Microwave and millimeter wave dielectric permittivity and magnetic permeability of Epsilon-Gallium-iron-oxide nano-powders

Abstract

The hexagonal ferrite materials such as Ba- and Sr-Fe12O19 show strong ferromagnetic absorption in the 40–60 GHz range. However, in the higher millimeter wave frequency range beyond 60 GHz, the natural ferromagnetic ferrite materials with high performance are lacking due to the limitation of material science and synthetic complexity. The synthesis of single phase of e- iron oxide (e-Fe 2 O 3 ) enables the development of magnetic materials which have natural ferromagnetic resonance from 50 to 200 GHz. Among the four polymorphs of α-, β-, γ-, e-Fe 2 O 3 , the β- and e- Fe 2 O 3 are rare and must be synthesized in the laboratory. The pure e- Fe 2 O 3 shows the largest coercive field value (Hc) of 20 kOe among metal oxide-based magnets at room temperature. Multiple factors contribute to the gigantic H c in e-Fe 2 O 3 . One is the small e-Fe 2 O 3 crystal size. A large H c value is expected when the particle size is sufficiently small to form a single magnetic domain. A particle size of ca. 100 nm in the material is suitable to realize a single magnetic domain. Another is the intrinsic magnetic property of the e-Fe 2 O 3 phase. The H c value depends on the magnetocrystalline anisotropy constant (K) and saturated magnetization (M), i.e., H c K/M s . Analysis of the initial magnetization process estimates the K value in the present material as 2–4 × 10 erg cm−3, which greatly exceeds the K values of γ-Fe 2 O 3 (ca. 104 erg cm-3) and α-Fe 2 O 3 (ca. 10 erg cm-3). In addition, the observed M value is small, 15 emu g−1 (15 A m2 kg−1) at 7.0 T. Therefore, it is concluded that the large H c value of 20 kOe is due to (1) the suitable nanoscale size of particles that form a single magnetic domain and (2) the large K and small M s values of e-Fe 2 O 3 . Such high H c is very attractive in millimeter applications. By employing metal substitution method, the metal-substituted e-iron oxide can exhibit an adjustable ferromagnetic resonant frequency at 35–182 GHz depending on the degree of metal substitution. These nano-sized materials can be further made into composite material and applied in millimeter wave devices such as phase shifter, isolator and circulator. The gallium substituted e-iron oxide (e-Ga x Fe 2−x O 3 ) in this paper with x=0.22 and 0.29 are characterized by the vector network analyzer from 6 to 40 GHz and by free space quasi optical method (30–120 GHz). The tunability of the ferromagnetic resonance frequency depends on the x-parameter constituents. The ferromagnetic absorption peak moves to higher frequencies with decreasing x-parameter value. These absorptions are due to the natural resonance achieved by the large magnetic anisotropies in this series with change in x-parameter. The materials have ferromagnetic resonant frequencies in the frequency range from 30 GHz to 150 GHz relies on the concentration of gallium ions [1]. The e-GaxFe2-xO3 is synthesized by sol-gel techniques [2]. The particle sizes are observed to be smaller than 100 nanometer. The nano-powders are characterized by vector network analyzer employing transmission and refection method [3]. The complex permittivity and permeability spectra over the X-band frequency range are shown in Figure 1. The measurement was made in powder state. However the deensity value was recorded. The data seem to be mostly flat in this frequency range. Figure 2 shows the imaginary permeability in the millimeter wave frequency range. Such materials have great potential in developing millimeter wave absorber and other devices such as isolator and circulator [4]. The real part of the magnetic permeability displays dispersive-shaped lines at 98 GHz and 113 Hz for the E-Ga 0.29 Fe 1.71 O 3 and e-Ga 0.22 Fe 1.78 O 3 , respectively. The μ values reach a maximum around 98 GHz and 113 GHz; μ max = 0.78 (98 GHz) and μ max = 0.85 (113 GHz) which are expected as the ferromagnetic absorptions present.