Simply put, permanent magnetic materials and soft magnetic materials are a pair of complementary “partners” that often work together. They serve different roles due to their fundamentally opposite properties, yet cooperate closely because they share the same functional goal.
Their relationship can be understood as follows:
- Permanent magnetic materials: like a stable magnetic field source (such as a neodymium iron boron magnet). Their primary function is to provide a constant and long-lasting magnetic field, and their magnetization state is difficult to change.
- Soft magnetic materials: like an efficient magnetic field channel and controller (such as a transformer core). Their main function is to guide, concentrate, amplify, or rapidly switch magnetic fields, and their magnetization state is very easy to change.
From the perspective of the hysteresis loop, the key differences in their core characteristics are mainly as follows:
| Characteristic | Permanent Magnetic Materials | Soft Magnetic Materials |
| B–H Hysteresis Loop | Broad hysteresis loop with large enclosed area | Narrow hysteresis loop with small enclosed area |
| Coercivity (Hc) | Very high coercivity (typically > 10 kA·m⁻¹) | Very low coercivity (typically < 1 kA·m⁻¹) |
| Remanent Flux Density (Br) | High remanent flux density | Low remanent flux density (ideally approaching zero) |
| Magnetic Permeability (μ) | Low relative permeability | Very high relative permeability |
| Magnetization Behavior | Difficult to magnetize and highly resistant to demagnetization | Easily magnetized and easily demagnetized |
| Energy Characteristics | Designed to maximize maximum energy product, (BH)_max, enabling efficient magnetic energy storage | Designed to minimize magnetic losses and enable efficient magnetic flux conduction and energy conversion |
| Representative Materials | NdFeB, SmCo, Alnico, Ferrite | Electrical steels, soft magnetic ferrites, soft magnetic composites (SMCs), amorphous and nanocrystalline alloys |
Application Overlap and Synergy: How Do They Work Together?
In modern electromagnetic devices, these materials are often integrated together to achieve more complex and highly efficient functionalities. Typical scenarios of their synergistic application include:
| Application Area | Role of Permanent Magnetic Materials | Role of Soft Magnetic Materials | Explanation of Synergistic Operation |
| Permanent Magnet Motors / Generators | Provide a constant excitation magnetic field (rotor or stator) without the need for continuous electrical power. | Form the stator and rotor magnetic cores (commonly electrical steels or SMCs), efficiently guiding and concentrating magnetic flux to complete the magnetic circuit. | Soft magnetic materials act as low-reluctance magnetic pathways, minimizing magnetic losses and enabling the magnetic field generated by permanent magnets to effectively drive motor or generator rotation. |
| Loudspeakers and Headphones | Ring-shaped permanent magnets provide a stable and strong magnetic field. | Magnetic pole pieces and yokes (e.g., voice-coil formers and magnetic circuits) guide and homogenize the magnetic field, ensuring uniform motion of the voice coil. | Soft magnetic materials optimize the magnetic field distribution in the magnetic gap, enabling the voice coil to operate more linearly, thereby reducing distortion and improving acoustic efficiency. |
| Magnetic Resonance Imaging (MRI) | Generate an ultra-strong static magnetic field using permanent magnets or superconducting magnets. | Used in gradient coils and shim cores to enable rapid and precise local modulation of the magnetic field. | Permanent magnets establish the main static field, while soft magnetic materials enable fine magnetic field shaping and gradient control, working together to achieve high-resolution imaging. |
| Magnetic Coupling and Transmission | Installed on input and output shafts to provide interacting magnetic poles. | Used to guide, shield, or strengthen magnetic flux paths, depending on design requirements. | In applications requiring contactless power transmission (e.g., sealed systems), permanent magnets provide torque, while soft magnetic materials optimize magnetic flux coupling efficiency. |
In summary, the relationship between permanent magnetic materials and soft magnetic materials is one of synergy rather than competition. Permanent magnets act as “magnetic field engines,” while soft magnetic materials serve as “magnetic field routers.” Through their precise cooperation, they together form the foundation of nearly all devices for electromagnetic energy and signal conversion, ranging from household appliances to cutting-edge technologies.
