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Development of Organic-Based Thin Film Magnets for Spintronics
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the growing field of spintronics, development of semiconducting magnets is a high priority. Organic-based molecular magnets are attractive candidates since their properties can be tailor-made by organic chemistry. Other advantages include low weight and low temperature processing. Vanadium tetracyanoethylene, V(TCNE)x, x~2, is particularly interesting since it is one of very few semiconducting magnets with magnetic ordering above room temperature.

The aim of the research presented in this thesis was to prepare and characterize thin film organic-based magnets with focus on V(TCNE)x. Photoelectron and absorption spectroscopy studies were performed leading to a more complete picture of the electronic and chemical structure of the material. Depending on the preparation method of V(TCNE)x, the material contains varying amounts of disorder which among other things makes it very air sensitive. In this thesis, a new preparation method for organic-based magnets based on physical vapor deposition is presented and the first result shows that it generates less air sensitive V(TCNE)x than previous methods reported. A new spin valve design based on V(TCNE)x was proposed where the material delivers both spin-filtering and spin-transporting functionality, making use of its fully spin-polarized transport levels. In such devices, the interface of V(TCNE)x with ferromagnetic metals is of great importance and was hence studied. As vanadium ions always are very reactive towards oxygen, substituting vanadium by a less reactive ion would be desirable from both an interface engineering and device packaging perspective. Very few alternatives exist however that orders magnetically above room temperature. In order to find out what are the key design criteria for preparing thin film semiconducting room temperature magnets, we have begun to study systems which order magnetically much below room temperature and compared them with V(TCNE)x.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2010. , 49 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1317
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-56262ISBN: 978-91-7393-385-8 (print)OAI: oai:DiVA.org:liu-56262DiVA: diva2:317838
Public defence
2010-05-25, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2010-05-05 Created: 2010-05-05 Last updated: 2010-05-05Bibliographically approved
List of papers
1. Near-edge x-ray absorption studies of Na-doped tetracyanoethylene films: A model system for the V(TCNE)x room-temperature molecular magnet
Open this publication in new window or tab >>Near-edge x-ray absorption studies of Na-doped tetracyanoethylene films: A model system for the V(TCNE)x room-temperature molecular magnet
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2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, 054420- p.Article in journal (Refereed) Published
Abstract [en]

V(TCNE)x, with TCNE=tetracyanoethylene and x~2, is an organic-based molecular magnet with potential to be used in spintronic devices. With the aim of shedding light on the unoccupied frontier electronic structure of V(TCNE)x we have studied pristine TCNE and sodium-intercalated TCNE by near edge x-ray absorption fine structure (NEXAFS) spectroscopy as well as with theoretical calculations. Sodium-intercalated TCNE was used as a model system of the more complex V(TCNE)x and both experimental and theoretical results of the model compound have been used to interpret the NEXAFS spectra of V(TCNE)x. By comparing the experimental and theoretical C K-edge of pristine TCNE, the contributions from the various carbon species (cyano and vinyl) could be disentangled. Upon fully sodium intercalation, TCNE is n doped with one electron per molecule and the features in the C and N K-edge spectra of pristine TCNE undergo strong modification caused by partially filling the TCNE lowest unoccupied molecular orbital (LUMO). When comparing the C and N K-edge NEXAFS spectra of fully sodium-doped TCNE with V(TCNE)x, the spectra are similar except for broadening of the features which originates from structural disorder of the V(TCNE)x films. The combined results from the model system and V(TCNE)x suggest that the lowest unoccupied molecular orbital with density on the nitrogen atoms in V(TCNE)x has no significant hybridization with vanadium and is similar to the so-called singly occupied molecular orbital of the TCNE anion. This suggests that the LUMO of V(TCNE)x is TCNE or vanadiumlike, in contrast to the frontier occupied electronic structure where the highest occupied molecular orbital is a hybridization between V(3d) and cyano carbons. The completely different nature of the unoccupied and occupied frontier electronic structure of the material will most likely affect both charge injection and transport properties of a spintronic device featuring V(TCNE)x.

