Multibit MRAM Cell With Synthetic Antiferromagnetic Storage Layer
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Solution Overview
Problem
As MRAM cell size decreases, the writing field amplitude requirements increase exponentially due to magnetostatic interactions between the sense layer and storage layer, making it difficult to align the storage magnetization away from the magnetocrystalline easy axis, especially with synthetic antiferromagnetic storage layers which have reduced magnetic moment and larger anisotropy energy.
Innovation Solution
A multibit MRAM cell design using a synthetic antiferromagnetic storage layer with a freely orientable sense magnetization, a tunnel barrier layer, and a storage coupling layer, where the sense magnetization induces a dipolar field above the spin-flop field to align the second storage magnetization at high temperature, allowing alignment in various orientations, and the antiferromagnetic storage layer is heated and cooled to pin and unpin the first storage magnetization, respectively, for low read and write field amplitudes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the MRAM cell size is decreased, then the storage density is improved, but the writing field amplitude requirements increase exponentially due to magnetostatic interactions
Solution Approach 1:
A non-magnetic spacer layer is introduced between the sense layer and storage layer to act as an intermediary that reduces magnetostatic interactions. This spacer layer with perpendicular magnetic anisotropy minimizes the coupling between layers, allowing smaller cell sizes without requiring exponentially higher writing field amplitudes.
Solution Approach 2:
The invention changes the magnetic anisotropy parameter from in-plane to perpendicular orientation in the sense layer. This parameter change modifies the magnetostatic interaction characteristics, reducing the writing field requirements even as cell size decreases, thereby enabling higher storage density without exponential increase in writing field amplitude.
2Force
If a synthetic antiferromagnetic storage layer is used, then the magnetostatic coupling is reduced and writing field amplitude is reduced, but the storage magnetization becomes very difficult to align away from the magnetocrystalline easy axis
Solution Approach 1:
A thermal field is applied preliminarily to heat the storage layer above its Curie temperature before applying the writing magnetic field. This preliminary heating action temporarily reduces the magnetic anisotropy energy barrier, enabling the storage magnetization to be aligned away from the magnetocrystalline easy axis with lower writing field amplitudes, while maintaining the benefits of reduced magnetostatic coupling.
Solution Approach 2:
The invention utilizes the phase transition of the storage layer from ferromagnetic to paramagnetic state by heating above the Curie temperature. This phase transition temporarily eliminates the magnetocrystalline anisotropy constraint, allowing easy alignment of storage magnetization with the writing field, after which cooling freezes the magnetization in the desired orientation.
3Stability of the object's composition
If the storage layer magnetization is pinned by an antiferromagnetic layer, then the storage magnetization stability is improved, but the writing field amplitude requirements increase
Solution Approach 1:
The invention changes the temperature parameter of the storage layer during writing operations. By temporarily increasing the temperature above the Curie point, the magnetic anisotropy and exchange coupling are reduced, allowing the pinned storage magnetization to be switched with lower field amplitudes. After writing, cooling restores the stable pinned state.
Solution Approach 2:
The invention introduces dynamic temperature control to the otherwise static pinned storage layer. The temperature is dynamically adjusted during write operations to temporarily reduce the pinning strength, enabling low-amplitude writing fields, while maintaining stable pinning during read and retain operations.
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
The solution enables scalability to low patterning dimensions with very low read and write field amplitudes, achieving maximum multibit capacity by allowing the second storage magnetization to be aligned in multiple orientations, thus reducing the writing field amplitude requirements and enhancing storage density.
Implementation Method 1
the sense magnetization is arranged for inducing a dipolar field having a magnitude above a spin-flop field of the storage layer and capable of aligning the second storage magnetization when the magnetic tunnel junction is at the high threshold temperature
Implementation Method 2
the first storage magnetization being pinned by an antiferromagnetic layer at a low threshold temperature of the magnetic tunnel junction and unpinned at a high threshold temperature of the magnetic tunnel junction
Implementation Method 3
heating the antiferromagnetic storage layer to the high threshold temperature such as to unpin the first storage magnetization
Implementation Method 4
cooling the antiferromagnetic storage layer to the low threshold temperature to freeze the first storage magnetization
Implementation Method 5
the sense magnetization is arranged for inducing a dipolar field having a magnitude above a spin-flop field of the storage layer
Data Source
Figure 1
Figure 2(a)~3
Figure 4(a)~4(d)
AI summary
The present disclosure concerns a multibit MRAM cell (1) comprising a magnetic tunnel junction (2) including a sense layer (21) having a freely orientable sense magnetization (211); a tunnel barrier layer (22), a synthetic antiferromagnet storage layer (23) having a first and second storage layer (231, 232); wherein the sense magnetization (211) induces a dipolar field (212) having a magnitude above a spin-flop field (HSF) of the storage layer (23); the MRAM cell (1) further comprising aligning means for aligning the sense magnetization (211) in a plurality of distinct orientations such as to encode a plurality of distinct logic states in the MRAM cell (1). The present disclosure also concerns a method for operating the multibit MRAM cell (1).