STT MRAM Free Layer Stack With Spacers for Low Switching Current
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Solution Overview
Problem
Existing STT MRAM devices face challenges in reducing the critical switching current density while maintaining thermal stability, due to the trade-offs in film thickness, damping constant, and saturation magnetization.
Innovation Solution
The STT magnetoresistive memory device incorporates a free layer stack with a total thickness of less than 2 nm, comprising a proximal ferromagnetic layer, non-magnetic metal sub-monolayers, an intermediate ferromagnetic layer, and a distal ferromagnetic layer, to minimize increases in damping and reduce saturation magnetization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the free layer thickness is reduced to lower critical switching current density, then switching current density decreases, but thermal stability deteriorates
Solution Approach 1:
The free layer is segmented into multiple thin ferromagnetic layers (first, second, and third ferromagnetic layers) separated by nonmagnetic metal spacers. This segmentation allows each layer to contribute to the overall magnetic moment while maintaining a total thickness less than 2 nm, achieving low switching current density. The segmented structure also enables independent optimization of each layer's properties to maintain thermal stability through enhanced perpendicular magnetic anisotropy at the interfaces.
Solution Approach 2:
The patent employs a composite structure combining ferromagnetic layers with nonmagnetic metal spacers (such as ruthenium, rhodium, or iridium). This composite architecture creates perpendicular magnetic anisotropy at the ferromagnetic/nonmagnetic interfaces, which enhances thermal stability while keeping the total free layer thickness minimal. The nonmagnetic spacers prevent magnetic coupling between ferromagnetic layers while contributing to the anisotropy effect.
2Reliability
If perpendicular magnetic anisotropy is enhanced by adding nonmagnetic metal layers, then thermal stability improves, but damping constant increases
Solution Approach 1:
The nonmagnetic metal spacers are strategically positioned only at specific interfaces within the free layer stack, creating localized perpendicular magnetic anisotropy where needed. The spacers are placed between ferromagnetic layers to generate interface anisotropy, while the bulk ferromagnetic layers maintain their original magnetic properties with lower damping. This localized approach enhances thermal stability without uniformly increasing damping across the entire free layer.
Solution Approach 2:
The patent optimizes the thickness and material composition of nonmagnetic metal spacers to achieve the desired perpendicular magnetic anisotropy while minimizing damping increases. By controlling the spacer thickness to be very thin (sub-nanometer scale) and selecting specific materials (Ru, Rh, Ir), the interface anisotropy is maximized while the negative impact on damping is minimized. The ferromagnetic layer compositions (CoFeB, CoFe) are also tuned to balance anisotropy and damping properties.
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 enhances perpendicular magnetic anisotropy without increasing damping or degrading magnetic moment, resulting in improved thermal stability and reduced switching current density.
Implementation Method 1
Spin-transfer torque (STT) refers to an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve is modified by a spin-polarized current. When the spin-polarized current flows through a free layer of a magnetic tunnel junction or a spin valve, the electrons in the spin-polarized current can transfer at least some of their angular momentum to the free layer, thereby producing a torque on the magnetization of the free layer.
Implementation Method 2
A resistance differential of a magnetic tunnel junction between different magnetization states of the free layer can be employed to store data within the magnetoresistive random access memory (MRAM) cell depending if the magnetization of the free layer is parallel or antiparallel to the magnetization of the polarizer layer
Implementation Method 3
enhances perpendicular magnetic anisotropy without increasing damping or degrading magnetic moment, resulting in improved thermal stability
Data Source
AI summary
A spin-transfer torque (STT) magnetoresistive memory device includes a first electrode, a second electrode, and a magnetic tunnel junction located between the first electrode and the second electrode. The magnetic tunnel junction includes a reference layer having a fixed magnetization direction, a free layer stack, and a nonmagnetic tunnel barrier layer located between the reference layer and the free layer stack. The free layer stack has a total thickness of less than 2 nm, and contains in order, a proximal ferromagnetic layer located proximal to the nonmagnetic tunnel barrier layer, a first non-magnetic metal sub-monolayer, an intermediate ferromagnetic layer, a second non-magnetic metal sub-monolayer, and a distal ferromagnetic layer.