Magnetoresistive Sensor Free Layer Using Cobalt Boron Alloy

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Solution Overview

Problem

Tunnel magnetoresistance (TMR) sensors face challenges in reducing magnetic reader gap spacing below 25 nm due to thick free layers with high magnetostriction and high Gilbert damping constants, leading to increased magnetic noise and limited areal density in hard disk drives.

Innovation Solution

A multilayered free layer structure is introduced, comprising a first magnetic layer with positive spin polarization and magnetostriction, and a second magnetic layer with negative magnetostriction and low Gilbert damping constant, using cobalt and boron alloys to reduce overall thickness and noise.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Stability of the object's composition

If a thick second magnetic layer (NiFe alloy) is used to counteract magnetostriction, then the desired low magnetostriction is achieved, but the overall thickness increases and Gilbert damping constant increases

Engineering Contradiction:

ImprovemagnetostrictionVSAvoidfree layer thickness

Core Design Contradiction:

Stability of the object's compositionVSLength of stationary object

Solution Approach 1:

The patent changes the material composition parameter from conventional NiFe alloy to CoB (Cobalt-Boron) alloy for the second magnetic layer. This parameter change enables achieving the desired negative magnetostriction with a thinner layer thickness and lower Gilbert damping constant, directly resolving the contradiction between magnetostriction compensation and thickness reduction

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent uses a composite free layer structure combining a first magnetic layer (CoFe) with a second magnetic layer (CoB alloy). This composite structure allows the first layer to provide high spin polarization and positive magnetostriction while the second layer provides negative magnetostriction, achieving net low magnetostriction with reduced overall thickness and damping

Inventive Principle:

Principle #40Composite materials

2Stability of the object's composition

If Ni-rich NiFe alloy is used for the second magnetic layer, then magnetostriction counteraction is achieved, but magnetic noise increases due to high Gilbert damping constant

Engineering Contradiction:

ImprovemagnetostrictionVSAvoidmagnetic noise

Core Design Contradiction:

Stability of the object's compositionVSObject-generated harmful factors

Solution Approach 1:

The patent changes the material composition from NiFe alloy to CoB alloy, which fundamentally alters the Gilbert damping constant parameter. The CoB alloy provides a lower Gilbert damping constant than NiFe, thereby reducing magnetic noise while maintaining the necessary negative magnetostriction for compensation

Inventive Principle:

Principle #35Parameter changes

3Productivity

If the free layer thickness is reduced to achieve smaller reader gap spacing, then areal density increases, but achieving desired magnetostriction becomes more difficult

Engineering Contradiction:

Improveareal densityVSAvoidmagnetostriction

Core Design Contradiction:

ProductivityVSStability of the object's composition

Solution Approach 1:

The patent changes the material parameters of the second magnetic layer to CoB alloy, which has superior magnetostriction compensation properties. This allows achieving the desired net low magnetostriction with a thinner free layer, enabling smaller reader gap spacing and higher areal density

Inventive Principle:

Principle #35Parameter changes

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 improved multilayered structure achieves a reduced sensor stack thickness, lower Gilbert damping constant, and decreased magnetic noise, enabling higher areal density and reduced magnetostriction in TMR sensors.

Implementation Method 1

a first magnetic layer having a positive spin polarization and a positive magnetostriction and a second, counteracting, magnetic layer having a negative magnetostriction

Methodology Applied

Scientific EffectMagnetostriction: Magnetostriction

Implementation Method 2

The second magnetic layer can have a negative magnetostriction. The second magnetic layer can comprise at least cobalt (Co) and boron (B). The second magnetic layer can further have a magnetization greater than 600 emu/cm3. The second magnetic layer can further have a Gilbert damping constant less than 0.02.

Methodology Applied

Scientific EffectMagnetism: Magnetism

Implementation Method 3

Tunnel magnetoresistance (TMR) sensors are frequently employed as magnetoresistive sensors in the magnetic heads of hard disk drives

Methodology Applied

Scientific EffectTunnel magnetoresistance: Magnetoresistance

Data Source

PatentUS8498084B1Magnetoresistive sensors having an improved free layer

Publication Date: 2013.07.30 WESTERN DIGITAL TECHNOLOGIES INC

AI summary

A magnetoresistive sensor having a novel free layer and a method of producing the same are disclosed. The magnetoresistive sensor comprises a pinned layer, a barrier layer disposed over the pinned layer, and a free layer disposed over the barrier layer. The free layer comprises a first magnetic layer disposed over the barrier layer. The first magnetic layer has a positive spin polarization, a positive magnetostriction, and a polycrystalline structure. The free layer further comprises a second magnetic layer disposed over the first magnetic layer. The second magnetic layer has a negative magnetostriction and comprises at least cobalt (Co) and boron (B).