报告题目：Ferromagnetic semiconductors and heterostructures: Recent progress and future prospects
报告人：Prof. Masaaki Tanaka Center for Spintronics Research Network (CSRN), The University of Tokyo
报告时间：2018年11月5日(星期一)下午3:00
报告地点：2号楼303A室
内容简介：
Semiconductors are used for many electronic devices, in which we use charge transport of carriers. On the other hand, ferromagnets are used for magnetic devices, in which we use spin and magnetization. By integrating these material functionalities, we can create a new class of devices using both charge and spin degrees of freedom, which will be useful for next-generation information technology. Two examples of such new devices are1) spin MOSFET proposed by Sugahara and Tanaka [1,2], and 2) spin bipolar transistor proposed by Fabian and Zutic [3]. In the talk, first we give a brief introduction on the concept of “spin transistors”. These spintronics devices can be used for nonvolatile memory and reconfigurable logic, non-volatile logic, and possibly nueromorphic computing applications. To realize such spin-transistor devices, we need ferromagnetic semiconductors that have both the properties of semiconductors and ferromagnets.
Ferromagnetic semiconductors (FMSs) have been intensively studied for decades,since they have novel functionalities that cannot be achieved with conventional metallic materials, such as the ability to control magnetism by electrical gating or light irradiation [4-6]. Prototype FMSs such as (Ga,Mn)As, however, are always p-type, making it difficult to be used in real spin devices. Here, we demonstrate that by introducing Fe into InAs, it is possible to fabricate a new n-type electron-induced FMS with the ability to control ferromagnetism by both Fe and independent carrier doping. The studied (In1-x,Fex)As layers were grown by low-temperature molecular beam epitaxy on semi-insulating GaAs substrates. Electron carriers in these layers are generated by independent chemical doping of donors. The ferromagnetism was investigated by magnetic circular dichroism (MCD), superconducting quantum interference device (SQUID), and anomalous Hall effect (AHE) measurements. With increasing the electron concentration (n = 1.8×1018 cm-3 to 2.7×1019 cm-3) and Fe concentration (x = 5 - 8%), the MCD intensity shows strong enhancement at optical critical-point energies of InAs, indicating that the band structure of (In,Fe)As is spin-split due to sp-d exchange interaction between the localized d states of Fe and the electron sea. SQUID and AHE measurements are also consistent with the MCD results. The Hall and Seebeck effects confirm the n-type conductivity of our (In,Fe)As samples. The electron effective mass is estimated to be as small as 0.03-0.175m0, depending on the electron concentration. These results reveal that the electrons are in the InAs conduction band rather than in the impurity band, allowingus to use the conventional mean-field Zenermodel of carrier-induced ferromagnetism [7]. This band picture is different from that of (Ga,Mn)As [8][9]. Our results open the way to implement novel spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as help understand the mechanism of carrier-mediated ferromagnetism in FMSs [10-17].
Furthermore, we have found new phenomena in (In,Fe)As and its quantum heterostructures: Novel crystalline anisotropic magnetoresistance with two fold and eight fold symmetry[10], and control of ferromagnetism by strain, quantum confinement, gate electric field and wave-function engineering in quantum heterostructures with a (In,Fe)As quantum well[13-15]. Very recently, we have found very intriguing phenomena; sudden restoration of the band ordering associated with the ferromagnetic phase transition in the prototypical ferromagnetic semiconductor (Ga,Mn)As [18], and control of the bias-voltage dependence of tunneling anisotropic magneto-resistance using quantization in (Ga,Mn)As quantum wells [19]. Also, we have successfully grown new narrow-gap p-type III-V-based FMS (Ga,Fe)Sb and n-type III-V-based FMS (In,Fe)Sb with Curie temperatureshigher than room temperature (TC > 300K) [20][21]. Combining different n-type and p-type FMSs with high TC will lead to new spin-related functionalities and devices.
References
[1] S. Sugahara and M. Tanaka, Appl. Phys. Lett. 84, 2307 (2004).
