Nanocrystalline Soft Magnetic Materials

March 15th, 2021 | Mike Guthrie 

Quadrant offers soft magnetic materials with a high saturation induction to carry the maximum flux through the smallest area in a DC magnetic circuit. If that material doesn’t constantly carry high flux, it should also have a high permeability so that it’ll yield more flux even when not pushed to saturation. While these properties are still required, materials in a high-frequency AC magnetic circuit have dissimilar priorities. While an AC material’s required attributes still include high saturation induction (Bs) and high magnetic permeability (µ), low core losses become very important, so that it’ll yield high flux with the least eddy current and hysteresis losses.  We shall address materials for an AC magnetic circuit.

Alloy chemistry and grain structure are key to superior properties and the ability to tailor micro-structural features allows specific application optimization. Intrinsic magnetic properties, Curie temperatures (Tc) and Bs are determined by alloy composition and crystal structure. The magnetic response function to an applied magnetic field, µ, is not only determined by chemistry and crystal structure, but also by microstructure and morphology. Alloys with small magneto-crystalline anisotropies and magnetostrictive coefficients produce especially soft magnetic materials.

High Bs materials include body centered cubic (BCC) Fe & Fe-Si, (FCC) α-Co and FeCo alloys. FeCo alloys are especially attractive because of their very high Bs, but their permeabilities are less than those of amorphous or nano-crystalline alloys and Si steels. Nano-crystalline means materials with the large majority of their grain diameters in the range of 1-50 nm.  These nano diameters are produced via rapid solidification techniques or by solid-state reactions to an amorphous material that subsequently crystallizes.  

Amorphous metallic alloys are made by rapid solidification process techniques in alloy systems where the liquid phase remains stable to low temperatures with competing crystalline phases below the liquidus. These are typical metastable phases.  Amorphous alloys can be produced by a variety of rapid solidification processes including splat quenching, melt spinning, gas atomization and condensation from the gas phase.  These typically require cooling rates greater than 104 K/s for binary eutectic alloys.  The most common rapid solidification processing method is melt-spinning which yields amorphous metallic ribbons typically 20 µm thick and gas atomization processes that yield mm to sub-micron powders which can be made amorphous.  The most attractive soft magnetic properties of nano-crystalline materials are gained by controlled crystallization from this amorphous state. 

While there are single phase nano-crystalline alloys, the more common are the biphase materials where the nano-crystalline ferromagnetic phase is surrounded by an amorphous grain-boundary phase. These alloys have:  relatively high electrical resistivity (yielding low eddy-current losses), low magneto-crystalline anisotropy (yielding high Bs), and high mechanical strength.  Other desired properties include:  Low magnetostriction so that mechanical stresses have less effect on the magnetic properties, low temperature dependence of µ, low coercivity (yielding lower hysteresis losses) and a large range where µ remains high over a larger frequency band. 

Three types of B-H curve squareness are produced by different annealing processes and are tailored for different applications.  Materials annealed without a magnetic field, such as Quadrant’s FNMB7-N, have superior permeability:  µ > 19,000 @ 100 kHz.  These are suited for EMI filters, common mode chokes, high precision inductors and AC shielding.  








FNMB7-Nanocrystalline Magnetization Curve

The materials annealed with a transverse magnetic field, our FNMB7-L, have the lowest hysteresis area and so the lowest hysteresis losses: Ps <20W/kg @ 20 kH, 0.5 T and Ps < 30W/kg @ 50 kHz, 0.3 T, DC-bias µ = 20,000. These have low magnetostriction and are used for very high-frequency power transformers and transmitters.  They are also requested for active filters and smoothing choke cores.  

Materials annealed in a longitudinal magnetic field such as FNMB7-N, have the greatest squareness. While they have slightly greater hysteresis area than the previous two, they exhibit similar µ properties over a greater frequency range and can better absorb pulse frequencies. They are used for high-voltage pulse transformers, magnetic amplifiers and work well to protect from high voltage noise caused by lightning, sparking or other frequency spikes.











FNMB7-Nanocrystalline vs. Ferrite vs. Amorphous

Frequency Dependence of Permeability

Quadrant’s high performance nano-crystalline materials are indispensable for the low-loss electrical circuits required to satisfy today’s energy savings demands. They are necessary for space savings, optimal for the weight reduction in EV, and vital for aeronautic and aerospace applications, please contact us to learn more.

FNMB7-N, FNMB7-L, FNMB7-H. Three types of B-H curve squareness (flexibility to control magnetic properties during annealing) corresponding to various applications.
Quadrant FNMB7-Nanocrystalline vs. Ferrite vs. Amorphous Frequency Dependence of Permeability

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