The Interface Effect on the Perpendicular Magnetic Anisotropy in MgO/CoFeB/Ta

• Main Authors: Shen, Hsin-Huang, 申新煌
• Other Authors: Chern, Gung
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/65069236583559609738

碩士 === 國立中正大學 === 物理學系暨研究所 === 100 === In develop of magnetic random access memory (MRAM) device, using the perpendicular recording media has become a trend in the future. In hence, the study magnetic perpendicular anisotropy has become popular. Recently, it is found that MgO-CoFeB based MRAM structure is important because these systems have high effective anisotropy (Ku), high magnetoresistive ratio (MR%) and high thermal stabilization these advantage. A surface/interface effect, which is usually ignored in a bulk material, becomes crucial as the size reduces to nanometer scale. However, the mechanism of PMA is not yet clear and more experimental results will be needed. In this thesis, we fabricated a series of top CoFeB-MgO based structures by sputtering system and did a detailed study on the interface structure and magnetic characterizations. Two sets of samples are prepared: Si/SiO2/Ta(5)/MgO(1)/ Co20Fe60B20(x)/Ta(y) (units: nm.) and these samples are post-annealed at different annealing temperatures. Set A consists of fixed y =1.0 or 5.0 and changed x=1.0 - 1.7. Set B consists of fixed x=1.2 or 1.5 and changed y=0.5 - 5.0. In order to realize magnetic characterizations of films, we did detailed measurements of the magnetic hysteresis (M-H) curve analysis and the results are divided into three parts: 1. The magnetic dead layer (MDL) induced by depends upon the thickness of Ta capping layer: For the samples in set A, we used m/A=M(t-td) formula to calibrate the MDL thickness, where m is magnetic moment (emu), M is magnetization (emu/cm3), A is substrate area (cm2), t is deposited thickness of CoFeB and td is the thickness of MDL. We observed that the MDL is 0.1 nm when Ta is 1.0 nm while the MDL is 0.5 nm when Ta is 5.0 nm, and M is approximately 1190 emu/cm3 in both cases. For set B for the dependence of MDL on thickness of Ta capping layer is calibrated. We found that the thickness of MDL linearly drops from ~ 0.56 nm to 1 nm Ta decreases from 5 nm to 1 nm. In addition, the effect of annealing temperature on MDL is not significant. 2. The effect of thickness of magnetic layer on the effective anisotropy: In set A, the effective anisotropy of ferromagnetic films is calibrated. An effective anisotropy constant may be defined as Keff =Kv+Ki/teff, where Keff =MsHk/2 is the effective anisotropy (erg/cm3), Kv=-2πMs2 is the bulk anisotropy (erg/cm3), Ki is the interface anisotropy (erg/cm2), teff is the effective thickness of CoFeB, Ms is magnetization and Hk is saturation field of hard axis. In our experiments, we found that the Kv is 8.8 Merg/cm3, optimal Ki is 1.5 erg/cm2 and M is 1180 emu/cm3 when capping layer Ta is 1.0 nm while the Kv is 8.2 Merg/cm3, optimal Ki is 0.9 erg/cm2 and M is 1150 emu/cm3 when Ta is 5.0 nm. In addition, the optimal annealing temperature of 315℃ with CoFeB 1.2 nm, the effective anisotropy reaches 2.5 Merg/cm3. 3. The effect of thickness of cap layer on the effective anisotropy : The impact of the thickness of capping layer Ta on the perpendicular anisotropy is investigated. In set B, we show that increasing the thickness of the cap layer Ta also leads to the increase of the MDL. As a result, the Ms, Hc and Hk all become smaller. The effective anisotropy and interface anisotropy constant reach maxima when Ta is 0.8 nm. In addition, we found the Ki(MgO/CoFeB) is approximately 1.3 erg/cm2. The Ki(MgO/CoFeB) degrades as the capping layer Ta thickness increasing. This may be due to the Ta diffusion into CoFeB layer and indirectly introduce defects and affect the crystalline quality of MgO layer.