MTJ Magnetic Sensor with SOT Reference Switching for Low 1/f Noise
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Solution Overview
Problem
Tunnel magnetoresistance (TMR)-based sensors suffer from pink noise (1/f noise) that limits their magnetic detectivity at low frequencies, and existing methods to reduce this noise, such as increasing sensor size or modulating the magnetic field, result in higher costs, complexity, or reduced linearity and spatial resolution.
Innovation Solution
A magnetic sensor element with a reference layer and sense layer configuration that uses a spin-orbit torque (SOT) electrode to switch the reference magnetization, allowing differential resistance measurement to sense external magnetic fields without pink noise, and a sensing device with a SOT electrode to pass a current that switches the reference magnetization in different directions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the sensor size is increased to decrease 1/f noise, then the noise level is reduced, but the die cost increases and spatial resolution decreases
Solution Approach 1:
The sensor is divided into multiple sensing elements arranged in a bridge configuration, where each element contributes to noise cancellation through differential measurement. This segmentation allows noise reduction without increasing the overall sensor size, as the noise cancellation is achieved through the combination of multiple smaller elements rather than a single large element.
Solution Approach 2:
The patent uses identical or matched sensing elements replicated in a bridge configuration. By copying the same structure multiple times and combining their outputs differentially, the system achieves noise rejection without requiring larger individual elements, thereby maintaining spatial resolution while reducing 1/f noise.
2Object-affected harmful factors
If a full bridge configuration is used to reduce 1/f noise, then the noise performance is improved, but the device complexity increases
Solution Approach 1:
The patent combines multiple sensing elements into a unified bridge structure that shares common components such as biasing circuits and readout electronics. This merging approach reduces the overall complexity compared to using four completely independent sensing circuits, while still achieving the noise cancellation benefits of a full bridge configuration.
3Measurement precision
If magnetic field modulation is applied to improve detectivity, then the sensor detectivity is enhanced, but the linearity of the output signal is reduced and the sensing range is limited
Solution Approach 1:
The patent employs periodic switching of the bias voltage polarity to modulate the sensing operation. This periodic action allows the sensor to operate in alternating high and low states, enabling noise cancellation through differential measurement while maintaining a linear response to the external magnetic field. The periodic switching frequency is chosen to be well above the noise spectrum, avoiding distortion of the magnetic signal.
Solution Approach 2:
The bias voltage is dynamically switched between positive and negative values, allowing the sensor to adapt its operating point. This dynamic operation enables the sensor to maintain linearity across a wider range of magnetic field strengths compared to static biasing, while still achieving noise reduction through the differential measurement of the switched states.
4Measurement precision
If the sensor operates in a full bridge configuration with bias voltage modulation, then the detectivity is improved, but the circuit complexity and fabrication costs increase
Solution Approach 1:
The bridge configuration is designed so that the same structural elements serve multiple functions: they provide both the sensing function and the noise cancellation function. The differential arrangement allows a single fabrication process to create all sensing elements with matched characteristics, eliminating the need for separate trimming or calibration steps and reducing overall fabrication complexity and cost.
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 solution provides a magnetic sensor element and device that senses external magnetic fields with good linearity and operating range, eliminating pink noise and resistance offsets, without requiring a full-bridge architecture.
Implementation Method 1
a SOT electrode, in direct contact with the first reference sublayer and configured to pass a SOT current adapted to switch the first reference magnetization by a spin orbit torque (SOT) interaction
Implementation Method 2
A tunnel magnetoresistance (TMR)-based sensor has high sensitivity and therefore is suitable for sensing an external magnetic field
Data Source
AI summary
A magnetic sensor element is disclosed, comprising a magnetic tunnel junction (MTJ) comprising a reference layer, a tunnel barrier layer, a sense layer having a sense magnetization freely orientable in the presence of the external magnetic field. The reference layer has a reference magnetization and comprises a reference SAF structure and an in-plane sensitivity axis. A SOT electrode configured to pass a SOT current adapted to switch the first reference magnetization in two opposed directions along the sensitivity axis by a spin orbit torque interaction. Also disclosed is a sensing device comprising at least one sensing branch including at least one magnetic sensor element and a sensing operation using the sensing device for sensing an external magnetic field. The magnetic sensor element allows for sensing the external magnetic field with low 1/f noise.