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The research interests are focused in the following areas:
1. Spintronics and Novel Magnetic Oxide Systems, Diluted Magnetic semiconductors, Multiferroics
The field of Spintronics [1,2] requires development of novel magnetic materials and the class of materials called diluted magnetic semiconducting oxides has gained significance over the last few years on account of their potential for integrating optical, electronic and magnetic phenomena in the same system . Applications range from spin FETs [1], to novel magnetically controlled optical polarization in emitters [3] to GHz emitters based on the spin torque effect [4,5]. Attempts to make DMS materials from conventional semiconductors has resulted so far in a maximum Curie temperature of about 150K in the Mn doped GaAs system while applications require a Curie temperature in excess of 300K. A number of DMS oxide systems have been identified over the last few years, TiO2 [6-12], LaSrTiO3 [13], Cu2O [14], SnO2 [15] and so on [16-19], with Curie temperatures significantly in excess of 300K. Our group has done pioneering work in this area and we continue to discover new materials in this area.
With magnetic oxide systems exhibiting a lack of center of symmetry another phenomenon has been discovered whereby magnetism and ferroelectricity, ferroeleasticity or piezo electricity are seen to co-exist in the same system and these are called Multiferroic materials. We have produced and studied epitaxial films of GaFeO3 [20] and various doped versions of these materials and are also studying nanoparticles of materials such TbMnO3 [21]. Layered structures of magnetic oxides and ferroelectric materials could also be used to synthesize multiferroic systems and we are looking at various candidates here as well.
2. Engineering novel Optical, Conducting and Magnetic systems from Wide Bandgap Oxides
By doping at a few percent level wide band gap oxides with large dielectric constant with dopants having a single electron/hole contribution one may be able to synthesize novel materials with unusual optical, magnetic or electronic properties. For examples, it may be possible to make ferromagnets out of materials that have no magnetic constituents. The magnetism in these materials is mediated by both cationic and anionic defects and these effects get enhanced by the large dielectric constants of these materials which enhance the Bohr radius of the localized defects causing an overlapping band at relatively low concentrations.
3. ZnMgO system, the widest band gap material accessible today
ZnMgO is a system with tremendous potential to replace GaAlN in terms its potential for optical emitters and electronic devices. The fact that the exciton binding energy in this material is the highest (61 meV vs 29meV for GaN), the range of band gap is from 3.25-7.8 eV one of the largest span, availability of two inch single ZnO crystals for homo epitaxy and the large terrestrial availability of Zn make this a very attractive alternative to the AlGaN system. We are currently involved in the fabrication of solar blind UV detectors based on MSM structures, doping of ZnO to get p-type dopants, understanding the surface of ZnO single crystal to get good homoepitaxy and also the growth of nano fibers of ZnO for sensor applications. [22-24].
4. Electronic Transport at Oxide Interfaces
We are looking at the junction characteristics of various p and n type oxides interfaces. Many such pairs of oxides have exhibited outstanding pn junction characteristics and these may be potential candidates for non silicon based electronics such as flexible or transparent electronics or for nm sized transistors. We have studied junctions of YBCO/Nb-STO [25], Fe3O4/Nb-STO [26] and so on. We are also interested in superlattice structures of materials such as STO/LAO where very high mobilities have been observed at room and low temperatures.
Related Publications:
1. S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. Von Molnar, M. Roukes, A. Y. Chtchelkanova and D. M. Treger, "Spintronics: A spin based electronics vision for the future," Science 294, 1488 (2001).
2. S. Das Sarma, "Spintronics," Am. Sci. 89, 516 (2001).
3. Y. Ohno, D. K. Young, B. Beschoten, F. Matuskura, H. Ohno and D. D. Awschalom, "Electrical spin injection in a ferromagnetic semiconductor heterostructure," Nature 402, 790 (1999).
4. Mutual phase-locking of icrowave spin torque nano-oscillators, S. Kaka, M.R. Pufall, W.H. Rippard, T.J. Silva, S.E. Russek and J.A. Katine, Nature 437, 389 (2005).
5. Phase-locking in double-point-contact spin-transfer devices, F.B. Mancoff, N.D. Rizzo, B.N. Engel and S. Tehrani, Nature 437, 393 (2005).
6. Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S.-Y. Koshihara and H. Koinuma, "Room-Temperature Ferromagnetism in Transparent Transition Metal-Doped Titanium Dioxide," Science 291, 854 (2001).
7. S. R. Shinde, S. Ogale, S. Das Sarma, J. R. Simpson, H. D. Drew, S. Lofland, C. Lanci, J. P. Buban, N. D. Browning, V. N. Kulkarni, J. Higgins, R. P. Sharma, R. L. Greene and T. Venketesan, "Ferromagnetism in laser deposited anatase
Ti1-xCoxO2-δ films," Phys. Rev. B 67, 115211 (2003).
8. S. Guha, K. Ghosh, J. G. Keeth, S. Ogale, S. R. Shinde, J. R. Simpson, H. D. Drew and T. Venketesan, "Temperature-dependent optical studies of Ti1-xCoxO2," Appl. Phys. Lett. 83, 3296 (2003).
9. S. R. Shinde, S. Ogale, J. Higgins, H. Zheng, A. J. Millis, R. Ramesh, R. L. Greene and T. Venketesan, "Co-occurence of Superparamagnetism and Anomalous Hall Effect in Highly Reduced Cobalt Doped Rutile TiO2-δ Films," Phys. Rev. Lett. 92, 166601 (2004).
