Soft Magnetic Materials

Introduction to Soft Magnetic Materials

Soft magnetic materials are essential components in many devices and are indispensable in modern electrical engineering and electronics. The wide range of applications, steadily rising demands on component quality, and increasing specialization necessitate careful material selection to achieve optimal performance. Development in this field has accelerated significantly in recent years.

The term "soft" distinguishes these materials from "hard" magnetic materials used for permanent magnets. Soft magnetic materials can be easily magnetized and demagnetized, making them ideal for applications with changing magnetic fields.

Today, advanced soft magnetic materials serve as key components in numerous critical devices across electrical engineering and electronics sectors. Their significant role in these applications demands thorough consideration when selecting the appropriate alloy for each specific use case.

Recent Advances in Soft Magnetic Materials

In recent years, researchers have made significant progress in developing new soft magnetic materials. Amorphous and nano-crystalline metals represent an important expansion and enhancement of the available range of soft magnetic alloys. Their unique structure and composition provide numerous advantages and innovative properties compared to conventional materials.

Despite these advances, silicon steels and ferrites continue to dominate the soft magnetic materials market. However, powder metallurgy processing offers distinct advantages over traditional lamination silicon steels and low carbon steels, particularly in providing net shape capabilities and eliminating secondary operations such as punching, grinding, honing, and drilling.

Powder Metallurgy in Magnetic Materials

Over the years, iron powder produced through both iron ore reduction and water atomization has found extensive applications in the magnetic materials industry. Three areas of particular interest include:

1. Powder cores
2. Sintered iron and iron alloys
3. Insulated iron powder compacts

Powder cores utilize iron powder dispersed in plastic or polymer, compacted into various shapes. These cores maintain consistent permeability across a wide frequency range. They represent a cost-effective alternative to ferrites while providing higher induction. Common applications include switch-mode power supplies, inductors, and high-frequency broadband applications.

Sintered iron powder and iron-phosphorus alloy powders compete effectively with low carbon steel across various applications.

Table 1. Typical Properties of PM Magnetic Materials showing alloy systems, density, relative cost, permeability, coercive force, maximum induction, and resistivity

Iron-Phosphorus Sintered Products

Iron-phosphorus powder premixes are typically sintered at 1120°C in a hydrogen or nitrogen-hydrogen atmosphere. Carbon contamination during sintering should be avoided as it negatively impacts magnetic properties. Phosphorus content typically remains around 0.45% but can reach up to 0.8% with careful processing to prevent embrittlement.

Applications for iron-phosphorus materials include:

• Components requiring higher induction and moderate resistivity
• Stators and rotors for low-speed stepper motors
• Electric starter motors
• Pole caps
• Actuators for valve control and anti-lock braking system (ABS) sensors

These materials offer good strength and hardness, with sufficient ductility to allow riveting operations.

Iron-Silicon Alloy Properties and Applications

Iron-silicon alloys typically contain approximately 1.5% to 3.0% silicon. To preserve compressibility, silicon is not pre-alloyed into the iron. Instead, manufacturers mix a master alloy of iron pre-alloyed with silicon (up to 33%) with pure iron powder. This mixture is sintered at 1260°C in a 100% hydrogen atmosphere to achieve silicon diffusion.

Key benefits and applications of iron-silicon alloys include:

• Better performance than iron-phosphorus materials at moderate frequencies
• Suitability for impact-related applications such as actuators
• Use in impact printer heads due to their mechanical and magnetic properties

Table 2. Iron-Silicon PM material properties showing alloy designations, induction values, remanence, coercive force, and maximum permeability

Iron-Nickel Alloys for Specialized Applications

Iron-nickel alloys typically contain 50% nickel in pre-alloyed form. These powders exhibit low compressibility and require sintering at 1260°C in either hydrogen or vacuum atmospheres.

These alloys offer several advantages for specialized applications:

• Lower induction values compared to other iron-based materials
• Higher permeability, allowing actuation at very low applied fields
• Permendur variant provides the highest saturation, though at higher cost
• 409L and 434L stainless grades serve well in magnetic sensor applications where corrosion resistance is important

Despite their beneficial properties, iron-nickel alloys have several disadvantages:

• Expensive compared to other soft magnetic materials
• Difficult to process due to their low compressibility
• Generally have lower induction than iron or iron-phosphorus systems

For magnetic applications using these materials, proper stress relief is essential after any post-sintering operations such as machining or coining. Annealing parts at 815°C for 15 minutes effectively relieves such stress.

Toshiba Materials specializes in manufacturing nickel and iron-based high-permeability alloys (Permalloy) and degaussing alloys for specialized applications.

Table 3. Iron-Nickel alloys properties chart