Magnetic field problems are deceptively expensive. A nearby transformer, an MRI fringe field bleeding into an adjacent suite, or DC traction current from a light-rail line can distort electron microscope images, corrupt sensitive measurements, or fail a sign-off inspection long after the building is finished. Unlike high-frequency RF interference, low-frequency magnetic fields pass straight through ordinary metal walls and most conductive paints as if they were not there — and the fix is rarely cheap once the structure is built.
Choosing the right EMF shielding materials is therefore a design decision, not a shopping decision. The correct material depends on the frequency of the offending field, its strength, and how much attenuation the application actually requires. This guide compares the workhorses of magnetic field shielding — silicon (electrical) steel, mu-metal and its cousins, copper for eddy-current control, and active compensation coils — so facility managers, lab directors, and architects can specify with confidence rather than discover the gap at commissioning.
Magnetic Shielding Is Not RF Shielding
The single most common and costly misconception in this field is treating all "shielding" as one problem. It is two physically distinct problems that happen to share a vocabulary.
High-frequency RF shielding — the kind used in a SCIF, a TEMPEST enclosure, or an EMC test chamber — works primarily by conductivity. A conductive skin reflects incoming electromagnetic waves and dissipates induced currents, and even thin copper, aluminum, or steel performs well at MHz and GHz frequencies. Continuity at seams matters enormously, but raw thickness does not.
Magnetic field shielding at low frequencies — DC up to a few hundred hertz, including the 50/60 Hz powerline band — works on a completely different principle. Here a low-impedance magnetic path is created so field lines prefer to travel through a high-permeability metal rather than through the protected volume. This is passive magnetic shielding, and it depends on relative permeability and material thickness, not conductivity. A solid copper sheet that gives 100 dB of RF attenuation may give almost no useful attenuation of a static or 60 Hz magnetic field.
Two mechanisms govern low-frequency magnetic field control:
- Flux shunting — high-permeability ferromagnetic metals (steel, mu-metal) provide an easy path that diverts field lines around the protected space. This is the dominant mechanism at DC and very low frequencies.
- Eddy-current cancellation — in good conductors (copper, aluminum), a changing magnetic field induces circulating currents that generate an opposing field. This grows more effective as frequency rises, so copper contributes increasingly above roughly 1 kHz.
Get the mechanism wrong and you specify the wrong metal. The sections below map each material to the job it actually does well.
The High-Permeability Metals: Steel and Mu-Metal
Silicon (Electrical) Steel
Silicon steel — also called electrical steel — is the practical baseline for heavy magnetic shielding. Its relative permeability is modest compared with nickel alloys (typically in the low thousands), but it has two decisive advantages: a high saturation flux density (around 1.5–2.0 tesla) and low cost per pound. High saturation means it keeps working in strong fields where exotic alloys would saturate and effectively stop shielding.
This is why MRI rooms are almost always built around steel. The static main field of a 1.5T or 3T magnet is enormous, and the fringe field outside the bore can be far too strong for thin mu-metal alone. Heavy welded steel plate, sized by a magnetic model of the specific magnet and room, carries the bulk of the fringe field. Our magnetic shielding design process starts here, modeling field strength before any material is cut.
Mu-Metal, Co-Netic, and HyMu80
Mu metal shielding uses nickel-iron alloys (roughly 80% nickel) sold under names such as Co-Netic, HyMu80, and Permalloy. Their relative permeability is extraordinary — often 20,000 to 100,000 or higher after proper heat treatment — which makes them superb at attenuating weak, low-level fields. A thin mu-metal liner can knock down a residual field that steel alone leaves behind.
The trade-offs are real and must be respected:
- Low saturation (around 0.7–0.8 T). In a strong field, mu-metal saturates and its permeability collapses, so it is used as an inner layer behind steel, never as the sole defense against a large magnet.
- Mechanical sensitivity. Bending, cutting, or even dropping mu-metal degrades its permeability. It typically requires annealing in a hydrogen atmosphere after fabrication, which constrains design and adds cost.
- High material cost — many times that of steel by weight.
The classic high-performance solution is a layered approach: an outer steel shell handles the strong field and prevents the inner layer from saturating, while an inner mu-metal layer mops up the remaining weak field. Proper magnetic shielding installation protects that hard-won permeability, because a beautifully engineered mu-metal liner that is stressed during fitting will underperform its datasheet.
Copper and Eddy-Current Shielding
Copper and aluminum are not high-permeability metals — their relative permeability is essentially 1 — so they do nothing useful against a static (DC) magnetic field. Their role is eddy-current shielding of time-varying fields. As frequency climbs, induced currents in a thick copper sheet generate a strong opposing field, and attenuation improves with both thickness and frequency.
In MRI specifically, copper does double duty. It is the standard RF shield for the megahertz-range pulses the scanner transmits and receives, and a sufficiently thick copper layer also helps damp the rapidly switching gradient fields. That is why an MRI room is fundamentally a copper RF enclosure with steel (and sometimes mu-metal) added for the static and low-frequency magnetic problem — a combination covered in our MRI shielding design service. For a deeper comparison of conductive materials, see our guide to comparing RF shielding materials.
Active Shielding: Compensation Coils
Passive metal is not always the answer. When the disturbing field is large, slowly varying, or comes from a moving source — an elevator, a passing train, traffic-generated DC fields — the practical metal mass needed for passive attenuation can become absurd. Active shielding offers an alternative.
