Permanent Magnet With Antiferromagnetic Layer and Ferromagnetic Sublayers
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Solution Overview
Problem
Permanent magnets used in magnetic-field sensors are sensitive to heat treatments, leading to significant modifications in their magnetic properties, particularly a substantial decrease in the Hex/Hc ratio, which affects their thermal stability.
Innovation Solution
A permanent magnet design incorporating a stack of ferromagnetic and antiferromagnetic layers with sublayers made of specific ferromagnetic materials (T1 and T2) that are thermally stable, where T1 has a strong exchange field but is thermally unstable, and T2 has a stable coercive field but weaker exchange field, improving thermal stability by optimizing the crystal structure and deposition rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If a permanent magnet is formed by a stack of ferromagnetic layers and antiferromagnetic layers to generate a strong magnetic field, then the magnetic field strength is improved, but the magnet becomes sensitive to heat treatments causing substantial decrease in the Hex/Hc ratio
Solution Approach 1:
The ferromagnetic layer is segmented into multiple sublayers with different materials (CoFeB, CoFe, CoFeNi) and different thicknesses. Each sublayer contributes differently to the magnetic properties, allowing the overall structure to maintain strong magnetic field while improving thermal stability through the combined effect of various materials with complementary properties.
Solution Approach 2:
The patent uses composite ferromagnetic materials consisting of multiple sublayers with different compositions (CoFeB, CoFe, CoFeNi) stacked together. This composite structure combines the advantages of different materials: CoFeB provides strong exchange coupling, while CoFe and CoFeNi sublayers contribute to thermal stability and coercive field maintenance at high temperatures.
2Force
If the first sublayer thickness is increased to improve exchange coupling with the antiferromagnetic layer, then the exchange field Hex is improved, but the magnetization direction control becomes more difficult and thermal stability decreases
Solution Approach 1:
Different sublayers are assigned different local qualities: the first sublayer (CoFeB) with thickness 2-5 nm provides strong exchange coupling locally at the interface with the antiferromagnetic layer, while subsequent sublayers (CoFe, CoFeNi) with larger thicknesses provide bulk magnetic stability and thermal resistance. This local differentiation allows simultaneous optimization of exchange coupling and thermal stability.
Solution Approach 2:
The patent optimizes the thickness parameters of each sublayer within specific ranges: first sublayer 2-5 nm for strong exchange coupling, second sublayer 3-10 nm for magnetic stability, and third sublayer 2-8 nm for thermal protection. These parameter changes allow balancing exchange field strength and magnetization stability.
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 proposed design significantly reduces the degradation of magnetic properties during heat treatments, maintaining the strength of the magnetic field and squareness of the hysteresis cycle, thereby enhancing the thermal stability of the permanent magnet.
Implementation Method 1
a ferromagnetic layer, the magnetization direction of the ferromagnetic layer being set by exchange coupling to the antiferromagnetic layer
Implementation Method 2
a first sublayer making contact with the antiferromagnetic layer or solely separated from the antiferromagnetic layer by an intermediate ferromagnetic sublayer the thickness of which is smaller than 2 nm
Data Source
AI summary
A permanent magnet comprising an antiferromagnetic layer and a ferromagnetic layer having a first sub-layer made of a first type of ferromagnetic material, the first type of ferromagnetic material being an at least partially crystallized alloy of iron and cobalt, and a second sub-layer made of a second type of ferromagnetic material, this second type of ferromagnetic material also being an alloy of iron and cobalt in which the proportion of face-centered cubic crystals is less than the proportion of face-centered cubic crystals in the first type of ferromagnetic material.