Cryogenic Magnetic Shielding at 4 K
We cover the complete shielding process
Custom CRYOPHY® magnetic shields for quantum computing, superconducting RF systems, SQUIDs and other sensitive equipment operating at cryogenic temperatures.
Magnetic Shields USA provides application support, design, fabrication and specialist hydrogen heat treatment for custom cryogenic magnetic shielding systems.
Cryogenic magnetic shielding for low-temperature systems
The magnetic properties of high-permeability nickel-iron alloys vary with temperature. A material that provides excellent shielding at room temperature may not retain the same permeability when cooled to cryogenic temperatures.
For applications operating below approximately −40 °C (−40 °F), and particularly for systems operating around 4 K, we generally recommend CRYOPHY®.
CRYOPHY® is a nickel-iron-molybdenum soft magnetic alloy specifically developed for magnetic shielding at cryogenic temperatures. It can be formed and fabricated into custom shields for integration into cryostats, cryomodules, vacuum vessels, dilution refrigerators and other low-temperature systems.
Material selection by temperature
| Operating temperature | Typical material selection |
|---|---|
| Above approximately −40 °C (−40 °F) | MuMetal |
| Below approximately −40 °C (−40 °F) | CRYOPHY® |
This is a practical material-selection guide rather than an abrupt temperature boundary. Magnetic permeability changes continuously with temperature, and the final material selection must also consider the applied field, required attenuation, shield geometry, available space and surrounding magnetic environment.
CRYOPHY® Magnetic Properties at 4 K
CRYOPHY® has a typical composition of:
- 81% nickel
- 5% molybdenum
- Balance iron
Published typical magnetic properties at 4 K include:
- Relative permeability of approximately 70,000 at an applied field of approximately 0.4 A/m
- Saturation induction of approximately 0.8 T at an applied field of approximately 160 A/m
- Coercive force of approximately 0.8 A/m
These values were measured on 1 mm-thick ring samples after heat treatment at 1,150 °C in pure, dry hydrogen followed by appropriate cooling.
They are indicative material values and should not be interpreted as guaranteed attenuation values for a completed shield. Finished shielding performance depends on the material condition, heat treatment, shield geometry, openings, joints, applied field and surrounding magnetic environment.
CRYOPHY® is supplied as cold-rolled strip or sheet and can be manufactured into cylindrical shields, shielding cans and application-specific assemblies.
Hydrogen annealing for CRYOPHY® shields
Forming, welding, machining and mechanical deformation introduce stress into high-permeability nickel-iron alloys. Published testing on CRYOPHY® has demonstrated that plastic deformation can significantly reduce its magnetic performance.
The principal fabrication operations should therefore be completed before the final magnetic heat treatment.
Magnetic Shields Limited uses a specialist CRYOPHY® furnace program lasting more than 36 hours. The cycle incorporates controlled heating in a pure, dry hydrogen atmosphere, treatment at temperatures up to approximately 1,150 °C and a carefully managed cooling period.
This final post-fabrication heat treatment develops the material’s required soft-magnetic properties. Mechanical work after treatment should be minimized because subsequent bending, impact, machining or distortion can reduce permeability and compromise shielding performance.
Combined warm and cryogenic magnetic shielding
A cryogenic magnetic shield does not always have to operate alone. Some systems benefit from a nested arrangement consisting of a CRYOPHY® shield within the cryogenic vessel and a separate high-permeability magnetic shield outside the vessel at ambient temperature.
The external shield reduces the surrounding magnetic field before it reaches the cryogenic assembly. The internal CRYOPHY® shield then provides further attenuation close to the sensitive component or experimental region.
This type of combined arrangement has been used in superconducting RF cavity systems, with an ambient-temperature shield outside the cryostat and a cryogenic high-permeability shield positioned closer to the cavity.
The optimum material arrangement, spacing and geometry must be determined for the particular magnetic and thermal environment.
Cryogenic magnetic shield design
Successful cryogenic magnetic shielding requires more than selecting the correct alloy. The shield must be designed around the magnetic, mechanical and thermal requirements of the complete system.
Important design considerations include:
- Required residual magnetic field or shielding factor
- Magnitude and direction of the external magnetic field
- Operating temperature and cooldown profile
- Thermal contraction of the shield and supporting structure
- Available space within the cryostat or vacuum vessel
- Openings for wiring, cooling lines, optical access and mechanical supports
- Joints, seams, lids and removable sections
- Proximity of magnetic components and structural materials
- Thermal anchoring and mechanical support
- Assembly sequence and installation access
- Interaction with additional ambient-temperature or cryogenic magnetic shields
Open ends, penetrations and mechanical discontinuities can substantially affect shielding performance. Their position and geometry should therefore be considered during the initial design rather than added after manufacture.
Cryogenic Magnetic Shielding Applications
Superconducting RF cavities and cryomodules
Niobium superconducting RF cavities can trap magnetic flux as they cool through their superconducting transition temperature. Trapped flux can increase residual resistance, reduce the cavity quality factor and impair RF performance.
Cryogenic high-permeability magnetic shields are used around cavities and within cryomodules to reduce the magnetic field present during cooldown and operation.
