MTJ Magnetic Angular Sensor Stack for Low- and High-Field Accuracy
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Solution Overview
Problem
Magnetic angular sensors suffer from angular errors due to finite magnetic anisotropy and stray fields, limiting their operational margin at low and high magnitudes of external magnetic fields.
Innovation Solution
A magnetic angular sensor element with a ferromagnetic pinned layer and a ferromagnetic sensing layer coupled antiferromagnetically, combined in a half-bridge or full-bridge configuration, to compensate for angular errors by stabilizing the reference layer magnetization and reducing resistance variation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the sensing layer magnetization is made soft to align easily with external magnetic field, then the sensing layer can be easily aligned by external magnetic field, but angular errors increase at low magnitudes of external magnetic field due to finite magnetic anisotropy and stray fields
Solution Approach 1:
The sensing layer is divided into two separate sensing layers (first sensing layer and second sensing layer) with different magnetization characteristics. The first sensing layer has softer magnetization for easy alignment, while the second sensing layer has harder magnetization for stability. This segmentation allows each layer to fulfill different functional requirements that were previously conflicting in a single sensing layer.
Solution Approach 2:
Different regions of the sensing structure are given different magnetic properties. The first sensing layer is designed with specific anisotropy for easy alignment, while the second sensing layer is designed with different anisotropy for stability. This local differentiation of magnetic properties resolves the contradiction between ease of alignment and measurement precision.
2Stability of the object's composition
If the reference layer magnetization is made rigid to remain pinned, then the reference layer maintains stable orientation, but angular errors increase at high magnitudes of external magnetic field due to finite magnetic stiffness
Solution Approach 1:
The reference layer is segmented into multiple sublayers (first reference layer, second reference layer, third reference layer) with different pinning strengths and orientations. This segmentation allows the reference layer structure to maintain overall stability while distributing the magnetic field response across multiple layers, reducing the angular error at high field magnitudes.
Solution Approach 2:
The reference layer sublayers are configured with asymmetric properties - different thicknesses, different pinning strengths, and different orientations relative to the external magnetic field. This asymmetric configuration allows the reference layer to maintain stability while responding differently to various components of the external magnetic field, reducing angular errors.
3Device complexity
If a single MTJ stack is used for sensing, then the device structure is simple, but the operational margin is limited at both low and high magnitudes of external magnetic field due to angular errors
Solution Approach 1:
The single MTJ stack is segmented into multiple sensing layers and reference layer sublayers, each performing specific functions. This internal segmentation improves the operational margin and reduces angular errors without requiring multiple separate MTJ stacks, thus maintaining relative structural simplicity while enhancing reliability.
Solution Approach 2:
Multiple sensing layers with different magnetic properties are merged within a single MTJ stack structure, along with multiple reference layer sublayers. This merging approach achieves the benefits of complex multi-layer structures while maintaining a compact single-stack configuration, improving reliability without proportionally increasing device complexity.
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
The proposed configuration significantly reduces angular errors across a wide range of external magnetic fields, improving sensor accuracy and operational margin.
Implementation Method 1
The metallic spacer 24 is configured to provide an antiferromagnetic coupling between the first sensing magnetization 230a and the second sensing magnetization 230b through the RKKY coupling
Implementation Method 2
Tunnel magnetoresistance (TMR) effect is employed to sense the relative angle between the magnetization of the sensing layer and the magnetization of the reference layer
Implementation Method 3
One of the ferromagnetic layers (sensing layer) is magnetically soft and is easily aligned by an external magnetic field, the other one is magnetically rigid (reference layer) with a pinned direction of its magnetization
Data Source
Figure 1~2
Figure 3
Figure 4~5(b)
AI summary
Magnetic angular sensor element (20) destined to sense an external magnetic field (60), comprising a magnetic tunnel junction (2) containing a ferromagnetic pinned layer (21) having a pinned magnetization (210), a ferromagnetic sensing layer (23), and a tunnel magnetoresistance barrier layer (22); the ferromagnetic sensing layer (23) comprising a first sensing layer (23a) being in direct contact with the barrier layer (22) and having a first sensing magnetization (230a), a second sensing layer (23b) having a second sense magnetization (230b), and a metallic spacer (24) between the first sensing layer (23a) and the second sensing layer (23b); wherein the metallic spacer (24) is configured to provide an antiferromagnetic coupling between the first sensing magnetization (230a) and the second sensing magnetization (230b) such that the first sensing magnetization (230a) is oriented substantially antiparallel to the second sensing magnetization (230b); the second sensing magnetization (230b) being larger than the first sensing magnetization (230a), such that the second sensing magnetization (230b) is oriented in accordance with the direction of the external magnetic field (60).