Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. Unlike typical volatile Random Access Memory (RAM) technologies which store data as electric charge, data in MRAM is stored by magnetoresistive elements. Generally, the magnetoresistive elements are made from two magnetic layers, each of which holds a magnetization. The magnetization of one layer (the “pinned layer”) is fixed in its magnetic orientation, and the magnetization of the other layer (the “free layer”) can be changed by an external magnetic field generated by a programming current. Thus, the magnetic field of the programming current can cause the magnetic orientations of the two magnetic layers to be either parallel, giving a lower electrical resistance across the layers, or antiparallel, giving a higher electrical resistance across the layers. The switching of the magnetic orientation of the free layer and the resulting high or low resistance states across the magnetic layers controls the state of a typical MRAM cell.
A type of MRAM cell is a spin torque transfer (STT) cell. A conventional STT cell includes a magnetic tunnel junction (MTJ) that functions as a magnetoresistive data storing element with a pinned magnetic layer and a free magnetic layer, and an insulating layer between the pinned and the free magnetic layers. An example of an insulating layer is magnesium oxide (MgO). The STT cell is coupled between an access device and a data line. The MTJ can be viewed as a multi-state resistor due to different relative orientations (e.g., parallel and antiparallel) of the magnetic moments, which can change the magnitude of a current flowing (e.g., passing) through the cell. Magnetic fields caused by currents flowing through the MTJ can be used to switch a magnetic moment direction of the free magnetic layer of the MTJ, which can place the device in a high or low resistance state. The pinned layer polarizes the electron spin of the programming current, and torque is created as the spin-polarized current flows through the MTJ. The spin-polarized electron current interacts with the free layer by exerting a torque on the free layer. When the spin-polarized electron current flowing through the MTJ is greater than a critical switching current density (JC) for writing the cell, the torque exerted by the spin-polarized electron current is sufficient to switch the magnetization of the free layer and thus change the resistance state across the MTJ. A read process can then be used to determine the state of cell, using a read pulse that causes a current to flow through the MTJ that has a sufficiently small magnitude to not disturb the state of the MTJ.
STT technology has some advantageous characteristics compared to other MRAM technology. The STT cell does not need an external magnetic field to switch the free layer but rather uses the spin-polarized electron current to switch the free layer. Further, scalability is improved with STT technology as the programming current decreases with decreasing cell sizes. Additionally, STT technology can provide a larger ratio between high and low resistance states, which improves read operations.
In writing a STT cell, the amplitude of a programming signal, such as a current or voltage pulse (referred to herein as a write pulse), is selected to be high enough to reduce or minimize the bit error rate (BER). However, this write pulse may cause some over stress of the tunnel barrier between the free and pinned magnetic layers of the MTJ. The tunnel barrier may be a tunnel oxide such as magnesium oxide (MgO). This stress may be attributable to the array topology, or to process spreads that reflect variability in the manufacturing process that may affect the MTJ and/or the access device. This stress may reduce the endurance of the memory cell as the stress may cause writing and reading failures after fewer write cycles.