TMR Sensor Free Layer Noise Reduction via Composite Structure
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Solution Overview
Problem
Current TMR sensors with CoFeB free layers face issues of high noise, positive magnetostriction, and reduced signal-to-noise ratio, which hinder the achievement of low coercivity and high TMR ratios required for advanced magnetic recording applications.
Innovation Solution
A composite free layer structure comprising alternating CoFe or CoFe alloy layers with CoB or CoFeB layers is introduced, which reduces noise and magnetostriction while maintaining a high TMR ratio and low coercivity, achieved through a specific layer configuration and deposition process involving natural oxidation of MgO tunnel barriers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a CoFeB free layer is used in TMR sensor, then high TMR ratio and low RA value are achieved, but noise and magnetostriction increase causing signal-to-noise ratio degradation
Solution Approach 1:
The free layer is segmented into multiple alternating sub-layers of CoFeB and CoFe, where each layer has optimized thickness (CoFeB: 3-7 nm, CoFe: 1-3 nm). This segmentation allows the CoFeB layers to provide high TMR ratio while the CoFe layers suppress noise and magnetostriction through their different magnetic properties, resolving the contradiction between high TMR ratio and low noise.
Solution Approach 2:
A composite free layer structure is created by combining CoFeB and CoFe materials in alternating layers. The CoFeB material contributes to high TMR ratio due to its spin-polarized electron transport properties, while the CoFe material provides low magnetostriction and noise suppression. This composite structure achieves both high TMR ratio and low noise simultaneously.
2Reliability
If free layer thickness is reduced to achieve low RA value, then resistance decreases, but noise increases and TMR ratio degrades
Solution Approach 1:
Different regions of the free layer are assigned different local qualities through the alternating CoFeB and CoFe layers. The CoFeB layers provide high spin polarization for low resistance, while the CoFe layers provide noise suppression. This local quality differentiation allows the thin free layer to achieve low RA value without excessive noise, as each layer performs its specialized function.
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 solution effectively reduces noise and maintains a high TMR ratio, achieving low coercivity and a low RA value, thereby enhancing the performance of TMR sensors in magnetic recording applications.
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
achieved through a specific layer configuration and processing techniques such as natural oxidation of Mg layers
Data Source
AI summary
A TMR sensor that includes a free layer having at least one B-containing (BC) layer made of CoFeB, CoFeBM, CoB, CoBM, or CoBLM, and a plurality of non-B containing (NBC) layers made of CoFe, CoFeM, or CoFeLM is disclosed where L and M are one of Ni, Ta, Ti, W, Zr, Hf, Tb, or Nb. One embodiment is represented by (NBC/BC)n where n≧2. A second embodiment is represented by (NBC/BC)n/NBC where n≧1. In every embodiment, a NBC layer contacts the tunnel barrier and NBC layers each with a thickness from 2 to 8 Angstroms are formed in alternating fashion with one or more BC layers each 10 to 80 Angstroms thick. Total free layer thickness is <100 Angstroms. The free layer configuration described herein enables a significant noise reduction (SNR enhancement) while realizing a high TMR ratio, low magnetostriction, low RA, and low Hc values.