Stacked Toggle MRAM Cells with Opposite Biasing
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Solution Overview
Problem
Conventional MRAM architectures face challenges with 'half-select' issues, where non-selected cells are susceptible to magnetic state switching due to energized word and bit lines, leading to reduced selectivity and increased power consumption.
Innovation Solution
The proposed MRAM design features memory stacks with toggle memory cells stacked vertically, where each pair of cells has their easy axes of magnetization aligned parallel and biased in opposite directions, allowing for selective writing without toggling adjacent cells, thereby reducing the required write field and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional MRAM architectures use half-select writing scheme, then writing operation can be performed, but half-selected cells are susceptible to magnetic state switching leading to reduced selectivity
Solution Approach 1:
The patent applies local quality by assigning different easy-axis orientations to different memory cells within the same stack. Specifically, alternating cells have their easy axes oriented in opposite directions (e.g., +X and -X), creating locally differentiated magnetic properties that enable selective writing. This allows the writing field to be tailored to affect only the intended cell while leaving others unaffected, thus resolving the half-select problem.
Solution Approach 2:
The patent employs asymmetry by creating an asymmetric magnetic configuration where cells in the stack have non-uniform easy-axis orientations. This asymmetric arrangement means that the magnetic field response differs from cell to cell, allowing selective activation of specific cells during writing operations. The asymmetric orientation pattern (+X, -X, +X, -X) ensures that no two adjacent cells respond identically to the writing field.
2Reliability
If higher write current is applied to switch selected cell, then switching is achieved, but power consumption increases
Solution Approach 1:
By assigning different easy-axis orientations to different cells, the patent enables lower write currents to be used for selected cells. The writing field only needs to overcome the anisotropy energy of the selected cell's easy axis orientation, rather than requiring sufficient field to affect all cells in the stack. This localized magnetic property assignment reduces the energy required for successful switching operations.
Solution Approach 2:
The patent changes the magnetic parameter (easy-axis orientation) of different cells to optimize writing efficiency. By varying the easy-axis direction as a cell-specific parameter, the system reduces the write field requirement for selected cells. This parameter differentiation allows the writing current to be optimized for each cell's specific magnetic characteristics, thereby reducing overall power consumption while maintaining reliable switching.
3Quantity of substance
If multiple toggle memory cells are stacked vertically, then storage capacity increases, but switching field requirements increase
Solution Approach 1:
The patent applies local quality by orienting the easy axes of stacked cells in alternating directions. This creates a pattern where adjacent cells have opposite magnetic susceptibilities. When a writing field is applied, only the cells with easy axes aligned with the field direction respond, while cells with opposite orientations remain unaffected. This localized response enables selective writing in vertically stacked configurations without requiring proportionally higher switching fields.
Solution Approach 2:
The patent segments the magnetic response of stacked cells by assigning different easy-axis orientations to different segments (cells) within the stack. This segmentation creates distinct magnetic domains that can be independently addressed. By dividing the stack into magnetically distinct segments, the system achieves high storage capacity while maintaining manageable switching field requirements through selective activation of individual segments.
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
This design enhances cell selectivity and reduces power consumption by allowing any cell in a stack to be written without affecting others, achieving lower switching fields and more efficient toggling operations.
Implementation Method 1
The SAF free layer responds to applied magnetic fields differently than a conventional single ferromagnetic free layer. Writing occurs by a process called 'toggle' writing in which a two-phase programming pulse sequence incrementally rotates the SAF free layer moment or magnetization direction 180 degrees
Implementation Method 2
the SAF free layer is biased by a magnetic bias field in the in-plane direction along its easy axis
Implementation Method 3
The write currents generate orthogonal magnetic fields in the X and Y directions that switch the magnetization direction of the selected MTJ
Data Source
Figure 1~2
Figure 3A~3B
Figure 4
AI summary
A "toggling" type of magnetic random access memory (MRAM) has memory stacks arranged in the X-Y plane on the MRAM substrate with each memory stack having a plurality of toggle memory cells stacked along the Z axis. Each stack is located at an intersection region between the two orthogonal write lines. The cells are stacked in pairs, with the cells in each pair having their easy axes of magnetization aligned substantially parallel to one another and nonparallel with the X and Y axes. The cells in each pair have their free layers magnetically biased in opposite directions. Because the free layer of each cell in a pair is biased in a direction opposite to the bias direction of the free layer of the other cell, one cell in a pair can be toggle written without toggle writing the other cell in the pair. The bias fields on the free layers reduces the required switching field for each cell, which results in less write current and a lower-power toggling MRAM.