Asymmetric Synthetic Ferrimagnetic Reference Layer for MRAM Stability
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Solution Overview
Problem
Magnetic random access memory (MRAM) devices with perpendicular magnetization suffer from reduced recording retention time in the anti-parallel state due to instability, which hinders miniaturization and requires large reference layers to maintain stability, complicating the control of stray fields and film thickness.
Innovation Solution
The implementation of a synthetic ferrimagnetic structure with different areas for the two magnetic layers in the reference layer, optimizing the relationship between the recording and reference layers to minimize stray fields and enhance retention time, allowing for miniaturization without compromising stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the area of the reference layer is increased to maintain stability in the anti-parallel state, then the recording retention time is improved, but the device size increases and miniaturization is hindered
Solution Approach 1:
The patent applies asymmetry by making the reference layer have a different area than the recording layer. Specifically, the reference layer is designed with a larger area to provide sufficient magnetic field for stabilizing the anti-parallel state, while the recording layer maintains a smaller area for miniaturization. This asymmetric area design allows the reference layer to generate adequate stray field for stability without forcing the entire device to be large, as only the reference layer needs the larger footprint.
Solution Approach 2:
The patent segments the magnetic layers into distinct functional regions with different areas. The reference layer is separated from the recording layer in terms of spatial extent, allowing independent optimization of each layer's area. This segmentation enables the reference layer to be larger for stability while the recording layer remains compact for miniaturization, resolving the contradiction between retention time and device size.
2Reliability
If the area of the reference layer is increased to stabilize the anti-parallel state, then the stray field control becomes more difficult and manufacturing precision is reduced
Solution Approach 1:
By implementing asymmetric areas between reference and recording layers, the patent simplifies the manufacturing process. The reference layer can be formed with a larger area using standard lithography and deposition techniques without requiring atomic-level thickness control. This asymmetric design naturally provides the necessary stray field for stability while avoiding the need for extremely precise film thickness control that would be required if the entire structure needed to be uniformly large.
3Productivity
If perpendicular magnetization is used to enable miniaturization, then the resistance change is improved, but the thermal stability difference between parallel and anti-parallel states causes recording retention time to decrease
Solution Approach 1:
The patent resolves the contradiction between miniaturization and retention time by applying perpendicular magnetization to enable small device size, while simultaneously using asymmetric area design where the reference layer has a larger area than the recording layer. This asymmetric configuration provides sufficient stray field from the larger reference layer to stabilize the anti-parallel state, compensating for the thermal stability issue inherent in perpendicular magnetization structures, thereby maintaining both miniaturization and adequate retention time.
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 stabilizes the anti-parallel state, preventing a decrease in recording retention time and enabling the miniaturization of MRAM devices by effectively managing stray fields and maintaining high resistance changes in both parallel and anti-parallel states.
Implementation Method 1
A feature of the MTJ device is a point that the device resistance is changed depending on a relative angle of the magnetization of two ferromagnetic layers
Implementation Method 2
A general method for writing information is magnetization reversal of the recording layer realized by a spin-transfer torque which is generated by applying an electrical current to the MTJ device
Implementation Method 3
by making an area of a reference layer larger than the area of a recording layer, a stray field which is applied to a first ferromagnetic layer 101 (recording layer) from a second ferromagnetic layer 103 (reference layer) is decreased
Data Source
AI summary
A magnetic tunnel junction device includes a first ferromagnetic layer, a tunnel barrier that is in contact with the first ferromagnetic layer, and a synthetic ferrimagnetic reference layer that is in contact with the tunnel barrier while being in the other side of the first ferromagnetic layer, in which the synthetic ferrimagnetic reference layer includes a second ferromagnetic layer that has a first magnetization direction while being in contact with the tunnel barrier, a magnetic layer that has a second magnetization direction which is anti-parallel to the first magnetization direction, and a first nonmagnetic layer that is interposed between the second ferromagnetic layer and the magnetic layer, and lateral dimensions of the magnetic layer of the synthetic ferrimagnetic reference layer are made larger than lateral dimensions of the first ferromagnetic layer and the second ferromagnetic layer.