As given in figure 3 of ref paper attached, that magnetic nanparticles of single domain and superparamagnetic type are magnetized in the same direction.
My question is than what is the difference in single domain and superparamagnetic (I know superparamagnetic are less size) but what other properties relevant to them differentiate them ?
What is the dominant relaxation (Neel's or Brownian) heating mechanism in single domain and superparamagnetic ?
thanks
Dear Irfan,
The main characteristic and condition for superparamagnetism is to be formed by single domain magnetic nanoparticles.Thus these conditions are joined and no possible to be separated.
Single domain particles at certain size be superparamagnetic particles, but not any nanoparticles be superparamagnetic particles. The size guide the behaviour.
Single-domain particle consists of just one magnetic domain and when you neglect possible effects connected with the demagnetization field, its magnetization will equal the spontaneous magnetization Ms (at the given temperature) and its magnetic moment will be either Ms. V or σ . m where M is (volume) magnetization and V the volume of the particle and σ the specific magnetization (per mass unit) and m its mass. In principle and highly simplified, the particle becomes single-domain when its size is diminished to such an extent that its volume would not support the energy wall - in other words the difference in magnetostatic energy for the one- domain and multidomain state will be smaller than the energy of the domain wall.
Superparamagnetism is an effect similar to paramagnetism with the difference that the moments that are changing their directions due to thermal motion are not atomic moments but the moments of the (single-domain) particles. When speaking about superparamagnetism of a system, it is necessary to state the temperature and the time window of your observation, within which the moment will average to zero (in zero external field). The blocking temperature of the superparamagnetic particle is such temperature, at which the energy of the thermal motion equals the energy barrier dividing the states of different orientations of the particle moment. In particle with uniaxial anisotropy the barrier equals Ku V (where Ku is the anisotropy constant (energy density) and V the particle volume.
Let us note that the blocking temperature changes with the time window - for D.C. magnetic measurements the time window is of the order of 1-10 s and e.g. for a Mössbauer spectrum it is typically 10-9 s.
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