Nanoceramics-SUPERPARAMAGNETISM
Superparamagnetism is the enhanced version of paramagnetism which specifically occurs in nanostructured magnetic materials. In paramagnetism, the magnetic behaviour of the materials becomes weakly magnetized in the presence of an external magnetic field, but do not retain magnetism once the field is removed. This temporary magnetic effect is due to the presence of unpaired electrons in the atoms or ions of the material, which align their magnetic moments with the applied magnetic field. Superparamagnetism is the “collective version” of paramagnetism.
What is Superparamagnetism?
Superparamagnetism is a magnetic phenomenon observed in nanoscale ferromagnetic or ferrimagnetic materials. It is a nanoscale effect where magnetic nanoparticles behave like paramagnets with enhanced magnetic properties. Each nanoparticle behaves like single magnetic domains and can spontaneously flip their magnetic direction under the influence of thermal energy. This means that, although the material is magnetic, it does not retain permanent magnetization when an external magnetic field is removed—much like a paramagnet, but with much stronger magnetic susceptibility.
Why It Happens:
In bulk ferromagnetic materials, domains align in a certain direction, and magnetic memory (remanence) is retained even after the field is removed.
But in nanoparticles below a critical size (typically < 20–30 nm), the entire particle acts as one magnetic domain, and thermal fluctuations become strong enough to flip the direction of magnetization randomly.
As a result:
- No hysteresis loop is observed.
- Zero coercivity and zero remanence are seen at room temperature.
Key Characteristics of Superparamagnetic Materials:
- High magnetic susceptibility (respond strongly to external magnetic fields)
- No residual magnetism after field removal
- No energy loss due to hysteresis
- Fast response to magnetic field changes
- Behaviour depends strongly on particle size and temperature
Examples of Superparamagnetic Materials:
Iron Oxide Nanoparticles (Fe₃O₄, γ-Fe₂O₃)
- Most common example used in biomedical applications
- Superparamagnetic when particle size is below ~20 nm
- Used in MRI contrast agents, magnetic hyperthermia for cancer treatment, and magnetic drug delivery systems
- Cobalt Ferrite Nanoparticles (CoFe₂O₄)
- Exhibit Superparamagnetism at smaller sizes
- Used in magnetically responsive smart materials, recording media, and magnetic biosensors
- Manganese Ferrite (MnFe₂O₄) and Nickel Ferrite (NiFe₂O₄) Nanoparticles
- Used in magnetic fluid hyperthermia and enzyme immobilization
- Strong superparamagnetic behaviour under optimized synthesis conditions
Applications of Superparamagnetism:
- Biomedicine
- MRI contrast agents: Provide high contrast with no residual magnetism
- Magnetic hyperthermia: Particles heat up under an AC magnetic field to kill cancer cells
- Targeted drug delivery: Nanoparticles can be directed using a magnetic field to specific body sites
- Data Storage & Spintronics
- Superparamagnetic limits are considered when designing ultra-dense magnetic memory
- Used in advanced read/write heads of hard drives
- Magnetic Separation
- Separation of biomolecules, cells, or pollutants using magnetic nanoparticle tags
- Sensors and Diagnostics
- Magnetic biosensors detect small biomolecules using superparamagnetic labels