Spinel ferrites field

R. Bauminger, S. G. Cohin, A. Marinov, S. Offer, E. Segal, Study of the low-temperature transition in magnetite and the internal fields acting on iron nuclei in some spinel ferrites, using Mossbauer absorption, Phys. Rev. 122 (1961) 1447-1450. 

It turns out that crystal field theory accounts adequately for practically all the experimental results in spinels. The fact that all the known chromites have a normal distribution is consistent with the high octahedral field stabilisation energy value calculated for Cr (S = 3/2). In ferrites, the arrangement is very dependent on the divalent cation, since Fe has no crystal field stabilisation energy. When the divalent cation also shows no clear preference, ferrites with 3 values between zero and one (mixed ferrites) are obtained. 

A magnetic material which is easy to magnetise (and demagnetise) is known as a soft material, and permanent magnets are referred to as hard magnets. The boundary in coercive field value for these two cases is somewhere between 1 kA/m (soft if // < 1 kA/m) and 10 kA/m (hard for k 10 kA/m). In ferrites, Mn-Zn spinels have the lowest values (//< 16 A/m) and BaFe 120,9 particles the highest 420 kA/m, Mee... 

Ferrofluids are a class of important ternary ferrite spinels are of the formula MFc204 (M = Co, Mn, Ni), synthesized as nano-sized particulates dispersed in a solvent. These solutions have applicatians that span electronic devices e.g., forming seals around spinning drive shafts in computer hard drives heat conduction in speaker tweeters) to medicine (hyperthermia-based cancer treatment - i.c., directing nanoparticles to a cancerous area and raising their temperature via an external magnetic field). For a review of ferrofluids, see Odenbach, S. J. Phys. Condens. Matter 2004,16, R1135. 

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