MTJ Sensor Bridge With 3D Coil for Low Hysteresis
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Solution Overview
Problem
Magnetic field sensors with magnetic tunnel junctions (MTJ) face challenges in achieving high sensitivity while minimizing hysteresis, which affects their performance and resolution, particularly in linear sensor applications.
Innovation Solution
A magnetic field sensor design incorporating a Wheatstone bridge configuration with active and passive MTJ elements, magnetic shield elements, and a three-dimensional coil to concentrate magnetic flux and control magnetic history, reducing hysteresis and enhancing sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the sensor is designed to have no hysteresis (as shown in FIG. 1C), then the sensor provides a unique resistance value at each applied field value, but the slope in the middle region of the curve becomes even less steep, resulting in low resolution
Solution Approach 1:
The sensor is divided into multiple MTJ elements arranged in a bridge configuration, with each element having different magnetic properties (different coercive fields). This segmentation allows each element to contribute differently to the overall response, enabling the sensor to achieve both low hysteresis and high resolution simultaneously by combining the responses of multiple elements with staggered switching characteristics
Solution Approach 2:
Different MTJ elements are designed with locally optimized magnetic properties through varying the composition or thickness of magnetic layers. This creates elements with different coercive fields and switching characteristics, allowing the sensor to maintain low hysteresis while achieving steep slope through the cumulative effect of multiple elements switching at different field values
2Measurement precision
If the sensor is designed to have higher slope in the middle region (as shown in FIG. 1B), then the resolution is improved, but hysteresis is introduced, making the sensor double valued over its sensitive region
Solution Approach 1:
The bridge configuration segments the sensing function across multiple MTJ elements, where elements with different coercive fields switch at different field values. This segmentation allows the sensor to achieve high slope through the cumulative response of multiple elements while maintaining single-valued output by preventing simultaneous switching of all elements
Solution Approach 2:
The sensor dynamically adapts its response characteristics by utilizing elements with different magnetic properties. As the applied field changes, different elements become active or inactive, allowing the sensor to maintain optimal sensitivity across a wide field range while avoiding hysteresis through the staggered switching behavior of the segmented elements
3Device complexity
If the sensor uses a conventional planar configuration (as shown in FIG. 3A), then the device structure is simple, but the sensitivity is limited and hysteresis cannot be effectively controlled
Solution Approach 1:
The invention transitions from a conventional planar two-dimensional layout to a three-dimensional stacked configuration. MTJ elements are arranged in multiple layers vertically, with magnetic shield elements positioned between layers. This dimensional change enables improved magnetic flux concentration and control while maintaining compact footprint, achieving both high sensitivity and effective hysteresis control without excessive complexity
4Volume of moving object
If the sensor is made very small in size, then the form factor is improved, but achieving high sensitivity and low hysteresis becomes more difficult
Solution Approach 1:
By stacking MTJ elements and magnetic shield elements in multiple vertical layers, the sensor achieves three-dimensional integration. This allows the sensor to maintain a very small planar footprint while accommodating multiple functional elements that contribute to high sensitivity and low hysteresis. The vertical stacking enables compact design without sacrificing 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
The design achieves low hysteresis and high sensitivity, allowing for precise magnetic field measurements with improved resolution and power efficiency, even in small form factors.
Implementation Method 1
magnetic shield elements, and a three-dimensional coil to concentrate magnetic flux
Implementation Method 2
a three-dimensional coil to concentrate magnetic flux
Implementation Method 3
MTJ devices are well known to show resistance changes as a function of applied magnetic fields
Implementation Method 4
Hysteresis in a magnetic-field sensor further hinders good performance
Data Source
Figure 1A~1C
Figure 2
Figure 3A~3B
AI summary
An MTJ sensor having low hysteresis and high sensitivity is disclosed. The MTJ sensor includes, in one embodiment, a bridge with first and second active MTJ elements and first and second passive MTJ elements connected in a Wheatstone bridge configuration. First and second magnetic shield elements are located over the first and second passive MTJ elements and form a gap therebetween that concentrates magnetic flux toward the first and second active MTJ elements. A three-dimensional coil is wound around the first and second magnetic shield elements with over-windings located over the first and second magnetic shield elements and under-windings located under the first and second magnetic shield elements, connected together by a plurality of vias adjacent the first and second magnetic shield elements. The MTJ sensor may be operated with circuitry configured to supply a reset current pulse through the three-dimensional coil sufficient to magnetically saturate the first and second active MTJ elements and the first and second passive MTJ elements, supply a monotonically increasing current with a polarity opposite to the reset current pulse through the three-dimensional coil, and determine a value of current through the three-dimensional coil when an output voltage of the bridge reaches an endpoint, such as zero volts.