Mu‑metal is a commercially produced nickel–iron alloy known for its exceptionally high magnetic permeability, which makes it especially effective for shielding sensitive electronic equipment from low‑frequency magnetic fields. The material is classified as a soft ferromagnetic alloy because it can be magnetized and demagnetized easily, with minimal hysteresis loss.
Composition and Structure
Typical formulations of mu‑metal contain approximately 77 % nickel, 16 % iron, 5 % copper, and 2 % chromium, although exact percentages may vary among manufacturers. The alloy is engineered to have a highly ordered crystalline structure after a specialized annealing process, which reduces internal stresses and maximizes its magnetic permeability. The presence of copper and chromium contributes to the alloy’s corrosion resistance and mechanical stability.
Magnetic Properties
Mu‑metal exhibits a relative magnetic permeability (µ_r) in the range of 20 000 to 100 000, far exceeding that of ordinary steel (µ_r ≈ 100–200). Its low coercivity (typically less than 0.1 A m⁻¹) and low magnetic loss make it suitable for applications where weak magnetic fields must be attenuated without introducing significant distortion.
Manufacturing and Treatment
The alloy is usually supplied as rolled sheets, foils, or thin strips. After shaping, mu‑metal undergoes a high‑temperature annealing step (approximately 1100 °C) in a hydrogen atmosphere, followed by a controlled cooling regimen in the presence of a weak magnetic field. This annealing relieves mechanical stresses introduced during fabrication and aligns the magnetic domains, thereby enhancing permeability.
Applications
- Magnetic shielding – Enclosures, housings, and feedthroughs for scientific instruments (e.g., magnetometers, superconducting quantum interference devices – SQUIDs), medical imaging equipment (MRI), and precision electronic components are often fabricated from mu‑metal to reduce ambient magnetic interference.
- Magnetic shielding in laboratories – Mu‑metal is employed to line walls of rooms or within cryogenic environments where external magnetic fields could affect experimental results.
- Cable shielding – Conductors for low‑frequency or DC power lines may be wrapped in mu‑metal to limit magnetic field emission.
- Magnetic field control – In certain designs, mu‑metal components are used to shape or concentrate magnetic fields, such as in magnetic‑field shunting or return paths in electric motors and transformers.
Limitations
While mu‑metal provides superior low‑frequency shielding, its effectiveness diminishes at higher frequencies (above a few megahertz) where eddy‑current losses become significant. In such regimes, alternatives such as high‑conductivity copper or specialized ferrite materials are preferred. Additionally, the alloy’s high permeability can be degraded by mechanical deformation; any bending, cutting, or welding typically necessitates re‑annealing to restore performance.
Historical Development
The term “mu‑metal” originated from the Greek letter μ (mu), representing magnetic permeability. The alloy was first developed in the 1930s by the Westinghouse Electric Corporation for use in nuclear instrumentation. Over subsequent decades, its production was refined by several specialty metal firms, and the commercial name “Mu‑metal” is now a registered trademark of the specialty alloys division of the corporation formerly known as International Nickel Company (INCO), currently part of the Carpenter Technology Corporation.
Physical Data (representative values)
| Property | Approximate Value |
|---|---|
| Density | 8.7 g cm⁻³ |
| Electrical Resistivity | 0.60 µΩ cm (room temperature) |
| Curie Temperature | ~ 600 °C |
| Relative Permeability (µ_r) | 20 000 – 100 000 (post‑anneal) |
| Tensile Strength | 250–350 MPa (varies with processing) |
Standards and Specifications
Mu‑metal products are often supplied in accordance with ASTM B150 (Standard Specification for Nickel–Iron Alloys for Magnetic Shielding) and IEC 60384‑9 (Electromagnetic Shielding Materials). Compliance with these standards ensures consistent magnetic performance across batches.
Safety and Handling
The alloy is non‑toxic and chemically stable under normal atmospheric conditions. However, during high‑temperature annealing, appropriate protective equipment is required to handle hot gases and prevent exposure to hydrogen, which is used as a reducing atmosphere.
See also
- Soft magnetic material
- Magnetic shielding
- Permalloy
- Superconducting quantum interference device (SQUID)
References
- S. R. L. Bramwell, “Magnetic Shielding Materials,” Journal of Applied Physics, vol. 68, no. 2, 1990, pp. 823‑828.
- D. J. Griffiths, Introduction to Electrodynamics, 4th ed., Pearson, 2017, Chapter 7.
- Carpenter Technology Corporation, “Mu‑metal Product Data Sheet,” 2022.
- ASTM International, “Standard Specification for Nickel–Iron Alloys for Magnetic Shielding (ASTM B150),” 2021 edition.