Composition: Made of ferrite (iron oxides) or metals like nickel, iron, cobalt.
Coating: Typically passivated with silica, ligands, or surfactants to prevent clumping and functionalize the surface for applications.
Magnetization Curve (Hysteresis Curve)
Key Parameters:
Coercivity:
Indicates the magnetic field strength needed to reduce magnetization to zero.
Important for applications like data storage; requires balance between large coercivity (prevention of spontaneous bit flipping) and small coercivity (lower writing energy).
Remnants:
Value of magnetization when the external magnetic field is zero.
Large remnants needed for magnetic recording; small remnants preferred for applications like cancer treatment.
Size Dependence of Coercivity
Behavior in Ferromagnetic Materials:
As particle size decreases, coercivity initially increases, peaking when the particle becomes a single magnetic domain.
Further size reduction leads to coercivity falling to zero, transitioning to superparamagnetic behavior.
Critical Diameter and Competing Effects
Critical Diameter: Changes based on material type, shape, and other properties like magnetic saturation.
Competing Effects:
Demagnetizing Field:
Magnetic field generated by magnetization; energy cost associated with larger volumes increases with size.
Exchange Interaction:
Quantum mechanics leads to energetically favorable alignment of neighboring atoms; minimizes energy costs related to domain walls.
Behavior Under External Magnetic Field
Bulk Ferromagnet Behavior:
Initially random domains align under external magnetic field.
Domains grow in size toward alignment, propagating from domain walls outward.
Results in permanent magnetism when external field is removed due to coercivity.
Superparamagnetic Particles
Characteristics:
Behave like paramagnetic materials in the absence of an external field; can revert to random orientation.
Larger susceptibility than typical paramagnets due to ferromagnetic properties in bulk.
Applications of Superparamagnetic Nanoparticles
Biomedical Applications:
Contrast agents for MRIs.
Targeted drug delivery.
Hyperthermia for cancer treatment.
Storage Density Limitations:
Superparamagnetism limits the size of magnetic storage particles; critical for data retention.
Ferrofluids:
Colloidal suspensions of magnetic nanoparticles that behave like fluids in magnetic fields.
Composed of iron oxide nanoparticles coated to prevent agglomeration; lose magnetic properties when the field is zero.
Conclusion
Superparamagnetic nanoparticles exhibit unique properties that can be harnessed for various applications but also pose challenges, especially in data storage.