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Electronic and magnetic structure of GaxFe1-x thin films

Description: The electronic as well as magnetic properties of Ga{sub x}Fe{sub 1-x} films were studied by soft x-ray measurements. Using x-ray magnetic circular dichroism the Fe majority-spin band was found to be completely filled for x {approx} 0.3. With further enhanced Ga content, the Fe moment as well as the angular dependence of the x-ray magnetic linear dichroism decrease strongly, which we attribute to the formation of D0{sub 3} precipitates. Moreover, the magnetocrystalline anisotropy drops significantly.
Date: September 8, 2010
Creator: Arenholz, E.; van der Laan, G.; McClure, A. & Idzerda, Y.
Partner: UNT Libraries Government Documents Department

Linear dichroism and resonant photoemission in Gd 011

Description: Magnetic Linear Dichroism in Angular Distributions (MLDAD) from Photoelectron Emission was used to probe the nature of Resonant Photoemission. Gd 5p and Gd 4f emission were investigated. Using novel theoretical simulations, we were able to show that temporal matching is a requirement for ``True`` Resonant Photoemission, where the Resonant Photoemission retains the characteristics of Photoelectron Emission.
Date: May 13, 1998
Creator: Mishra, S.R.; Cummins, T.R.; Gammon, W.J.; van der Laan, G.; Goodman, K.W. & Tobin, J.G.
Partner: UNT Libraries Government Documents Department

An investigation of resonant photoemission in Gd with x-ray linear dichroism

Description: The constructive summing of direct and indirect channels above the absorption threshold of a core level can cause a massive increase in the emission cross section, leading to a phenomenon called "resonant photoemission". Using novel magnetic linear dichroism in angular distribution photoelectron spectroscopy experiments and theoretical simulations, we have probed the nature of the resonant photoemission process in Gd metal. It now appears that temporal matching as well as energy matching is a requirement for true resonant photoemission.
Date: July 1, 1998
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Magnetic structure near the Co/NiO(001) interface

Description: We investigate the magnetic coupling at the Co/NiO interface using soft x-ray magnetic linear dichroism (XMLD) and circular dichroism taking explicitly into account the recently observed angular dependence of the XMLD with respect to the crystallographic axes. We find that the Co moments are aligned perpendicular to the NiO moments. We discuss the impact of the anisotropic XMLD on the intensity ratio of the two peaks at the Ni L{sub 2} edge, which is commonly employed to determine the spin orientation in antiferromagnets using XMLD.
Date: August 29, 2008
Creator: Arenholz, Elke; van der Laan, Gerrit & Nolting, Frithjof
Partner: UNT Libraries Government Documents Department

Order from Chaos: (alpha)-Fe(001) Growth on GaAs(001)

Description: The growth of Fe upon GaAs(001) has been studied with Spin-Resolved Photoelectron Spectroscopy (SRPES), Photoelectron Spectroscopy (PES) and X-ray Magnetic Linear Dichroism (XMLD) in PES. Despite evidence of atomic level disorder such as intermixing, an over-layer with the spectroscopic signature of {alpha}-Fe(001), with a bcc real space ordering, is obtained. The results will be discussed in light of the possibility of using such films as a spin polarized source in device applications.
Date: February 8, 2008
Creator: Tobin, J G; Yu, S W; Morton, S A; Waddill, G D; Thompson, J D; Neal, J R et al.
Partner: UNT Libraries Government Documents Department

Element-specific magnetometry with linear dichroism in photoemission

Description: In this paper, we investigate the magnetic linear dichroism in the core-level photoemission spectra of the binary alloys Co<sub>x</sub>Ni<sub>l-x</sub> and Fe<sub>x</sub>Ni<sub>1-x</sub>/Cu(100). These epitaxial films have fee structures, but very different magnetic behavior.We show that the magnetic linear dichroism in x-ray photoemission (XMLD) signal tracks the magnetization in these alloys. Comparison with recent SQUID data provides a quantitative check and endorses the view that XMLD monitors the element-specific magnetometry.
Date: July 1, 1998
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Orbital Moment Determination in (MnxFe1-x)3O4 Nanoparticles

