(Invited) Room Temperature Ferromagnetism Induced by Electric Field in Cobalt-Doped TiO₂

Abstract

Electric field induced ferromagnetism at room temperature in cobalt-doped titanium dioxide was demonstrated by means of electric double layer transistor. This result represents that a carriermediated exchange coupling plays a principal role in the high temperature ferromagnetism in this compound. Accordingly, this compound is a promising material for room temperature semiconductor spintronics. IntroductionFerromagnetic semiconductor is a semiconductor doped with small amount of transition metal, and the possession of both ferromagnetic and semiconducting characters owing to the spin and charge degrees of freedom is useful for semiconductor spintronics (1). For representative ferromagnetic semiconductors like (Ga,Mn)As and (In,Mn)As, various spintronic devices have been demonstrated such as the spin polarized light emitting diode (2), the electric field control of ferromagnetism (3), the optical control of the ferromagnetism (4), and the electrical magnetization rotation (5). However, their Curie temperatures lower than 200 K hampered the operation of those spintronic devices at room temperature. In order to raise the Curie temperature, a larger exchange interaction between band carriers and localized spins of doped transition metal ions is required. In wide gap semiconductors, the larger exchange interaction could be expected because of the heavy carrier mass and the large electron carrier density, that are generally seen in oxide semiconductors. Such expectation led us to propose various magnetic oxide semiconductors, (Zn,Mn)O, (Sn,Mn)O 2 , and so on (6 − 8). A combinatorial material exploration finally led to a discovery of room temperature ferromagnetism in (Ti,Co)O 2 (9). In the high temperature ferromagnetism, the role of carriers was uncertain and the origin has been of long debate. If the ferromagnetism is originated from an extrinsic source like the segregation of Co metal, this compound will not be useful for semiconductor spintronics. Generally, the band carriers mediate ferromagnetic exchange coupling between spatially separated magnetic ions in ferromagnetic semiconductors. Thus, the electric field control of ferromagnetism at room temperature in (Ti,Co)O 2 is a 10.1149/05004.0053ecst ©The Electrochemical Society ECS Transactions, 50 (4) 53-57 (2012) 53 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 131.215.225.9 Downloaded on 2015-06-22 to IPfirm evidence of a ferromagnetic semiconductor, as was demonstrated in (In,Mn)As and (Ga,Mn)As at low temperature (3). Such demonstration is important not only for evidencing the carrier mediated ferromagnetism but for the implementation of spintronic devices. Experiments and resultsThin film growth and the electric measurements Epitaxial thin films of anatase (Ti,Co)O 2 were grown with pulsed laser deposition. Ti 0.90 Co 0.10 O 2 (001) films were grown on LaAlO 3 (100) substrates buffered with 5 unit cell thick insulating TiO 2 . In order to control the electron carrier density, the ...