Measurements of Complex Magnetic Permeability of Nano-Size $\varepsilon$ -Al$_{\rm x}$ Fe$_{2-{\rm x}}$ O$_{3}$ Powder Materials at Microwave and Millimeter Wavelengths

The imaginary part of complex magnetic permeability spectrum confirms the strong ferromagnetic resonance absorptions to appear at 89 GHz for the ε-Al
_x
Fe
_2-x
O
₃
nanomagnets prepared by sol-gel techniques for the x value to be 0.66. A strong anomalous dispersion corresponding to 89 GHz resonance is also observed in the real part of magnetic permeability spectrum. The 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 transmittance, real and imaginary parts of complex dielectric permittivity and real and imaginary parts of complex magnetic permeability properties of this nano magnet materials. Complex dielectric permittivity and Complex magnetic permeability spectra over six waveguide bands are almost flat in the 8-40 GHz range The vector network analyzer and in-waveguide measurements were employed for the measurements over 8-40 GHz range. 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-Al
_x
Fe
_2-x
O
₃
nanomagnets lies between 25 to 50 nm. The absorption and dispersion characteristics are repeatable for the same density of the powder material.