Description: Nanoparticles of (Mn{sub x}Fe{sub 1-x}){sub 3}O{sub 4} with a concentration ranging from x = 0 to 1 and a crystallite size of 14-15 nm were measured using X-ray absorption spectroscopy and X-ray magnetic circular dichroism to determine the ratio of the orbital moment to the spin moment for Mn and Fe. At low Mn concentrations, the Mn substitutes into the host Fe{sub 3}O{sub 4} spinel structure as Mn{sup 2+} in the tetrahedral A-site. The net Fe moment, as identified by the X-ray dichroism intensity, is found to increase at the lowest Mn concentrations then rapidly decrease until no dichroism is observed at 20% Mn. The average Fe orbit/spin moment ratio is determined to initially be negative and small for pure Fe{sub 3}O{sub 4} nanoparticles and quickly go to 0 by 5%-10% Mn addition. The average Mn moment is anti-aligned to the Fe moment with an orbit/spin moment ratio of 0.12 which gradually decreases with Mn concentration.
Date: October 22, 2010
Creator: Pool, V. L.; Jolley, C.; Douglas, T.; Arenholz, E. & Idzerda, Y. U.
Partner: UNT Libraries Government Documents Department

Determination of rotatable and frozen CoO spins and their relationship to exchange bias in CoO/Fe/Ag(001)

Description: The exchange bias of epitaxially grown CoO/Fe/Ag(001) was investigated using X-ray Magnetic Circular Dichroism (XMCD) and X-ray Magnetic Linear Dichroism (XMLD) techniques. A direct XMLD measurement on the CoO layer during the Fe magnetization reversal shows that the CoO compensated spins are rotatable at thinner thickness and frozen, i.e. fixed in direction to the lattice, at larger thickness. By a quantitative determination of the rotatable and frozen CoO spins as a function of the CoO film thickness, we find the remarkable result that the exchange bias is well established before frozen spins are detectable in the CoO film, contrary to the common assumption that the majority of antiferromagnetic spins need to be frozen to generate the exchange bias. We further show that the rotatable/frozen CoO spins are uniformly distributed in the CoO film.
Date: February 10, 2010
Creator: Wu, J.; Park, J.; Kim, W.; Arenholz, E.; Liberati, M.; Scholl, A. et al.
Partner: UNT Libraries Government Documents Department

A direct measurement of rotatable and frozen CoO spins in exchange bias system of CoO/Fe/Ag(001)

Description: The exchange bias of epitaxially grown CoO/Fe/Ag(001) was investigated using x-ray magnetic circular dichroism and x-ray magnetic linear dichroism (XMLD) techniques. A direct XMLD measurement on the CoO layer during the Fe magnetization reversal shows that the CoO compensated spins are rotatable at thinner thickness and frozen at larger thickness. By a quantitative determination of the rotatable and frozen CoO spins as a function of the CoO film thickness, we find the remarkable result that the exchange bias is well established before frozen spins are detectable in the CoO film. We further show that the rotatable and frozen CoO spins are uniformly distributed in the CoO film.
Date: March 10, 2010
Creator: Wu, J.; Park, J. S.; Kim, W.; Arenholz, E.; Liberati, M.; Scholl, A. et al.
Partner: UNT Libraries Government Documents Department

Stereochemical Studies of Nitrosamines: The Induced Circular Dichroism of Achiral Nitrosasmines

Description: The induced circular dichroism (ICD) of several chiral nitrosamines and various chiral reagents has been investigated. The interaction is attributed to a 1:1 hydrogen bonded complex between the NO group of the nitrosamine and the hydroxyl groups of alcohols and polyols, or the amino group of amines. Only those chiral reagents possessing large differences in size of the groups about the hydrogen bonding site contributed to CD anomalies. The acyclic 2-octanols did not give observable Cotton effects, presumably due to the similarity in size of the methyl and methylene groups and rotational freedom of the acyclic system. The signs of the Cotton effects could be correlated with the absolute configuration of the sterically hindered alcohols and amines. Only the alpha, axial hydrogens of conformationally biased, heterocyclic nitrosamines were found to undergo selective hydrogen-deuterium exchange, suggesting that this feature is critical for nitrosamine carcinogenicity.
Date: August 1980
Creator: Fribush, Howard M.
Partner: UNT Libraries