The shield design must account for the cavity geometry, cryomodule structure, support system, penetrations and available space around the cavity.
Quantum computing and superconducting circuits
Superconducting qubits, resonators and associated readout components can be sensitive to unwanted magnetic fields and magnetic-field variations.
CRYOPHY® shields can be incorporated into dilution refrigerators and positioned around sensitive experimental volumes or cold-stage components. Depending on the application, they may be used independently or as part of a multilayer high-permeability shielding system.
CRYOPHY® shielding has been used experimentally within dilution refrigerators to reduce ambient fields and magnetic interaction between adjacent superconducting and spin-based quantum systems.
SQUID systems and cryogenic magnetic instrumentation
Superconducting quantum interference devices, commonly known as SQUIDs, are highly sensitive magnetic sensors. They must operate below the critical temperature of their superconducting material.
Conventional low-temperature SQUID systems commonly operate at approximately 4 K or below, while some high-temperature superconducting SQUIDs operate at higher cryogenic temperatures. The SQUID sensor is cryogenic, but the sample or system being measured does not necessarily have to be at the same temperature.
Cryogenic high-permeability shielding can be positioned around the SQUID, sample region or cold stage to reduce unwanted ambient fields and magnetic interference. The shield must preserve the intended magnetic signal path to the sensor.
SQUID systems remain relevant in scanning magnetic microscopy, superconducting-circuit research, cryogenic detector readout, materials characterization and precision magnetic measurement.
Custom CRYOPHY® shield design and manufacturing
Magnetic Shields USA provides design and manufacturing support for custom cryogenic magnetic shields, from initial concept through fabrication and final hydrogen heat treatment.
To assess an application, we typically require:
- Operating and cooldown temperatures
- Maximum available space
- Drawings or models of the cryogenic assembly
- External magnetic-field information, where available
- Required residual field or shielding performance
- Details of penetrations, supports and access requirements
- Materials and components located close to the shield
- Installation and assembly constraints
- Details of any additional ambient-temperature or cryogenic magnetic shielding
Early involvement allows the shield geometry, mounting arrangement, penetrations and heat-treatment requirements to be considered before the cryogenic system design becomes fixed.
Cryogenic Shielding FAQs
What is cryogenic magnetic shielding?
Cryogenic magnetic shielding reduces unwanted magnetic fields around sensitive equipment operating at very low temperatures. It uses materials specifically selected to retain suitable magnetic properties within the intended cryogenic environment.
When should CRYOPHY® be used instead of MuMetal?
As a practical guide, MuMetal is generally used above approximately -40 °C (-40 °F), while CRYOPHY® is recommended below this temperature and particularly for applications operating around 4 K.
This is not an abrupt material boundary. The correct selection also depends on the applied field, required attenuation, shield geometry and operating environment.
What temperature is CRYOPHY® designed for?
CRYOPHY® is designed for magnetic shielding at cryogenic temperatures, with published magnetic properties measured at 4 K.
For systems operating at other temperatures, the material and shield design should be reviewed against the specific application requirements.
What permeability does CRYOPHY® provide at 4 K?
Published material data gives a typical relative permeability of approximately 70,000 at 4 K with an applied field of approximately 0.4 A/m.
This value was measured using a heat-treated ring sample and does not represent a guaranteed shielding factor for a completed shield. Finished performance depends on the complete shield design and operating conditions.
Does CRYOPHY® require hydrogen heat treatment?
For maximum magnetic performance in a fabricated CRYOPHY® shield, yes. Forming, welding and machining introduce mechanical stresses that reduce permeability, so the principal fabrication operations should be completed before the final hydrogen anneal.
Our specialist CRYOPHY® furnace program lasts more than 36 hours and uses a controlled, pure, dry hydrogen atmosphere followed by carefully managed cooling.
Can CRYOPHY® be combined with an ambient-temperature magnetic shield?
Yes. Some systems use a CRYOPHY® shield close to the cryogenic component together with a separate high-permeability shield outside the cryogenic vessel at ambient temperature.
The outer shield reduces the surrounding field before it reaches the cold assembly, while the internal shield provides further attenuation close to the sensitive region.
Is CRYOPHY® suitable for quantum computing systems?
CRYOPHY® can be incorporated into dilution refrigerators and other cryogenic quantum systems to reduce unwanted magnetic fields around superconducting circuits, qubits and associated instrumentation.
The required material thickness, number of layers, mounting position and shield geometry must be determined for the particular system.
What information is needed to design a cryogenic magnetic shield?
We typically require the operating and cooldown temperatures, available space, external field conditions, required residual field or attenuation, assembly drawings, details of penetrations and information about nearby materials and components.
Providing this information early allows the magnetic shield, mountings and cryogenic assembly to be developed together.
CRYOPHY® is a registered trademark of Aperam Alloys Imphy.
Talk to us about your cryogenic magnetic shielding application.
Get in touch with your application details and any available drawings or dimensions.
To help us advise quickly, it is useful to include:
- required material form (foil or sheet) if known
- thickness or size requirements
- application / shielding objective
- whether the part will be cut, formed or fabricated
- any annealing or performance requirements