Figure 2. Classification of magnetic materials and characteristics of magnetic particles with different sizes and shapes.

(a-c) Magnetic component of magnetic soft materials can be divided into three categories (soft-magnetic, hard-magnetic, and superparamagnetic) depending on their magnetization characteristics. In general, soft-magnetic materials are characterized by their high saturation magnetization (M_s), low coercivity (H_c), and low remanence (M_r) with narrow hysteresis curves, whereas hard-magnetic materials are characterized by large hysteresis due to their high coercivity and remanence. Superparamagnetic materials exhibit no hysteresis and become quickly saturated under relatively low fields. (d) Qualitative behavior of coercivity of magnetic particles depending on their size. The coercivity increases as the particles become smaller to approach the single-domain regime, but the coercivity disappears below a certain critical size to enter the superparamagnetic regime as the smaller particles become more susceptible to thermal fluctuation and hence cannot retain stable magnetism in the absence of external fields. (e) Magnetic particles can also be classified into magnetically isotropic or anisotropic particles depending on their particle morphology and preferred magnetization direction.