M. Ozdemir et al., EVIDENCE OF SUPERPARAMAGNETIC BEHAVIOR IN AN AMORPHOUS NI62MN38 FILM BY ESR MEASUREMENTS, Physica. B, Condensed matter, 252(1-2), 1998, pp. 138-148
Electron spin resonance (ESR) measurements were carried out on an amor
phous Ni62Mn38 film in the temperature range of 4-300 It. The spectra
comprise multi-peak resonance absorptions forming mainly two groups se
parated in the magnetic field; one of which corresponds to unresolved
usual spin-wave resonance (SWR) modes, the other occurs at about H = 3
kG for the applied field parallel to the film surface and shows a rat
her less dependence on the field direction compared to those of the ma
in SWR modes. It has been observed that the latter peak becomes more a
nd more apparent (the amplitude intensifies and the linewidth decrease
s) with decreasing temperature and then starts again to broaden as the
temperature is further decreased. This overall behaviour leads us to
suggest that the sample consists of a large number of smaller clusters
coupled weakly to each other. Macroscopic ferromagnetic state average
d over all these clusters gives us the usual SWR peaks. However, each
weakly coupled cluster behaves as an almost free individual magnetic e
ntity and therefore the latter peak can be attributed to the superpara
magnetic resonance absorbtions. By fitting the theoretical SWR modes s
imultaneously to the corresponding experimental ones for parallel and
perpendicular geometries, the exchange stiffness constant D, the excha
nge unidirectional anisotropy field H-exe, the bulk magnetic anisotrop
y constant 2K(eff)/M-0 and the surface pinning parameter QL have been
obtained as a function of temperature. The results have been compared
with the existing data for the NiMn alloys with different compositions
. The role of the local spin fluctuations (LSFs) on the temperature de
pendence of the magnetic parameters mentioned above has been discussed
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