High-performance millimeter-wave absorber composed of ε-AlxFe2-xO3 nanomagnets is prepared by sol-gel techniques. Free space quasi-optical spectrometer equipped with a set of tunable backward-wave oscillators as power sources is used in this work to study the absorption property of this series of materials. Special aluminum-disc sample container with thickness of 2 mm is fabricated. Transmittance spectra of ε-AlxFe2-xO3 with different x values (x = 0.66 and 0.49) are recorded using our free space spectrometer. Strong magnetic absorption at 95 GHz (corresponding to x = 0.66) and 105 GHz (x = 0.49) are found. Combining the data obtained in this and previous work, second order polynomial curve is simulated to study the x parameter dependence of absorption frequencies. The simulation results are in good agreement with the experimental ones.