SPM particles exhibit superparamagnetic properties above so called blocking temperature TB. Below TB they are in blocked state, which means no relaxation of magnetic moment. Blocking temperature depends on magnetic anisotropy K, volume of the sample V and time of measurement tm: TB=KV/kBln(tm/t0) where t0 is attempt time characteristic for material and kB is Boltzmann constant. Usually t0=10−9–10−10 second.

For simplicity, SPM materials are in superparamagnetic state above TB=KV/kB. This temperature depends both on anisotropy constant and volume of a nanoparticle. Thus there is no universal size of a particle, regardless of a material to be paramagnetic. Please see Fig.4 in the article attached which presents critical sizes for various materials.

Best regards, Bartek

Superparamagnetic - single domain particles are achieved in the range of ferromagnetic particle diameter from 10 to 100 nm. Single domain particle formation depends on chemical and phase composition of the particles.

Dear Sajid bhai,

For SPM the main condition is that the particles should behaves as a single domain. And the SPM behavior is observed at a particular temperature which should be lower than the activation energy because of the criteria of non-interaction between nano-domains in ensemble. Also, the worth point of notice is that there is no critical size for all the materials, they varies. I hope the attached review article will be helpful for you and satisfied you question up to the mark.

Best Regards,

A. Ahad

Thank you all for your reply.

thanks for sharing MRSBull-38-899-3t 8- size effec.pdf