Magnetic Random Access Memory Cell With Perpendicular Anisotropy
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Solution Overview
Problem
In MRAM based on the spin transfer magnetization switching method, there is a conflict between reducing the write threshold current density and improving the magnetoresistance ratio, leading to variations in the magnetization state and reference level during data reading, which affects the reliability of data storage.
Innovation Solution
The MRAM design includes a memory cell with a first magnetoresistance element having perpendicular magnetic anisotropy and a reference cell with an in-plane magnetization, where the magnetization free layers are magnetically coupled, allowing for independent optimization of write and read characteristics, and a second magnetoresistance element with a fixed intermediate resistance value to stabilize the reference level.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a perpendicular magnetization film is used to reduce write threshold current density, then write current is reduced, but magnetoresistance ratio decreases
Solution Approach 1:
The magnetoresistance element is divided into two separate elements: a first magnetoresistance element optimized for writing (with perpendicular magnetization film and low threshold current density) and a second magnetoresistance element optimized for reading (with high magnetoresistance ratio). This segmentation allows each element to be independently optimized for its specific function without compromise.
Solution Approach 2:
The first magnetization free layer serves dual purposes: it acts as the storage layer in the first magnetoresistance element for writing operations, and simultaneously functions as the magnetization source that magnetically couples to the second magnetization free layer to enable reading operations. This multi-functionality resolves the contradiction by allowing one component to serve both writing and reading needs.
2Ease of operation
If magnetization free layers are optimized for low threshold current, then write characteristics improve, but variations in magnetization state increase
Solution Approach 1:
The first magnetization free layer acts as an intermediary between the write current and the second magnetization free layer. It receives the write current directly (enabling low threshold current writing) and magnetically couples to the second magnetization free layer (providing stable magnetization state for reading). This intermediary role resolves the contradiction between ease of writing and stability of magnetization state.
3Use of energy by moving object
If perpendicular magnetization film is used, then write threshold current density is reduced, but reference level stability deteriorates
Solution Approach 1:
The reference cell is segmented from the storage cell. The reference cell uses a second magnetoresistance element with in-plane magnetization that provides stable reference levels, while the storage cell uses the first magnetoresistance element with perpendicular magnetization optimized for low threshold current writing. This segmentation allows independent optimization of reference stability and write efficiency.
Solution Approach 2:
Different magnetic anisotropy types are applied locally to different parts of the system: perpendicular magnetic anisotropy is used in the first magnetization free layer where low threshold current is needed, while in-plane magnetic anisotropy is used in the second magnetization free layer where stable reference levels are required. This local differentiation resolves the contradiction between write efficiency and reference stability.
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 approach effectively reduces the write threshold current density while enhancing the read-out signal and stabilizing the reference level, thereby improving the reliability and accuracy of data reading in MRAM.
Implementation Method 1
the first magnetization free layer and the second magnetization free layer are magnetically coupled to each other
Implementation Method 2
interaction between spin-polarized conduction electrons of the write current and local electrons in the first magnetic layer causes switching of the magnetization of the first magnetic layer
Implementation Method 3
An MRAM is a nonvolatile random access memory that utilizes a magnetoresistance element such as a magnetic tunnel junction (Magnetic Tunnel Junction; MTJ) element as a memory element
Data Source
AI summary
An MRAM has: a memory cell including a first magnetoresistance element; and a reference cell including a second magnetoresistance element. The first magnetoresistance element has a first magnetization fixed layer, a first magnetization free layer, a first nonmagnetic layer sandwiched between the first magnetization fixed layer and the first magnetization free layer, a second magnetization fixed layer, a second magnetization free layer and a second nonmagnetic layer sandwiched between the second magnetization fixed layer and the second magnetization free layer. The first magnetization fixed layer and the first magnetization free layer have perpendicular magnetic anisotropy, and the second magnetization fixed layer and the second magnetization free layer have in-plane magnetic anisotropy. The first magnetization free layer and the second magnetization free layer are magnetically coupled to each other. Center of the second magnetization free layer is displaced in a first direction from center of the first magnetization free layer in a plane parallel to each layer. Whereas, the second magnetoresistance element has: a third magnetization free layer whose magnetization easy axis is parallel to a second direction; a third magnetization fixed layer whose magnetization direction is fixed in a third direction perpendicular to the second direction; and a third nonmagnetic layer sandwiched between the third magnetization fixed layer and the third magnetization free layer. The third magnetization fixed layer and the third magnetization free layer have in-plane magnetic anisotropy.