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To inhibit electromagnetic interference problems in next generation wireless communications using millimeter waves, the development of millimeter wave absorbers is crucial. Herein we theoretically designed a millimeter wave absorbing sheet composed of ε-GaxFe2-xO3 nanomagnets based on impedance matching calculations. From the previously reported magnetic permeability (μ) of ε-GaxFe2-xO3, the artificial frequency dependences of the real (μ') and imaginary parts (μ") were constructed using the Landau-Lifshitz equation. The impedance matching calculation showed that a 217.2 μm thick metal-backed ε-Ga0.537Fe1.463O3 sheet perfectly absorbs 60 GHz millimeter wave, while a 201-233 μm thick sheet can absorb 99% of the millimeter wave, i.e., a reflectance loss (RL) of -20 dB. The frequency dependence of RL indicated that the frequency range where RL exceeds -20 dB is as broad as 1 GHz (59.5-60.5 GHz). Such theoretical calculations will be useful for fabricating a 60 GHz-band millimeter wave absorber sheet from the viewpoint of industrial applications.