An active compensation system uses magnetometers to sense the ambient field in real time and drives current through a set of coils to generate an equal and opposite field, canceling the disturbance at the point of interest. It excels precisely where passive shielding struggles: very low frequencies (including near-DC), large but slow fluctuations, and environments where adding tons of steel is impractical.
Active systems carry their own costs: power, electronics, periodic calibration, ongoing maintenance, and they cancel the field only within a defined volume. In demanding installations — high-field MRI, electron microscopy suites, magnetometry labs — the best result is often a hybrid: passive steel and mu-metal handle the bulk and the high-frequency content, while active compensation nulls the slow, large-amplitude fluctuations passive metal cannot economically catch.
Material Comparison at a Glance
The table below frames typical, representative figures. Actual permeability, saturation, and cost depend on grade, thickness, heat treatment, and supplier, and any real project should be sized by a magnetic model rather than a datasheet alone.
| Material / Approach | Typical Relative Permeability | Saturation (T) | Best Frequency / Use Case | Relative Cost |
|---|---|---|---|---|
| Silicon / electrical steel | ~2,000–10,000 | ~1.5–2.0 | Strong DC & low-freq fields; MRI fringe field; primary bulk shield | Low |
| Mu-metal (Co-Netic, HyMu80, Permalloy) | ~20,000–100,000+ | ~0.7–0.8 | Weak low-level fields; inner layer behind steel; DC to low-freq | High |
| Copper (eddy-current) | ~1 | N/A | Time-varying fields above ~1 kHz; RF shield; MRI gradient damping | Medium |
| Aluminum (eddy-current) | ~1 | N/A | Lighter eddy-current shielding of AC fields; weight-sensitive uses | Low–Medium |
| Active compensation coils | N/A | N/A | Very low freq / near-DC; large, slow fluctuations; moving sources | High (system + upkeep) |
Choosing the Right Approach for Your Facility
A defensible specification follows from three questions, answered in order:
- What is the frequency? Static or 50/60 Hz points to high-permeability metals. Megahertz RF points to conductive copper. Mixed environments — MRI is the classic example — need both.
- How strong is the field? Strong fields demand high-saturation steel as the primary shield. Weak residual fields are where mu-metal earns its premium.
- Is the source steady or fluctuating? Steady, predictable fields suit passive metal. Large, slow, or moving-source fluctuations are candidates for active compensation.
For MRI in particular, the interplay of static field, switching gradients, and RF makes a single-material answer impossible; our overview of MRI shielding design with passive steel, mu-metal, and active compensation walks through how the layers work together, and the MRI shielding cost guide sets budget expectations. The right starting point is always a site survey and a magnetic model — measure the real environment, model the real magnet, then specify the minimum material that meets the requirement with margin.
Frequently Asked Questions About Magnetic Shielding Materials
What is the difference between magnetic shielding and RF shielding?
RF shielding blocks high-frequency electromagnetic waves using conductive materials that reflect and dissipate the energy, and it works well even with thin metal. Magnetic shielding controls low-frequency and static magnetic fields by providing a high-permeability path that diverts field lines around the protected space. They rely on different physics, so a material that excels at one may be nearly useless at the other.
Why is mu-metal not used by itself for MRI shielding?
Mu-metal has extremely high permeability but very low saturation, around 0.7–0.8 tesla. The fringe field of a 1.5T or 3T MRI magnet easily saturates it, at which point its permeability collapses and it stops shielding effectively. The standard solution uses high-saturation steel to carry the strong field, with mu-metal as an inner layer to attenuate the remaining weak field.
Does copper shield magnetic fields?
Copper does essentially nothing against a static (DC) magnetic field because its relative permeability is about 1. It shields time-varying magnetic fields through eddy currents, and that effect improves as frequency rises. This is why copper is the standard RF shield in MRI rooms and contributes to damping switching gradient fields, while steel handles the static field.
When is active shielding better than passive shielding?
Active compensation coils are advantageous for very low frequency or near-DC disturbances, for large but slowly fluctuating fields, and for sources that move, such as elevators or passing trains. In these cases the steel mass required for passive attenuation can be impractical. Active and passive approaches are often combined so each handles the part of the problem it addresses best.
What is the role of silicon steel in magnetic shielding?
Silicon steel, also called electrical steel, provides high saturation flux density at low cost, making it the practical bulk shield for strong magnetic fields. It carries the heavy field load and, in layered designs, prevents an inner mu-metal layer from saturating. It is the standard primary material for MRI fringe-field control.
How does seam continuity affect shielding performance?
For passive magnetic shielding, gaps and poorly mated joints interrupt the low-impedance magnetic path and let flux leak into the protected space, so welds, overlaps, and joint design directly determine real-world attenuation. The same principle applies to RF copper shields, where every seam, penetration, and door must maintain electrical continuity. Field performance is usually limited by joints and penetrations, not by the sheet material itself.
How do I know how much shielding my facility needs?
Start with a site survey to measure the actual ambient field and its frequency content, then model the specific equipment and room geometry. The required attenuation comes from the difference between the measured environment and the equipment manufacturer's limit, with margin added. Specifying material without measurement and modeling is the most common cause of under- or over-built shielding.