Design of room temperature spintronics nanodevices: Investigations of buried interfaces by Spin Polarized Hard X-ray photoemission
Zusammenfassung der Projektergebnisse
Recent advances in innovative technology fields like spintronics imply the search and elaboration of new materials exploiting the spin degree of freedom for the construction of electronic devices. The characterization of new magnetic materials for spintronics devices requires the measurement of the spin and charge for the elucidation of transfer processes in these materials and their multilayer stacks. The activity within this project was focused at the development of advanced techniques for spin-resolved investigation of the electronic structure of buried magnetic layers and interfaces. The HArd X-ray PhotoElectron Spectroscopy (HAXPES) is a non-destructive bulk sensitive probe of the electronic band structure. This technique has been successfully employed to elucidate the influence of atom diffusion on tunneling magnetoresistance (TMR) in FexCoyBz based tunnel junctions. The dependence of the TMR of FexCoyBz based junctions on the annealing conditions is explained by the combined effects of an improved crystalline structure together with a change in the spin polarization at the Fermi edge due to the decrease of boron content in the FexCoyBz layer. Thereafter, we have made one step further and implemented the Spin-HAXPES technique. Combination of HAXPES and spin polarimetry results in the Spin-HAXPES technique (a new method world-wide), facilitating the complete studies of the electronic band structure on buried layers resolving electron energy, momentum and spin. The spin-resolved HAXPES experiment has been performed on buried magnetic layers. The measurements prove that a spin polarization of about 50% is retained during the transmission of the photoelectrons through a 3 nm thick oxide capping layer. The developed Spin-HAXPES experiment paves the way to spin-resolved spectroscopy on buried layers and buried interfaces, issues being inaccessible by the classical low-energy approach. The direct measurement of photoelectron spin is highly demanding because it implies an extremely strong reduction of detection efficiency at the spin-discriminating stage by 2-4 orders of magnitude. Fortunately, the spin polarization of electronic states may also be exploited and studied with magnetic linear and circular dichroism in photoelectron emission. The magnetic dichroism explored in photoelectron emission from buried CoFe, Co2FeAl and Co2FeAl0.5Si0.5 magnetic layers shows asymmetries up to 58% upon excitation with circularly and linearly polarized hard X-rays and is thus much stronger as compared to that in the case of excitation by soft X-rays. The high bulk sensitivity of HAXPES combined with circularly and linearly polarized photons provides a major impact in the study of the magnetic phenomena and state symmetry in deeply buried magnetic layers and bulk samples. The obtained asymmetry values quantifying the dichroic effects show rather large dynamic range and can therefore be used not only for indicative studies on magnetic structure, but for its element-specific quantitative analysis as well. The implementation of dichroism studies with recently proposed standing wave technique makes feasible the element-specific investigation of magnetic layers at buried interfaces in a selected depth regime. The results of Spin-HAXPES experiments agree well with the features of magnetic circular and linear dichroism in photoelectron emission. The information contents of the spin polarization and magnetic dichroism signals are different and their combination provides a detailed insight into the dynamics of photoemission from a ferromagnetic material. One can elucidate the magnetic states from the measurements of magnetic dichroism in photoelectron emission providing much higher detecting efficiency than achieved in spin-resolved measurements with a spin detector. However, this does not exclude the necessity in direct spin-resolved measurements with spin detectors since the technique of spin-polarized state restoration from MCD and MLD is not well established. The developed new type of multichannel spin polarimeter shows an efficiency improved by about four orders of magnitude in comparison to state-of-the-art single channel spin detectors. This gain in measuring time paves the way to new experiments in various fields of current and emerging research, in particular spin-resolved hard X-ray photoelectron spectroscopy in the valence band region.
Projektbezogene Publikationen (Auswahl)
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Hard X-ray photoelectron spectroscopy of buried Heusler compounds, J. Phys. D: Appl. Phys. 42 (2009) 084010
Siham Ouardi, Benjamin Balke, Andrei Gloskovskii, Gerhard H. Fecher, Claudia Felser, Gerd Schoenhense, Takayuki Ishikawa, Tetsuya Uemura, Masafumi Yamamoto, Hiroaki Sukegawa, Wenhong Wang, Koichiro Inomata, Yoshiyuki Yamashita, Hideki Yoshikawa, Shigenori Ueda, and Keisuke Kobayashi
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A nondestructive analysis of the B diffusion in Ta–CoFeB–MgO–CoFeB–Ta magnetic tunnel junctions by hard X-ray photoemission, Appl. Phys. Lett. 96 (2010) 072105
Xeniya Kozina, Siham Ouardi, Benjamin Balke, Gregory Stryganyuk, Gerhard H. Fecher, Claudia Felser, Shoji Ikeda, Hideo Ohno, and Eiji Ikenaga
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Exploring the details of the martensite–austenite phase transition of the shape memory Heusler compound Mn2NiGa by hard X-ray photoelectron spectroscopy, magnetic and transport measurements. Appl. Phys. Lett. 98 (2011) 252501
Christian G. F. Blum, Siham Ouardi, Gerhard H. Fecher, Benjamin Balke, Xeniya Kozina, Gregory Stryganyuk, Shigenori Ueda, Keisuke Kobayashi, Claudia Felser, Sabine Wurmehl and Bernd Buechner
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Highly efficient multichannel spin detection, Phys. Rev. Lett. 107 (2011) 207601
Michaela Kolbe, Pavel Lushchyk, Bernd Petereit, Hans-Joachim Elmers, Gerd Schoenhense, Andreas Oelsner, Christian Tusche, and Juergen Kirschner
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Magnetic dichroism in angle-resolved hard X-ray photoemission from buried layers, Phys. Rev. B 84 (2011) 054449
Xeniya Kozina, Gerhard H. Fecher, Gregory Stryganyuk, Siham Ouardi, Benjamin Balke, Claudia Felser, Gerd Schoenhense, Eiji Ikenaga, Takeharu Sugiyama, Naomi Kawamura, Motohiro Suzuki, Tomoyuki Taira, Tetsuya Uemura, Masafumi Yamamoto, Hiroaki Sukegawa, Wenhong Wang, Koichiro Inomata, and Keisuke Kobayashi
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Spin Polarimetry and Magnetic Dichroism on a buried magnetic layer using Hard X-ray Photoelectron Spectroscopy, Jpn. J. Appl. Phys. 51 (2011) 016602
Gregory Stryganyuk, Xeniya Kozina, Gerhard H. Fecher, Siham Ouardi, Stanislav Chadov, Claudia Felser, Gerd Schoenhense, Pavel Lushchyk, Andreas Oelsner, Pasqual Bernhard, Eiji Ikenaga, Takeharu Sugiyama, Hiroaki Sukegawa, Zhenchao Wen, Koichiro Inomata, and Keisuke Kobayashi
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Symmetry of Valence States of Heusler Compounds Explored by Linear Dichroism in Hard-X-Ray Photoelectron Spectroscopy, Phys. Rev. Lett. 107 (2011) 036402
Siham Ouardi, Gerhard H. Fecher, Xeniya Kozina, Gregory Stryganyuk, Benjamin Balke, Claudia Felser, Eiji Ikenaga, Takeharu Sugiyama, Naomi Kawamura, Motohiro Suzuki, and Keisuke Kobayashi
