Unlike conventional random access memory (RAM) chip technologies, in magnetic RAM (MRAM) data is not stored as electric charge, but is instead stored by magnetic polarization of storage elements. The storage elements are formed from two ferromagnetic layers separated by a tunneling layer. One of the two ferromagnetic layers, which is referred to as the fixed layer or pinned layer, has a magnetization that is fixed in a particular direction. The other ferromagnetic magnetic layer, which is referred to as the free layer, has a magnetization direction that can be altered to represent either a “1” when the free layer magnetization is anti-parallel to the fixed layer magnetization or “0” when the free layer magnetization is parallel to the fixed layer magnetization or vice versa. One such device having a fixed layer, a tunneling layer, and a free layer is a magnetic tunnel junction (MTJ). The electrical resistance of an MTJ depends on whether the free layer magnetization and fixed layer magnetization are parallel or anti-parallel with each other. A memory device such as MRAM is built from an array of individually addressable MTJs.
To write data in a conventional MRAM, a write current, which exceeds a critical switching current, is applied through an MTJ. The write current exceeding the critical switching current is sufficient to change the magnetization direction of the free layer. When the write current flows in a first direction, the MTJ can be placed into or remain in a first state, in which its free layer magnetization direction and fixed layer magnetization direction are aligned in a parallel orientation. When the write current flows in a second direction, opposite to the first direction, the MTJ can be placed into or remain in a second state, in which its free layer magnetization and fixed layer magnetization are in an anti-parallel orientation.
To read data in a conventional MRAM, a read current flows through the MTJ via the same current path used to write data in the MTJ. If the magnetizations of the MTJ's free layer and fixed layer are oriented parallel to each other, the MTJ presents a resistance that is different than the resistance the MTJ would present if the magnetizations of the free layer and the fixed layer were in an anti-parallel orientation. In a conventional MRAM, two distinct states are defined by two different resistances of an MTJ in a bitcell of the MRAM. The two different resistances represent a logic “0” and a logic “1” value stored by the MTJ.
MRAM is an inherently scalable type of memory because the MTJ critical switching current (Ic) scales linearly with device area. However, as the critical switching current for writing to an MTJ decreases, the sensing current used to measure the resistance of the MTJ for reading its logical state (magnetization orientation) also decreases to prevent a read disturbance. For example, due to process variations the sensing current used to measure the resistance of an MTJ could exceed the critical switching current and change the logical state of the MTJ being read. Thus, increased scaling of MRAM leads to a reduced sensing margin. The reduction of sensing current may also causes reduced switching speed of the MTJ.
The critical switching current of an MTJ increases sharply when the pulse width of the switching current is decreased. Thus, one technique for preventing read disturbance in MRAM involves the application of an MTJ sensing current with a shortened pulse width. For example a sensing current having a pulse width of less than about 10 nanoseconds may be used for measuring the resistance of an MTJ. The sensing current pulse width may be reduced to a lower limit of about 3 nanoseconds with reliable sensing. With technology scaling, however, the switching current of an MTJ will continue to decrease. Thus, as the sensing current pulse width approaches its lower limit with increased MTJ scaling, sense current should still be reduced to prevent read disturbance.