Date: Tuesday, June 19, 2012
Time: 10:00 - 11:00
Place: 1st floor 2nd seminar room, Sengen
A magnetic tunnel junction (MTJ) consisting of a thin insulating layer
(a tunnel barrier) sandwiched between two ferromagnetic electrodes exhibits
the tunnel magnetoresistance (TMR) effect due to spin-dependent electron
tunneling. Since the discovery of room-temperature TMR in the mid-1990s,
MTJs with an amorphous aluminum oxide (Al?O) tunnel barrier have been studied
extensively. Such MTJs exhibita magnetoresistance (MR) ratio of several
tens ofpercent at room temperature (RT) and have been applied to magnetoresistive
random access memory (MRAM) and the read heads of hard disk drives. MTJs
with MR ratios substantially higher than 100%, however, are desired for
next-generation spintronic devices. In 2001, first-principle theories predicted
that the MR ratios of epitaxial Fe/MgO/Fe MTJs with a crystalline MgO(001)
barrier would be over 1000% due to the coherent tunneling of specific Bloch
states. In 2004, MR ratios of about 200% were obtained for MgO-based MTJs
[1]. MTJs with a CoFeB/MgO/CoFeB structurewere developed for practical
application and found to have MR ratios of above 200% and other practical
properties [1,2]. This lecture focuses on the physics of magnetoresistance
and spin-transfer torque inMTJs and the application of MTJs to various
spintronic devices such as magnetic sensors, spin-transfer-torque MRAM
(STT-RAM or spin-RAM) with perpendicular magnetization, and novel spin-torque
oscillators [3]. In addition, new types of MTJs such as spin-filter junctions
with a ferromagnetic tunnel barrier will be discussed.
[1] S. Yuasa and D. D. Djayaprawira, J. Phys. D: Appl. Phys. 40, p.R337
(2007).
[2] D. D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata,
N. Watanabe, S. Yuasa, Y. Suzuki and K. Ando, Appl. Phys. Lett. 86, 092502
(2005).
[3] PDF file of the lecture slides can be downloaded from http://unit.aist.go.jp/src/cie/ieee.htmlˇˇwith a password given at the lecture.