xxiii πανελληνιο ÏƒÏ Î½ÎµÎ´ÏÎ¹Î¿ Ï†Ï ÏƒÎ¹ÎºÎ·Ï‚ στερεας καταστασης & επιστημης ...

Room Temperature Doped Ferromagnetic Oxides:
- A current Scenario Towards Magnetic p-n Junctions
K.V.Rao
Dept of Materials Science-Tmfy, Royal Institute of Technology,
Stockholm, Sweden
A goal of spintronics is to develop a semiconductor that can manipulate the magnetism of an electron. Adding the
spin degree of freedom to electronics will provide significant versatility and functionality to future electronic
products. Although many possible electronic components, like for example the magnetic switch (Ohno et al) have
been demonstrated, practical usefulness of the doped GaMnAs is limited by their Curie temperatures which currently
lie below 300K. From this point of view, recently, ZnO based materials have been of growing interest following the
first report [Sharma, P et al., Nature Mater.2, 673 (2003) with over 330 citations it is now the most cited paper from
Nature Materials at present) of room temperature ferromagnetism in Mn doped ZnO thin films. With a band gap in
the ultraviolet at 3.4 eV, ZnO based materials open up a variety of potential applications, especially for spinoptoelectronics,
some of which already exist but currently independent of the spin considerations. Mn 2 + ion has the
largest magnetic moment, 5 μ B, but often the moments measured experimentally have been rather small although the
Tc’s have been above room temperature. Because the properties of ZnO are extremely sensitive to processing
conditions, and the additional fact that Mn has many allotropes of oxides that are magnetic, it was shown by Sharma
et al that the key to obtain above room temperature ferromagnetism is the deposition and processing at temperatures
below 400 o C in a controlled atmosphere. These results have now been extensively reproduced often with
considerably larger magnetic moments per Mn atom. Gamelin and his coworkers found that although the ‘as
obtained’ nanocrystals produced by the colloidal approach were paramagnetic at room temperature, the films
obtained by spincoating on to fused silica with a 2 min aerobic anneal at 500 o C became ferromagnetic with Tc above
RT. In a subsequent experiment (JACS 127, 5292 (2005); PRL 94, 147209 (2005) they also showed that on
Nitrogen-capping the colloidal particles during the processing of the thin films, gave a strong evidence for
ferromagnetism with Mn in 2+ state, while on O-capping it was paramagnetic at room temperature. This result
supports the prediction expected from a theoretical approach that the ferromagnetic order is carrier induced, and is
via the holes in the p-state of ZnO. However, there are many recent experiments which evidence that, although
carrier mediated, ferromagnetic order above RT is also obtained on n-type doping [Narayan et al APL 88, 242503
(2006)]. Another strong evidence for ferromagnetism in Mn doped ZnO is the significant magnetic circular
dischroism at the ZnO band edge observed at room temperature. Along with such a result on doping ZnO with other
TM elements it implies that ferromagnetism is an intrinsic property of the bulk ZnO lattice.(Neal et al PRL 96,
197208 (2006). Neal et al also observed that the MCD data at the ZnO band edge shows room temperature hysteretic
behaviour.
Recently, room temperature ferromagnetism in Mn doped ZnO has also been reported in nanorods, and
freestanding single crystal nanowires, [e.g. APL 88, 263101 (2006)]. All the above facts and many more recent
studies only confirm that we have robust ferromagnetism in Mn doped ZnO at room temperature and can be
exploited for spintronic applications because of its high intrinsic Mn moment. Another challenging support and
promise for device application comes from a recent observation that on Al co-doping Mn doped ZnO, Xu, X.H. et al
[New J. Phys 8, 135 (2006)] obtained in their samples the full moment of Mn and in addition co-doping of Al
resulted in n-type ZnO matrix. This is a significant observation, because we can now think of designing
ferromagnetic n-p junctions at room temperature! Needless to stay that such an electronic element would not be
possible with doped GaMnAs materials.
It is always of interest to look for new and less complex materials suitable for applications in spintronics. In this
context I will also present some highlights of our extensive studies of Room Temperature Ferromagnetism in
semiconductors like ZnO, GaP, GaN In2O3.. doped with non-magnetic elements like Cu, V, Ti, Cr to mention a few
among many others. Theoretical calculations based on the density functional theory (DFT) suggest that the most
stable compounds of Tm(V, Cr Fe, and Ti) doped In 2 O 3 are all ferromagnetic at room temperature and specially in
the case of V doping it gives rise to a rather strong ferromagnetic coupling, and the moment at the V site for example
can be as large 2μ B The magnetic interactions are believed to be mediated by the hybridization between d-states of
V and p-bands of oxygen. In Stockholm we have fabricated Cu doped ZnO films into patterned MRAM type
structures with the patterned elements of size ranging from microns to nanometer scale by using ink-jet printing
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