An investigation of the switching threshold of multilayer thin film magneto-resistive memory elements
For any type of computer memory element, knowledge of the switching threshold is crucial for memory development. This determines the magnitude of read currents which can be applied without destroying the information contained in the storage element and is used to determine reliable operating levels for the complete memory. Initial tests of a new multilayer thin film magneto-resistive memory element have shown the switching threshold of these elements to be quite sharp. Such a sharp threshold in a thin film structure, which are normally plagued by the presence of multiple domains, had not been previously reported. The multilayer structure, though much larger than a single domain particle, appeared to behave effectively as a single domain. A more carefully controlled investigation of the switching threshold was thus warranted. This investigation consisted of a computer controlled experiment in which the word current amplitude was set using a digital/analog converter and the mean time between failures was measured by pulsing the word current and monitoring the output. The threshold was defined as the point at which switching occurs after the application of one pulse. Behavior between 94 and 100% of this threshold was studied at room temperature, at 100°C, and at -21° or -11°C by pulsing the word current at a value near the switching threshold. The word current was reduced in steps to determine a relationship between the applied word field and the mean time between failures. In all cases, with a high degree of correlation, the relationship between the applied word field and the mean time between failures was an exponential one, indicating that the rotational threshold was under investigation. As expected, the rapidity at which a reliable operating level was approached decreased with increasing temperature. All results were normalized to a percentage of the switching threshold at the temperature at which the measurements were taken. Even at higher temperatures the elements could be operated quite reliably, i.e., P(no failures after 10,000 hrs) =.999, at word current levels up to 88.8% of the threshold level. Results in the investigation were also shown to be consistent with results obtained in previous tests of the same type of memory elements.