10. J. R. Simpson, H. D. Drew, S. R. Shinde, R. J. Choudhary, Y. G. Zhao, S. Ogale and T. Venketesan, "Optical band edge shift of anatase Ti1-xCoxO2-δ ," Phys. Rev. B 69, 193205 (2004).
12. Electric field effect in diluted magnetic insulator anatse Co:TiO2, T. Zhao, S. R. Shinde, S. Ogale, H. Zheng, T. Venketesan, R. Ramesh and S. Das Sarma, Phys. Rev. Lett. 94, 126601 (2005).
13. Y. G. Zhao, S. R. Shinde, S. Ogale, J. Higgins, R. J. Choudhary, V. N. Kulkarni, R. L. Greene, T. Venketesan , S. Lofland, C. Lanci, J. P. Buban, N. D. Browning, S. Das Sarma and A. J. Millis, "Co-doped La0.5Sr0.5TiO3-δ : Diluted magnetic oxide system with high Curie temperature," Appl. Phys. Lett. 83, 2199 (2003).
14. S. N. Kale, S. Ogale, S. R. Shinde, M. Sahasrabuddhe, V. N. Kulkarni, R. L. Greene and T. Venketesan, "Magnetism in cobalt-doped Cu2O thin films without and with Al, V, or Zn codopants," Appl. Phys. Lett. 82, 2100 (2003).
15. S. B. Ogale, R. J. Choudhary, J. P. Buban, S. Lofland, S. R. Shinde, S. N. Kale, V. N. Kulkarni, J. Higgins, C. Lanci, J. R. Simpson, N. D. Browning, S. Das Sarma, H. D. Drew, R. L. Greene and T. Venketesan, "High Temperature Ferromagnetism with a Giant Magnetic Moment in transparent Co-doped SnO2-δ," Phys. Rev. Lett. 91, 077205 (2003).
16. D. C. Kundaliya, S. Ogale, S. Lofland, S. Dhar, C. J. Metting, S. R. Shinde, Z. Ma, B. Varughese, K. V. Ramanujachary, L. Salamanca-Riba and T. Venketesan, "On the origin of high temperature ferromagnetism in low temperature processed Mn-Zn-O system," Nature Materials 3, 709 (2004).
17. Search for ferromagnetism in undoped and cobalt-doped HfO2-δ, M.S.R. Rao, Darshan C. Kundaliya, S.B. Ogale, L.F. Fu, S.J. Welz, N.D. Browning, V. Zaitsev, B. Varughese, C.A. Cardoso, A. Curtin, S. Dhar, S.R. Shinde, T. Venkatesan, S.E. Lofland and S. A. Schwarz, Applied Physics Letter 88, 142505 (2006).
18. Search for ferromagnetism in conductive Nb:SrTiO3 with magnetic transition element(Cr, Co, Fe, Mn) dopants, S.X. Zhang, S.B. Ogale, Darshan C. Kundaliya, L.F. Fu, N.D. Browning, S. Dhar, W. Ramadan, J.S. Higgins, R.L. Greene, T. Venkatesan, Applied Physics Letter 89, 012501 (2006).
19. Consequences of niobium doping for the ferromagnetism and microstructure of anatase Co:TiO2 films S. X. Zhang, S. B. Ogale, L. F. Fu, S. Dhar, D. C. Kundaliya, W. Ramadan, N. D. Browning, and T. Venkatesan, Applied Physics Letter 88, 012513 (2006).
20. Darshan C. Kundaliya, S. B. Ogale, S. Dhar, K. F. McDonald, E. Knoesel, T. Osedach, S. E. Lofland, S. R. Shinde and T. Venkatesan, “Large Second Harmonic Kerr rotation in GaFeO3 thin films on YSZ buffered Silicon.” Journal of Magnetism and Magnetic Materials 299, 307 (2006).
21. Synthesis and properties of the nanoparticles of Multiferroic TbMnO3, S. Kharrazi, S. W. Gosavi, S. K. Kulkarni, Darshan C. Kundaliya, S. B. Ogale, T. Venkatesan and J. Urban, Solid State Communication (In Press).
22. W. Yang, S. S. Hullavarad, B. Nagaraj, I. Takeuchi, R. P. Sharma, T. Venkatesan, R. D. Vispute and H. Shen , Appl. Phys. Lett. 82, 3424 (2003).
23. W. Yang, R. D. Vispute, S. Choopun, R. P. Sharma, T. Venkatesan and H. Shen, Appl. Phys. Lett. 78, 2787 (2001).
24. Realization of Mg((x=0.15))Zn((1-x=0.85))O-based metal-semiconductor-metal UV detector on quartz and sapphire, S. Hullavarad, S. Dhar, B. Varughese, I. Takeuchi, T. Venkatesan , R.D. Vispute, J. Vac. Sci. & Tech. A 23, 982-985 (2005).
25. Electrical properties of epitaxial junctions between Nb:SrTiO 3 and optimally doped, underdoped and Zn-doped YBa2Cu3O7-δ . W. Ramadan, S. B. Ogale, S. Dhar, S. R. Shinde, Darshan C. Kundaliya, M. S. R. Rao and T. Venkatesan, Phys. Rev. B 72, 205333 (2005).
26. Interfacial characteristics of a Fe3O4/Nb(0.5%):SrTiO3 oxides junction,D.C. Kundaliya, S.B. Ogale, L. Fu, S.J. Welz, J.S. Higgins, G. Langham, S. Dhar, N.D. Browning, T. Venkatesan, J. Appl. Phys. 99, 08K304 (2006).
Center for Superconductivity Research, University of Maryland, College Park, MD 20742-4111
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