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| Home > Publications database > The Quest for and Demonstration of direct Experimental Evidence for half Metallicity in Heusler Compounds |
| Home > Publications database > The Quest for and Demonstration of direct Experimental Evidence for half Metallicity in Heusler Compounds |
| Conference Presentation (Invited) | PUBDB-2015-01317 |
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2014
Abstract: Spin transport electronics (spintronics) is commonly considered as a promising future information technology, which motivates the search for new materials for optimum device performance. The key parameter for the magnitude of spin transport effects is the spin polarization, which ideally amounts to 100% in so-called half-metals. However, clear room temperature evidence for this property has been reported so far only for some oxides, which are not suitable for the incorporation into electronic devices. On the other hand, for Heusler compounds half-metallicity was predicted theoretically already more than ten years ago (e.g. [Gal02]), but direct experimental evidence for an exceptionally high spin polarization, especially at room temperature, has proven elusive and no reports exist up to now. In tunneling juctions with spin-filtering epitaxial MgO barriers Heusler electrodes show large magnetoresistance effects (e.g. [Liu12]), but still are inferior to conventional CoFeB electrodes (e.g. [Ick08]).Our exclusion of possible extrinsic effects which reduce the experimentally determinable spin polarization started with the optimization of growth conditions of epitaxial Heusler thin films, which we first used as electrodes of tunneling magnetoresistance (TMR) junctions with AlOx barrier. It could be demonstrated, that the barrier properties are crucial and probably limiting the obtainable TMR values [Her09]. However, x-ray magnetic circular dichroism (XMCD) experiments indicated a high spin polarization of the Heusler thin films [Kal09]. Thus we developed a new experimental set-up, which allows for the in-situ investigation of epitaxial Heusler thin films by highly efficient spin-polarized photoemission spectroscopy [Kol11]. After a systematic study of the suitability of several compounds [Jou11] this approach soon produced a record value at that time of the directly observed spin-polarization of 55% at room temperature for the compound Co2MnGa [Kol12].The breakthrough was achieved very recently [Jou14]: For the first time half-metallicity at room temperature was demonstrated for a Heusler compound. Investigating thin films of the compound Co2MnSi deposited in optimized growth conditions by in-situ spin-resolved UV-photoemission spectroscopy, it was possible to reveal the high spin polarization: In the surface region of Heusler thin films (93+7−11) % spin polarization at room temperature was measured directly. This experimental result is in excellent agreement with theoretical computations of the electronic properties of the investigated compound, which include surface effects in a novel way. Ex-situ spin-integrated high energy x-ray photoemission spectroscopy (HAXPES) experiments on capped Co2MnSi thin films were explained by the same band structure and photoemission calculation including all surface related effects. It shows that the observation of a high spin polarization in a wide energy range below the Fermi energy is related to a stable surface resonance in the majority band of Co2MnSi extending deep into the bulk of the material.Our results show that careful thin film preparation can indeed result in a high spin polarization with a sufficient degree of stability in a surface region of several atomic layers. In particular they demonstrate that the observed room temperature tunneling magnetoresistance values are not limited by the intrinsic spin polarization of the Heusler alloy and that potentially much larger TMR values can be obtained by carefully optimized device preparation.References: [Gal02] Galanakis, I., Dederichs, P. H. & Papanikolaou, Phys. Rev. B 66, 174429 (2002).[Liu12] Liu, H. et al. Appl. Phys. Lett. 101,132418 (2012).[Ick08] Ikeda, S. et al. Appl. Phys. Lett. 93,082508 (2008).[Her09] Herbort, C., Arbelo Jorge, E. & Jourdan, M. Appl. Phys. Lett. 94, 142504 (2009).[Kal09] Kallmayer, M. et al. Phys. Rev. B 80, 020406 (2009).[Kol11] Kolbe, M. et al. Phys. Rev. Lett. 107,207601 (2011).[Jou11] Jourdan, M. et al. J. Phys. D: Appl. Phys. 44, 155001 (2011).[Kol12] Kolbe, M. et al. Phys. Rev. B 86, 024422 (2012).[Jou14] Jourdan, M. et al. Nat. Commun. 5, 3974
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