The exchange coupling between ferromanetic layer in CoFeB/MgO/CoFeB tunnel structure with perpendicular magnetic anisotropy

• Main Authors: Weng, Yi-chien, 翁怡鍵
• Other Authors: Chern, Gung
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/09230966250146087835

碩士 === 國立中正大學 === 物理學系暨研究所 === 100 === In this thesis we study the interlayer-exchange-coupling, as a function of the thickness of MgO spacer layers and the thickness of CoFeB, in the Co40Fe40B20/MgO/Co20Fe60B20 perpendicular magnetic tunnel junction structures. The main work is divided into two parts: (1) In Co40Fe40B20(1.2)/MgO(x)/Co20Fe60B20(1.2) structure (all units are in nm), the MgO thickness varies from 0.9 to 1.5 nm at annealing temper -atures are 225℃ and 255℃, respectively. We found that the coupling strength and sign (corresponded to par -allel or antiparallel coupling) varied depending upon MgO layer thick -ness. These samples show parallel(ferromagnetic) coupling at the thickness of MgO ~ 1.25nm while show antiparallel (antiferromagnetic) coupling at the thickness of MgO ~ 1.2 nm, and show no coupling at the thickness of MgO ~ 1nm. Samples with MgO layer thickness of 1, 1.1, 1.2, 1.3, 1.4, and 1.5nm are chosen for further annealing test and the annealing temperature is in the range of 225℃- 315℃. The results show clear variation of the coup -ling field as a function of annealing temperatures. The samples of MgO 1, 1.1, 1.3, 1.4nm show ferromagnetic coupling at 225℃ but change to antiferromagnetic after the annealing temperature is increased. The sample of MgO 1.2nm show antiferromagnetic coupling at 225℃ and the antiferromagnetic coupling strength is increased after the annealing temp -erature is increased. The sample of MgO 1.5nm show antiferromagnetic coupling at 225℃ and changes to ferromagnetic at the annealing temper -ature increases from 225℃ to 285℃. However, it changes back to antiferromagnetic at T > 285℃. In addition, we also found that the anisotropy energy constant (Ku) and the exchange coupling energy (J) shows inverse relation as the annealing temperature varies. The anisotropy energy constant increases as temperature increases while the exchange coupling decreases from ferromagnetic to antiferromagnetic as temperature increases. This effect seems to be consistent with the recent report of the calculation which is based on an Orange peel Model. (2) In Co40Fe40B20(1.2)/MgO(1)/Co20Fe60B20(x) and Co40Fe40B20(x)/MgO(1)/Co20Fe60B20(1.2) structures, the ferromagnetic layer thickness is changed in the range of 0.9 - 1.5nm and annealing temperatures of 225 - 255℃, respectively. We measure the variation of the coupling field as a function of ferro -magnetic layer thickness and find an oscillatory behavior of exchange coupling as a function of the ferromagnetic layer thickness at annealing temperature 225℃. However, this oscillatory behavior disappears after the annealing reaches 255℃. This effect may be explained by a Fabry-Perot- type interferences model.