Amorphous and nanocrystalline alloys are generally classified as soft magnetic materials rather than permanent magnetic materials. Their performance objectives are different from those of Ndfeb, Smco, and hard ferrite magnets.
Permanent magnets require relatively high coercivity to maintain magnetization after the external magnetic field is removed. Amorphous and nanocrystalline soft magnetic materials, by contrast, are usually designed to provide low coercivity, high permeability, and low power loss under alternating magnetic fields.
These materials are widely used in high-frequency transformers, common-mode chokes, current sensors, reactors, inverters, and power electronic systems for electric vehicles. Their magnetic properties are not fixed. Composition, cooling rate, tratamiento térmico, estrés mecánico, frequency, magnetic flux density, and operating temperature can all produce significant changes in performance.
1. Structural Differences Between Amorphous and Nanocrystalline Materials
Atoms in conventional crystalline metals are arranged in an ordered structure and form grains with clear grain boundaries. Amorphous alloys are produced through rapid solidification, which prevents conventional crystal nucleation and growth. The atoms therefore remain in a long-range disordered state.
Because amorphous alloys do not contain conventional grains and grain boundaries, the local obstacles to magnetic domain-wall movement can be reduced or controlled. This structural characteristic makes it possible to achieve low coercivity and useful soft magnetic performance.
Nanocrystalline materials are commonly produced by first preparing an amorphous ribbon and then applying controlled annealing. During annealing, very small and uniformly distributed crystalline grains precipitate from the amorphous matrix.
When the grain size remains below the characteristic exchange-interaction length, the magnetocrystalline anisotropy of individual grains can be averaged through exchange coupling. Como resultado, the material can achieve low coercivity and high permeability.
JLMAG Innovation supplies Fe-based amorphous and Fe-based nanocrystalline ribbons. Por ejemplo, the reference properties of 1K107B nanocrystalline ribbon include a saturation magnetic induction of approximately 1.2 t, coercivity below 20 A/m, and a thickness range of 12–27 μm. The reference saturation magnetic induction of the Fe-based amorphous ribbon is approximately 1.56 t. Actual performance should always be confirmed according to the specific grade, magnetic core structure, and test conditions.
2. Which Factors Cause Magnetic Properties to Change?
2.1 Alloy Composition
Iron content, metalloid elements, and small additions of elements such as copper and niobium affect glass-forming ability, crystallization temperature, and nanocrystal precipitation.
If the composition deviates from the designed range, the ribbon may experience localized crystallization, increased brittleness, or unstable magnetic properties.
Increasing the iron content can help produce a higher saturation magnetic induction. Sin embargo, it may also make it more difficult to form a completely amorphous structure. Alloy design must therefore balance saturation induction, amorphous-forming ability, estabilidad térmica, and soft magnetic performance.
2.2 Rapid Solidification Quality
Amorphous ribbon is commonly manufactured by rapid solidification. Melt temperature, nozzle gap, cooling-wheel speed, presión, and ribbon tension all affect thickness, surface quality, and the degree of amorphization.
If the cooling rate is insufficient, unwanted crystalline phases may form. These phases can act as pinning sites for magnetic domain walls, increasing coercivity and magnetic loss.
Ribbon thickness must also remain consistent. Large thickness variations may result in uneven magnetic flux distribution, inconsistent core filling factors, and variations in power loss after the ribbon is wound into a core.
2.3 Heat Treatment Conditions
The performance of nanocrystalline alloys depends strongly on annealing temperature, holding time, heating and cooling rates, and protective atmosphere.
If the annealing temperature is too low, the required nanocrystalline phase may not precipitate sufficiently. If the temperature is too high or the holding time is too long, excessive grain growth may occur, resulting in increased coercivity and core loss.
Stress-relief annealing is also important for amorphous materials. It can reduce residual stresses introduced during ribbon casting, slitting, winding, or forming.
Annealing under a controlled magnetic field can induce magnetic anisotropy in a selected direction. This process can be used to adjust the hysteresis loop, permeability, and DC bias characteristics of the finished magnetic core.
2.4 Processing and Assembly Stress
Ribbon winding, stamping, corte, clamping, resin curing, and housing assembly can all introduce mechanical stress.
Because of magnetoelastic effects, residual stress may restrict domain-wall movement, increasing coercivity and reducing permeability. A magnetic core that performs well before packaging may therefore show increased loss after being tightly clamped or encapsulated in a high-shrinkage resin.
Differences in thermal expansion between the magnetic core, adhesive, and housing can generate additional stress during temperature changes. Mechanical fixation should therefore provide sufficient structural strength without placing excessive pressure on the core.
2.5 Frequency and Magnetic Flux Density
Soft magnetic material loss generally consists of hysteresis loss, eddy-current loss, and excess loss. Total loss usually increases as operating frequency or magnetic flux density rises.
The small thickness and relatively high electrical resistivity of amorphous and nanocrystalline ribbons help suppress eddy currents. Sin embargo, this does not mean the materials will maintain low loss under every operating condition.
Magnetostriction, domain-wall movement, waveform, temperature, and core geometry can all affect actual loss. A material measured at 20 kHz and 0.2 T cannot be compared directly with another material tested at 100 kHz and 0.05 t.
3. Comparison Between Amorphous and Nanocrystalline Materials
| Comparison factor | Fe-based amorphous material | Fe-based nanocrystalline material |
| Typical structure | Long-range disordered amorphous structure | Nanometer-scale grains distributed in an amorphous matrix |
| Saturation magnetic induction | Generally high | Moderate to high, depending on composition |
| coercitividad | Bajo | Can be extremely low |
| Permeability | Alto | Usually higher and more adjustable |
| High-frequency loss | Bajo | Often lower within suitable frequency ranges |
| Main purpose of heat treatment | Relieve stress and stabilize the structure | Control crystallization, grain size, and anisotropy |
| Aplicaciones típicas | Distribution transformers, reactors, magnetic shielding | High-frequency transformers, common-mode chokes, sensores |
| Main risks | Local crystallization, ribbon embrittlement, residual stress | Excessive crystallization, grain growth, uneven annealing |
4. How Can Magnetic Performance Be Optimized?
The first requirement is to stabilize raw material composition and the rapid-solidification process. Ribbon thickness, surface defects, amorphization, and compositional uniformity should be monitored carefully.
The second requirement is to develop a suitable annealing curve for each material grade. Annealing temperature and holding time should be established through testing rather than simply selecting the highest temperature that the alloy can tolerate.
For magnetic cores requiring a particular hysteresis-loop shape, longitudinal, transverse, or other controlled magnetic-field annealing methods may be considered.
Encapsulation materials should have limited curing shrinkage and suitable thermal expansion characteristics. The clamping force should also be controlled to prevent assembly stress from degrading permeability and increasing core loss.
Finished-product testing should include:
- coercitividad;
- Initial and effective permeability;
- Saturation magnetic induction;
- Core loss at different frequencies;
- Temperature rise;
- Insulation performance;
- DC bias characteristics;
- Thermal stability.
Frequency, waveform, magnetic flux density, and temperature should be clearly stated in every test report. Comparing core-loss values without consistent test conditions can lead to incorrect material selection.
JLMAG Innovation offers Fe-based amorphous and nanocrystalline ribbons, as well as sintered NdFeB, Falso, Smco, ferrito, and magnetic assemblies. Before selecting a material, engineers should first determine whether the product must maintain a stable permanent magnetic field or respond rapidly to an alternating magnetic field with low energy loss. This distinction determines whether a imán permanente or soft magnetic material is the appropriate solution.
