Preparation and characterization of an organic-based magnet Papers included in the thesis Paper I Near edge x-ray absorption studies of Na-doped tetracyanoethylene films: a model system for the V(TCNE) x room temperature molecular magnet Paper II The unoc

Abstract In the growing field of spintronics there is a strong need for development of flexible lightweight semi-conducting magnets. Molecular organic-based magnets are attractive candidates since it is possible to tune their properties by organic chemistry, making them socalled "designer magnets". Vanadium tetracyanoethylene, V(TCNE) x , is particularly interesting since it is a semiconductor with Curie temperature above room temperature (T C~4 00 K). The main problem with these organic-based magnets is that they are extremely air sensitive. This thesis reports on the frontier electronic structure of the V(TCNE) x by characterization with photoelectron spectroscopy (PES) and near edge x-ray absorption fin

Electronic structure of thin film cobalt tetracyanoethylene, Co(TCNE)x

V(TCNE)x, TCNE = tetracyanoethylene, x ∼ 2, is a semiconducting organic-based 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. 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, but in line with the results of Fe(TCNE)x (TC ∼ 121 K, derived from its powder form) for which the HOMO also is TCNE−-derived. Moreover, the results propose Co(TCNE)x to contain large amounts of local bonding disorder in contrast to V(TCNE)x which can be grown virtually defect free. We speculate that cobalt binds to vinyl – instead of cyano groups, hence creating a disordered bonding arrangement which deviates from octahedral. The very weak (<1 eV) crystal field splitting and a large spread in the local ligand field strengths due to disorder in Co(TCNE)x are in agreement with the absence of fine structure in the Co L-edge spectra

Electronic structure of thin film iron-tetracyanoethylene: Fe(TCNE)x

Thin film iron-tetracyanoethylene Fe(TCNE) x , x∼2, as determined by photoelectron spectroscopy, was grown in situ under ultra-high vacuum conditions using a recently developed physical vapor deposition-based technique for fabrication of oxygen- and precursor-free organic-based molecular magnets. Photoelectron spectroscopy results show no spurious trace elements in the films, and the iron is of Fe2+ valency. The highest occupied molecular orbital of Fe(TCNE) x is located at ∼1.7 eV vs. Fermi level and is derived mainly from the TCNE− singly occupied molecular orbital according to photoelectron spectroscopy and resonant photoelectron spectroscopy results. The Fe(3d)-derived states appear at higher binding energy, ∼4.5 eV, which is in contrast to V(TCNE)2 where the highest occupied molecular orbital is mainly derived from V(3d) states. Fitting ligand field multiplet and charge transfer multiplet calculations to the Fe L-edge near edge X-ray absorption fine structure spectrum yields a high-spin Fe2+ (3d6) configuration with a crystal field parameter 10Dq∼0.6 eV for the Fe(TCNE) x system. We propose that the significantly weaker Fe-TCNE ligand interaction as compared to the room temperature magnet V(TCNE)2 (10Dq∼2.3 eV) is a strongly contributing factor to the substantially lower magnetic ordering temperature (T C ) seen for Fe(TCNE) x -type magnets