When discussing magnetic materials, we must first address the concept of "magnetism" itself. Experiments have demonstrated that any substance, when placed within an external magnetic field, can be magnetized to some degree—though the extent of this magnetization varies. Based on the characteristics substances exhibit in the presence of an external magnetic field, they can be classified into five categories: paramagnetic substances, diamagnetic substances, ferromagnetic substances, ferrimagnetic substances, and antiferromagnetic substances.
Paramagnetic Substances: These are substances that, when brought near a magnetic field, become magnetized in the direction of that field; however, this magnetization is extremely weak—so faint, in fact, that it can only be detected using precision instruments. Furthermore, if the external magnetic field is removed, the internal magnetic field within the substance reverts to zero, resulting in a complete loss of magnetism. Examples include aluminum and oxygen.
Diamagnetic Substances: These are substances characterized by a negative magnetic susceptibility. When subjected to an external magnetic field, induced electron currents are generated within their molecules; these currents create a magnetic moment that opposes the direction of the external field. In other words, following magnetization, the direction of the internal magnetic field runs counter to that of the external field. All organic compounds exhibit diamagnetism; other examples of diamagnetic substances include graphite, lead, and water.
Ferromagnetic Substances: These are substances that, once magnetized under the influence of an external magnetic field, retain their magnetized state—and thus their magnetic properties—even after the external field has been removed. Iron, cobalt, and nickel are all ferromagnetic substances.
Ferrimagnetic Substances: These substances exhibit macroscopic magnetic properties similar to those of ferromagnetic substances, differing primarily in having a lower magnetic susceptibility. Ferrites are typical examples of ferrimagnetic substances. The most significant distinction between ferrimagnetic and ferromagnetic substances lies in the differences in their internal magnetic structures.
Antiferromagnetic Substances: Within antiferromagnetic substances, the spins of adjacent valence electrons tend to align in opposite directions. Consequently, the net magnetic moment of such substances is zero, meaning they do not generate a magnetic field. These substances are relatively uncommon, and most antiferromagnetic materials exist only under low-temperature conditions. If the temperature exceeds a certain threshold, they typically transition into a paramagnetic state. Examples of antiferromagnetic substances include chromium and manganese.
We classify paramagnetic and diamagnetic substances as "weakly magnetic materials," while ferromagnetic and ferrimagnetic substances are termed "strongly magnetic materials." Generally speaking, when the term "magnetic materials" is used in a common context, it refers specifically to these strongly magnetic substances. Based on their application, magnetic materials can be classified into the following categories:
Soft Magnetic Materials: These are materials capable of achieving maximum magnetization intensity with a minimal external magnetic field; they are characterized by low coercivity and high permeability. Soft magnetic materials are easily magnetized and, conversely, easily demagnetized. Examples include soft ferrites, as well as amorphous and nanocrystalline alloys.
Hard Magnetic Materials: Also known as permanent magnet materials, these refer to materials that are difficult to magnetize and, once magnetized, are equally difficult to demagnetize. Their primary characteristic is high coercivity. This category includes rare-earth permanent magnets, metallic permanent magnets, and permanent magnet ferrites.
Functional Magnetic Materials: This category primarily encompasses magnetostrictive materials, magnetic recording materials, magnetoresistive materials, magnetic bubble materials, magneto-optical materials, and magnetic thin-film materials, among others.
Magnetic Materials
│
├── Weakly Magnetic Materials
│ ├── Diamagnetic Materials
│ └── Paramagnetic Materials
│
└── Strongly Magnetic Materials
│
├── Soft Magnetic Materials
│ │
│ ├── Ferrite Soft Magnets
│ │ ├── Mn-Zn Ferrite
│ │ ├── Ni-Zn Ferrite
│ │ └── Mg-Zn Ferrite
│ │
│ ├── Metallic Soft Magnets
│ │ ├── Electrical Pure Iron
│ │ ├── Silicon Steel
│ │ ├── Permalloy
│ │ └── Other Soft Magnetic Alloys
│ │ ├── Fe-Al Alloy
│ │ ├── Fe-Si-Al Alloy
│ │ └── Fe-Co Alloy
│ │
│ └── Nanocrystalline Soft Magnets
│ ├── Amorphous Soft Magnetic Materials
│ └── Nanocrystalline Soft Magnetic Materials
│
├── Hard Magnetic Materials
│ │
│ ├── Metallic Permanent Magnets
│ │ ├── Quenched Hardened Magnetic Steel
│ │ │ └── Carbon Steel, Tungsten Steel, Chromium Steel,
│ │ │ Aluminum Steel, Cobalt Steel, etc.
│ │ ├── Precipitation-Hardened Magnetic Steel
│ │ │ └── Fe-Ni Alloy, Fe-Co Alloy, Al-Ni-Co Alloy
│ │ ├── Age-Hardened Permanent Magnets
│ │ │ └── Fe-Ni-Ti, Fe-Ni-Al, Cu-Ni-Fe,
│ │ │ Fe-Co Alloy
│ │ └── Ordered Hard Magnetic Alloys
│ │ └── Ag-Mn, Co-Pt, Fe-Pt, Mn-Al,
│ │ Mn-Bi Alloys
│ │
│ ├── Ferrite Permanent Magnets
│ │ ├── Barium Ferrite
│ │ └── Strontium Ferrite
│ │
│ └── Rare Earth Magnets
│ ├── SmCo Magnets
│ │ ├── SmCo5
│ │ └── Sm2Co17
│ ├── NdFeB Magnets
│ │ ├── Sintered NdFeB
│ │ ├── Bonded NdFeB
│ │ └── Hot-Pressed NdFeB
│ └── Rare Earth Iron Nitride Magnets
│
└── Functional Magnetic Materials
├── Magnetostrictive Materials
├── Magnetic Recording Materials
├── Magnetoresistive Materials
├── Magnetic Bubble Materials
├── Magneto-Optical Materials
└── Magnetic Thin Film Materials
