| Thin-film magnetoresistive (MR) sensors are used in hard drives to read magnetically stored information. An understanding of thermally activated reactions in these sensors is crucial to proper device fabrication and subsequent long-term stability. Three thermally activated reaction types were studied in detail: (1) interdiffusion, which degrades device performance; (2) paramagnetic to antiferromagnetic phase transformations, required for proper device performance; and (3) the fabrication of thermally stable oxide thin-films for use in magnetic tunnel junctions (MTJs), critical components used in an advanced MR sensor type. (1) The operating temperatures of current and future sensors (projected by thermal modeling) are high enough to warrant a detailed investigation of thin-film interdiffusion as a cause of long-term sensor performance degradation. Co-Cu and Fe-Cu phase equilibria suggest that these materials are insoluble. However, in Co,Fe/Cu thin-films, significant instability in the form of phase separation and interdiffusion is observed experimentally and supported with thermodynamic rationalizations. Another interface of interest is the Co,Fe/PtMn interface. Experimental results provided primarily by three-dimensional atom probe support thermodynamic calculations, which predict a relatively stable interface between a Co,Fe alloy and the PtMn compound. (2) NiMn and PtMn thin-films are used as antiferromagnets in MR sensors. However, their sputter deposited states are paramagnetic, and a heat treatment is needed to induce the antiferromagnetic structure. Experiments on this phase transformation in PtMn are used to produce a model for the heat treatment procedure. Controversy surrounding the stability range of the antiferromagnetic NiMn phase exists in the literature. Therefore, the NiMn phase diagram was experimentally determined in the equiatomic region before phase transformation analysis. Thermodynamic parameters extracted from the phase transformation experiments agree with thermodynamic modeling. (3) MTJs consist of an insulating tunnel barrier layer sandwiched between ferromagnetic electrodes. Thermodynamic calculations were used to identify several stable oxide candidates for use in MTJs. However, these candidates will only be stable if they can be deposited with uniform composition. Current oxidation methods cannot selectively oxidize only the tunnel barrier layer. A method is proposed which can thermodynamically control the partial pressure of O2, enabling selective oxidation of the tunnel barrier. |
