Spin Valve MRAM with Cutouts for Unipulse Switching
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Solution Overview
Problem
Current magnetic memory technologies face challenges in achieving small cell areas and high-density multivalue recording due to issues with cell size, rewrite cycles, and crosstalk in existing MRAM designs, particularly with the one diode-one MTJ configuration and current-induced magnetization switching methods.
Innovation Solution
A magnetic memory element with a spin valve structure, including a free layer, pinning layer, and a nonmagnetic layer, utilizing cutouts and a magnetic change layer with temperature-dependent magnetization, allows for unipolar electrical switching and multivalue recording by generating a vortex magnetization mode without requiring separate magnetic field wiring, using unipolar electric pulses to control resistance states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If magnetic-field rewrite type MRAM is used, then nonvolatile storage is achieved, but cell area becomes large (20 to 30 F2)
Solution Approach 1:
The patent replaces the magnetic-field rewrite mechanism with a current-induced magnetization switching mechanism. By injecting spin-polarized current through the MTJ, the free layer's magnetization is switched via spin transfer torque, eliminating the need for external magnetic field wiring and reducing cell area to 6 to 8 F2 while maintaining nonvolatile storage capability.
Solution Approach 2:
The invention extracts and removes the magnetic field generation components (separate magnetic field wiring) from the memory cell structure. The magnetization switching is achieved purely through current injection via the MTJ, simplifying the cell structure and reducing the footprint to make it comparable to DRAM cell sizes.
2Area of stationary object
If one diode-one MTJ configuration is used, then cell area is reduced to 4 F2, but crosstalk and disturbance occur during write, erase, and read operations
Solution Approach 1:
The patent introduces asymmetry in the spin valve structure by providing cutouts at specific positions (e.g., only at the north or south position) in the free layer or nonmagnetic layer. This asymmetric structure creates a preferred magnetization switching direction, enabling selective switching of the targeted MTJ while preventing unintended switching of adjacent cells, thus eliminating crosstalk in the one diode-one MTJ configuration.
Solution Approach 2:
The invention applies local quality by creating position-specific structural features (cutouts) in the spin valve structure. The cutouts are placed at specific angular positions to create localized magnetic anisotropy, ensuring that only the intended MTJ responds to the applied current while neighboring MTJs remain unaffected, thereby preventing disturbance during operations.
3Use of energy by moving object
If current-induced magnetization switching is used, then write energy is reduced through scaling, but switching control becomes difficult without separate magnetic field wiring
Solution Approach 1:
The patent makes the MTJ structure multi-functional by using it for both data storage and magnetization switching. The same current path that reads the memory also performs the write operation through spin transfer torque, eliminating the need for separate magnetic field wiring while maintaining easy control through standard CMOS-compatible current drivers.
Solution Approach 2:
The asymmetric spin valve structure acts as an intermediary that translates isotropic current injection into anisotropic magnetization switching. The cutout structure mediates between the applied current and the free layer magnetization, providing deterministic switching control without requiring external magnetic fields, thus simplifying the control circuitry.
4Ease of operation
If spin precession method is used, then unipolar pulse switching is achieved, but error susceptibility increases due to variation in element shapes and pulse widths
Solution Approach 1:
The patent changes the structural parameters of the spin valve by introducing cutouts with specific geometries and positions. This structural modification creates a dominant magnetic anisotropy direction that makes the switching process less sensitive to variations in pulse width and element shape, thereby reducing error susceptibility while maintaining unipolar pulse switching capability.
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 enables a cell area comparable to DRAM and flash memory, achieving high-density multivalue recording with reduced error susceptibility and simplified circuitry, facilitating the integration of high-performance nonvolatile storage devices at lower costs.
Implementation Method 1
magnetic change layer, arranged so as to sandwich the separate nonmagnetic layer together with the free layer, and having magnetic characteristics which change according to temperature
Implementation Method 2
a magnetic field generated from the magnetic change layer by a driving method described below, which is a magnetic field comprising an in-plane component parallel to or antiparallel to a cutout and a perpendicular component
Implementation Method 3
due to the fact that there is a cutout with a different shape, a vortex portion is generated in the magnetization of the free layer by spin injection from the pinning layer
Implementation Method 4
a vortex portion is generated in the magnetization of the free layer
Implementation Method 5
MTJs (magnetic tunneling junctions) using MgO tunnel barrier film, for which a magnetoresistive ratio of 200% or higher can easily be obtained
Data Source
AI summary
In accordance with one aspect of the invention, a magnetic memory element records information in a spin valve structure having a free layer, a pinning layer, and a nonmagnetic layer sandwiched therebetween. The magnetic memory element further has, on the free layer, a separate nonmagnetic layer and a magnetic change layer having magnetic characteristics which change according to temperature. Multiple cutouts, including one cutout with a different shape, are provided in a peripheral portion of the spin valve structure. A method of driving the magnetic memory element is characterized in that information is recorded by applying unipolar electric pulses.