Magnetoresistive Element Spin-Transfer Torque Write Mechanism
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Solution Overview
Problem
Conventional magnetoresistive random access memory (MRAM) technologies face challenges in simultaneously reducing write current, overcoming thermal agitation, and minimizing cell area, especially as microfabrication advances, due to increased magnetic field requirements and thermal agitation issues.
Innovation Solution
A magnetoresistive element utilizing a spin-polarized current to reverse the magnetization of a magnetic material, with a layered structure including a pinned layer, a free layer, and nonmagnetic layers, where the saturation magnetization of the free layer is optimized to satisfy a specific relationship involving write current density, thickness, and magnetic anisotropy energy density, and optionally incorporating soft magnetic films to absorb leaked magnetic fields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If microfabrication progresses to reduce cell area, then cell size is reduced, but magnetic field requirement increases making current reduction difficult
Solution Approach 1:
The patent replaces the conventional field-write mechanism (using magnetic fields generated by current) with a spin-polarized current mechanism. This substitution allows direct manipulation of magnetization through spin transfer torque, eliminating the need for high magnetic fields and enabling both microfabrication and current reduction simultaneously
Solution Approach 2:
The patent changes the magnetization direction from in-plane to perpendicular magnetization. This parameter change enables the use of shape anisotropy and magnetocrystalline anisotropy to maintain stable magnetization at small dimensions while allowing spin-polarized current to efficiently reverse magnetization, thus supporting both miniaturization and low write current
2Reliability
If magnetic anisotropy energy density is increased to overcome thermal agitation, then thermal stability is improved, but reversal field increases making current reduction difficult
Solution Approach 1:
The patent replaces field-driven magnetization reversal with spin-polarized current-driven reversal. This substitution allows the system to overcome thermal agitation through enhanced spin polarization rather than increasing magnetic field strength, thus maintaining thermal stability while reducing write current
Solution Approach 2:
The patent employs composite magnetic layer structures with specific materials (CoFeB, CoFe, etc.) that provide both high magnetocrystalline anisotropy for thermal stability and high spin polarization for efficient current-driven reversal. The composite structure optimizes both thermal stability and reversal efficiency simultaneously
3Use of energy by moving object
If yoke structure is used to concentrate magnetic field and reduce current, then write current is reduced, but cell area increases
Solution Approach 1:
The patent eliminates the need for yoke structures by substituting field-write with spin-polarized current write. This substitution removes the magnetic circuit components entirely, enabling cell area reduction while maintaining low write current through direct spin transfer torque
Solution Approach 2:
The patent extracts and removes the yoke structure from the MTJ cell design. By eliminating this external magnetic field concentration component, the cell area can be minimized while the spin-polarized current mechanism provides sufficient reversal capability without requiring field concentration
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 efficient microfabrication with reduced write current and cell size, while maintaining stability against thermal agitation, allowing for high-density MRAM implementation, such as 256 Mbits or more, without increasing reversal current density.
Implementation Method 1
magnetization reversal by a spin polarized current is theoretically predicted and also confirmed by experiments
Implementation Method 2
the magnetization of a magnetic material can be reversed only by flowing a spin polarized current to the magnetic material
Implementation Method 3
A magnetoresistive random access memory (MRAM) using a tunneling magnetoresistive (TMR) effect is characterized in storing data in accordance with the magnetization state of a magnetic tunnel junction (MTJ) element
Implementation Method 4
They basically employ a field write scheme of reversing the magnetization direction of a magnetic layer by using a magnetic field generated by a current
Data Source
AI summary
A magnetoresistive element which records information by supplying spin-polarized electrons to a magnetic material, includes a first pinned layer which is made of a magnetic material and has a first magnetization directed in a direction perpendicular to a film surface, a free layer which is made of a magnetic material and has a second magnetization directed in the direction perpendicular to the film surface, the direction of the second magnetization reversing by the spin-polarized electrons, and a first nonmagnetic layer which is provided between the first pinned layer and the free layer. A saturation magnetization Ms of the free layer satisfies a relationship 0≦Ms<√{square root over ( )}{Jw/(6πAt)}. Jw is a write current density, t is a thickness of the free layer, A is a constant.