SOT-MRAM Free Layer Magnetization Control

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Solution Overview

Problem

Conventional RAMs face challenges in writing speed and reliability due to high energy and time-consuming initialization procedures, and large occupied volume in spin-orbit torque magnetic random access memories (SOT-MRAMs) which affect storage density and writing efficiency.

Innovation Solution

A spin-orbit torque magnetic random access memory (SOT-MRAM) with a magnetic free layer comprising ferromagnetic first and second metal films, where the first metal film has a thickness allowing a remanent-to-saturated magnetization ratio of -0.9 to 0.9, and a writing method involving external magnetic fields and electric pulses to generate a spin-Hall effect, reversing magnetic moments without initialization, achieving multilevel storage with reduced volume.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Quantity of substance

If conventional RAMs use two-terminal approach with multilevel structures, then storage capacity increases, but initialization procedure becomes energy and time consuming

Engineering Contradiction:

Improvestorage capacityVSAvoidinitialization time

Core Design Contradiction:

Quantity of substanceVSLoss of time

Solution Approach 1:

The patent extracts and eliminates the initialization procedure from the writing process by using spin-orbit torque to directly switch magnetic states without requiring the memory cell to be in a specific initial state, thereby removing the time-consuming initialization step while maintaining multilevel storage capability

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent applies a preliminary magnetic field to set the magnetic free layer in a desired state before the writing operation, which eliminates the need for initialization during the actual writing process and enables direct data writing at high speed

Inventive Principle:

Principle #10Preliminary action

2Reliability

If conventional SOT-MRAM uses three-terminal configuration, then writing reliability improves, but occupied volume increases and areal density decreases

Engineering Contradiction:

Improvewriting reliabilityVSAvoidoccupied volume

Core Design Contradiction:

ReliabilityVSVolume of moving object

Solution Approach 1:

The patent merges the bit line and write line functions into a single terminal structure, combining the advantages of spin-transfer torque and spin-orbit torque approaches in a unified two-terminal device, thereby reducing occupied volume while maintaining writing reliability

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The patent designs the magnetic tunnel junction to serve multiple functions simultaneously - it acts as both the storage element and the writing element, eliminating the need for separate write line infrastructure and reducing the overall occupied volume of the memory cell

Inventive Principle:

Principle #6Universality (Multi-functionality)

3Speed

If high writing voltage is applied in STT-MRAM, then writing speed improves, but damage to free layer and MgO tunnel barrier layer occurs

Engineering Contradiction:

Improvewriting speedVSAvoiddamage to magnetic layers

Core Design Contradiction:

SpeedVSObject-affected harmful factors

Solution Approach 1:

The patent replaces the direct electrical switching mechanism with a spin-orbit torque mechanism that uses spin current generated by heavy metal layer to switch the magnetic state, substituting high voltage electrical stress with lower current density spin transfer, thereby achieving fast writing without damaging the magnetic tunnel junction

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent introduces a heavy metal layer as an intermediary that converts charge current into spin current through the spin Hall effect, which then acts on the magnetic free layer to achieve switching. This intermediary mechanism allows writing at lower current densities that do not damage the MgO tunnel barrier layer

Inventive Principle:

Principle #24Intermediary (Mediator)

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 SOT-MRAM achieves multilevel storage with improved storage density and increased writing speed without the need for initialization, enhancing reliability and efficiency by manipulating magnetic domains through controlled current density and magnetic fields.

Implementation Method 1

a second metal film contacting the first metal film for generating a spin-Hall effect

Methodology Applied

Scientific EffectSpin-Hall effect: Hall Effect

Data Source

PatentUS9666256B1Spin-orbit torque magnetic random access memory and method of writing the same

Publication Date: 2017.05.30 NATIONAL TSING HUA UNIVERSITY

AI summary

An SOT-MRAM comprises a substrate, and an SOT memory cell disposed on the substrate and including a magnetic free layer. The magnetic free layer includes a first metal film exhibiting ferromagnetic characteristics, and a second metal film for generating a spin-Hall effect. The first metal film has a thickness sufficient to allow the magnetic free layer, after being applied with a first external magnetic field which is subsequently removed, to have a magnetization ratio ranging from −0.9 to 0.9. The first metal film, upon being applied with a second external magnetic field and an electric pulse, has multiple magnetic domains when a current density resulting from the electric pulse is greater than a critical value.