Dual MTJ Stack With SOT/STT Switching for Two-Bit MRAM
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Solution Overview
Problem
Current non-volatile magnetic memory devices face challenges in forming spin-orbit torque materials on top of magnetic free layers, leading to damaged magnetic free properties and limited storage capacity and bit density, especially in vertical RAM units and STT-MRAM devices with complex device structures and high manufacturing costs.
Innovation Solution
A non-volatile memory device with a material layer stack comprising two magnetic tunnel junctions, where the second top electrode is partially exposed to form a shoulder, allowing for bi-directional current injection and reducing contact resistance, enabling independent switching of magnetic polarization in both junctions without damaging the magnetic free layer, and allowing for two-bit storage and efficient power dissipation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If spin-orbit torque material is formed on top of magnetic free layer, then spin-orbit torque control is achieved, but magnetic free property is damaged or destroyed
Solution Approach 1:
The patent inverts the conventional stacking order by placing the spin-orbit torque material below the magnetic free layer instead of above it. This allows the magnetic free layer to maintain its properties while still receiving spin-orbit torque control from the underlying material, effectively resolving the contradiction between achieving torque control and preserving magnetic freedom.
Solution Approach 2:
The patent introduces a non-magnetic conductive layer as an intermediary between the spin-orbit torque material and the magnetic free layer. This intermediate layer facilitates the transmission of spin-orbit torque while protecting the magnetic free layer from direct contact that would damage its properties, thus enabling both torque control and magnetic property preservation.
2Ease of operation
If vertical wire-access to non-magnetic conductive layer is obtained, then current injection is improved, but device structure becomes complex
Solution Approach 1:
The non-magnetic conductive layer serves multiple functions: it acts as an electrode for current injection, provides a platform for forming spin-orbit torque material, and maintains electrical connectivity between layers. This multi-functionality eliminates the need for additional access structures, improving current injection while avoiding increased device complexity.
3Device complexity
If single bit storage is implemented, then device structure is simple, but storage capacity and bit density are limited
Solution Approach 1:
The patent combines two magnetic tunnel junctions into a single vertical stack, allowing two bits of information to be stored in one device structure. This merging approach doubles the storage capacity without proportionally increasing device complexity, as both junctions share common layers and can be accessed through the same electrodes.
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 solution enables efficient writing and reading of two bits independently, improving endurance and reducing power dissipation, while simplifying the device structure and reducing manufacturing costs, and is suitable for machine learning applications with in-memory computing capabilities.
Implementation Method 1
a direction of magnetization of the magnetic free layer of the first magnetic tunnel junction is controlled by the spin-orbit torque exerted by a current flowing between the end portions of a first cell electrode
Implementation Method 2
a direction of magnetization of the magnetic free layer of the third magnetic tunnel junction is controlled by spin-transfer torque exerted by a current flowing through it
Implementation Method 3
non-volatile magnetic memory devices, in particular to those based on spin-orbit and spin-transfer torque, and their use in magneto-resistive memory storage technology
Data Source
Figure 1~2
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Figure 5
AI summary
A material layer stack, a non-volatile memory device comprising the stack, and arrays thereof are described. The material layer stack (11; 21; 31) comprises a first and a second magnetic tunnel junction (14, 15) and a first top electrode (13a) formed on a top face (17b) of the stack. A shoulder (18a) is formed on a lateral face of the stack and divides the stack into a lower portion and an upper portion, wherein a tunnel barrier (14b) of the first magnetic tunnel junction is comprised by the lower stack portion and a tunnel barrier (15b) of the second magnetic tunnel junction by the upper stack portion. A second top electrode (13b) is formed on the shoulder. Each magnetic tunnel junction is adapted to store a bit as a reconfigurable magnetoresistance of its magnetic electrodes (14a, 14c, 15a, 15c). Preferably, a bottom face (17a) of the stack is connected to a conductor (12) supporting current induced magnetic polarization switching for the first magnetic tunnel junction by spin-orbit torque; magnetic polarization switching for the second magnetic tunnel junction is preferably achieved by spin-transfer torque.