Weakly Coupled Magnetic Layers for Low Current MRAM

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Solution Overview

Problem

Conventional spin-torque magnetic random access memory (STMRAM) devices require high write currents, which can exceed the breakdown voltage of the tunnel barrier and limit memory density due to large pass transistors, necessitating a reduction in programming currents.

Innovation Solution

The use of weakly coupled magnetic layers in the magnetic tunnel junction structure, where thin nonmagnetic interlayers between magnetic layers spread out the switching event over multiple layers, reducing the peak torque and current required for magnetization reversal, allowing for lower programming currents and smaller pass transistors.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If spin-torque effect is used to program MRAM bits, then write speed is improved, but write current becomes excessively large exceeding tunnel barrier breakdown voltage

Engineering Contradiction:

Improvewrite speedVSAvoidwrite current

Core Design Contradiction:

SpeedVSUse of energy by moving object

Solution Approach 1:

The free magnetic layer is divided into multiple sublayers (first free magnetic sublayer, second free magnetic sublayer, third free magnetic sublayer) with different magnetization directions. This segmentation allows the spin torque to be distributed across multiple interfaces, reducing the current density required at each interface while maintaining effective switching capability.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different sublayers are assigned different magnetization directions (first sublayer: upward, second sublayer: downward, third sublayer: upward) to create local variations in magnetic properties. This local quality differentiation enables selective switching at specific interfaces with lower current requirements compared to uniform magnetization structures.

Inventive Principle:

Principle #3Local quality

2Reliability

If large write current is used in STMRAM, then magnetization switching is achieved, but pass transistor size must be large limiting memory density

Engineering Contradiction:

Improvemagnetization switchingVSAvoidpass transistor size

Core Design Contradiction:

ReliabilityVSArea of stationary object

Solution Approach 1:

Dividing the free magnetic layer into multiple sublayers with alternating magnetization directions creates multiple switching interfaces. This segmentation reduces the current density requirement for each interface, enabling the use of smaller pass transistors that can accommodate lower current while still achieving reliable magnetization switching.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention changes the magnetic configuration parameter by creating a specific pattern of upward and downward magnetization directions in adjacent sublayers. This parameter change optimizes the spin torque efficiency at interfaces, reducing the current threshold for switching and enabling smaller transistor dimensions.

Inventive Principle:

Principle #35Parameter changes

3Device complexity

If conventional MRAM structure is used, then device simplicity is maintained, but scalability and density are limited

Engineering Contradiction:

Improvestructure simplicityVSAvoidmemory density

Core Design Contradiction:

Device complexityVSProductivity

Solution Approach 1:

The free magnetic layer is segmented into multiple thin sublayers with alternating magnetization directions, while maintaining the overall simplicity of the MRAM structure. This segmentation enables higher density and better scalability without fundamentally changing the device architecture, allowing conventional fabrication processes to be adapted.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention uses a composite magnetic structure combining multiple ferromagnetic sublayers with alternating magnetization directions within the free layer. This composite structure provides enhanced functionality for low-current switching while maintaining compatibility with standard MRAM device designs and fabrication methods.

Inventive Principle:

Principle #40Composite materials

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 approach achieves lower programming currents, preventing tunnel barrier breakdown and enabling higher memory density and lower power consumption while maintaining data stability.

Implementation Method 1

the bits are programmed by allowing a spin-polarized electron current to impinge upon a magnetic free layer. The change of angular momentum associated with the spin-polarized current generates a torque on the free layer that can change its magnetization direction

Methodology Applied

Scientific EffectSpin transfer torque: Angular Momentum Conservation

Implementation Method 2

Recalling data is accomplished by sensing the resistance of a current tunneling between the free and fixed magnetic layers

Methodology Applied

Scientific EffectQuantum tunneling:

Data Source

PatentEP3232440B1Magnetic element having reduced current density

Publication Date: 2020.05.27 EVERSPIN TECHNOLOGIES INC

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AI summary

A memory device includes a fixed magnetic layer, a tunnel barrier layer over the fixed magnetic layer, and a free magnetic structure formed over the tunnel barrier layer, wherein the free magnetic structure has layers or sub-layers that are weakly magnetically coupled. Thus, a low programming voltage can be used to avoid tunnel barrier breakdown, and a small pass transistor can be used to save die real estate.