Question

Which of the following materials exhibits superparamagnetism at the nanoscale?

• A. Gold nanoparticles
• B. Silicon nanowires
• C. Iron oxide nanoparticles
• D. Carbon nanotubes

Hint:

Superparamagnetism is a phenomenon observed in nanoparticles where they exhibit magnetic behavior only in the presence of an external magnetic field.
Iron oxide nanoparticles (Fe$_3$O$_4$ and $\lambda$-Fe$_2$O$_3$) are a common example of materials exhibiting superparamagnetism at the nanoscale.
Other materials like gold nanoparticles and carbon nanotubes do not show superparamagnetism.

Answer 1

The Correct Option is C

Superparamagnetism is a phenomenon that occurs in nanoparticles, where the material exhibits magnetic properties in the absence of an external magnetic field. The magnetic moments of individual nanoparticles randomly align, but when an external magnetic field is applied, they quickly align with the field. Let’s analyze the options to determine which material exhibits superparamagnetism at the nanoscale.
Step 1: Understand superparamagnetism
Superparamagnetism typically occurs in magnetic nanoparticles when their size is reduced to the nanoscale. At this size, the thermal energy becomes comparable to the energy required to reorient the magnetic moments of the particles, causing the magnetic properties to be highly sensitive to external magnetic fields.
Step 2: Analyze Option A - Gold nanoparticles
Gold nanoparticles are not known to exhibit superparamagnetism. While gold is a noble metal and has many interesting properties at the nanoscale, it does not show magnetic behavior like superparamagnetism.
Thus, option A is incorrect.
Step 3: Analyze Option B - Silicon nanowires
Silicon nanowires are semiconductor materials and do not exhibit superparamagnetism. They have electronic properties but are not typically magnetic at the nanoscale.
Thus, option B is incorrect.
Step 4: Analyze Option C - Iron oxide nanoparticles
Iron oxide nanoparticles, particularly magnetite (Fe$_3$O$_4$) and maghemite ($\lambda$-Fe$_2$O$_3$), are known to exhibit superparamagnetism at the nanoscale. These nanoparticles have magnetic properties that allow them to behave as superparamagnetic materials, especially when their size is reduced to below 30 nm.
Thus, option C is the correct answer.
Step 5: Analyze Option D - Carbon nanotubes
Carbon nanotubes are not typically magnetic and do not exhibit superparamagnetism. They are made of carbon atoms arranged in a cylindrical structure, and while they have unique mechanical and electrical properties, they do not exhibit superparamagnetism.
Thus, option D is incorrect.
Step 6: Conclusion
Superparamagnetism is exhibited by materials that can rapidly align with an external magnetic field when reduced to the nanoscale. Iron oxide nanoparticles (Fe$_3$O$_4$ and $\lambda$-Fe$_2$O$_3$) are well-known examples of such materials. Therefore, option C is the correct answer.