Nonvolatile SRAM Using Current-Induced MTJ Magnetization Reversal

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Solution Overview

Problem

SRAMs operate at high speed but lose data when power is shut down, while nonvolatile memories retain data but struggle with high-speed operations, and latch circuits consume power due to leak currents during standby, making it difficult to achieve both low power consumption and high-speed data access.

Innovation Solution

A memory circuit incorporating a bistable circuit and a ferromagnetic tunnel junction device that nonvolatilely stores data based on the magnetization direction of a ferromagnetic electrode free layer, allowing for high-speed data writing and reading, and restoring data even after power is shut down.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If SRAM is used for high-speed data access, then data access speed is improved, but data is lost when power is shut down

Engineering Contradiction:

Improvedata access speedVSAvoiddata retention

Core Design Contradiction:

SpeedVSReliability

Solution Approach 1:

The memory system is divided into two functional parts: a volatile bistable circuit for high-speed data access and a nonvolatile ferromagnetic tunnel junction device for data retention. The bistable circuit handles read/write operations at high speed, while the MTJ device preserves data when power is shut down, resolving the contradiction between speed and reliability through functional segmentation.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The ferromagnetic tunnel junction device acts as an intermediary between the bistable circuit and external storage. It receives data from the bistable circuit during operation and restores it when power is shut down, mediating between the volatile fast memory and the need for nonvolatile data retention.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If nonvolatile memory is used for data retention, then data retention is improved, but data access speed deteriorates

Engineering Contradiction:

Improvedata retentionVSAvoiddata access speed

Core Design Contradiction:

ReliabilityVSSpeed

Solution Approach 1:

The memory system separates retention functions from access functions. The ferromagnetic tunnel junction device provides nonvolatile data retention, while the bistable circuit provides high-speed access. This segmentation allows each component to optimize for its specific function without compromising the other.

Inventive Principle:

Principle #1Segmentation

3Reliability

If latch circuit is used for data storage, then data retention is improved, but power consumption increases due to leak currents during standby

Engineering Contradiction:

Improvedata retentionVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The invention extracts the data retention function from the active latch circuit into a separate nonvolatile ferromagnetic tunnel junction device. This allows the latch circuit to be powered down during standby, eliminating leak current power consumption, while the MTJ device maintains data retention without requiring power.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The system operates in periodic cycles: during active periods, the bistable circuit processes data at high speed; during standby periods, power is shut down to eliminate leakage, and data is retained in the MTJ device. This periodic operation pattern reduces average power consumption while maintaining data retention capability.

Inventive Principle:

Principle #19Periodic action

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

Enables high-speed data access and reduced power consumption by nonvolatilely storing data in the ferromagnetic tunnel junction device, which can be restored to the bistable circuit when power is resumed, ensuring data integrity and efficiency.

Implementation Method 1

a ferromagnetic tunnel junction device that nonvolatilely stores data stored in the bistable circuit according to a magnetization direction of a ferromagnetic electrode free layer

Methodology Applied

Scientific EffectMagnetization: Magnetism

Implementation Method 2

The ferromagnetic tunnel junction device may change the magnetization direction of the ferromagnetic electrode free layer by a current-induced magnetization reversal method

Methodology Applied

Scientific EffectCurrent-induced magnetization reversal: Magnetism

Data Source

PatentUS8295079B2Nonvolatile SRAM/latch circuit using current-induced magnetization reversal MTJ

Publication Date: 2012.10.23 THE JAPAN SCI & TECH AGENCY

AI summary

The present invention is a memory circuit that includes a bistable circuit that stores data; and a ferromagnetic tunnel junction device that nonvolatilely stores the data stored in the bistable circuit according to a magnetization direction of a ferromagnetic electrode free layer, the data nonvolatilely stored in the ferromagnetic tunnel junction device being able to be restored in the bistable circuit. According to the present invention, writing data to and reading data from the bistable circuit can be performed at high speed. In addition, even though a power source is shut down, it is possible to restore data nonvolatilely stored in the ferromagnetic tunnel junction devices to the bistable circuit.