Spin-Orbit Torque Memory Bits With Lateral Asymmetry
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Solution Overview
Problem
Current spintronic devices require an external magnetic field to switch magnetization in perpendicularly magnetized films, which reduces thermal stability and scalability, making them unsuitable for high-density memory applications.
Innovation Solution
Introducing lateral structural asymmetry in NM/F/I heterostructures, which breaks mirror symmetry and generates a current-induced spin-orbit torque that allows for zero-field switching of perpendicular magnetization, eliminating the need for external magnetic fields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an external magnetic field is applied to switch magnetization in perpendicularly magnetized films, then magnetization switching is achieved, but thermal stability is reduced and scalability is limited
Solution Approach 1:
The patent extracts and eliminates the external magnetic field component from the magnetization switching process. By using spin-orbit torque generated within the device structure itself, the solution removes the need for external field application, thereby maintaining thermal stability while achieving deterministic switching.
Solution Approach 2:
The patent introduces a non-magnetic metal layer with strong spin-orbit coupling as an intermediary between the current and the ferromagnetic layer. This intermediary converts charge current into spin current via the spin Hall effect or Rashba effect, which then exerts torque on the magnetization without requiring external magnetic fields.
2Reliability
If spin transfer torque is used to reorient magnetization, then magnetization switching is achieved, but current density is too high causing rapid aging of tunnel barriers
Solution Approach 1:
The patent substitutes the direct spin transfer torque mechanism with a spin-orbit torque mechanism. Instead of relying on high current density to transfer spin angular momentum directly across the tunnel barrier, the solution uses spin-orbit coupling in a non-magnetic layer to generate the necessary torque, thereby reducing current density and preventing tunnel barrier degradation.
3Reliability
If conventional STT-MRAM devices are used, then magnetization switching is achieved, but tunnel barriers experience rapid aging from high writing current densities
Solution Approach 1:
The patent introduces a non-magnetic metal layer with strong spin-orbit coupling as an intermediary that protects the tunnel barrier from high current density. The write current flows through this intermediary layer rather than directly through the tunnel barrier, generating spin-orbit torque to switch the magnetization while preserving the tunnel barrier integrity.
4Device complexity
If lateral structural asymmetry is introduced to enable zero-field switching, then deterministic switching without external fields is achieved, but device structure becomes more complex
Solution Approach 1:
The patent deliberately introduces lateral structural asymmetry in the form of a wedge-shaped non-magnetic layer with varying thickness. This asymmetry creates a gradient in spin-orbit coupling strength across the device, which generates a net spin current that produces deterministic switching without external magnetic fields. The asymmetry is engineered to provide the necessary torque while maintaining manufacturability.
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 deterministic switching of perpendicular magnetization without external magnetic fields, reducing the write current density and improving energy efficiency and scalability for SOT-based devices like SOT-MRAM.
Implementation Method 1
Spin current generation may arise from the metal layer by the spin Hall effect
Implementation Method 2
Spin current generation may arise from the metal layer by the spin Hall effect or it may arise at the interface by current induced spin polarization (the Rashba-Edelstein effect)
Implementation Method 3
Introducing lateral structural asymmetry in NM/F/I heterostructures, which breaks mirror symmetry and generates a current-induced spin-orbit torque that allows for zero-field switching of perpendicular magnetization
Implementation Method 4
at least one ferromagnetic layer having magnetization with two stable states perpendicular to a layer plane
Data Source
AI summary
A basic Spin-Orbit-Torque (SOT) structure with lateral structural asymmetry is provided that produces a new spin-orbit torque, resulting in zero-field current-induced switching of perpendicular magnetization. More complex structures can also be produced incorporating the basic structure of a ferromagnetic layer with a heavy non-magnetic metal layer having strong spin-orbit coupling on one side, and an insulator layer on the other side with a structural mirror asymmetry along the in-plane direction. The lateral structural asymmetry and new spin-orbit torque, in effect, replaces the role of the external in-plane magnetic field. The direction of switching is determined by the combination of the direction of applied current and the direction of symmetry breaking in the device.