Co-Fe-B TMR Sensor Free Layer with Negative Saturation Magnetostriction
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Solution Overview
Problem
Current current-perpendicular-to-plane (CPP) tunneling magnetoresistive (TMR) sensors with Co—Fe—B free layers exhibit high positive saturation magnetostriction, leading to unstable read performance and requiring impractically high temperatures for fabrication, which deteriorates ferromagnetic properties.
Innovation Solution
A Co—Fe—B free layer with reduced Fe and B content, annealed at temperatures below 300°C, achieves negative or zero saturation magnetostriction while maintaining superior TMR properties, and a free-layer structure with specific composition and thickness adjustments allows tuning of saturation magnetostriction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a Co-Fe-B free layer with high Fe content is used to achieve high TMR coefficient, then the TMR coefficient increases, but the saturation magnetostriction becomes highly positive causing unstable read performance
Solution Approach 1:
The patent changes the composition parameters of the Co-Fe-B free layer by reducing Fe content to ≤10 at% and B content to ≤10 at%, which fundamentally alters the saturation magnetostriction from highly positive to negative or zero, thereby resolving the contradiction between high TMR coefficient and stable read performance
Solution Approach 2:
The patent creates a composite magnetic layer structure combining Co-Fe-B free layer with Co-Pt or Co-Pt-Alm longitudinal bias layers, where the Co-Pt layer provides negative magnetostriction to compensate for the Co-Fe-B layer's positive magnetostriction, achieving both high TMR and stable read performance
2Manufacturing precision
If high annealing temperature is used to achieve desired microstructure, then the microstructure quality improves, but the ferromagnetic properties deteriorate
Solution Approach 1:
The patent changes the annealing temperature parameter from conventional high temperature (360°C) to low temperature (≤300°C), which prevents ferromagnetic property deterioration while still achieving the desired microstructure quality through the optimized low-Fe, low-B composition
3Ease of manufacture
If a free layer with positive saturation magnetostriction is used, then the TMR sensor can be fabricated, but high longitudinal bias fields are required causing low read sensitivity
Solution Approach 1:
The patent inverts the magnetostriction sign from positive to negative or zero by changing the free layer composition, which reverses the magnetization bias direction from transverse to longitudinal, thereby eliminating the need for high bias fields and improving read sensitivity
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 approach results in a TMR sensor with high TMR coefficient and low junction resistance-area product, achieving stable read performance without the need for high bias fields and at a lower annealing temperature, ensuring manufacturability and improved ferromagnetic properties.
Implementation Method 1
it is desired that the free layer has a negative, or at least zero, saturation magnetostriction, λS. After receiving compressive stresses induced by mechanical lapping in the fabrication process of the write and read heads, a free layer with a negative λS longitudinally biases its own magnetization in a longitudinal direction parallel to the ABS
Implementation Method 2
After annealing at a temperature of less than 300° C., the Co—Fe—B free layer exhibits a negative or zero saturation magnetostriction, λS
Implementation Method 3
The relative orientations of the magnetizations of the pinned and free layers change the electrical resistance of the TMR sensor based on the spin-dependent tunneling of conduction electrons through the barrier layer
Data Source
AI summary
A tunneling magnetoresistive (TMR) sensor with a free layer made of a Co—Fe—B alloy is disclosed. The Co—Fe—B free layer has an Fe content of not greater than 10 atomic percent, and a B content of not greater than 10 atomic percent. The free-layer structure can include a first free layer lying on a barrier layer and a second free layer lying on the first free layer. The first free layer is made of an alloy selected from Co—Fe, Co—B and Co—Fe—B alloys, while the second free layer is made of an alloy selected from Co—B and Co—Fe—B alloys.