Study of microstructures and magnetoresistance of MgO based magnetic tunnel junctions
Date Issued
2006
Date
2006
Author(s)
Chou, Chun-Yuan
DOI
zh-TW
Abstract
Magnetic tunnel junctions are deposited on Si substrate by dc and rf magnetron sputtering in an ultra-high vacuum chamber. The micro-sized spin-dependent tunnel junction is patterned by e-beam lithography and ion-milling. The pseudo-spin valve magnetic tunnel junction is fabricated in this work. Due to the difference in the coercivity between two ferromagnetic layers, the parallel and antiparallel spin states of the two ferromagnetic layers can be obtained as the magnetic field is varied. The effects of underlayer materials, working pressure and annealing temperatures on the microstructure and magnetoresistance of the MgO film are investigated.
TEM analysis shows that the as-deposited MgO film with Co or CoFe underlayer has nanocrystalline structure or mixing phases of nanocrystalline and amorphous. Besides, the MgO grains are random-oriented. The as-deposited MgO thin film with (001) preferred orientation is obtained as the MgO film is deposited on CoFeB、CoFeC or SiO2 underlayer. X-ray diffraction analysis shows that the (001) preferred orientation of the MgO thin film does not enhance significantly after post-annealing at 400 ℃ for 30 minutes. However, that is beneficial for relaxing the stress in the MgO thin film.
Magnetoresistance measurements indicate that the tunneling magnetoresistance ratio of about 25 % is obtained by annealing the Os (2 nm)/CoFeB (5 nm)/MgO (2.5 nm)/CoFe (5 nm)/Ta (5 nm) thin film at 300 ℃ for 30 minutes. The MgO (001)[100]//CoFeB (001) [110] orientation relationship can be obtained according to the TEM analysis. Because the CoFe grains are random-oriented as it is deposited on the MgO layer, the larger tunneling magnetoresistance ratio of about 90 % can be obtained for the magnetic tunnel junction with better epitaxial growth of (001) CoFe grains on (001) MgO grains. The as-deposited CoFe thin film with (111) preferred orientation can be obtained by inserting the CoFeB thin layer between the MgO and CoFe thin layers. Therefore, the tunnel magnetoresistance ratio for the as-deposited magnetic tunnel junction is increased from 1 % for the Os (2 nm)/CoFeB (6 nm)/MgO (2.5 nm)/CoFe (5 nm)/Ta (5 nm) thin film to 12 % for the Os (2 nm)/CoFeB (6 nm)/MgO (2.5 nm)/CoFeB (1.2 nm)/CoFe (3 nm)/Ta (5 nm). It is observed that the insertion of the ferromagnetic spacer does not improve the magnetoresistance ratio after 300 ℃ annealing. The tunnel magnetoresistance ratio for the Os (2 nm)/CoFeB (6 nm)/MgO (2.5 nm)/CoFeB (1.2 nm)/CoFe (5 nm)/Ta (5 nm) thin film is about 25 % after annealing at 300 ℃ for 30 minutes. The tunnel magnetoresistance for the as-deposited Os (2 nm)/CoFeC (6 nm)/MgO (2.5 nm)/CoFe (3 nm)/Ta (5 nm) thin film is increased from 9 % to 12 % after annealing at 150 ℃. However, the tunnel magnetoresistance and the resistance-area product are decreased as the annealing temperature is increased above 150 ℃. From the I-V curve measurement of the Os/CoFeC/MgO/CoFe/Ta thin film, it can be see that the barrier properties is deteriorated after annealing at 350 ℃ for 30 minutes.
Subjects
氧化鎂
磁性穿隧接面
顯微結構
磁阻
MgO
magnetic tunnel junction
microstructure
magnetoresistance
Type
thesis
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