Multi-bit MRAM Cell with Perpendicular Magnetization
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Solution Overview
Problem
Conventional magnetic random access memory (MRAM) cells face challenges with high write current, complex cell design, and thermal stability issues due to in-plane magnetization, which limits scalability and storage density.
Innovation Solution
A multi-bit MRAM cell design utilizing perpendicular magnetization with free layers having reversible magnetization directions, separated by tunnel barrier layers and a pinned layer with fixed perpendicular magnetization, employing a hybrid switching mechanism combining bias magnetic fields and spin-polarized currents for efficient data writing with reduced switching current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If in-plane magnetization is used in conventional MRAM cells, then the magnetization switching can be achieved, but thermal stability is poor and scalability is limited
Solution Approach 1:
The patent transitions from in-plane magnetization to perpendicular magnetization, changing the dimension of magnetization direction from parallel to the layer plane to perpendicular to the layer plane. This dimensional change enables both high thermal stability through perpendicular magnetic anisotropy and scalability to smaller dimensions, directly resolving the technical contradiction between thermal stability and scalability
2Device complexity
If conventional 1T-1MTJ design is used, then simple cell structure is achieved, but storage density is low due to large length-to-width aspect ratio
Solution Approach 1:
The patent introduces multi-bit storage capability by encoding multiple bits in a single MTJ element through controlled magnetization states of the free layer. This parameter change in storage capacity per cell allows reduction of the overall cell area while maintaining structural simplicity, resolving the contradiction between simple structure and high storage density
3Ease of operation
If field induced switching mechanism is used, then magnetization switching is achieved, but write current is high and cell design becomes complicated
Solution Approach 1:
The patent replaces the conventional field-induced switching mechanism with spin-transfer torque (STT) switching. Instead of using external magnetic fields to switch magnetization, spin-polarized current directly transfers angular momentum to the magnetic moments in the free layer, achieving magnetization switching with lower current and simplified cell design without requiring magnetic field generation structures
4Reliability
If conventional MTJ elements are used, then basic memory function is achieved, but switching current is high
Solution Approach 1:
The patent employs synthetic antiferromagnetic (SAF) pinned layer structure composed of multiple thin magnetic layers separated by non-magnetic spacer layers. This composite structure provides enhanced spin polarization and controlled magnetic properties that reduce the switching current requirement while maintaining reliable memory function. The SAF structure creates a tailored magnetic environment that facilitates efficient spin-transfer torque switching
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
The solution achieves high thermal stability, small cell size, excellent scalability, and low switching current, while avoiding the half-selected cells problem, enabling improved storage density and performance.
Implementation Method 1
driving a switching current pulse through the magnetoresistive element along an easy axis of the pinned, first free and second free layers for producing a spin momentum transfer
Implementation Method 2
driving a bias current pulse through a bit line in a proximity to but not through the magnetoresistive element for producing a bias magnetic field along a hard magnetic axis of the pinned, first free and second free layers
Implementation Method 3
first and second tunnel barrier layers, and a pinned layer comprising a fixed magnetization direction directed substantially perpendicular to the layer plane, the pinned layer is disposed between the first and second free layers and is separated from the free layers by one of the tunnel barrier layers
Data Source
AI summary
A multi-bit cell of magnetic random access memory comprises a magnetoresistive element including first and second free layers, each free layer comprising a reversible magnetization direction directed substantially perpendicular to a layer plane in its equilibrium state and a switching current, first and second tunnel barrier layers, and a pinned layer comprising a fixed magnetization direction directed substantially perpendicular to the layer plane, the pinned layer is disposed between the first and second free layers and is separated from the free layers by one of the tunnel barrier layers; a selection transistor electrically connected to a word line, and a bit line intersecting the word line; the magnetoresistive element is disposed between the bit line and the selection transistor and is electrically connected to the bit line and the selection transistor, wherein the first and second free layers have substantially different switching currents.