Multilayered Free Ferromagnetic Layer for MRAM Stability
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Solution Overview
Problem
Magnetic memory devices, such as MTJs, face challenges in maintaining the stability of the magnetization direction of free ferromagnetic layers due to thermal fluctuations and external disturbances as the size of the cells decreases, leading to potential data loss.
Innovation Solution
Incorporating a magnetic biasing layer that is magnetically coupled to the free ferromagnetic layer to increase its coercivity, allowing the magnetization direction to be changed while maintaining stability against thermal fluctuations and external fields, without requiring asymmetric cell shapes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the size of MTJ cell is reduced to increase storage capacity, then the number of MTJ cells per wafer area increases, but the magnetization direction becomes increasingly sensitive to thermal fluctuations and external field disturbances
Solution Approach 1:
The free ferromagnetic layer is divided into multiple sublayers (first free ferromagnetic layer and second free ferromagnetic layer) separated by a nonmagnetic spacer layer. Each sublayer has its own magnetization direction that can be independently controlled, allowing the system to maintain stability through distributed magnetic moments while keeping the overall cell size small for high storage capacity.
Solution Approach 2:
The patent uses a composite structure combining ferromagnetic layers with different magnetic properties (CoFeB, CoFe, CoFeNi) and a nonmagnetic spacer layer (Ru). This composite approach allows optimization of each layer's contribution to magnetic stability while maintaining small cell dimensions, resolving the contradiction between high storage capacity and magnetization stability.
2Reliability
If a magnetic biasing layer is added to increase coercivity of the free layer, then thermal stability is improved, but device complexity increases
Solution Approach 1:
The nonmagnetic spacer layer serves multiple functions: it provides exchange coupling between the two ferromagnetic layers to enable independent magnetization control, acts as a diffusion barrier, and contributes to the overall structural stability. This multi-functionality reduces the need for additional specialized layers, minimizing device complexity while achieving thermal stability.
Solution Approach 2:
The patent optimizes the thickness of each layer (first free ferromagnetic layer: 3-7 nm, nonmagnetic spacer: 1-3 nm, second free ferromagnetic layer: 3-7 nm) and the composition parameters to achieve the desired coercivity and thermal stability. By carefully controlling these parameters, the system achieves improved reliability without requiring excessive structural complexity.
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 magnetic biasing layer enhances the thermal stability of the free layer, enabling high-density magnetic memory devices with low power consumption and fast switching, compatible with standard CMOS fabrication processes.
Implementation Method 1
a magnetic biasing layer formed to be in contact with and magnetically coupled to the free ferromagnetic layer to increase coercivity of the free ferromagnetic layer
Implementation Method 2
when the thickness of the insulator is sufficiently thin, e.g., a few nanometers or less, electrons in the two ferromagnetic layers can 'penetrate' through the thin layer of the insulator due to a tunneling effect under a bias voltage applied to the two ferromagnetic layers across the barrier layer
Implementation Method 3
the resistance to the electrical current across the MTJ structure varies with the relative direction of the magnetizations in the two ferromagnetic layers... The magnitude of this effect is commonly characterized by the tunneling magnetoresistance (TMR)
Implementation Method 4
by a current flowing across the MTJ based on the spin transfer effect... the current required for changing the magnetization of the free layer can be small (e.g., 0.1 mA or lower) and can be significantly less than the current used for the field switching
Data Source
AI summary
Magnetic multilayer structures, such as magnetic or magnetoresistive tunnel junctions (MTJs) and spin valves, having a magnetic biasing layer formed next to and magnetically coupled to the free ferromagnetic layer to achieve a desired stability against fluctuations caused by, e.g., thermal fluctuations and astray fields. Stable MTJ cells with low aspect ratios can be fabricated using CMOS processing for, e.g., high-density MRAM memory devices and other devices, using the magnetic biasing layer. Such multilayer structures can be programmed using spin transfer induced switching by driving a write current perpendicular to the layers. Each free ferromagnetic layer can include two or more layers and may be a multilayered free ferromagnetic stack that includes first and second ferromagnetic layers and a non-magnetic spacer between the first and second ferromagnetic layers.