Modified composition of barium ferrite to act as a microwave absorber in X-band frequencies

Abstract

The magnetic and electromagnetic wave absorbing properties of the Co–Mn–Ti substituted barium ferrites, BaCo_xMn_xTi_2xFe_12−4xO₄ (0≤x≥0.5), prepared by a solid-state technique, were investigated. The grain size uniformity and densification improved with increasing concentration of Co–Mn–Ti, while the permeability increased and the coercivity decreased remarkably. The reflection coefficients $(S_{11},S_{12})$ of the ferrite samples were measured in the X-band frequency range (8–12 GHz) and their absorbing properties were calculated according to transmission line theory. The maximum reflection loss (−14.7 dB) is observed for the $x=0.4$ sample. A ferromagnetic resonance frequency shift has been observed to depend on the Co–Mn–Ti concentration, which is an important phenomenon of barium ferrite for absorbing microwaves at high frequencies. Based on the magnetic and microwave measurements, BaCo_xMn_xTi_2xFe_12−4xO₄ may be a good potential candidate for electromagnetic compatibility and for other practical applications at high frequencies.

Introduction

With the advancement of electronic technologies, special attention has focused on the development of new microwave materials and their composites for applications in the field of shielding and stealth technology. Therefore, demands to develop thinner electromagnetic (EM) wave absorbers with wider absorbing band-widths are ever increasing [1], [2]. EM wave energy can be completely absorbed and dissipated into heat through magnetic losses and dielectric losses if the characteristic impedance of free space is matched with the input characteristic impedance of an absorber. The microwave absorber can be used to minimize the electromagnetic reflection from metal plates such as aircraft, ships, tanks and walls of anechoic chambers and electronic equipment [3], [4]. For the purpose of preparing a low-reflecting absorber in the desired frequency range, two fundamental conditions must be satisfied; the first is that the incident wave can enter the absorber by the greatest extent (impedance matching characteristic), and the second is that the electromagnetic wave entering into the materials can be almost entirely attenuated and absorbed within the finite thickness of the absorber (attenuation characteristic). Among the materials used in microwave absorbing applications, ferrites exhibit an interesting behaviour. Ferrites have been used as absorbing materials in various forms, e.g. sheets, paints, films, ceramic tiles and powders loaded in matrix composites or mixed with conducting materials [5]. The hexagonal ferrites like barium ferrite are suitable as a radar absorbing material (RAM) due to their large values of permeability and magnetization as well as their good dielectric properties at microwave frequencies. X-band frequency radar has been of great interest to the military sector because it allows for high resolution imaging and greater precision target identification. There are different EM wave absorber materials being used, including ferrites, conducting fibers, ferromagnets, and carbon nanotubes [6], [7], [8]. Substitution for the Fe³⁺ and Ba²⁺ is an effective method to vary the magnetic properties of barium ferrite. After the Fe or Ba ions are substituted with Co–Ti, Zn–Ti, Zn–Sn, Co–Sn, Ni–Zr and Co–Mo [9], [10], [11], [12], [13], [14], the saturation magnetization, coercivity, anisotropy constant and ferromagnetic resonant frequency of barium ferrite are changed. The electromagnetic wave absorption by a particular material is limited to a narrow band of frequencies. This is one of the serious constraints in an absorbing material. If several kinds of barium ferrite with different resonant frequencies are combined into a multilayer structure, the material may absorb electromagnetic waves in a wide microwave band. The relationship between the ferromagnetic resonant frequency of the barium ferrite and the substitution elements requires further research. It is well known that some special phenomena possibly appear during ferromagnetic resonance at high frequency, such as multipeaks [15], [16]. A multipeak may increase the microwave absorption capability of barium ferrite. In the work carried out by Kagotani [17], after Fe and Ba ions were substituted with La, Zn and Mn ions, the microwave absorption peaks of barium ferrite varied gradually in the range of frequency from 4 to 16 GHz. Moreover, no secondary peak of the ferromagnetic resonance arose. When Meshram [18] substituted Fe ions with Co, Ti and Mn ions, the microwave absorption properties of barium ferrite were improved. But several peaks appeared on the absorption spectra. The factors resulting in the multipeak have been researched for a long time. They are related with not only the high frequency, but also the grain size, crystalline structure and magnetic domains [15]. In this paper, the magnetic and microwave absorption properties and the ferromagnetic resonant frequencies of Co–Mn–Ti substituted barium ferrites are explored. The factors resulting in the multipeak during ferromagnetic resonance are also discussed.