Research articles
Suppressed domain wall damping in planar BaM hexaferrites for miniaturization of microwave devices

https://doi.org/10.1016/j.jmmm.2020.167172Get rights and content

Highlights

  • CoRu substitution enhanced the permeability of planar BaM ferrite for GHz application.

  • Bi2O3 doping CoRu-BaM has lower magnetic loss with retaining a high permeability of 3.

  • Reduced loss arises from domain wall damping suppression by the control of grain size.

Abstract

CoRu-substituted BaM hexaferrites, with additives of Bi2O3, demonstrate low losses and high permeability above 1 GHz. The RuCo- and BiRuCo-doped BaM ferrites were prepared by conventional solid-state reaction methods. Magnetic spectra indicate that there were two peaks in the imaginary part of the permeability spectra which originate from domain wall motion and spin rotation. Compared to the RuCo-BaM sample, BiRuCo-doped BaM samples exhibit slight decreases in permeability (~2.9), but the magnetic loss tangents decrease markedly by 47% to ~0.05 at 1 GHz while retaining a low magnetic loss ~0.1 up to 1.5 GHz. From fitting to experimental data, it’s clear that the reduction of magnetic loss stems predominantly from a suppressed domain wall damping due to the introduction of Bi2O3. It is demonstrated that Bi2O3 can effectively control the microstructure of polycrystalline ferrites, and in turn tailor the contribution of domain wall resonance or spin rotation to permeability and magnetic loss. The results are valuable to the engineering of ferrite materials for use in the miniaturization of wide band microwave antenna at operating frequencies up to 1 GHz.

Introduction

Antenna substrate materials, made of low-loss magnetic materials, ideally have high permeability at applied frequencies to decrease antenna size without degradation to performance. With increasing demands from telecommunication devices and systems, the resonance frequency of ferrites must be shifted well above 5 GHz with high permeability and low losses [1].

Limited by the Snoek’s relationship, attaining high permeability in ferrites has been a longstanding challenge for high frequency applications. It is difficult for spinel ferrites to possess high permeability in the gigahertz range due to low anisotropy fields and low resonance frequencies.

Hexagonal ferrites have attracted more attention for gigahertz application because planar anisotropy of hexagonal ferrites extends Snoek’s limit [2], which is useful to shift the resonance frequency beyond GHz. Many methods used to adjust the planar anisotropy field or improve sintering process to lower the damping coefficient have been reported for BaM [3], [4], [5], Co2Y [6], [7] and Co2Z [8], [9], [10] ferrites to increase the permeability and decrease magnetic loss. However, the permeability is usually lower than 2.5 when the working frequency is above 1 GHz. It is difficult to meet the requirement of matching impedance. In general, there are two resonance frequencies observed in the permeability spectrum with frequency: one is domain wall motion at low frequency; while the other is resonance at high frequency stemming from spin rotation. It is clear that the domain wall resonance may limit lower operating frequencies for antenna operations. One of the solutions to this limiting magnetic loss is to have polycrystalline ferrites with finer grains, which enables the reduction of domain wall losses to the total loss. The other solution is to make polymer-based composites by mixing ferrites, or magnetic alloy particles, within dielectric polymer hosts [11], [12], [13]. However, the composite path leads to a marked reduction in permeability. A number of published works have revealed that it remains difficult to realize a permeability above 3 while retaining a magnetic loss tangent of <0.1 over a frequency range of 1–5 GHz.

Here, we present a path to realizing both high permeability and low loss by means of the introduction of Bi2O3 to CoRu-doped BaM type ferrites. The addition of Bi2O3 effectively tailors the microstructure of polycrystalline ferrites thus modifying the role of domain wall resonance to both permeability and magnetic loss. The experimental results indicate that the ferrite materials have a permeability of ~3 at a frequency >1 GHz with a magnetic loss tangent of ~0.05 and 0.1 at 1 GHz and 1.5 GHz, respectively. The results demonstrate great potential for application as substrates for miniaturized antenna at f  1 GHz.

Section snippets

Experimental

The BaCo1.2Ru0.2Fe10.9O19 M-type hexaferrite were prepared via the conventional solid-state reaction method. Starting materials of BaCO3, Co3O4, RuO2 and Fe2O3 of reagent grade purity were mixed, dried and calcined at 1100 °C for 4 h. The calcined powders were then ground by a planetary ball mill. During ball milling, one of the calcined powders was milled without any additive as a control sample (i.e., Sample A), whereas the other was milled with 1 wt-% Bi2O3 (i.e., Sample B). After

Results and discussion

Fig. 1 presents permeability and magnetic loss values at 1 GHz for Samples A and B sintered at different temperatures. The permeability of Sample A, sintered in air, increases slightly with sintering temperature, but the magnetic loss exhibits different behavior and experiences a minimum at a sintering temperature of 1260 °C. To further lower magnetic losses, the sample was sintered in an oxygen gas atmosphere while an additive (Bi2O3) is incorporated. Fig. 1(b) indicates that the additive

Conclusion

We have demonstrated that Ru substituted BaM-type hexaferrite has excellent high frequency magnetic characteristics, but the magnetic loss properties induced by domain wall motion remains a hindrance to practical applications. Our attempt to lower magnetic losses by tailoring grain morphology, magnetic domain structure and cut-off frequency by the introduction of Bi2O3 additive in conjunction with co-doping of Co and Ru ions in the BaM ferrite led to marked success. Experimental data revealed a

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Qifan Li: Conceptualization, Methodology, Formal analysis, Writing - original draft. Yajie Chen: Resources, Writing - review & editing. Qifan Li: Formal analysis, Validation. Lezhong Li: Data curation, Investigation. Kun Qian: Formal analysis, Investigation. Vincent G. Harris: Supervision, Writing - review & editing.

Acknowledgement

This research is funded by Rogers Corporation.

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