Impact Factor
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ABSTRACT: We report the experimental results of Fe and Fe<sub>3</sub>O<sub>4 </sub> nanostructures on GaAs(100) surfaces and hybrid Ferromagnetic/Semiconductor/Ferromagnetic (FM/SC/FM) spintronic devices. Element specific x-ray magnetic circular dichroism (XMCD) measurements have shown directly that Fe atoms on the GaAs(100)-4times6 surface are ferromagnetic. Within coverages of 2.5 to 4.8 ML superparamagnetic nanoclusters are formed and exhibiting strong uniaxial anisotropy, of the order of 6.0times10<sup>5</sup> erg/cm<sup>3</sup>. The coercivities of epitaxial Fe dot arrays films grown on GaAs(100) were observed to be dependent on the separation and size of the dots indicating that interdot dipolar coupling affects the magnetization processes in these dots. In addition Fe<sub>3</sub>O<sub>4</sub> films grown on deformed GaAs(100) substrates have been observed to form nanostripes following the topography of the substrate and magneto-optical Kerr effect (MOKE) measurements showed that these nanostripes have uniaxial magnetic anisotropy with easy axis perpendicular to the length of the nanostripes. Meanwhile the FM/SC/FM vertical device has exhibited a biasing current dependent on MR characteristics, with a maximum change of 12% in the MR observed, indicating for the first time a large room temperature spin injection and detection
IEEE Transactions on Nanotechnology 10/2006; · 2.29 Impact Factor
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ABSTRACT: An ultrahigh vacuum chamber that enables the in situ growth of thin films and multilayers by magnetron sputtering techniques is described. Following film preparation, x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements are performed by utilizing an in vacuum electromagnet. XMCD measurements on sputtered thin films of Fe and Co yield spin and orbital moments that are consistent with those obtained previously on films measured in transmission geometry and grown in situ by evaporation methods. Thin films of FeN prepared by reactive sputtering are also examined and reveal an apparent enhancement in the orbital moment for low N content samples. The advantages of producing samples for in situ XAS and XMCD studies by magnetron sputtering are discussed.
Review of Scientific Instruments 07/2006; 77(7):073903-073903-6. · 1.37 Impact Factor
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ABSTRACT: Hugely anisotropic Mn L2,3 x-ray magnetic linear dichroism (XMLD) is reported for the ferromagnetic semiconductor (Ga,Mn)As. The sign, magnitude, and line shape of the dichroism changes with azimuthal rotation of the samples between the in-plane ⟨110⟩ and ⟨100⟩ directions. The strong anisotropy is confirmed by multiplet calculations for a Mn 3d5 configuration where charge anisotropy and magnetocrystalline anisotropy are absent. This allows a precise determination of the Mn 3d crystal-field splitting, and establishes the anisotropy in the XMLD as a general feature for localized moment systems with crystal-field symmetry.
Phys. Rev. B. 06/2006; 73(23).
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ABSTRACT: Remarkably anisotropic Mn L2,3 x-ray magnetic circular dichroism spectra from the ferromagnetic semiconductor (Ga,Mn)As are reported. States with cubic and uniaxial symmetry are distinguished by careful analysis of the angle dependence of the spectra. The multiplet structures with cubic symmetry are qualitatively reproduced by calculations for an atomiclike d5 configuration in tetrahedral environment, and show zero anisotropy in the orbital and spin moments within the experimental uncertainty. However, hybridization with the host valence bands is reflected by the presence of a preedge feature with a uniaxial anisotropy and a marked dependence on the hole density.
Physical Review Letters 04/2006; 96(11):117207. · 7.37 Impact Factor
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ABSTRACT: We present measurements of the magnetic circular dichroism in x-ray resonant emission in the perpendicular geometry (circularly polarized x rays at normal incidence to the magnetization) for L(2,3) the absorption region in Fe, Co, and Ni metal. The results show that spin-dependent screening of the core hole takes place within the scattering time scale, which is supported by the absence of the effect in ionic systems. This allows an assessment of the time scale for the screening process (up to a few femtoseconds). The process is almost complete within the scattering time for Fe and Co, but this is not the case for the narrow band metal Ni which shows a much slower dynamics.
Physical Review Letters 01/2006; 95(26):267402. · 7.37 Impact Factor
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ABSTRACT: The information of the W- and Ag-induced magnetic moments in Fe/W and NiFe/Ag multilayers was determined separately from the huge 3d magnetic moments with X-ray magnetic circular dichroism. Moreover, their spatial distribution has been extract from X-ray resonant magnetic scattering experiments. It was found that both W and Ag atoms are magnetically polarized and that this polarization extends through all the W and Ag layers, respectively. However, although W is found to be oscillating, the Ag polarization is exponentially decreasing from the interface.
