Magnetic Memory Data Writing via Spin Orbit Torque
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Solution Overview
Problem
Current magnetic random access memory (MRAM) writing methods, such as spin transfer torque (STT) and spin orbit torque (SOT), face challenges in device reliability, high write power consumption, and limited storage density due to high current requirements and shared read/write paths.
Innovation Solution
A data writing method for magnetic memory that combines spin transfer torque and spin orbit torque effects, using two currents to switch the magnetization direction of magnetic tunnel junctions, reducing current requirements and eliminating incubation delays, thereby improving reliability and storage density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If spin transfer torque (STT) is used for writing data in magnetic memory, then the write operation can be performed with a single bidirectional current, but the write current passes through the oxide barrier layer causing device breakdown and the read path shares the same path as write current, influencing both read and write performance
Solution Approach 1:
The patent segments the write current path from the read path by introducing a separate heavy metal layer for generating spin-orbit torque. The write current flows through the heavy metal layer rather than through the magnetic tunnel junction, while the read current continues to flow through the MTJ. This segmentation eliminates the shared path problem and prevents barrier breakdown during write operations.
Solution Approach 2:
The heavy metal layer acts as an intermediary that converts charge current into spin current via the spin Hall effect or Rashba effect. This intermediary mechanism allows the write operation to be performed without direct current flow through the oxide barrier layer, thus protecting the barrier from breakdown while still achieving magnetization switching in the adjacent ferromagnetic layer.
2Device complexity
If spin transfer torque (STT) is used for writing data, then the write operation can be performed with simple current structure, but the write operation is triggered by thermal fluctuations requiring long incubation delay, limiting write speed
Solution Approach 1:
The patent replaces the thermal fluctuation-based switching mechanism of STT with a deterministic spin-orbit torque mechanism. The spin-orbit torque provides a direct torque on the magnetization through the spin Hall effect or Rashba effect, eliminating the need for thermal activation and long incubation delays. This substitution enables faster and more reliable magnetization switching.
3Reliability
If spin orbit torque (SOT) is used for writing data, then the read path can be separated from the write path reducing barrier breakdown risk, but the memory cell requires three terminals and usually two access transistors, severely limiting storage density
Solution Approach 1:
The patent merges the SOT write mechanism with a simplified memory cell structure that uses only one access transistor per bit. The heavy metal layer is integrated alongside the MTJ, and the write current is applied through the heavy metal layer while the read current flows through the MTJ and access transistor. This merging achieves path separation for reliability while reducing the transistor count for high density.
4Reliability
If write current is increased to ensure reliable magnetization switching, then the write operation becomes more reliable, but the power consumption increases significantly
Solution Approach 1:
The heavy metal layer serves as an intermediary that amplifies the effectiveness of the write current through the spin Hall angle or Rashba effect. A small charge current flowing through the heavy metal layer generates a large spin current that exerts strong torque on the magnetization. This intermediary mechanism achieves reliable magnetization switching with much lower power consumption compared to direct current through the MTJ.
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 method reduces the risk of barrier breakdown, lowers write current, and enhances write speed and storage density by utilizing spin orbit torque to assist spin transfer torque, allowing for independent optimization of read and write performance.
Implementation Method 1
applying a current, called as a spin orbit torque write current, between the first bottom electrode and the second bottom electrode, producing a spin orbit torque, disturbing magnetization states of the first ferromagnetic metals of all the magnetic tunnel junctions through the spin orbit torque
Implementation Method 2
applying a current, called as a spin transfer torque write current, between the first bottom electrode and the Xth top electrode, or between the Xth top electrode and the second bottom electrode, producing a spin transfer torque, writing the data value based on a magnetization direction of the first ferromagnetic metal of the Xth magnetic tunnel junction under an action of the spin transfer torque
Implementation Method 3
a resistance of every magnetic tunnel junction is dependent on a magnetization direction of the first ferromagnetic metal and the second ferromagnetic metal; if the magnetization direction of the first ferromagnetic metal is as same as the magnetization direction of the second ferromagnetic metal, the resistance of every magnetic tunnel junction is smaller
Data Source
AI summary
A magnetic memory includes one or more magnetic tunnel junctions, a heavy metal or anti-ferromagnetic strip film, a first bottom electrode and a second bottom electrode. Every magnetic tunnel junction is located on the strip film and represents a memory cell; the first bottom electrode and the second bottom electrode are respectively connected with two ends of the heavy metal or anti-ferromagnetic strip film; every magnetic tunnel junction includes a first ferromagnetic metal, a first oxide, a second ferromagnetic metal, a first synthetic antiferromagnetic layer and an Xth top electrode from bottom to top in sequence, wherein X is a serial number of the memory cell. A data writing method combines spin orbit torque with spin transfer torque to write data, and respectively applies two currents to the magnetic tunnel junction and the heavy metal or anti-ferromagnetic strip film. Only one current is unable to complete data writing.