SOT-MRAM Cell Design with Shared Current Paths
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Solution Overview
Problem
Magnetic memories, specifically SOT-MRAMs, face challenges in minimizing the area occupied by memory cells due to the need for separate read and write current paths, leading to increased cell size and difficulty in maintaining sufficient magnetic anisotropy energy as magnetoresistive elements are miniaturized.
Innovation Solution
The design incorporates a structure with obliquely arranged nonmagnetic layers and shared write transistors connected to different bit and word lines, utilizing vertical transistors and magnetoresistive elements with in-plane crystal magnetic anisotropy to reduce cell size and enhance write efficiency while maintaining memory retention energy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If separate read and write current paths are provided in SOT-MRAM, then write efficiency is improved, but the area occupied by memory cells increases
Solution Approach 1:
The patent merges the read and write current paths by having both operations share the same current path through the magnetoresistive element. The write current flows through the nonmagnetic layer to generate spin-orbit torque, while the read current flows through the magnetoresistive element to detect resistance changes. This integration eliminates the need for completely separate paths while maintaining functional distinction between read and write operations.
Solution Approach 2:
The nonmagnetic layer serves multiple functions: it acts as a spin current source for write operations through spin-orbit interaction, and as part of the magnetoresistive element for read operations. The same current path structure is used for both reading and writing, making the system multi-functional and reducing the number of dedicated components needed.
2Area of stationary object
If magnetoresistive elements are miniaturized to reduce cell size, then area is reduced, but magnetic anisotropy energy becomes insufficient
Solution Approach 1:
The patent changes the magnetic anisotropy parameter from perpendicular magnetic anisotropy to in-plane crystal magnetic anisotropy. This parameter change allows the magnetoresistive element to maintain sufficient magnetic anisotropy energy even when miniaturized, as in-plane anisotropy scales better with reduced dimensions. The easy axis of magnetization is reoriented to lie in the plane of the storage layer, parallel to the film surface.
Solution Approach 2:
The patent introduces local quality by creating specific crystallographic orientations and material compositions in the storage layer to generate in-plane crystal magnetic anisotropy. The storage layer is designed with specific crystal structures that provide strong in-plane anisotropy, allowing stable magnetization at smaller sizes. This local structural optimization enables miniaturization without sacrificing magnetic stability.
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 effectively minimizes the area occupied by memory cells and maintains high memory retention energy, even in smaller sizes, by optimizing the arrangement of transistors and magnetoresistive elements, thus addressing the challenge of scaling down SOT-MRAMs.
Implementation Method 1
A spin-orbit interaction is a phenomenon in which current is applied to a nonmagnetic layer so that electrons having spin angular momenta (hereinafter also referred to simply as the spin) of the opposite orientations from each other are scattered in the opposite directions, and a spin current Is is generated.
Implementation Method 2
Another one of the known techniques for writing is a technique using a spin Hall effect or a spin-orbit interaction (spin-orbit coupling).
Implementation Method 3
It is possible to determine whether the magnetization directions of the reference layer and the storage layer are parallel or whether the magnetization directions are antiparallel, using a magnetoresistive effect.
Data Source
AI summary
A magnetic memory of an embodiment includes: a first nonmagnetic layer including a first and second faces; a first and second wirings disposed on a side of the first face; a third wiring disposed on a side of the second face; a first transistor, one of the source and the drain being connected to the first wiring, the other one being connected to the first nonmagnetic layer; a second transistor, one of source and drain being connected to the second wiring, the other one being connected to the first nonmagnetic layer; a magnetoresistive element disposed between the first nonmagnetic layer and the third wiring, a first terminal of the magnetoresistive element being connected to the first nonmagnetic layer; and a third transistor, one of source and drain of the third transistor being connected to the second terminal, the other one being connected to the third wiring.