IGNOU MPHE 27 2026 - What is the difference between single domain and superparamagnetic par

Question

What is the difference between single domain and superparamagnetic particles? A superparamagnetic material has a characteristic time of 10^-11 seconds and a Néel relaxation time of 2.0 second at 300 K.

Calculate

i) the energy barrier (in eV); and

ii) the blocking temperature if the measurement time is 120s.

Take $k_B = 8.62 \times 10^{-5}\text{ eVK}^{-1}$ .

19 Jan 2026

Answer :

Word Count : 255

Magnetic nanoparticles exhibit different magnetic behaviors depending on their size. A single-domain particle is one in which the entire particle is uniformly magnetized, forming only one magnetic domain. In such particles, the magnetization is stable over time and does not spontaneously reverse at ordinary temperatures because the magnetic anisotropy energy barrier is sufficiently large compared to thermal energy. In contrast, superparamagnetic particles are extremely small single-domain particles in which thermal fluctuations are strong enough to randomly flip the direction of magnetization. As a result, they show no remanent magnetization or coercivity in the absence of an external

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