Multibit MRAM Cell Using Synthetic Storage Layer
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Solution Overview
Problem
Conventional magnetic random access memory (MRAM) cells require large magnetic fields and currents for writing and reading, limiting their efficiency and scalability, especially when trying to store more than two data bits.
Innovation Solution
A method for writing and reading more than two data bits to an MRAM cell using a magnetic tunnel junction with a read magnetic layer, a tunnel barrier layer, and a storage layer that can be freely oriented at a high temperature threshold, allowing for the orientation of storage magnetizations to achieve multiple resistance states without the need for an additional current line, by applying an external magnetic field and cooling to freeze the magnetization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional MRAM cells use magnetic tunnel junction with two ferromagnetic layers, then writing and reading operations can be performed, but large magnetic fields and currents are required, limiting efficiency and scalability
Solution Approach 1:
The patent changes the magnetic anisotropy parameter by introducing a synthetic antiferromagnetic storage layer with perpendicular magnetic anisotropy, replacing the conventional in-plane anisotropy. This allows the storage layer magnetization to be freely oriented in three-dimensional space, enabling multilevel storage with lower writing currents and fields while maintaining stable read operations
Solution Approach 2:
The patent employs a composite magnetic structure consisting of a pinned layer, tunnel barrier, and synthetic antiferromagnetic storage layer with two coupled ferromagnetic sublayers. This composite structure enables simultaneous achievement of stable reference magnetization and freely orientable storage magnetization, resolving the contradiction between readability and writability efficiency
2Quantity of substance
If conventional MRAM cells store only two data bits, then device structure remains simple, but storage capacity is limited
Solution Approach 1:
The patent introduces dynamic magnetization orientation capability in the storage layer, allowing the magnetization vector to be freely oriented in three-dimensional space rather than being constrained to two fixed states. This dynamic freedom enables encoding of multiple data bits (at least four distinct state levels) within the same physical structure without adding more cells or complex control logic
Solution Approach 2:
The synthetic antiferromagnetic storage layer serves multiple functions simultaneously: it provides perpendicular magnetic anisotropy for stable magnetization, enables three-dimensional orientation freedom for multilevel storage, and maintains exchange coupling between sublayers for controlled switching. This multi-functionality achieves enhanced storage capacity without proportionally increasing device complexity
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 the storage of at least four distinct state levels in an MRAM cell using a single current line for generating a magnetic field, improving writing efficiency and reducing the need for large magnetic fields and currents, thereby enhancing the performance and scalability of MRAM technology.
Implementation Method 1
magnetic tunnel junction demonstrated a strong magnetoresistance at ambient temperature
Implementation Method 2
Each ferromagnetic layer can be coupled with an anti-ferromagnetic layer (not shown), whose function is to trap the ferromagnetic layer it couples, so that the magnetization of the coupled ferromagnetic layer is pinned
Implementation Method 3
A first field current is passed in the first current line generating a first magnetic field, and a second field current is passed in the second current line generating a second magnetic field. The first and second magnetic fields are adapted such as to switch the magnetization direction of the second magnetic layer
Data Source
Figure 1
Figure 2~4(d)
Figure 5(a)~6
AI summary
The present disclosure concerns a method for writing and reading a plurality of data bits to a magnetic random access memory (MRAM) cell (1) comprising a magnetic tunnel junction (2) formed from a read magnetic layer (21) having a read magnetization (210), a tunnel barrier layer (22), and a storage layer (23); the storage layer (23) comprising a first storage ferromagnetic layer (230) having a first storage magnetization (233), a second storage ferromagnetic layer (231) having a second storage magnetization (234) that can be freely oriented at a high temperature threshold, and a storage anti-parallel coupling layer (232) magnetically coupling the first and second storage magnetization (233, 234), an antiferromagnetic layer (24) which pins at the functioning temperature the storage layer (23) and a field line (41); comprising: heating the magnetic tunnel junction (2) over a high temperature threshold; orienting the first and second storage magnetization (233, 234) until the first storage magnetization (233) forms a predetermined angle (α) with respect to the second storage magnetization (234) such as to reach a predetermined resistance state level of the magnetic tunnel junction (2) determined by the orientation of the first storage magnetization (233) relative to the read magnetization (210); and cooling the magnetic tunnel junction (2) to a low temperature threshold to freeze the second storage magnetization (234) at the predetermined angle (α). The method allows for storing at least four distinct state levels in the MRAM cell using only one current line to generate a writing field.