Place, publisher, year, edition, pages
Institutionen för teknik och naturvetenskap, 2008
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-11753 (URN)10.1103/PhysRevB.77.054420 (DOI)
Note
Original publication: E. Carlegrim, B. Gao, A. Kanciurzewska, M. P. de Jong, Z. Wu, Y. Luo and M. Fahlman, Near-edge x-ray absorption studies of Na-doped tetracyanoethylene films: A model system for the V(TCNE)x room-temperature molecular magnet, 2008, Physical Review B, (77), 054420. http://dx.doi.org/10.1103/PhysRevB.77.054420. Copyright: The America Physical Society, http://prb.aps.org/Available from: 2008-05-07 Created: 2008-05-07 Last updated: 2017-12-13
2. The unoccupied electronic structure of the semi-conducting room temperature molecular magnet V(TCNE)2
Open this publication in new window or tab >>The unoccupied electronic structure of the semi-conducting room temperature molecular magnet V(TCNE)2
2008 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 452, no 1-3, 173-177 p.Article in journal (Refereed) Published
Abstract [en]

The unoccupied electronic structure of the organic-based magnet V(TCNE)x (TCNE = tetracyanoethylene, x~2) has been studied with near edge x-ray absorption fine structure (NEXAFS) and with photoelectron spectroscopy (PES). By studying V(TCNE)x upon sodium-doping, the electron-accepting state, i.e. the lowest unoccupied molecular orbital (LUMO) of V(TCNE)x was shown mainly to be localized on (TCNE)--units in contrast to the hole-accepting state, i.e. the highest occupied molecular orbital (HOMO), which previously was assigned primarily to be V(3d)-derived. This study also showed that there are trap states for electron transport located below the (TCNE)2- level, likely leading to decreased electron mobility.

Place, publisher, year, edition, pages
Institutionen för teknik och naturvetenskap, 2008
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-11143 (URN)10.1016/j.cplett.2007.12.049 (DOI)
Note
Original publication: Elin Carlegrim, Anna Kanciurzewska, Michel P. de Jong, Carl Tengstedt and Mats Fahlman, The unoccupied electronic structure of the semi-conducting room temperature molecular magnet V(TCNE)2, 2008, Chemical Physics Letters, (452), 1-3, 173-177. http://dx.doi.org/10.1016/j.cplett.2007.12.049. Copyright: Elsevier Ltd, http://www.elsevier.com/Available from: 2008-02-27 Created: 2008-02-27 Last updated: 2017-12-13
3. Air-stable organic-based semiconducting room temperature thin film magnet for spintronics applications
Open this publication in new window or tab >>Air-stable organic-based semiconducting room temperature thin film magnet for spintronics applications
2008 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 16, 163308- p.Article in journal (Refereed) Published
Abstract [en]

Herein, we report on a preparation method of vanadium tetracyanoethylene, V(TCNE)x, an organic-based semiconducting room temperature thin film magnet. Previously, this compound has been reported to be extremely air sensitive but this preparation method leads to V(TCNE)x, which can retain its magnetic ordering at least several weeks in air. The electronic structure has been studied by photoelectron spectroscopy and the magnetic properties by superconducting quantum interference device. The properties mentioned above, in combination with complete spin polarization, makes this air-stable V(TCNE)x a very promising material for spintronic devices.

Place, publisher, year, edition, pages
Institutionen för teknik och naturvetenskap, 2008
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-11869 (URN)10.1063/1.2916901 (DOI)
Note
Original publication: Elin Carlegrim, Anna Kanciurzewska, Per Nordblad and Mats Fahlman, Air-stable organic-based semiconducting room temperature thin film magnet for spintronics applications, 2008, Applied Physics Letters, (92), 163308. http://dx.doi.org/10.1063/1.2916901. Copyright: American Institute of Physics, http://apl.aip.org/apl/top.jspAvailable from: 2008-05-21 Created: 2008-05-21 Last updated: 2017-12-13
4. Characterization of the Ni/V(TCNE)x interface for hybrid spintronics applications
Open this publication in new window or tab >>Characterization of the Ni/V(TCNE)x interface for hybrid spintronics applications
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2010 (English)In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 11, no 6, 1020-1024 p.Article in journal (Refereed) Published
Abstract [en]

