Domain Wall Motion MRAM Cell Structure
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Solution Overview
Problem
Domain wall motion type Magnetic Random Access Memory (MRAM) faces challenges in reducing write current while maintaining thermal stability and scalability, and in achieving independent improvement of write and read characteristics without increasing variation in magnetization state between memory cells.
Innovation Solution
The MRAM employs a structure with a first magnetoresistance element having a magnetization free layer with perpendicular magnetic anisotropy and a second magnetoresistance element with in-plane magnetic anisotropy, allowing for separate optimization of write and read characteristics through magnetic coupling, reducing write current density, and stabilizing thermal stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a current magnetic field is used to switch magnetization direction in the magnetoresistance element, then high speed operation (data write in one nanosecond or less) can be achieved, but a large write current (several miliamperes) is required which increases chip area and power consumption
Solution Approach 1:
The ferromagnetic layer is segmented into three distinct portions: a first magnetization fixed portion with fixed magnetization, a magnetization reversible portion with reversible magnetization for data storage, and a second magnetization fixed portion with fixed magnetization. This segmentation enables domain wall motion in the reversible portion when current is applied, achieving magnetization switching without requiring large current magnetic fields while maintaining high-speed operation capability
Solution Approach 2:
A domain wall is introduced as an intermediary mechanism between the current and the magnetization switching process. When current is supplied to the magnetoresistance element, the domain wall moves through the magnetization reversible portion, enabling magnetization switching at lower current levels compared to direct current magnetic field switching, thus reducing write current while maintaining speed
2Area of stationary object
If the size of memory cell is reduced to increase integration density, then chip area is reduced, but the write current is further increased which is undesirable from the viewpoint of scaling
Solution Approach 1:
The ferromagnetic layer is divided into three portions with different magnetization characteristics. The magnetization reversible portion is specifically designed to support domain wall motion, which enables scaling to smaller memory cell sizes without proportionally increasing write current, as the domain wall mechanism is more efficient than direct magnetic field switching at small dimensions
Solution Approach 2:
The invention changes the magnetization switching mechanism from direct current magnetic field switching to current-driven domain wall motion. This parameter change in the switching mechanism enables better scaling properties, where write current does not increase as rapidly with decreasing memory cell size compared to conventional approaches
3Use of energy by moving object
If spin transfer magnetization switching is used to decrease write current and improve scaling, then write current decreases with reduced memory cell size, but a write current must flow through the tunnel barrier layer causing rewriting durability and reliability problems
Solution Approach 1:
A domain wall is introduced as an intermediary mechanism that enables magnetization switching without requiring current to flow through the tunnel barrier layer. The domain wall moves in response to current applied to the ferromagnetic layer, transferring the switching function away from the tunnel barrier path and thereby improving rewriting durability and reliability
Solution Approach 2:
The ferromagnetic layer is segmented into portions with different magnetization characteristics, including a magnetization reversible portion that supports domain wall motion. This segmentation enables the switching function to be separated from the tunnel barrier current path, allowing low-current switching without compromising tunnel barrier integrity and thus improving reliability
4Device complexity
If a single magnetoresistance element structure is used for both write and read operations, then device complexity is reduced, but erroneous writing during reading may occur and independent optimization of write and read characteristics is prevented
Solution Approach 1:
The ferromagnetic layer is segmented into portions with different magnetization characteristics (fixed and reversible). This segmentation enables the element to respond differently to write currents and read currents, allowing accurate reading without erroneous writing while maintaining a relatively simple single-element structure
Solution Approach 2:
Different portions of the ferromagnetic layer are given different local properties: the first and second magnetization fixed portions have fixed magnetization for stability, while the magnetization reversible portion has reversible magnetization for data storage and domain wall motion. This local quality differentiation enables independent optimization of write and read characteristics within a single element structure
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 decreases the write current required for domain wall motion, enhances read signal quality, and simplifies manufacturing by allowing independent design of write and read layers, thereby improving both write and read characteristics and reducing external magnetic field disturbance sensitivity.
Implementation Method 1
a magnetoresistance element is integrated within a memory cell, and a data is stored as an orientation of magnetization of a ferromagnetic layer of the magnetoresistance element
Implementation Method 2
When a current is supplied in the direction that passes through the domain wall, the domain wall is moved in the direction of conduction electrons
Implementation Method 3
switch the magnetization direction of the ferromagnetic layer of the magnetoresistance element with a current magnetic field, which is generated by flowing the write current
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 free layer, a first magnetization fixed layer, a second magnetization free layer and a first nonmagnetic layer sandwiched between the first magnetization fixed layer and the second magnetization free layer. The first magnetization free layer has: first and second magnetization fixed regions; and a magnetization free region. The magnetization free region and the second magnetization free layer are magnetically coupled to each other. Whereas, the second magnetoresistance element has: a third magnetization free layer whose magnetization easy axis is parallel to a second direction; a second magnetization fixed layer whose magnetization direction is fixed in a third direction perpendicular to the second direction; and a second nonmagnetic layer sandwiched between the second magnetization fixed layer and the third magnetization free layer.