[2] M. Tanaka and S. Sugahara (invited paper), IEEE Trans. Electron Dev. 54, 961 (2007).
[3] J. Fabian and I. Zutic, Phys. Rev. B 69, 15314 (2004).
[4] S. Koshihara et al., Phys. Rev. Lett. 78, 4617 (1997).
[5] H. Ohno et al., Nature 408, 944 (2000).
[6] A. M. Nazmul, S. Kobayashi, S. Sugahara and M. Tanaka, Jpn. J. Appl. Phys. 43, L233 (2004).
[7] T. Dietl, H. Ohno, F. Matsukura, J. Cibert and D. Ferrand, Science 287, 1019 (2000).
[8] S. Ohya, I. Muneta, P. N. Hai, and M. Tanaka, Phys. Rev. Lett. 104, 167204 (2010).
[9] S. Ohya, K. Takata, and M. Tanaka, Nature Phys. 7, 342 (2011).
[10] P. N. Hai, L. D. Anh, S. Mohan, T. Tamegai, M. Kodzuka, T. Ohkubo, K. Hono, and M. Tanaka, Appl. Phys. Lett. 101, 182403 (2012).
[11] P. N. Hai, D. Sasaki, L. D. Anh, and M. Tanaka, Appl. Phys. Lett. 100, 262409 (2012).
[12] P. N. Hai, L. D. Anh, and M. Tanaka, Appl. Phys. Lett. 101, 252410 (2012).
[13] L. D. Anh, P. N. Hai, and M. Tanaka, Appl. Phys. Lett. 104, 042404 (2014).
[14] L. D. Anh, P. N. Hai, Y. Kasahara, Y. Iwasa, and M. Tanaka, Phys. Rev. B92, 161201(R) (2015).
[15] S. Sakamoto, L. D. Anh, P. N. Hai, G. Shibata, Y. Takeda, M. Kobayashi, Y. Takahashi, T. Koide, M. Tanaka, A. Fujimori,Phys. Rev. B93, 035203 (2016).
[16] L. D. Anh, P. N. Hai, and M. Tanaka, Nature Commun.7, 13810 (2016).
[17] M. Tanaka, S. Ohya, and P. N. Hai (invited paper), Appl. Phys. Rev. 1, 011102 (2014).
[18] I. Muneta, S. Ohya, H. Terada,and M. Tanaka, Nature Commun. 7, 12013 (2016).
[19] I. Muneta, T. Kanaki, S. Ohya, and M. Tanaka, Nature Commun. 8, 15387 (2017).
[20] N. T. Tu, P. N. Hai, L. D. Anh, and M. Tanaka, Appl. Phys. Lett. 108, 192401 (2016).
[21] N. T, Tu, P. N. Hai, L. D. Anh, and M. Tanaka, arXiv: 1706.00735 (2017);Appl. Phys. Exp. 11, 063005 (2018); N. T, Tu, P. N. Hai, L. D. Anh, and M. Tanaka, Appl. Phys. Lett. 112, 122409 (2018).
报告人简介：Masaaki Tanaka is a professor at the University of Tokyo, in which he is a Director of Center for Spintronics Research Network (CSRN). He also belongs to Department of Electrical Engineering & Information Systems, Graduate School of Engineering, of this university. He received his B.E., M.E. and Ph.D. degrees in electronic engineering from the University of Tokyo in 1984, 1986, and 1989, respectively. After that, he worked at Department of Electronic Engineering, the University of Tokyoas a research associate, lecturer, associate professor, and became a professor in 2004. He has received numerous awards, like IBM Japan Prize from IBM Japan in 2003, Fellow Award fromJapan Society of Applied Physicsin 2015. He has also been involved into many Society Activities, like Leader of the Spintronics Research Network of Japan from 2016 to present. He has served on several journal editorial boards and the organization committees of a number of spintronics-related international conferences.His research interests include ferromagnetic semiconductors, ferromagnet/semiconductor hybrid systems, and their applications to spintronic devices.