Permalloy (1J85) Cores: Properties and Applications

What Is 1J85 Permalloy?

1J85 is a nickel-iron soft magnetic alloy containing approximately 80% nickel and 20% iron, with small additions of molybdenum and other trace elements. It belongs to the family of alloys commonly called Permalloy, a name originally coined by Bell Labs in the 1910s. The "1J85" designation follows the Chinese alloy numbering system. Equivalent grades include Mu-metal (general term), Supermalloy, HyMu 80, Magnifer 7904, and similar trade names from various manufacturers around the world.

The defining characteristic of 1J85 is its extremely high magnetic permeability. Initial permeability typically exceeds 20,000, and maximum permeability can reach 100,000 to 300,000 or higher after proper heat treatment. This means that 1J85 cores can channel magnetic flux with extraordinary efficiency, producing strong magnetic coupling even at very low field strengths.

Composition and Metallurgy

Element	Content (%)	Purpose
Nickel (Ni)	79 to 82	Primary alloying element. Sets the high permeability.
Iron (Fe)	Balance (~14 to 17)	Base magnetic material.
Molybdenum (Mo)	3.8 to 4.2	Increases resistivity, reduces eddy current losses.
Manganese (Mn)	≤ 0.6	Deoxidizer, improves workability.
Silicon (Si)	≤ 0.3	Increases resistivity slightly.

The 80% nickel content places 1J85 at the peak of the permeability curve for nickel-iron alloys. At this composition, the magnetocrystalline anisotropy and magnetostriction both approach zero, which is what allows the extremely high permeability. Small deviations in composition, particularly the molybdenum content, significantly affect the magnetic properties, which is why tight alloy control during melting is essential.

Magnetic Properties

Property	Typical Value	Notes
Initial Permeability (µi)	20,000 to 50,000	After proper anneal. Highly sensitive to heat treatment.
Maximum Permeability (µmax)	100,000 to 300,000+	At optimal flux density, typically 0.3 to 0.5 T.
Saturation Flux Density (Bsat)	0.74 to 0.80 T	Lower than silicon-iron or ferrite. Limits power applications.
Coercivity (Hc)	0.4 to 1.6 A/m	Extremely low. Indicates soft magnetic behavior.
Resistivity	55 to 62 µΩ·cm	Higher than pure nickel due to Mo addition. Reduces eddy currents.
Curie Temperature	~460°C	Loses magnetic properties above this temperature.

Manufacturing Process

Melting and Casting

1J85 is typically vacuum-melted or vacuum-induction-melted (VIM) to achieve the purity levels required for high permeability. Impurities, particularly carbon, sulfur, and oxygen, degrade magnetic properties by pinning domain walls and increasing coercivity. Premium grades may undergo additional vacuum arc remelting (VAR) for further purification.

Rolling to Thin Strip

The cast ingot is hot-rolled and then cold-rolled to the final strip thickness, typically 0.05 mm to 0.35 mm (2 mil to 14 mil). Thinner strips reduce eddy current losses at higher operating frequencies. For toroidal cores used in current transformers, strip thicknesses of 0.05 to 0.10 mm are common.

Core Winding

Toroidal cores are produced by winding the thin strip into a tight spiral on a mandrel, building up the desired cross-sectional area layer by layer. The winding tension, alignment, and inter-layer insulation (if used) all affect the final magnetic properties. After winding, the core is secured with spot welds or adhesive to prevent unwinding.

Heat Treatment (Annealing)

This is the most critical step in producing high-permeability 1J85 cores. The wound core is annealed in a hydrogen atmosphere at 1,100 to 1,200°C for several hours, followed by a controlled slow cool. This process achieves several objectives simultaneously: it relieves mechanical stress from rolling and winding, promotes optimal crystal grain growth, establishes the ordered Ni3Fe superlattice structure, and removes residual carbon and sulfur through hydrogen reduction.

The cooling rate is critical. Too fast, and the ordered structure does not fully develop. Too slow, and grain growth becomes excessive. The exact annealing profile (temperature, hold time, cooling rate, atmosphere purity) is proprietary to each manufacturer and represents decades of accumulated process knowledge.

