Research On Electromagnetic Character Of Magnetic Tunnel Junction

The capacity of magnetic disk systems is growing year by year with the advance of the information-oriented society, and the areal density of magnetic recording is increasing by 60% every year. Currently the highest areal density in commercial HDDs is about 4 Gbit /in2, but this will surely grow to 10 to 20 Gbit/in2 in the next few years. To read recorded information from smaller areas, magnetic heads with higher sensitivities are needed. In the late 80s, people found that there existed interlayer coupling effect and giant magnetoresistance in FM/NM multilayer within nano-scale, on the base of which the spin valve resistor was developed to become the hot of the storing medium because of its high sensitivity among low magnetic fields. In 1975 Julliere found that there existed giant magnetoresistance effect in Fe/Ge/Co tunnel junction and then called it tunnel magnetoresistance(TMR). Compared with normal metal multilayer film, this kind of ferromagnet/insulator/ferromagnet structure tunnel junction has the characteristic of high inside resistance, low consumption and high out-voltage. And its magnetoresistance ratio is theoretically predicted to reach 20 to 50%, which is much larger than that of spin valve film. So the ferromagnetic tunnel junctions have good potential for use in high density magnetic random access memorizer.In this thesis, the samples of ferromagnet/insulator/ferromagnet structure tunnel junction were prepared on the common load glass slice with magnetron sputtering technique and their barriers were naturally oxidized at the atmosphere. Research was made on the electromagnetic character of the samples with the four-point-probe method. Through a series of experiments we found the non-linear characteristics of tunnel junction's volt-ampere characteristic under high pressure relatively and especially we found the negative resistance phenomenon of the NiFe/Al2O3/NiFe tunnel junction under certain high pressure. The analysis of the relationship between TMR and insulating barrier was made and the result shows that TMR value decreases progressively with the increase of thickness of insulating barrier within 3 to 7nm. And the analysis of the relationship between TMR and the thickness of ferromagnetic layer is also presented in this thesis. The result shows the vibration of tunnel magnetic resistance with the changes of thickness of ferromagnetic layer. The various annealing conditions also have some effect on the magneto- resistance of the magnetic tunnel junction and the result shows that tunnel magnetic resistance changes with the annealing temperature and it reaches the max near 230℃. The magnetoresistance values of the samples are relatively small and we deem it is caused by the low quality of the tunnel junctions. So at the end of the thesis we discuss the improvement on the quality of the tunnel junction. The research project related with this thesis was subsidized by the National Nature Science Foundation(approve number: 90207014).