Perpendicular MTJ Free-Layer Structure for Low-Current Switching
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Solution Overview
Problem
Conventional magnetic tunnel junction (MTJ) structures in spin transfer torque random access memories (STT-MRAMs) face challenges in achieving low switching current and thermal stability of the free layer, which are essential for improved performance and endurance.
Innovation Solution
The proposed solution involves an MTJ structure with perpendicular shape anisotropy, comprising a reference layer, a non-magnetic spacer, and a free layer with specific layer configurations and materials, such as CoFeB and CoFeX, to enhance saturation magnetization and tunnel magnetoresistance, thereby reducing the switching current and maintaining thermal stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional MTJ structures are used, then the device complexity is low, but the switching current is high and thermal stability is poor
Solution Approach 1:
The free layer is segmented into multiple sublayers (first free layer, second free layer, third free layer) with different magnetization directions and thicknesses. This segmentation allows each sublayer to contribute differently to the overall magnetic stability, enabling improved thermal stability while maintaining manageable device complexity through modular design
Solution Approach 2:
The patent employs composite magnetic layer structures combining different ferromagnetic materials (CoFeB, CoFe, CoFeTaOx) with specific perpendicular magnetic anisotropy properties. These composite structures achieve enhanced thermal stability by combining materials with complementary magnetic characteristics, resolving the contradiction between reliability improvement and complexity increase
2Use of energy by moving object
If conventional MTJ structures are used, then the manufacturing process is simple, but the switching current is high
Solution Approach 1:
Different regions of the free layer are assigned different local properties: the first free layer has higher saturation magnetization for spin filtering, while the second and third free layers have lower saturation magnetization for easier switching. This local quality differentiation reduces the overall switching current requirement while maintaining a relatively simple fabrication process through localized material property optimization
Solution Approach 2:
The patent optimizes critical parameters including the thickness ratios of free layers (tFL1/tFL2 between 0.5-2.0), saturation magnetization ratios (MsFL1/MsFL2 between 0.8-2.0), and perpendicular magnetic anisotropy energy densities. These parameter changes enable reduced switching current while keeping the manufacturing process within standard semiconductor fabrication capabilities
3Reliability
If the free layer thickness is increased to improve thermal stability, then the energy barrier increases, but the switching current also increases
Solution Approach 1:
The patent transitions from a single-layer free layer approach to a multi-layer free layer structure with vertical stacking. This dimensional change allows the system to achieve thermal stability through the cumulative effect of multiple thin layers rather than relying on a single thick layer, thereby maintaining low switching current while improving energy barrier
Solution Approach 2:
The multi-layer free layer structure introduces dynamic switching behavior where layers can switch at different current thresholds. The first free layer switches at lower current due to higher saturation magnetization, while the second and third layers switch at higher currents, creating a dynamic switching sequence that optimizes both thermal stability and switching efficiency
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 configuration increases the Figure of Merit (FOM) by up to 2×, providing higher energy barriers and reduced switching currents, leading to improved switching and stability of the magnetic states in STT-MRAMs.
Implementation Method 1
A spin-polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
Implementation Method 2
MTJ structure with perpendicular shape anisotropy, comprising a reference layer, a non-magnetic spacer, and a free layer with specific layer configurations and materials
Implementation Method 3
enhance saturation magnetization and tunnel magnetoresistance, thereby reducing the switching current and maintaining thermal stability
Implementation Method 4
a coupling layer disposed between the first free layer and the second free layer
Data Source
AI summary
A perpendicular shape anisotropy magnetic tunnel junction structure includes a reference layer, a non-magnetic layer, and a free layer. The reference layer includes a first side and a second side opposite the first side. The non-magnetic spacer includes a first side and a second side. The first side of the non-magnetic spacer is on the second side of the first reference layer. The free layer includes a first side and a second side. The first side of the free layer is on the second side of the non-magnetic spacer. The free layer includes a first layer on the first side of the free layer, a second layer on the second side of the free layer and a coupling layer disposed between the first layer and the second layer. A ratio of a saturation magnetization of the second layer to a saturation magnetization of the first layer ranges from 0.2-0.8 inclusive.