TMR Sensor Free Layer Trilayer Structure
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Solution Overview
Problem
Current TMR sensors face challenges in achieving high dR/R ratios while maintaining low RA values, magnetostriction, and coercivity, particularly with CoFeB free layers that introduce noise and high magnetostriction, limiting their performance in high-density recording applications.
Innovation Solution
A composite free layer structure comprising a trilayer configuration of Fe(100-X)CoX, CoFeB, and CoB layers, with specific thicknesses and compositions, is used to enhance the TMR ratio, reduce magnetostriction, and maintain low coercivity, even at elevated annealing temperatures, by inserting a thin CoFeB layer between the FeCo and CoB layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a CoFeB free layer is used to achieve high TMR ratio, then dR/R is improved, but magnetostriction and noise increase
Solution Approach 1:
The free layer is divided into three distinct segments: FeCo layer (2-15 Å), CoFeB layer (2-10 Å), and CoB layer (10-100 Å). Each segment serves a specific function: FeCo provides high TMR ratio, CoFeB reduces magnetostriction, and CoB suppresses noise. This segmentation allows optimization of each property independently while maintaining overall performance.
Solution Approach 2:
Different materials are assigned to different regions of the free layer based on their specific properties. The FeCo layer is positioned to maximize TMR ratio contribution, the CoFeB layer is placed to reduce magnetostriction, and the CoB layer is positioned to suppress noise. This local optimization of material properties resolves the contradiction between improving TMR ratio and reducing harmful effects.
2Measurement precision
If annealing temperature is increased to enhance TMR ratio, then dR/R is improved, but coercivity increases
Solution Approach 1:
The invention changes the material composition parameters of the free layer by introducing CoFeB with specific boron content (5-40 atomic%) and controlling the thickness parameters of each layer. This allows the system to achieve high TMR ratio at elevated annealing temperatures while maintaining low coercivity through the specific compositional and dimensional parameters of the trilayer structure.
3Reliability
If RA value is reduced to improve sensor performance, then sensitivity is improved, but manufacturing precision requirements increase
Solution Approach 1:
The invention uses a composite trilayer structure combining FeCo, CoFeB, and CoB materials with specific thickness ranges. This composite structure achieves low RA value (around 1.5 ohm/μm²) while providing manufacturing flexibility. The specific thickness ranges (FeCo: 2-15 Å, CoFeB: 2-10 Å, CoB: 10-100 Å) are designed to be manufacturable while achieving the desired electrical and magnetic properties.
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 structure achieves a TMR ratio above 60% with RA values around 1.5 ohm/μm² and low coercivity, improving signal-to-noise ratio and magnetostriction, making it suitable for high-performance magnetic recording devices.
Implementation Method 1
The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons
Implementation Method 2
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
Implementation Method 3
a free layer with low magnetostriction (λ) and low coercivity (Hc)
Data Source
AI summary
A TMR sensor with a free layer having a FL1/FL2/FL3 configuration is disclosed in which FL1 is FeCo or a FeCo alloy with a thickness between 2 and 15 Angstroms. The FL2 layer is made of CoFeB or a CoFeB alloy having a thickness from 2 to 10 Angstroms. The FL3 layer is from 10 to 100 Angstroms thick and has a negative λ to offset the positive λ from FL1 and FL2 layers and is comprised of CoB or a CoBQ alloy where Q is one of Ni, Mn, Tb, W, Hf, Zr, Nb, and Si. Alternatively, the FL3 layer may be a composite such as CoB/CoFe, (CoB/CoFe)n where n is ≧2 or (CoB/CoFe)m/CoB where m is ≧1. The free layer described herein affords a high TMR ratio above 60% while achieving low values for λ (<5×10−6), RA (1.5 ohm/μm2), and Hc (<6 Oe).