Design and development of Ga-substituted Z-type hexaferrites for microwave absorber applications: Mössbauer, static and dynamic properties

Abstract

Gallium substituted Z-type Sr₃Ga_xCo_2-xFe₂₄O₄₁ (x = 0.0–2.0 in steps of 0.4) hexaferrites were synthesised by the sol-gel auto-combustion process, and sintered at 1150 °C. The structural, morphology, magnetic, Mössbauer, dielectric and microwave absorption properties were examined. XRD results of x = 0.0, 0.4, 0.8, and 1.2 samples show the formation of a single Z-type hexagonal phase. The samples x = 1.6 and 2.0 show the formation of Z and M phases. Hysteresis loops analysis suggest that samples x < 1.6 possess a soft magnetic nature, while the samples x = 1.6 and 2.0 show a hard ferrite characteristics. All samples possess multi-domain microstructures. The composition x = 0.4 [maximum M_S = 97.94 Am²kg⁻¹] was fitted with seven sextets (Fe³⁺) and a paramagnetic doublet-A (Fe³⁺), while beyond x ≥ 0.8 two more doublets (Fe²⁺) were observed along with seven sextets in Mössbauer spectra. The maximum values of Fe²⁺ ions (1.26%) and relative area of paramagnetic doublets (1.91%) were observed for x = 1.6 composition, which is also responsible for the lowest value of M_S (69.99 Am²kg⁻¹) for this composition. The average hyperfine magnetic field was found to decrease, whereas average quadrupole splitting was found to increase, with Ga-substitution. The substitution of Ga ions enhanced permeability, dielectric constant, magnetic loss and dielectric loss, in a non-linear fashion. The reflection loss was maximum at lower frequencies for samples x = 0.0 and 0.8, and decreases with frequency. Sample x = 0.8 has maximum reflection loss of −12.44 dB at 8 GHz, a measured thickness of 3 mm, and a bandwidth of −10 dB at 1.18 GHz. The observed absorption has been discussed with the help of the input impedance matching mechanism and quarter wavelength mechanism. The observed coercivity in different samples also influenced microwave absorption which demonstrated potenial in microwave absorber applications.

Introduction

Recently, the enormous development in wireless technology has created severe electromagnetic pollution from electronic devices operating at high frequency ranges. Due to this pollution, electromagnetic interference (EMI)/radiation affects biological systems and the performance of electronic/electrical devices. Microwave absorbers are used to suppress GHz signals for military aircraft radar in stealth applications [1], and there has been a great deal of research in this area since World War II, particularly in the X-band, which covers around 8.2–12.4 GHz. Such materials are characterised by the absorption of electromagnetic waves in the GHz regime, which is determined by their structure, magnetic and dielectric properties [2]. Depending upon the application, these absorbing materials require low reflection loss over the wide/narrow bandwidth [2,3].

Numerous studies of hexaferrites have been reported for the development of microwave absorbing materials [4] and in this respect; they are superior to cubic spinel ferrites which typically operate in the vicinity of 1 GHz frequency. Hexaferrites are a family of complex ferrites discovered in the late 1950 [5], when they were of interest as microwave materials. The best known hexaferrites are the M-type ferrites (e.g., BaFe₁₂O₁₉), but owe complex phases too such as the Z-type hexaferrites [6]. These have the chemical composition A₃Me₂Fe₂₄O₄₁ where 'A' represents strontium (Sr), lead (Pb) or barium (Ba), and 'Me' represents a small divalent metal ion (typically a transition metal), a common example being Co²⁺ which typically forms a soft ferrite with high permeability [7]. Z-type Barium ferrites are known to show excellent microwave absorption properties in the low GHz region due to ferromagnetic resonance (FMR) [[8], [9], [10], [11], [12], [13]]. However, this range of frequency can be increased by tailoring the Fe³⁺ substitution with different dopants to have FMR frequencies between 3.5 GHz and 13.4 GHz [[14], [15], [16], [17], [18]]. The microstructure of the ferrites affect their properties, and flaky Cu and Zn Z-type ferrites containing variable grains showed a good reflection loss of < -10 dB from 2.0 to 9.6 GHz with a minimal of -17 dB reflection loss at ~2.5 GHz [14].

The Ba ion can be substituted with Sr to form SrZ-type (Sr₃Co₂Fe₂₄O₄₁) hexaferrite, first reported by Pullar and Bhattacharya [19] produced using the sol-gel process. The formation of monophase Z-type Sr hexaferrite (SrZ) took place over a very narrow temperature range of 1180–1220 °C, beyond which it decomposes to form SrCo₂Fe₁₆O₂₇ (SrW-phase) [20].

There are hardly any reports accessible in the literature on microwave properties of SrZ-type hexaferrites. High frequency magnetoelectric measurements were carried out on SrZ [21], looking at the effects of applied voltage on complex permeability at up to 4 GHz with a weak FMR peak between 2.5 and 3.5 GHz. Recently, Sr₃Co₂ZnFe₂₄O₄₁ was reported to have a reflection loss between −10 and −35 dB over the 8–12.5 GHz range [22], the best samples being 2.6 mm thick. A stable method with precise chemical composition is required for synthesising Z-type hexaferrites, typical techniques including sol-gel, sol-gel auto-combustion, solid-state reactions and chemical co-precipitation [9,21,[23], [24], [25], [26], [27], [28], [29], [30]]. A sol-gel auto-combustion process is considered to be one of the best methods, as the metal ions chelation eliminates the elemental homogeneities present in the gels that play a significant part in producing the desired single phase [31,32].

Ga³⁺substitution in M-type hexaferrites can enhance the absorption of electromagnetic energy which is appropriate for protective antiradar or EMI shielding from microwave radiation [33]. Trukhanov et al. have published several papers on the enhancing effect of Ga substitution on FMR frequencies in barium M ferrites (BaFe_12–xGa_xO₁₉) [[33], [34], [35], [36], [37], [38]], and Ihsan Ali et al. reported that the series obtained by substituting Cr–Ga in BaM hexaferrites are excellent materials for high-frequency applications [39]. Researchers have studied microwave absorption considering the simulated thickness of the material. For real-world applications as absorbers, EM absorption should be investigated as function of measured/actual thickness of the absorber. However, most of the reported work constitutes absorption investigation as a function of simulated thickness [22,40,41].

In the current study, the sol-gel auto-combustion process is used to synthesise Sr₃Ga_xCo_2-xFe₂₄O₄₁ hexaferrites (where x = 0.0, 0.4, 0.8, 1.2, 1.6, and 2.0), which were then sintered at 1150 °C. This is lower than the usual temperature required to forming the SrZ phase. The main purpose of the current work is to investigate the structural, morphological, magnetic, electrical, impedance and microwave absorbing properties of these Sr₃Ga_xCo_2-xFe₂₄O₄₁ (x = 0.0 to 2.0) hexaferrites when heated at 1150 °C, significantly lower than the usual temperatures required (1180–1220 °C), and their suitability for applications like EMI and radar absorbing materials (RAM). This work also considers microwave absorption as a function of the measured thickness.