Dual-Free Layer MTJ with Spacer for SOT Switching
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Solution Overview
Problem
Current SOT-based magnetic tunnel junction (MTJ) devices face a compromise between SOT-generating efficiency, spin-polarizing efficiency, and free layer retention performance, limiting their design flexibility and performance in MRAM technology.
Innovation Solution
The introduction of a dual-free layer design with a spacer layer separating the storage and sensing sub-stacks, allowing for independent optimization of materials and layer compositions, and utilizing magnetostatic stray field interaction for magnetization switching, which enhances SOT switching efficiency, TMR, and thermal robustness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a single free layer is used for both SOT switching and sensing, then the device structure is simple, but it is impossible to independently optimize SOT-generating efficiency, spin-polarizing efficiency, and retention performance
Solution Approach 1:
The patent divides the single free layer into two separate free layers: a first free layer optimized for SOT switching and a second free layer optimized for sensing. This segmentation allows independent optimization of each layer's properties (thickness, composition, magnetization) to achieve both high SOT efficiency and high TMR ratio simultaneously, resolving the contradiction between adaptability and structure complexity.
2Reliability
If the free layer is optimized for high TMR ratio, then spin-polarizing efficiency improves, but SOT switching efficiency and retention performance deteriorate
Solution Approach 1:
The patent segments the sensing function from the switching function by using separate free layers. The second free layer is optimized for high TMR ratio to ensure reliable sensing, while the first free layer is optimized for SOT switching efficiency. This resolves the contradiction by allowing each parameter to be optimized independently for its specific function.
Solution Approach 2:
The patent introduces a non-magnetic spacer layer as an intermediary between the first and second free layers. This spacer layer enables magnetic coupling (via stray field) while allowing independent optimization of the two free layers, facilitating both high TMR and high switching efficiency simultaneously.
3Duration of action of stationary object
If the free layer thickness is reduced to improve retention, then switching speed improves, but thermal robustness and manufacturing precision become problematic
Solution Approach 1:
The patent uses two free layers with different thickness optimizations: the first free layer can be thinner for good retention, while the second free layer can be thicker for better thermal robustness and manufacturing tolerance. This segmentation resolves the contradiction between retention and manufacturing precision.
Solution Approach 2:
The patent changes the magnetization direction parameter of the second free layer to be responsive to the first free layer via stray field coupling, rather than directly coupled to the reference layer. This parameter change enables better thermal stability and manufacturing precision while maintaining retention performance.
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 design improves SOT switching efficiency, TMR, and retention performance while providing flexibility in pinning layer design and thermal robustness, enabling better compatibility with BEOL processing and smaller critical dimensions.
Implementation Method 1
the magnetization dynamics occur via a current induced spin-orbit coupling
Implementation Method 2
The origin of the spin current may be described by the Spin hall-effect
Implementation Method 3
from Rashba interaction (at the interface between the SOT-layer and the free layer)
Implementation Method 4
the first free layer is adapted to generate a magnetic stray field acting on the second free layer
Data Source
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AI summary
According to an aspect of the present inventive concept there is provided an MTJ device (100) comprising a layer stack (108) comprising: a SOT-layer (120) and a first free layer (130), a second free layer (150), a reference layer (170) and a tunnel barrier layer (160) arranged between the second free layer and the reference layer, and a spacer layer (140)arranged as an interfacial layer between the first free layer and the second free layer, wherein the SOT-layer is adapted to switch a magnetization direction of the first free layer through SOT, and wherein the first free layer is adapted to generate a magnetic stray field acting on the second free layer such that a magnetization direction of the second free layer is responsive to a magnetization direction of the first free layer.