Composite Free Layer Structure for TMR Sensors
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Solution Overview
Problem
Current TMR sensors face challenges in achieving low magnetostriction, low coercivity, and high magnetoresistive ratio simultaneously, with existing free layers either degrading the TMR ratio or increasing magnetostriction beyond acceptable limits for high-density memory applications.
Innovation Solution
A composite free layer structure is introduced, comprising magnetic layers (FL1 and FL2) with an insertion layer (INS) containing magnetic and non-magnetic elements, such as Co, Fe, Ni, and Ta, to enhance magnetic coupling and adjust the TMR ratio, coercivity, and magnetostriction, while maintaining a low resistance area product.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional free layer structure is used, then the TMR ratio can be maintained, but magnetostriction increases beyond acceptable limits
Solution Approach 1:
The free layer is segmented into multiple sub-layers (first free layer, second free layer, third free layer) with distinct compositions and functions. The first free layer (CoFeB) provides high TMR ratio, the second free layer (NiFe) provides low magnetostriction, and the third free layer (CoFe) provides magnetic coupling. This segmentation allows each layer to optimize for its specific function, resolving the contradiction between maintaining high TMR ratio and reducing magnetostriction.
Solution Approach 2:
The free layer employs a composite structure combining multiple materials (CoFeB, NiFe, CoFe) with complementary properties. CoFeB contributes to high TMR ratio through its specific magnetic and electronic properties, NiFe contributes to low magnetostriction through its negative magnetostriction coefficient, and the combination creates a composite free layer that achieves both high TMR ratio and low overall magnetostriction.
2Ease of manufacture
If a conventional free layer structure is used, then manufacturing simplicity is maintained, but coercivity increases beyond acceptable limits
Solution Approach 1:
The free layer is divided into multiple sub-layers, each with optimized thickness and composition to control magnetic properties. The first free layer (CoFeB, 3-5 nm) provides soft magnetic characteristics with low coercivity, the second free layer (NiFe, 2-4 nm) further reduces coercivity through its magnetic properties, and the third free layer (CoFe, 1-3 nm) provides magnetic coupling. This multi-layer segmentation enables precise control of coercivity while maintaining compatibility with existing sputtering manufacturing processes.
3Object-affected harmful factors
If the free layer is optimized for low magnetostriction, then the TMR ratio degrades
Solution Approach 1:
Different regions of the free layer are assigned different local qualities and functions. The first free layer (CoFeB) is optimized for high TMR ratio with specific magnetic moment orientation, the second free layer (NiFe) is optimized for low magnetostriction with negative magnetostriction coefficient, and the third free layer (CoFe) is optimized for magnetic coupling. This local quality differentiation allows the overall free layer to achieve both low magnetostriction and high TMR ratio by optimizing each local region for its specific function.
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 achieves a high TMR ratio of over 60% with a low RA value and low magnetostriction, improving signal-to-noise ratio and device stability, while being cost-effective and compatible with existing manufacturing processes.
Implementation Method 1
an insertion layer (INS) containing magnetic and non-magnetic elements, such as Co, Fe, Ni, and Ta, to enhance magnetic coupling
Implementation Method 2
The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons
Implementation Method 3
The electrical resistance through the tunnel barrier layer (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby converts magnetic signals into electrical signals
Data Source
AI summary
A composite free layer having a FL1/insertion/FL2 configuration is disclosed for achieving high dR/R, low RA, and low λ in TMR or GMR sensors. Ferromagnetic FL1 and FL2 layers have (+) λ and (−) λ values, respectively. FL1 may be CoFe, CoFeB, or alloys thereof with Ni, Ta, Mn, Ti, W, Zr, Hf, Tb, or Nb. FL2 may be CoFe, NiFe, or alloys thereof with Ni, Ta, Mn, Ti, W, Zr, Hf, Tb, Nb, or B. The thin insertion layer includes at least one magnetic element such as Co, Fe, and Ni, and at least one non-magnetic element selected from Ta, Ti, W, Zr, Hf, Nb, Mo, V, Cr, or B. In a TMR stack with a MgO tunnel barrier, dR/R>60%, λ˜1×10−6, and RA=1.2 ohm-um2 when FL1 is CoFe/CoFeB/CoFe, FL2 is CoFe/NiFe/CoFe, and the insertion layer is CoTa or CoFeBTa.