TMR Free Layer Segmentation for High MR Ratio and Low Coercive Force

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Solution Overview

Problem

Tunnel magnetoresistive (TMR) elements face a challenge in achieving a higher MR ratio while maintaining a low coercive force, as using ferromagnetic materials with high spin polarization for the free layer deteriorates the soft magnetic property, leading to reduced magnetic field sensitivity and unstable output signals.

Innovation Solution

The TMR element incorporates a free layer comprising a first soft magnetic layer, a high polarization layer, and a second soft magnetic layer, where the high polarization layer is sandwiched between the first and second soft magnetic layers, each with a coercive force lower than the high polarization layer, to achieve a higher MR ratio and lower coercive force.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a ferromagnetic material with high spin polarization is used for the free layer, then the MR ratio is improved, but the coercive force increases and soft magnetic property deteriorates

Engineering Contradiction:

ImproveMR ratioVSAvoidmagnetic field sensitivity

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The free layer is divided into two distinct layers: a high spin polarization layer (FeCo alloy with 40-90 atomic % Fe) in contact with the tunnel barrier layer to achieve high MR ratio, and a soft magnetic layer (NiFe alloy or CoFeB alloy) adjacent to it to provide low coercive force and maintain soft magnetic properties. This segmentation allows each layer to fulfill its specific function without compromising the other.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The free layer is constructed as a composite structure combining two different ferromagnetic materials with complementary properties: FeCo alloy providing high spin polarization for high MR ratio, and NiFe or CoFeB alloy providing low coercive force for good soft magnetic properties. The composite structure enables simultaneous achievement of high MR ratio and low coercive force.

Inventive Principle:

Principle #40Composite materials

2Reliability

If the coercive force of the free layer is reduced to improve magnetic field sensitivity, then the soft magnetic property is improved, but the MR ratio decreases

Engineering Contradiction:

Improvemagnetic field sensitivityVSAvoidMR ratio

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The free layer is segmented into a soft magnetic layer (providing low coercive force) and a high spin polarization layer (providing high MR ratio). The soft magnetic layer is positioned adjacent to the tunnel barrier layer interface, ensuring that the low coercive force property is maintained while the high spin polarization layer maintains contact with the tunnel barrier for high MR ratio.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the free layer are assigned different material compositions and properties: the region adjacent to the tunnel barrier layer interface uses soft magnetic material (NiFe or CoFeB) for low coercive force, while the region in direct contact with the tunnel barrier layer uses high spin polarization material (FeCo alloy) for high MR ratio. This local differentiation of material quality resolves the contradiction between coercive force and MR ratio.

Inventive Principle:

Principle #3Local quality

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

This configuration enables a higher MR ratio and lower coercive force, resulting in improved magnetic field sensitivity and stable output signals for the TMR element.

Implementation Method 1

The tunnel barrier layer is a nonmagnetic insulating layer through which electrons are capable of passing while maintaining spins thereof by means of the tunnel effect

Methodology Applied

Scientific EffectTunnel effect:

Implementation Method 2

The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer

Methodology Applied

Scientific EffectExchange coupling:

Implementation Method 3

the direction of magnetization in the free layer changes in response to the signal magnetic field sent from a recording medium

Methodology Applied

Scientific EffectMagnetoresistive effect: Magnetoresistance

Data Source

PatentUS7372675B2Magnetoresistive element, thin-film magnetic head, head gimbal assembly, head arm assembly and magnetic disk drive

Publication Date: 2008.05.13 TDK CORP

AI summary

An MR element comprises: a tunnel barrier layer having two surfaces that face toward opposite directions; a free layer disposed adjacent to one of the surfaces of the tunnel barrier layer and having a direction of magnetization that changes in response to an external magnetic field; and a pinned layer that is a ferromagnetic layer disposed adjacent to the other of the surfaces of the tunnel barrier layer and having a fixed direction of magnetization. The free layer incorporates: a first soft magnetic layer disposed adjacent to the one of the surfaces of the tunnel barrier layer; a high polarization layer disposed such that the first soft magnetic layer is sandwiched between the tunnel barrier layer and the high polarization layer; and a second soft magnetic layer disposed such that the high polarization layer is sandwiched between the first and second soft magnetic layers.