Direct observation of imprinted antiferromagnetic vortex state in CoO/Fe/Ag(001) disks

Description: In magnetic thin films, a magnetic vortex is a state in which the magnetization vector curls around the center of a confined structure. A vortex state in a thin film disk, for example, is a topological object characterized by the vortex polarity and the winding number. In ferromagnetic (FM) disks, these parameters govern many fundamental properties of the vortex such as its gyroscopic rotation, polarity reversal, core motion, and vortex pair excitation. However, in antiferromagnetic (AFM) disks, though there has been indirect evidence of the vortex state through observations of the induced FM-ordered spins in the AFM disk, they have never been observed directly in experiment. By fabricating single crystalline NiO/Fe/Ag(001) and CoO/Fe/Ag(001) disks and using X-ray Magnetic Linear Dichroism (XMLD), we show direct observation of the vortex state in an AFM disk of AFM/FM bilayer system. We observe that there are two types of AFM vortices, one of which has no analog in FM structures. Finally, we show that a frozen AFM vortex can bias a FM vortex at low temperature.
Date: December 21, 2010
Creator: Wu, J.; Carlton, D.; Park, J. S.; Meng, Y.; Arenholz, E.; Doran, A. et al.
Partner: UNT Libraries Government Documents Department

Evidence of half-metallic interface magnetism via local moment formation in Co based Heusler alloys

Description: In this work we use a combination of x-ray magnetic circular and linear dichroism (XMCD and XMLD) techniques to examine the formation of local moments in Heusler alloys of the composition Co{sub 2}MnX (where X=Si or Al). The existence of local moments in a half-metallic system is reliant upon the band gap in the minority-spin states. By utilizing the element-specific nature of x-ray techniques we are able to explore the origin of the minority-spin band gap in the partial density of states (PDOS), via the degree of localization of moments on Co and Mn atoms. We observe a crucial difference in the localization of the Co moment when comparing Co{sub 2}MnSi (CMS) and Co{sub 2}MnAl (CMA) films that is consistent with the predicted larger minority-spin gap in the Co PDOS for CMS. These results provide important evidence for the dominant role of the Co minority-spin states in realizing half-metallic ferromagnetism (HMF) in these Heusler alloys. They also demonstrate a direct method for measuring the degree of interfacial HMF in the raw materials without the need for fabricating spin-transport devices.
Date: August 18, 2008
Creator: Telling, N. D.; Keatley, P.S.; van der Laan, G.; Hicken, R.J.; Arenholz, E.; Sakuraba, Y. et al.
Partner: UNT Libraries Government Documents Department

An analysis of the x-ray linear dichroism spectrum for NiO thin films grown on vicinal Ag(001)

Description: Antiferromagnetic (AFM) NiO thin films are grown epitaxially on vicinal Ag(118) substrate and investigated by x-ray linear dichroism (XLD). We find that the NiO AFM spin exhibits an in-plane spin reorientation transition from parallel to perpendicular to the step edges with increasing the NiO film thickness. In addition to the conventional L{sub 2} adsorption edge, x-ray linear dichroism (XLD) effect at the Ni L{sub 3} adsorption edge is also measured and analyzed. The result identifies a small energy shift of the L{sub 3} peak. Temperature-dependent measurement confirms that the observed XLD effect in this system at the normal incidence of the x-rays originates entirely from the NiO magnetic ordering.
Date: May 10, 2008
Creator: Wu, Y. Z.; Zhao, Y.; Arenholz, E.; Young, A. T.; Sinkovic, B. & Qiu, Z. Q.
Partner: UNT Libraries Government Documents Department

Remediation of Cr(VI) by biogenic magnetic nanoparticles: An x-ray magnetic circular dichroism study