IEEE Transactions on Magnetics 11/2005; · 1.36 Impact Factor
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ABSTRACT: The magnetic moments of ultrathin Fe films on three different III-V semiconductor substrates, namely GaAs, InAs and In<sub>0.2</sub>Ga<sub>0.8</sub>As have been measured with X-ray magnetic circular dichroism at room temperature to assess their relative merits as combinations suitable for next-generation spintronic devices. The results revealed rather similar spin moments and orbital moments for the three systems, suggesting the relationship between film and semiconductor lattice parameters to be less critical to magnetic moments than magnetic anisotropy.
IEEE Transactions on Magnetics 11/2005; · 1.36 Impact Factor
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ABSTRACT: Ultrathin Fe oxide films of various thicknesses prepared by post-growth oxidation on GaAs(100) surface have been investigated with X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray magnetic circular dichroism (XMCD). The XPS confirms that the surfaces of the oxide are Fe<sub>3</sub>O<sub>4</sub> rather than Fe<sub>2</sub>O<sub>3</sub>. XAS and XMCD measurements indicate the presence of insulating Fe divalent oxide phases (FeO) beneath the surface Fe<sub>3</sub>O<sub>4</sub> layer with the sample thickness above 4 nm. This FeO might act as a barrier for the spin injection into the GaAs.
IEEE Transactions on Magnetics 11/2005; · 1.36 Impact Factor
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T. Jungwirth,
J. Masek,
K. Y. Wang,
K. W. Edmonds,
M. Sawicki,
M. Polini,
Jairo Sinova,
A. H. MacDonald,
R. P. Campion,
L. X. Zhao,
N. R. S. Farley,
T. K. Johal, G van der Laan,
C.T. Foxon,
B. L. Gallagher
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ABSTRACT: We report on a comprehensive study of the ferromagnetic moment per Mn atom in (Ga,Mn)As ferromagnetic semiconductors. Theoretical discussion is based on microscopic calculations and on an effective model of Mn local moments antiferromagnetically coupled to valence band hole spins. The validity of the effective model over the range of doping studied is assessed by comparing with microscopic tight-binding/coherent-potential approximation calculations. Using the virtual crystal k.p model for hole states, we evaluate the zero-temperature mean-field contributions to the magnetization from the hole kinetic and exchange energies, and magnetization suppression due to quantum uctuations of Mn moment orientations around their mean-field ground state values. Experimental low-temperature ferromagnetic moments per Mn are obtained by superconducting quantum interference device and x-ray magnetic circular dichroism measurements in a series of (Ga,Mn)As semiconductors with nominal Mn doping ranging from ~2% to 8%. Hall measurements in as-grown and annealed samples are used to estimate the number of uncompensated substitutional Mn moments. Based on our comparison between experiment and theory we conclude that all these Mn moments in high quality (Ga,Mn)As materials have nearly parallel ground state alignment. Comment: 11 pages, 11 figures, submitted to Phys. Rev. B
08/2005;
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ABSTRACT: The magnetic moments of ultrathin 7 monolayer (ML) Fe films on GaAs, InAs and InGaAs were studied using the element specific X-ray magnetic circular dichroism (XMCD). The 7 ML Fe films were grown by molecular beam epitaxy (MBE) at room temperature and capped with 15 ML Cr to prevent oxidisation on removal from the vacuum system. The special substrates took the form of GaAs wafers topped with a 0.3μm epilayer of GaAs, InAs or InGaAs followed by an As capping layer. The lattice parameters of the GaAs, InAs and InGaAs epilayers were chosen for their lattice constants which, in a 2:1 relationship with Fe give mismatches of 1.56%, 5.32% and 0%, respectively. All samples revealed a reduced spin moment compared to bulk value and a marked enhancement in orbital moment. The similarity of the magnetic thin film features observed for the different substrates suggests that an unexpected degree of tolerance exists in these Fe films to surface issues such as strains induced by different lattice matches.
