Structure, Magnetic Properties, And Thermal Stability Of CoFeB/MgO Films

With the rapid development of electronic information technology, magnetic heads with higher sensitivities are needed. Magnetic tunnel junctions (MTJ) with structure of ferromagnetic/insulator/ferromagnet are considered suitable as a magnetic random memory (MRAM) cells because of high tunneling magnetoresistnce (TMR) at room temperature and the appropriate value of resistance. The integration of MTJs in the microelectronic process technology, however, requires a certain temperature stability in addition. In this thesis, MgO was used as insulator layer and FeCoB was used as ferromagnetic layer, the structure, magnetic properties and thermal stability of FeCoB/MgO films were studied.In this paper, we prepared a series of FeCoB/MgO films with different B contents on glass substrates by a magnetron sputtering system. X-ray diffractometer (XRD) were used to measure the structure of FeCoB/MgO films. The results showed that with increasing B content, the films become more amorphous. All the samples were vacuum annealed at different temperatures to investigate the thermal stability of FeCoB/MgO films. The XRD and Auger electronic spectroscopy (AES) were used to measure the structure and composition of FeCoB/MgO films after annealing. After annealing at 200℃and 300℃, the (110) diffraction peak of Fe₇Co₃ is present in the XRD patterns of the FeCoB/MgO films with B contents of 2.2% and 6.7%. For the FeCoB/MgO films with B contents of 12.1% and 16.9%, the films were still amorphous after annealing at 200℃and 300℃. When the samples were annealed at 400℃, all the XRD patterns of the FeCoB/MgO films with different B contents show the (110) diffraction peak of Fe₇Co₃. With the increase of the B content, the grain size of FeCoB/MgO films decreased and the position of (110) diffraction peaks shifted to the low diffraction angles. This is because the introduction of the B atoms into the FeCo lattice leads to the lattice expansion. In-depth AES analysis was used to investigate the composition change of the FeCoB/MgO films before and after annealing. The result shows that the B atoms diffused from FeCoB layer to the interface of FeCoB/MgO and MgO layer after annealing. The AES profile analysis shows that the B oxide was formed in the interface after annealing.We studied the magnetic properties of the FeCoB/MgO films with different B contents before and after annealing. For the as-deposited samples with low B content, the coercivity and saturation magnetizations decrease rapidly. For the as-deposited samples with higher B content, the coercivity and saturation magnetizations decrease slightly with the increase of the B content. The coercivity of FeCoB/MgO films decreases after the samples were annealed at 200℃. This is because the internal stress was released in the process of annealing. When the anneal temperature is above 300℃, the change of coercivity is negligible. The paramagnetic characteristic was observed from the magnetic hysteresis loops. After annealing at higher tempretures, B atoms diffused to the interface of FeCoB/MgO, the formation of B oxide have an influence on the magnetic properties of the FeCoB/MgO films.