A conventional MLU cell comprises a magnetic tunnel junction comprising a ferromagnetic reference layer and a ferromagnetic storage layer. When the magnetizations of the two layers are aligned the resistance of the stack is low, this could be a “0” (or arbitrarily a “1”). When the layers are anti-aligned the resistance is high, this could be a “1” (or vice versa).
In thermally-assisted-switching (TAS) MLU's the storage layer is blocked by an antiferromagnetic layer such as to achieve superior stability in normal operating temperatures. During a programming cycle, the temperature of the cell is momentarily locally raised above a blocking temperature of the antiferromagnetic layer (high threshold temperature), through resistive heating of the magnetic tunnel junction, allowing the magnetoresistance of the TAS-MLU cell to be varied. At normal operating temperatures the information (i.e., magnetic orientation) stored in TAS-MLU cells is thus not affected by external fields and noise.
Self-referenced MLU cells can be based on TAS MLU cells. Self-referenced MLU cells typically have the reference layer that is not pinned by an antiferromagnetic layer but is free to be varied. Such unpinned reference layer is often called “sense layer”. When a current is applied in a field line adjacent to the MLU cell, a magnetic field is generated such as to vary the sense layer magnetization during a read operation. A two-phase read operation utilizes the natural tendency of an un-driven field line's effect on a selected memory cell to create a momentary reference value that is compared to the cell's value when the field is driven. The stored information is thus read as this field is applied.
During logic operations the field lines are acting as controlling gates modulating the resistivity of the magnetic tunnel junction. The MLU cell behaves as a three-terminal device capable of performing native logical functions. The currents circulating in the field line can be modulated in direction, and intensity.
An MLU amplifier can be provided by electrically coupling an array comprising several (possibly tens of thousands) of MLU cells together. The gain of the resulting amplifier is largely increased device while the coupling capacitance remains very small. For each MLU cell, the magnetoresistance of the magnetic tunnel junction is modulated by the direction of a field current flowing through a field line which is set by an input bit to be matched. A high or low magnetoresistance at the output indicates whether the input bit matches the stored bit or not, respectively.
Operating MLU amplifier thus requires applying a heating current for heating the magnetic tunnel junction of each of the MLU cells and a field current for generating the magnetic field. The heating current and the field current must be accurately synchronized.
Moreover, in a MLU array comprising a plurality of MLU cells, the heating current passes through the magnetic tunnel junction of the MLU cells connected in series in a row or column. Depending on the number of MLU cells comprises in the MLU array, the magnitude of the heating current required for heating the magnetic tunnel junctions in the array can reach several hundreds of mA. The corresponding voltage can thus reach several tens of Volts resulting in very high voltage difference (equivalent to several hundreds of MV/m) in some area of the MLU cells, such as in the magnetic tunnel junction and in particular in the tunnel barrier layer or any dielectric layer between two metal levels. Such high voltage difference can irreversibly weaken and even destroy the MLU cell.