TOPIC Functional Oxide Nano-electronics and Spintronics using Vanadate and Ferrite 
AREA Microelectronic Technologies & Devices  
SPEAKER Prof Hidekazu Tanaka, Nanoscience and Nanotechnology Center,Institute of Scientific and Industrial Research, Osaka University 
DATE 8 October 2014, Wednesday 
TIME 10:00 am to 11:30 am 
VENUE E5-03-20, Engineering Blk E5, Faculty of Engineering, NUS 
FEES No Charge 

Transition metal oxides exhibit a rich variety of attractive properties such as a metal-insulator transition (MIT), colossal magnetoresistance, and high TC ferromagnetism due to their strongly correlated electrons. If we were to artificially control these behaviours, we can construct novel electronics devices. Among them, VO2 shows orders-of-magnitude changes in resistivity at around 340 K owing to MIT which is applicable for multifunctional switching device at room temperature [1], and the solid solution system of Fe3−xZnxO4 has high tunability in carrier concentration and high TC, with high spin polarization [2, 3], which it is one of the best candidates for advanced spintronics device working at room temperature. In this talk, I will introduce the control of MIT and ferromagnetism in 3D nano/micro structures and heterostructures towards functional oxide nanoelectronics and spintronics.

In VO2 system, VO2 thin films were deposited on TiO2 (001) and Al2O3 (0001) substrates using a pulsed laser deposition technique (PLD). The micro/nano-sized structures with variety of aspect ratio were prepared by photolithography and nanoimprint lithography. 1D alignment of metallic domains was clearly observed for the 1-m-wide VO2 wires on TiO2 (001). It was apparent that each domain was forced to be confined by the wire width and thus one-dimensionally aligned, and exhibited a much higher and clearer discrete drop in resistivity by two orders of magnitude than conventional 2D thin film [4, 5, 6]. By extending nano-imprint lithography technique, VO2 nano-wire with width of 200nm was fabricated on Al2O3 (0001) single crystal substrate, and it showed very sharp current induced insulator to metal transition which is applicable for correlated electron based switching devices [7].

In Fe3−xZnxO4 system, a field-effect transistor structure with a transition-metal oxide channel is an effective approach to externally controlling the density of charge carriers and the resulting electronic/ magnetic states. Ferroelectric field control of ferromagnetism at room temperature was achieved in the field effect heterostructure combining the Fe2.5Zn0.5O4 with ferroelectric PbZrTiO3 [8]. Field-effect gating with ionic liquid (IL) electrolytes has been shown to be a very powerful tool for inducing extremely high-density charge carrier accumulation, with the demonstration that the IL gating induces reversible and nonvolatile changes in the conductivity and magnetoresistance in a spinel ferrite system [9]. We will discuss their mechanisms from the point of view of electrostatic field effect and redox based effect [10]. As further development for enhancement of magnetic transport properties, epitaxial ferrite oxide artificial nanowire [11] /nano-dot [12] structures will be also introduced.

References [1] Adv. Materials 25 (2013) 6430, [2] Appl. Phys. Lett., 89 (2006) 242507, [3] Phys. Rev. B, 76 (2007) 205108, [4] Appl. Phys. Lett.100 (2012) 173112 [5] Appl. Phys. Lett.101 (2012) 263111, [6] Appl. Phys. Lett. 102 (2013) 153106, [7] Appl. Phys. Lett. 104 (2014) 023104, [8] Appl. Phys. Lett. 98 (2011) 102506, [9] Adv. Materials Interfaces 1 (2014) 1300108, [10] Sci. Rep. 4 (2014) 5818, [11] Nano Lett.10 (2010) 2772-2776, [12] Nano Lett.11 (2011) 343-347.

Professor Hedekazu Tanaka is a professor with Nanoscience and Nanotechnology Center, Institute of Scientific and Industrial Research, Osaka University. He received his Bachelor of Engineering (1993), Master of Engineering (1995) and PhD (1998) all from Osaka University. Since 1997, he has been working with Institute of Scientific and Industrial Research, Osaka University, as Research Associate (1997-2003), Associate Professor (2003-2008) and Professor (2008 present). His research interests include nanotechnology (thin film, heterostructure, nano-dot/wire) and functional materials (functional oxide, ferromagnetic, ferroelectric, strongly correlated system). He has received many awards including Young Scientist Award (The Japan Institute of Metals, 1993), Young Scientist Award (Japan Material Research Society, 2000), Young Scientist Award for the Presentation of an Excellent Paper (The Japan Society of Applied Physics, 2001) and Excellent Scientist Award (Uchiyama Prize, 2005). 


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