Coating and Protection

After annealing, 1J85 cores are extremely sensitive to mechanical stress. Even moderate clamping pressure or dropping can degrade the permeability by 50% or more. Cores are therefore encased in a protective coating, typically epoxy or a polymer case, immediately after annealing. This coating protects the core during handling, winding operations, and the life of the finished component.

Applications

Current Transformers

This is the primary application for 1J85 cores. Current transformers (CTs) use the high permeability to accurately transform a primary current into a proportional secondary current for measurement. The high permeability ensures low magnetizing current error, which directly translates into measurement accuracy. In power monitoring, energy metering, and protective relay applications, CT accuracy requirements of 0.1% to 0.5% demand core materials with permeability of 50,000 or higher.

Magnetic Shielding

The high permeability of 1J85 makes it an excellent magnetic shield material. Shields made from 1J85 (commonly called Mu-metal shields) redirect external magnetic fields around the protected volume, reducing the field inside by factors of 100 to 10,000 depending on the geometry and wall thickness. Applications include shielding sensitive electronics from stray magnetic fields in medical imaging equipment (MRI rooms), scientific instruments, and military systems.

Precision Sensors

Fluxgate magnetometers, ground fault circuit interrupters (GFCIs), and residual current detectors (RCDs) use 1J85 cores. These devices detect very small magnetic fields or very small currents (milliamps) and require core materials that respond to minimal excitation.

Signal-Level Transformers

Audio input transformers, impedance matching transformers, and low-level signal transformers benefit from the high permeability when operating at very low signal levels. The high permeability maintains inductance even at the small flux densities produced by millivolt signals.

Comparison with Other Core Materials

Property	1J85 (Permalloy)	Ferrite (MnZn)	Silicon-Iron (3% SiFe)
Max Permeability	100,000 to 300,000	2,000 to 15,000	30,000 to 50,000
Saturation (Bsat)	0.74 to 0.80 T	0.35 to 0.50 T	1.6 to 2.0 T
Coercivity	0.4 to 1.6 A/m	8 to 20 A/m	4 to 16 A/m
Frequency Range	DC to ~100 kHz	10 kHz to 2 MHz+	DC to ~50 kHz
Core Loss	Low at low frequency	Lowest at high frequency	Moderate
Cost per Core	High ($1 to $10+)	Low ($0.10 to $2)	Moderate ($0.50 to $5)
Primary Use	Current transformers, shielding	Power inductors, EMI filters	Power transformers, high-flux inductors

Cost Considerations

1J85 cores cost significantly more than ferrite or silicon-iron cores of similar dimensions. Several factors contribute to the higher cost.

• Raw material: Nickel is roughly 10 times the price of iron by weight. An 80% nickel alloy has fundamentally higher material cost.
• Melting: Vacuum melting is required, which is more expensive than conventional air melting used for silicon-iron.
• Thin strip rolling: Rolling to 0.05 mm thickness requires multiple passes and precision equipment.
• Heat treatment: High-temperature hydrogen annealing in controlled atmospheres is energy-intensive and time-consuming.
• Low yield: The sensitivity to mechanical stress means handling losses and reject rates are higher than for more robust core materials.

For applications that require permeability above 50,000, there is no substitute for 1J85 or its equivalents. The cost premium is justified by the performance requirements. For applications where permeability of 5,000 to 15,000 is sufficient, ferrite cores provide the same function at a fraction of the cost.

Specifying 1J85 Cores

When ordering 1J85 toroidal cores, the key specifications include the following.

Parameter	What to Specify
Dimensions	Outer diameter, inner diameter, and height, with tolerances
Strip Thickness	Typically 0.05 to 0.10 mm for high-frequency CT applications
Permeability	Minimum initial permeability after annealing (e.g., ≥ 50,000)
Coating	Epoxy, powder coat, or plastic case. Specify thickness and material.
Environmental	RoHS compliance, operating temperature range