Heusler Alloy Spin-Transfer-Torque Magnetic Tunnel Junctions
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Solution Overview
Problem
Current magnetic random access memory (MRAM) devices using magnetic tunnel junctions (MTJs) face challenges in achieving high tunnel magnetoresistance (TMR) and low switching current, especially as device sizes scale down.
Innovation Solution
The development of Heusler alloy-based spin-transfer-torque magnetic tunnel junctions, which include a substrate, seed layer, nitride layer, templating layer, magnetic layer with perpendicular magnetic anisotropy (PMA), tunnel barrier, and additional magnetic layer, facilitating multi-state storage for flash memory applications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional MRAM devices use standard MTJ structures with ferromagnetic layers, then the device structure is simple, but the tunnel magnetoresistance (TMR) is insufficient and switching current is high
Solution Approach 1:
The patent employs Heusler alloy compounds (such as Co2MnSi, Co2FeAl) as magnetic layers in the MTJ structure. These composite intermetallic materials with specific stoichiometric compositions provide enhanced spin polarization and TMR ratios compared to conventional ferromagnetic alloys, directly addressing the insufficient TMR problem while maintaining a layered composite structure
Solution Approach 2:
The patent introduces perpendicular magnetic anisotropy (PMA) by modifying the magnetic layer composition and interface structure. This parameter change from in-plane to perpendicular magnetization orientation enables smaller switching currents and higher TMR ratios, resolving the contradiction between reliability and device complexity through fundamental magnetic property modification
2Use of energy by moving object
If the magnetization is oriented in-plane, then the device structure is simpler, but the switching current is high
Solution Approach 1:
The patent changes the magnetic anisotropy parameter from in-plane to perpendicular orientation through careful selection of Heusler alloy composition and interface engineering. This parameter change reduces the switching current by enabling more efficient spin transfer torque action, directly resolving the energy consumption issue
Solution Approach 2:
The patent creates perpendicular magnetic anisotropy through specific interface structures between the Heusler magnetic layer and adjacent layers (such as oxide barriers or metal underlayers). This local interface quality enhancement provides the necessary PMA without requiring complex bulk material modifications, balancing energy efficiency with structural simplicity
3Productivity
If device size is scaled down for higher density, then the storage capacity increases, but the TMR ratio decreases and switching becomes unreliable
Solution Approach 1:
The patent uses Heusler alloy compounds with inherent perpendicular magnetic anisotropy that maintain stable magnetic properties at reduced dimensions. These composite materials provide robust spin polarization even in scaled-down devices, ensuring reliable switching and high TMR ratios at smaller sizes, thus enabling higher storage density without sacrificing reliability
Solution Approach 2:
The patent maintains perpendicular magnetic anisotropy through composition optimization of the Heusler layers, which provides size-independent magnetic stability. This parameter change ensures that switching reliability and TMR ratio remain high even as device dimensions are reduced for increased storage density
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 enables the creation of MTJ devices with reproducible switching between multiple states, improved stability, and reduced switching currents, making them suitable for multibit flash memory applications.
Implementation Method 1
The magnetic state of one of the layers is switched using Spin Transfer Torque (STT). The magnetic state of the MTJ is changed by passing a current through it. The current delivers spin angular momentum, so that once a threshold current is exceeded, the direction of the memory layer moment is switched.
Implementation Method 2
The magnetic layers have magnetization perpendicular to the film surface (i.e. have perpendicular magnetic anisotropy (PMA)) as smaller switching currents are needed than for in-plane magnetized MTJs.
Implementation Method 3
The resistance changes based on the magnetic orientation of the two magnetic layers, and the relative change in resistance is referred to as the tunnel magnetoresistance (TMR), which is related to the spin polarization.
Data Source
AI summary
A memory array includes a word lines intersecting bit line-complementary bit line pairs at a plurality of cell locations. Magnetic tunnel junction cells are located at each location, and each cell is electrically connected to a corresponding bit line and selectively interconnected to a corresponding one of the complementary bit lines under control of a corresponding one of the word lines. Each cell includes a substrate; a seed layer overlying the substrate; a nitride layer, overlying the seed layer, and having a thickness greater than 5 Angstroms; a templating layer, outward of the nitride layer, including a binary alloy having an alternating layer lattice structure, and having a thickness greater than 50 Angstroms; a magnetic layer overlying the templating layer, including a Heusler compound and exhibiting PMA; a tunnel barrier outward of the magnetic layer; and a magnetic layer outward of the tunnel barrier.