Voltage-Switching MTJ Cell With Independent Free Layers
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Solution Overview
Problem
Conventional STT-MRAM cells require high current for writing and lack efficient methods to independently control multiple free layers for multiple resistance states, limiting their functionality as logic devices.
Innovation Solution
A voltage-switching MTJ cell structure with two free magnetic layers and one fixed layer, utilizing electrical field-induced anisotropy combined with spin torque to control each free layer independently, allowing for four discrete resistance states and enabling logic operations with reduced power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high current is used for writing in conventional STT-MRAM cells, then the free layer magnetization can be switched, but the current requirement is high and power consumption increases
Solution Approach 1:
The patent divides the single MTJ cell into multiple sub-MTJs connected in series, each with its own free layer that can be independently controlled. This segmentation allows selective switching of individual sub-MTJs, reducing the current required for writing since only specific layers need to be switched rather than the entire stack.
Solution Approach 2:
The patent applies local quality by enabling independent control of each free layer's magnetization state through localized voltage pulses. Each sub-MTJ can be written independently with optimized current pulses, allowing precise control of individual layers while minimizing overall power consumption.
2Adaptability or versatility
If conventional STT-MRAM cells are used, then basic memory function is achieved, but efficient independent control of multiple free layers for multiple resistance states is lacking
Solution Approach 1:
The patent creates a multi-functional device where the same MTJ structure serves both as memory storage and as a logic device capable of performing logical operations. The multiple free layers enable the cell to represent multiple resistance states (3 or 4 states), providing universal functionality for both memory and logic applications without requiring separate dedicated circuits.
Solution Approach 2:
The patent adds a new dimension of control by introducing multiple independently controllable free layers within a single MTJ stack. This vertical stacking of controllable layers creates additional states beyond the conventional binary state, enabling multi-level cell (MLC) functionality and logic operations through combinations of layer states.
3Adaptability or versatility
If multiple free layers are added to create MLC or logic devices, then more resistance states are achieved, but the control mechanism becomes more complex
Solution Approach 1:
The patent segments the control mechanism into independent voltage pulse generators for each free layer. Each layer can be addressed and controlled separately through its own control line, simplifying the overall control architecture despite having multiple layers. This segmentation enables straightforward implementation of logic operations by selectively activating specific layers.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables efficient switching with lower current requirements and allows for complex logic operations using fewer and less complex circuitry, achieving four or three discrete resistance states depending on sub-MTJ configurations.
Implementation Method 1
the perpendicular anisotropy of magnetic layers in MgO based MTJ structures can be changed by the voltage applied to the magnetic layers
Implementation Method 2
the two magnetic layers adjacent to the MgO layer form a capacitor across the MgO layer
Implementation Method 3
the top magnetic layer to have a positive potential, i.e. electron depletion at the top layer's interface to the MgO layer
Implementation Method 4
Writing the cells requires a sufficiently high DC current flowing in the direction through the MTJ stack between the top and bottom metal contacts to induce a spin transfer torque (STT) that orients (switches) the free layer into the desired direction
Data Source
AI summary
Embodiments of the invention include a voltage-switching MTJ cell structure that includes two sub-MTJs in series. Each free layer can be switched independently from the other. Each sub-MTJ has a high and a low resistance state and the MTJ cell structure can have three or four discrete resistance states. By taking advantage of the electrical field induced anisotropy combining with the spin torque effect, free layer-1 and free layer-2 can be controlled individually by voltage pulses having selected sign (polarity) and amplitude characteristics. The MTJ cell structure can be used as a fully functional logic cell with two input bit values corresponding to the high or low resistance of the two sub-MTJ structures and the output of a logical operation, e.g. an XOR function, determined by the resistance state of each MTJ cell.