High throughput characterization of magnetic semiconductor thin films with a scanning SQUID microscope
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High throughput characterization of magnetic semiconductor thin films with a scanning SQUID microscope X. J. Fan,1,2 M. Murakami,1 R. Takahasi,1 T. Koida,1 Y. Matsumoto,1,2 T. Hasegawa,1,2 T. Fukumura,2,3 M. Kawasaki,2,3 P. Ahmet,2 T. Chikyow,2 and H. Koinuma,1,2 1
Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
2
Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba 305-0044,
Japan 3
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
ABSTRACT
Magnetic properties of Co-doped rutile (Ti1-xCoxO2) film in combinatorial composition-spread form have been surveyed by means of a Scanning Superconducting-quantum-interference-device Microscope (SSM). As a consequence, we found magnetic domains in the spatial regions with x>0.05 without external field, giving strong evidence for ferromagnetism with finite spontaneous magnetization. The magnetic moment was monotonously increased with increasing doping level x from 0.05 to ~ 0.13. On the other hand, it was almost unchanged for x > ~ 0.13, suggesting that Co does not dissolve into rutile film beyond x ~ 0.13. The SSM results on the rutile Ti0.95Co0.05O2 thin films with different thickness showed that the magnetic moment is proportional to film thickness, leading to a conclusion that the presently observed ferromagnetism does not result from Co or Co-based oxide particles on the film surface.
INTRODUCTION
Magnetic semiconductor (MS) is a key material in spin-electronics which utilizes both charge and spin degrees of freedom. However, conventional MSs, such as (Ga,Mn)As and (In,Mn)As, undergo ferromagnetic (FM) transition at temperatures far below room temperature, which substantially limits the practical application of MS. In contrast, very recent article [1] reported the discovery of room temperature ferromagnetism in Co-doped TiO2 anatase. This widens the candidates of MS to wide gap semiconductors doped with transition metals. Indeed, TiO2 with a S2.7.1
band gap of 3.06 eV has been widely used as high-efficiency photo-catalysts. TiO2 can be classified as a 3d0 oxide so that injected spins introducing new donor states into the band gap of the host material might largely modify its physics properties. Compared with the anatase phase of TiO2, the rutile structure is much more stable in natural environment and is easy to synthesize. It has been already demonstrated that the presence of a few percent Co in TiO2 rutile thin film destroys the antiferromagnetic interaction between charge carriers and gives rise to the ferromagnetic (FM) ground state [2]. In order to investigate the correlation between the ferromagnetism and Co doping level in rutile, we have fabricated composition-spread thin films of rutile Ti1-xCoxO2 by laser molecular beam epitaxy. In this paper, we present the local magnetic properties of the composition-spread chips probed by a scanning SQUID microscope, which can directly image spontaneous ferromagnetic domains in a micron scale.
EXPERIMENTS AND RESULTS
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