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Ferromagnetic resonance absorptions at 95 GHz (corresponding to x=0.66) and 105 GHz (x=0.49) are found for a series of ε-AlxFe2-xO3 nanomagnets prepared by sol-gel techniques. Free-space quasi-optical spectrometer equipped with a set of tunable backward-wave oscillators as high-power sources is used in this paper to study the absorption property of these nanomagnetic materials. Transmittance spectra of ε-AlxFe2-xO3 with different x values (x=0.66 and 0.49) are recorded using a free-space quasi-optical spectrometer. The absorption property is suggested to be caused by the natural resonance achieved by the large magnetic anisotropies in this series of aluminum substituted ferrite nanomagnets. The absorption peak shifts to the lower frequencies with increasing value of x-parameter. The average particle size of ε-AlxFe2-xO3 nanomagnets lies between 25 to 50 nm. The absorption characteristics are repeatable for these new specimens prepared one year later with absorption characteristics of earlier prepared samples.