Vanadium tetracyanoethylene, V(TCNE)x, is an organic-based magnet with properties suitable for spintronics applications, e.g. spin valves. In this paper we propose a new hybrid organic spin valve design where V(TCNE)x is used as a spin-transporting and spin-filtering layer sandwiched between two ferromagnetic (FM) metal contacts, i.e. FM/V(TCNE)x/FM. As the spin injection and detection of such a device occurs at the interfaces the quality of those are of crucial importance. Therefore, the Ni/V(TCNE)x interface has been investigated by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption spectroscopy (NEXAFS) as well as compared with XPS results from a model system, Ni/TCNE. Ni chemically interact with both the vinyl and cyano groups but there is no evidence for significant diffusion of Ni into the V(TCNE)x film. As the chemical interaction affects the spin injection and detection negatively by modifying the lowest unoccupied molecular orbital (LUMO) and destroying the magnetic ordering network at the surface, these results indicate that there is need for a buffer layer between V(TCNE)x and Ni, and in extension most likely between V(TCNE)x and any FM contact.

Keyword
Organic-based molecular magnets; Spintronics; Interfaces; Photoelectron spectroscopy; Magnetic semiconductors
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-56259 (URN)10.1016/j.orgel.2010.03.001 (DOI)000277935200008 ()
Note

Original Publication: Elin Carlegrim, Yiqiang Zhan, Fenghong Li, Xianjie Liu and Mats Fahlman, Characterization of the Ni/V(TCNE)x interface for hybrid spintronics applications, 2010, Organic electronics, (11), 6, 1020-1024. http://dx.doi.org/10.1016/j.orgel.2010.03.001 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com/

Available from: 2010-05-05 Created: 2010-05-05 Last updated: 2017-12-12
5. Electronic structure of thin film cobalt tetracyanoethylene, Co(TCNE)x
Open this publication in new window or tab >>Electronic structure of thin film cobalt tetracyanoethylene, Co(TCNE)x
2010 (English)In: Synthetic metals, ISSN 0379-6779, E-ISSN 1879-3290, Vol. 161, no 17-18, 1892-1897 p.Article in journal (Refereed) Published
Abstract [en]

V(TCNE)x, TCNE=tetracyanoethylene, x~2, is a semiconducting organicbased magnet and one of very few organic-based magnets with critical temperature above room temperature (RT). With the aim to understand the key design criteria for achieving RT organic-based magnets we have started to study the electronic and chemical structure of members of the M(TCNE)x family with significantly lower critical temperatures than V(TCNE)x. In this paper, Co(TCNE)x, x~2, (Tc~44 K, derived from its powder form) were prepared by a method based on physical vapor deposition, resulting in oxygen-free thin films. The results propose Co(TCNE)x to contain to local bonding disorder in contrast to V(TCNE)x thin films, which can be grown virtually defect free. In addition, the Co L-edge does not show any pronounced fine structure, suggesting the crystal field to be very weak. By using a variety of photoemission and X-ray absorption techniques the highest occupied molecular orbital (HOMO) of Co(TCNE)x was determined to mainly be TCNE-derived while the states originating from Co(3d) are localized at higher binding energies. This is in stark contrast to V(TCNE)x where V(3d) is mainly responsible for the HOMO. As the HOMO of Fe(TCNE)x (Tc~121 K, derived from its powder form) is TCNE-derived these results show that Co(TCNE)x is more similar to Fe(TCNE)x than to V(TCNE)x in terms of electronic structure.

Place, publisher, year, edition, pages
Elsevier, 2010
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-56261 (URN)10.1016/j.synthmet.2011.06.031 (DOI)000295564000016 ()
Note

Funding agencies|Swedish Research Council||Knut and Alice Wallenberg Foundation||

Available from: 2010-05-05 Created: 2010-05-05 Last updated: 2017-12-12

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