Magnetics Conference, 2005. INTERMAG Asia 2005. Digests of the IEEE International; 05/2005
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ABSTRACT: We demonstrate that carefully prepared (Ga,Mn)As films can show large magnetic moments per atom across a wide range of Mn concentrations, indicating almost full participation of the Mn in the ferromagnetism. Applying sum rules to Mn L2,3 x-ray magnetic circular dichroism (XMCD) spectra yields a magnetic moment per Mn of around 4.5μB, including a small positive orbital moment. We also present direct evidence for antiferromagnetic coupling between interstitial and substitutional Mn in unannealed (Ga,Mn)As. The Mn L2,3 x-ray absorption line shapes display no sizeable site or concentration dependence, but in unannealed (Ga,Mn)As the XMCD signal is significantly smaller, and increases linearly under high magnetic fields.
Phys. Rev. B. 02/2005; 71(6).
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ABSTRACT: An FePd thin film sample, showing magnetic stripe domains as imaged by magnetic force microscopy, has been measured by soft X-ray resonant magnetic scattering in reflection geometry. Illumination with coherent radiation, produced by inserting a 20 microm pinhole in front of the sample, leads to a magnetic speckle pattern in the scattered intensity that gives access to the domain morphology. Application of an in-plane magnetic field for a few seconds gives a strong change in the observed intensity fluctuations, which indicates a large degree of variation between the two patterns taken before and after field exposure. From the speckle pattern we calculate a degree of coherence of beta = 0.5 for the incident beam.
Journal of Synchrotron Radiation 12/2004; 11(Pt 6):469-75. · 2.73 Impact Factor
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ABSTRACT: We have used x-ray magnetic circular dichroism, which offers a unique capability to give element specific information at submonolayer sensitivity, to determine the spin and orbital magnetic moments at the Fe/GaAs(100) interface. The wedge samples, grown by molecular beam epitaxy at room temperature, consisted of 0.25-1 monolayer (ML) Fe on GaAs(100)-4x6 capped with 9 ML Co and have shown Fe spin moments of (1.84-1.96)micro(B) and a large orbital enhancement. Our results demonstrate unambiguously that the Fe/GaAs(100)-4x6 interface is ferromagnetic with a bulklike spin moment, which is highly promising for spintronics applications.
Physical Review Letters 08/2004; 93(3):037206. · 7.37 Impact Factor
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ABSTRACT: A detailed characterization of magnetic oxide films is essential to enable their use in magnetoresistive devices since their properties depend critically on stoichiometry and structural order. Here, the composition and magnetic properties of ultrathin iron oxide films grown epitaxially on Al <sub>2</sub> O <sub>3</sub>(0001) have been characterized using x-ray magnetic circular dichroism XMCD and magnetoresistance (MR) measurements. The XMCD data show by comparison with theoretical calculations that we have successfully found growth conditions for well ordered epitaxial films with Fe <sub>3</sub> O <sub>4</sub> stoichiometry. Nonstoichiometric films exhibit, in addition to a relative reduction in Fe <sup>2+</sup> ions, a net transfer of Fe <sup>3+</sup> from tetrahedral to octahedral sites. The in-plane MR for both these films is found to be 1% at room temperature in a field of 1 T even though the electrical conductivity differs by a factor of 5. © 2004 American Institute of Physics.
Journal of Applied Physics 08/2004; · 2.17 Impact Factor
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ABSTRACT: We present Mn L<sub>3,2</sub> x-ray absorption measurements from two dilute magnetic semiconductor systems: p-type (Ga,Mn)As, which is ferromagnetic with T<sub>C</sub>=140 K ; and n-type (Ga,Mn)N, in which the magnetic impurities are predominantly paramagnetic. After removing a Mn-rich oxide surface layer by chemical etching, the Mn L<sub>3,2</sub> spectra from (Ga,Mn)As appear less localized than in previous reports, which is ascribed to screening due to p–d hybridization. Our results suggest that previous studies may have been influenced by the quality of the surface. In contrast, in the (Ga,Mn)N film the Mn ground state is closer to pure d<sup>5</sup>. © 2004 American Institute of Physics.
Journal of Applied Physics 07/2004; · 2.17 Impact Factor
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ABSTRACT: Soft X-ray resonant magnetic scattering studies on a magnetically modulated, but nominally chemically homogenous, 5 nm Ni layer in a Cu/Ni/Cu/Co/NiO/GaAs(110) system are reported. It was possible to estimate the main chemical structure of the sample on the basis of the results from specular reflectivity and rocking scans, probing the scattering vector components in the perpendicular and in-plane direction to the Ni wires, respectively. The magnetic scattering using polarized X-rays demonstrates the magnetic modulation of the Ni layer.
Journal of Synchrotron Radiation 06/2004; 11(Pt 3):254-60. · 2.73 Impact Factor