Description: Biologically synthesized magnetite (Fe{sub 3}O{sub 4}) nanoparticles are studied using x-ray absorption and x-ray magnetic circular dichroism following exposure to hexavalent Cr solution. By examining their magnetic state, Cr cations are shown to exist in trivalent form on octahedral sites within the magnetite spinel surface. The possibility of reducing toxic Cr(VI) into a stable, non-toxic form, such as a Cr{sup 3+}-spinel layer, makes biogenic magnetite nanoparticles an attractive candidate for Cr remediation.
Date: September 4, 2009
Creator: Telling, N. D.; Coker, V. S.; Cutting, R. S.; van der Laan, G.; Pearce, C. I.; Pattrick, R. A. D. et al.
Partner: UNT Libraries Government Documents Department

A [Cyclentetrakis(methylene)]tetrakis[2-hydroxybenzamide]Ligand That Complexes and Sensitizes Lanthanide(III) Ions

Description: The synthesis of a cyclen derivative containing four isophthalamide groups (L{sup 1}) is described. The spectroscopic properties of the Ln(III) complexes of L{sup 1} (Ln = Gd, Tb, Yb, Eu) reveal changes of the UV/visible absorption, circular dichroism absorption, luminescence and circularly polarized luminescence properties. It is shown that at least two metal complex species are present in solution, whose relative amounts are pH dependent. When at pH &gt; 8.0, an intense long lived emission is observed (for [L{sup 1}Tb] and [L{sup 1}Yb]) while at pH &lt; 8.0, a weaker, shorter-lived species predominates. Unconventional Ln(III) emitters (Pr, Nd, Sm, Dy and Tm) were sensitized in basic solution, both in the visible and in the near infra-red, to measure the emission of these ions.
Date: April 30, 2009
Creator: D'Aleo, Anthony; Xu, Jide; Do, King; Muller, Gilles & Raymond, Kenneth N.
Partner: UNT Libraries Government Documents Department

Antiferromagnetic exchange bias of a ferromagnetic semiconductor by a ferromagnetic metal

Description: We demonstrate an exchange bias in (Ga,Mn)As induced by antiferromagnetic coupling to a thin overlayer of Fe. Bias fields of up to 240 Oe are observed. Using element-specific x-ray magnetic circular dichroism measurements, we distinguish an interface layer that is strongly pinned antiferromagnetically to the Fe. The interface layer remains polarized at room temperature.
Date: November 5, 2009
Creator: Olejnik, K.; Wadley, P.; Haigh, J.; Edmonds, K. W.; Campion, R. P.; Rushforth, A. W. et al.
Partner: UNT Libraries Government Documents Department

Room-temperature photomagnetism in the spinel ferrite (Mn,Zn,Fe)3O4 as seen via soft x-ray magnetic circular dichroism

Description: We have used X-ray magnetic circular dichroism (XMCD) in conjunction with multiplet simulations to directly probe the origin of photomagnetism in nanocrystalline (Mn,Zn,Fe){sub 3}O{sub 4}. A photomagnetic effect at room temperature has been observed in these films with HeNe illumination. We have verified an intervalence charge transfer among octahedral Fe cations to account for the increase in magnetization observed at and above room temperature in small magnetic fields. Using XMCD, we demonstrate that the dichroism of Fe in octahedral sites increases by 18% at room temperature while the dichroism of Fe in tetrahedral sites does not change.
Date: August 1, 2009
Creator: Bettinger, J.S.; Piamonteze, C.; Chopdekar, R.V.; Liberati, M.; Arenholz, E. & Suzuki, Y.
Partner: UNT Libraries Government Documents Department

Correlation of magnetic dichroism in x-ray absorption and photoelectron emission using ultrathin magnetic alloy films

Description: We have begun a program to characterize magnetic alloy overlays using both magnetic x-ray circular dichroism (MXCD) and magnetic x-ray linear dichroism (MXLD). This will allow a direct comparison of MXCD-absorption and MXLD-photoelectron emission. First results from the Advanced Light Source will be presented.
Date: April 1, 1996
Creator: Tobin, J.G.; Goodman, K.W.; Mankey, G.J.; Willis, R.F.; Denlinger, J.D.; Rotenberg, E. et al.
Partner: UNT Libraries Government Documents Department

