In recent year, much attention has been paid to magnetic tunnel junctions for device applications such as a magnetoresistance(MR) head and magnetoresistive random access memory(MRAM). More interests have been aroused when large MR effects of more than 20% were observed at room temperature in the last 3 to 4 years. Although magnetic tunnel junctions have been studied for more than twenty years, it was not until 1995 that significant changes in MR(on the order of 20%) were reported in a simple junction at room temperature. Magnetic tunnel junctions basically consist of ferromagnet/tunnel barrier/ferromagnet trilayers. The tunneling current flows depending on the spin density of states at Fermi level of the two ferromagnets and the relative orientation of magnetizations. The success of magnetic tunnel junction depends critically on the quality of the insulating tunnel barrier in the trilayer structure. High values of MR can be achieved when interface is smooth and sharp, and insulator has trap-free structure.
In this study, the magnetic tunnel junction were deposited by DC magnetron sputtering on Si wafers with the following layer structure :Si/Ta(50Å)/NiFe(80Å)/FeMn(120Å)/FM(80Å)/$Al_2O_3$/FM(200Å), where FM represents Co, NiFe, and CoFe. The antiferromagnet FeMn layer was employed to provide exchange biasing for the bottom electrode.
The optimum condition of $Al_2O_3$ tunnel barrier formation was investigated. The barriers were formed by sputtering 8∼24Å Al and subsequently exposuring to $O_2$ or Ar/$O_2$ plasma at DC power of 30∼50W. The pressure of $O_2$ or Ar/$O_2$ gas was 50∼100mtorr. Exposure time was varied from 10s to 100s. The tunnel barrier that showed maximum MR of 30% was formed with a 16Å Al oxidized for 40s in 50mtorr of Ar/$O_2$ mixture gas with 50W applied power.
To study the dependence of ferromagnetic electrode, we have studied systematically the magnetotunneling properties of several magnetic tunnel junction system(Co-insulator-Co, NiFe-I-NiFe, CoFe-I-CoFe, NiFe/Co-I-Co/NiFe, NiFe/CoFe-I-CoFe/NiFe). The tunneling magnetoresistance value ranges between 10% and 30% depending on the ferromagnetic electrodes at the same oxidation condition.
We have investigated the chemical composition depth profile of plasma oxidized Al layers at different oxidation time by AES(auger electron spectroscopy) and XPS(x-ray photoelectron spectroscopy), and the surface roughness of each layer by AFM(atomic force microscopy). Also domain motion during magnetic reversal of the top layer was studied by a bitter method. It was found that it did not reversed in single domain fashion at junction area and this was thought to be one of the reason for the lower MR ratio.
