A Study On Magnetic And Spin Properties Of CoFe Based Spin Polarization Film In Magnetic Tunnel Junctions

The magnetic tunneling junction (MTJ) based magnetic random access memory (MRAM) is considered to be a potential candidate for non-volatile semiconductor memories because it compiles nonvolatility, large density and rapid read and write speed. The read and write of STT-MRAM is realized through spin-polarized current inducing magnetization reversal instead of the current magnetizing the recording layer, which could reduce the writing power consumption and improve the storage density. The magnetic tunneling junction is the key cell in magnetic random access memory, and the storage mechanism is the tunneling magenetoresistance ratio (TMR). On the one hand, the threshold current of in-plane STT-MRAM is too large. The perpendicular STT-MRAM could decrease the threshold current and avoid the superparamagnetic effect under the high desity. So we need to study the STT-MRAM with high perpendicular magnetic anisotropy (PMA). On the other hand, since the read and write of STT-MRAM is realized through spin-polarized current inducing magnetization reversal, we need to study the STT-MRAM with high polarization, which could improve the TMR of MTJ. In this paper, we take the spin-polarized layer of MTJ as the research object, studying the material and performance improvement project of CoFe-based spin-polarized layer from PMA and spin polarization.Firstly, in this work, the perpendicular magnetic anisotropy of CoFeB thin film was greatly improved through inserting a ultrathin metal layer into CoFeB. We experimentally inserted different kind of ultrathin metal layers into CoFeB thin films to enhance the interfacial effect and stronger perpendicular magnetic anisotropy was obtained. The microstructure and magnetic property testing results show that 1 nm Ag insertion layer and 1 nm Ta layer help enhance the perpendicular magnetic anisotropy in CoFeB thin film and this improvement is strongly dependent on the thickness of the insertion layer. While the thickness of Ag and Ta insertion is 0.5 nm, the perpendicular magnetic anisotropy in CoFeB thin film decreases. It is believed that this should attribute to the ultrathin Ag or Ta layer, which is too thin to form continuous Metal/CoFeB interface. The presence of an interface breaks this quasispherical symmetry so that the energy of the 3d orbitals pointing toward the interface is different from the energy of the 3d orbitals with planar symmetry. In order to give a reasonable explanation to the experimental results above, the first principle calculations were used to calculate the magnetic anisotropy energy (MAE) of the single CoFeB film and CoFeB thin films inserted with ultrathin metal layer. Structural relaxations and total energy calculations were performed using the pseudo potential plane-wave method implemented in the Vienna Ab Initio Simulation Package (VASP). 1nm Ag-inserted and 1nm Ta-inserted CoFeB film also have perpendicular magnetic anisotropy and the strength of perpendicular magnetic anisotropy is much stronger than that of the single-layer CoFeB film. These results indicate that inserting 1nm Ag and 1nm Ta ultrathin layer into CoFeB thin film can improve the perpendicular magnetic anisotropy of CoFeB film. The main reason is that the interfacial perpendicular anisotropy is successfully induced by introducing Metal/CoFeB interfaces in CoFeB film.Secondly, CoFe-based Heusler alloy Co2FeAl with high spin polarization are good candidates for spintronic applications due to their high Curie temperatures (Tc), wide band gaps and large magnetic moments. However, there was one big flaw of Co2FeAl that Co2FeAl doesn't have perpendicular magnetic anisotropy. In this paper, we experimentally inserted ultrathin metal layer Pt into Co2FeAl thin films to obtain the interfacial effect and strong PMA was obtained. Theoretically, based on the first-principles, the total energy calculations were employed to explain the PMA. Experimental and theoretical studies have been conducted to obtain perpendicular magnetic anisotropy in Co2FeAl thin film with 0.8 nm Pt insertion layer. Interfacial perpendicular anisotropy was successfully induced in Co2FeAl by using ultrathin inserted metal layer.Finally, in this paper, the stability of CoFe-based Heusler alloy Co2FeAl was enhanced by doping chalcogenide elements and GeTe. The Fermi level (EF) of Co2FeAl is located exactly at the upper edge of the gap, leading to the instability of its half-metallic properties. The reason is that EF, being initially situated at the top of the valence band, does not fall any longer inside of the gap at elevated temperatures. So, in this work we try to improve the stability of the Co2FeAl by doping chalcogenide element and GeTe into Co2FeAl Heusler alloy, using Vienna Ab Initio Simulation Package (VASP). On the one hand, the calculations indicate that when 25% of the number of Al atoms are substituted by chalcogenide element the half-metallic properties of Co2FeAl can be improved. The Fermi energy (EF) of the 25% chalcogenide element-doped Co2FeAl is located in the middle of the gap of the minority states instead of around the top of the valence band as in Co2FeAl. Moreover, the band gap of 25% Te-doped Co2FeAl (0.80 eV) is wider than that of Co2FeAl (0.74eV). These improved electronic structures will make the half-metallicity of the 25% chalcogenide element-doped Co2FeAl more stable against temperature fluctuation. On the other hand, The Fermi energy (EF) of GeTe-doped Co2FeAl is located in the middle of the gap of the minority states instead of around the top of the valence band as in Co2FeAl. Moreover, the band gap of GeTe-doped Co2FeAl (1.01 eV) is much wider than that of Co2FeAl (0.74eV). These improved electronic structures will make the half-metallicity of GeTe-doped Co2FeAl more stable against temperature fluctuation.