Among the nanocrystals, the cobalt ferrite possessing magneto crystalline anisotropy
is an apt material to investigate the super paramagnetic characteristics and inter particle interaction due to its changes in block temperature in a long range up to room temperature [2-3].
[f.sub.FMR] increases with increasing [H.sub.k], where [H.sub.k] is the magneto- crystalline anisotropy
field, [M.sub.s] the saturation magnetization, and [gamma]/2[pi] the gyromagnetic constant (2.8MHz/Oe).
M-type strontium hexaferrites are hard magnetic materials that exhibit versatile properties like, high coercive force, high saturation magnetization, large magneto crystalline anisotropy
, high chemical and magnetic stability and relatively high Curie temperature .Owing to these properties, these ferrites are extensively applied in microwave devices, high density recording media, loud speakers, permanent magnets and humidity sensors [2-3].
Let nanoparticle's magnetic phases be represented by the crystals of cubic symmetry with the constants of crystalline anisotropy of the first [K.sup.(1,2).sub.1] and the second [K.sup.(1,2).sub.2] order, respectively.
(1) Crystallographic directions , , and  of the phases coincide with the axes OX, OY, and OZ, respectively (see Figure 1), if the first constants [K.sup.(1,2).sub.1] of the phases' crystalline anisotropy are positive.
(i) crystalline anisotropy energy, which, according to , depending on the sign of the constant of anisotropy has the following form:
Because of low anisotropy and Snoek's limit (occurrences of FMR at about 1GHz); spinel ferrites can't be used at higher frequencies , so for efficient working of electronic and telecommunication system it is better to use hexagonal ferrites instead of spinel ferrite, due to their large value of magneto crystalline anisotropy
and higher resonance frequency .
which is consistent with results derived from Hypothesis I, thus confirming a weak crystalline anisotropy