Electric Dichroism Spectroscopy in the Vacuum Ultraviolet

Description: When a molecule, which possess a permanent dipole moment is exposed to an intense electric field, its absorption spectrum may be altered. These alterations are manifest as shifts in energy and as changes in band shape and intensity. The electric dichroism of absorption bands can be used to probe the excited state that is formed when a molecule undergoes a transition. The properties that may be investigated include transition polarization, excited state dipole moment and mean polarizability, and field-induced mixing of symmetrically equivalent excited states. The theoretical model and experimental devices that have been developed to determine these properties are presented and discussed. The data, taken in total, and its combination with other existing evidence, adds credence to the assignment of the second excited singlet of aldehydes and ketones to be extravalent, accompanied by relatively large delocalization of electronic charge, and polarized in-planiie and perpendicularly to the C-0 axis.
Date: May 1978
Creator: Causley, Gary C.
Partner: UNT Libraries

Crystal-field level inversion in lightly Mn-doped Sr3Ru2O7

Description: Sr3(Ru1-xMnx)2O7, in which 4d-Ru is substituted by the more localized 3d-Mn, is studied by x-ray dichroism and spin-resolved density functional theory. We find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+ acceptors; the extra eg electron occupies the in-plane 3dx2-y2 orbital instead of the expected out-of-plane 3d3z2-r2. We propose that the 3d-4d interplay, via the ligand oxygen orbitals, is responsible for this crystal-field level inversion and the material's transition to an antiferromagnetic, possibly orbitally ordered, low-temperature state.
Date: January 15, 2008
Creator: Hossain, M. A.; Hu, Z.; Haverkort, M. W.; Burnus, T.; Chang, C. F.; Klein, S. et al.
Partner: UNT Libraries Government Documents Department

Molecular orientation in soft matter thin films studied by resonant soft X-ray reflectivity

Description: We present a technique to study depth profiles of molecular orientation in soft matter thin films with nanometer resolution. The method is based on dichroism in resonant soft X-ray reflectivity using linear s- and p-polarization. It combines the chemical sensitivity of Near-Edge X-ray Absorption Fine Structure spectroscopy to specific molecular bonds and their orientation relative to the polarization of the incident beam with the precise depth profiling capability of X-ray reflectivity. We demonstrate these capabilities on side chain liquid crystalline polymer thin films with soft X-ray reflectivity data at the carbon K edge. Optical constants of the anisotropic refractive index ellipsoid were obtained from a quantitative analysis using the Berreman formalism. For films up to 50 nm thickness we find that the degree of orientation of the long axis exhibits no depth variation and isindependent of the film thickness.
Date: January 12, 2011
Creator: Mezger, Markus; Jerome, Blandine; Kortright, Jeffrey B.; Valvidares, Manuel; Gullikson, Eric; Giglia, Angelo et al.
Partner: UNT Libraries Government Documents Department

Enhanced Magnetism of Fe3O4 Nanoparticles with Ga Doping

Description: Magnetic (Ga{sub x}Fe{sub 1-x}){sub 3}O{sub 4} nanoparticles with 5%-33% gallium doping (x = 0.05-0.33) were measured using x-ray absorption spectroscopy and x-ray magnetic circular dichroism to determine that the Ga dopant is substituting for Fe{sub 3+} as Ga{sub 3+} in the tetrahedral A-site of the spinel structure, resulting in an overall increase in the total moment of the material. Frequency-dependent alternating-current magnetic susceptibility measurements showed these particles to be weakly interacting with a reduction of the cubic anisotropy energy term with Ga concentration. The element-specific dichroism spectra show that the average Fe moment is observed to increase with Ga concentration, a result consistent with the replacement of A-site Fe by Ga.
Date: October 22, 2010
Creator: Pool, V. L.; Klem, M. T.; Chorney, C. L.; Arenholz, E. & Idzerda, Y.U.
Partner: UNT Libraries Government Documents Department