Electron microscopy studies of magnetic tunnel junctions

This thesis reports work carried out to investigate some aspects of the structure and magnetic properties of magnetic tunnel junction (MTJ) films. Such MTJ films are of great technological importance since they can be used as high density magnetic read heads and non-volatile magnetic memory devices. The basic MTJ comprises simply two ferromagnetic layers separated by a thin insulating layer. In MTJ devices, current flows perpendicular to the layers. If the coercivities of the ferromagnetic layers are different, the MTJ will possess a two-stage magnetization reversal characteristic. Fabrication of the MTJ films using magnetron sputter deposition as well as patterning of the MTJ films using contact shadow mask and photolithographic techniques are presented. High resolution electron microscopy (HREM) and Lorentz transmission electron microscopy (LTEM) have been used to study the microstructure and the magnetic domain structures of the MTJ films respectively. LTEM in-situ magnetizing experiments have also been performed to observe the magnetization process of the MTJ films in real time. Hysteresis loops for the MTJ films have been measured using alternating gradient force magnetometry (AGFM). Four different Al-oxide layer preparation methods are presented and discussed. Observations of the microstructure, magnetic domain structure, and magnetization process of NiFe and Co thin films and NiFe/Al-oxide/Co junction films are reported. The two-stage magnetization reversal process and reversal mechanism of the NiFe/Al-oxide/Co junction system are particularly discussed. The effect of ferromagnetic bilayer (FMB) structure on the magnetization process of FMB/insulator/ferromagnet type junction film is reported; and for this study, two junction film structures, (NiFe/Co)/Al-oxide/NiFe and (Co/NiFe)/Al-oxide/NiFe, have been investigated. HREM results show that the FMBs possess different microstructures, therefore the magnetization reversal mechanisms of the FMBs as well as the magnetization process of the junction films are different. It has been reported that various seed layer materials can induce a strong <111> texture on the adjacent layer. The effect of Ti seed layer on the microstructure and thus on the magnetization process of Co/NiFe/Al-oxide/NiFe junction films is reported and discussed. The LTEM results are consistent with the hysteresis loop measurement using AGFM on the junction films studied. Simmons formula, which can be used to explain the electric tunnel effect in junction films, is described. Experimental current-voltage characteristic data for patterned NiFe/Al-oxide/Co junctions are modelled using the Simmons formula to obtain the effective junction barrier heights and widths. HREM studies confirm the effective junction barrier widths obtained and hence the validity of the MATLAB program prepared for data fitting. (author)