High-frequency Millimeter Wave Absorber Composed of a New Series of Iron Oxide Nanomagnets

Abstract

High-speed wireless communications using millimeter waves (30–300 GHz) have received much attention as a next-generation communication system capable of transmitting vast quantities of data such as high-definition video images. Due to the recent development of transistors composed of complementary metal-oxide semiconductors or double heterojunction bipolar transistors,1-5 electromagnetic (EM) waves in the millimeter wave range are beginning to be used in high-speed wireless communication.6-8 Especially, for 60 GHz-band wireless communication, Wigig alliance was established in December 2009, and televisions and local area network (LAN) using 60 GHz millimeter wave have been extensively researched and developed. Millimeter wave wireless communication is anticipated to realize a transmission rate that is several handred times greater than current wireless communication. On the other hand, in a wireless communication, electromagnetic interference (EMI) is a problem. In addition, the unnecessary EM waves should be eliminated to protect the human body, although the potential health effects due to the millimeter wave have not yet been understood.9 To solve these problems, millimeter wave absorbers need to be equipped with electronic devices such as isolators or be painted on a wall of building, etc. However, currently materials that effectively restrain EMI in the region of millimeter waves almost do not exist. Thus, finding a suitable material has received much attention. Insulating magnetic materials absorb EM waves owing to natural resonance. Particularly, a magnetic material with a large coercive field (Hc) is expected to show a highfrequency resonance. In recent years, we firstly succeeded to obtain a single phase of e-Fe2O3 nanomagnet (Figure 1), and found that e-Fe2O3 nanomagnet exhibited an extremely large Hc value of 20 kOe at room temperature, which is the highest Hc value for insulating magnetic materials.10-19 In this paper, we report a new millimeter wave absorber composed of eGaxFe2-xO3 (0.10 ≤ x ≤ 0.67) nanomagnets, which shows a natural resonance in the range of 35–147 GHz at room temperature.20 This is the first example of a magnetic material which shows a natural resonance above 80 GHz. In addition, the study of the magnetic permeability of e-GaxFe2-xO3 was performed in 60 GHz region (V-band). By analyzing electromagnetic wave absorption properties, the magnetic permeability (┤’-j┤”) and dielectric constant (e’-je”) of e-GaxFe2-xO3 were evaluated.21