Young-Min Kang

A cobalt-free compositional strategy was pursued to promote Ce incorporation in M-type SrFe₁₂O₁₉ and thereby enhance its hard magnetic performance. In Sr_1−xCe_xFe_12−xMg_xO₁₉ (x = 0–0.15), the highest coercivity and finest microstructure were obtained at x = 0.1; however, a residual CeO₂ phase persisted at all substitution levels, suggesting limited Ce incorporation. Partial replacement of Sr²⁺ by Ca²⁺ suppressed detectable CeO₂ reflections once y ≥ 0.2 in Sr_0.9−yCa_yCe_0.1Fe_11.9Mg_0.1O₁₉, providing indirect XRD evidence for enhanced Ce incorporation, although an α-Fe₂O₃ secondary phase remained. Reducing the nominal Fe content in Sr_0.7Ca_0.2Ce_0.1Fe_11.9−zMg_0.1O₁₉ (z = 0–0.75) further suppressed the α-Fe₂O₃ reflections, which were no longer detected at z = 0.75 (Sr_0.7Ca_0.2Ce_0.1Fe_11.15Mg_0.1O₁₉). Because the Ca-containing compositions exhibited reduced coercivity, potentially associated with grain coarsening, high-energy milling combined with sintering-temperature and additive optimization (SiO₂, CaO, SrCO₃, Cr₂O₃, ZnO, Mn₂O₃) was applied to restore a fine growth-controlled microstructure, lifting coercivity from 2714 Oe to 3552–3911 Oe. Additive sets sharing a common SiO₂–CaO–SrCO₃ base with ZnO, Cr₂O₃, or Mn₂O₃ addition yielded a closely matched, favorable remanence–coercivity balance. This work demonstrates a cobalt-free composition–processing route to hard magnetic M-type hexaferrites through Ca-assisted Ce incorporation, nominal Fe-content adjustment, and microstructural control.

Magnetic iron garnets hold great interest for magnonic, spintronic, and magnetooptical devices, but utilizing voltage-driven phenomena in insulating garnets has been prevented by the lack of a conductive epitaxial garnet underlayer. Beyond providing sufficient electrical conduction, the underlayer should exhibit low saturation magnetization to minimize magnetic interaction with the magnetic layers of the heterostructure, while maintaining high crystalline quality to enable epitaxial growth. Here, defect engineering via site-selective doping is employed to enhance the electrical conductivity of Y3Fe5O12 (YIG) while suppressing its magnetization. Epitaxial Fe-deficient Ca-doped YIG (Y2.7Ca0.3Fe4.7O12-δ, CaYIG) films, where Ca2+ substitutes Y3+ in the dodecahedral sites, exhibit electrical resistivities of 5.8 × 103 Ω cm and above and show saturation magnetization of 145 kA/m comparable to that of YIG, with perpendicular magnetic anisotropy or an in-plane easy axis depending on the substrate. Substitution of Al3+ into tetrahedral Fe3+ sites in Ca0.3Y2.7Fe3.5Al1O12-δ (CaAlYIG) reduces the saturation magnetization to below 25 kA/m and increases the resistivity to 1.65 × 104 Ω cm or higher. The functionality of CaAlYIG was demonstrated in a heterostructure of Bi1.2Y1.8Fe5O12 (BiYIG)/CaAlYIG/Gd3Sc2Ga3O12, where the CaAlYIG layer enables a voltage to be applied through the thickness of the BiYIG layer.

This study investigates the structural and magnetic properties of La–Mg co-substituted M-type strontium hexaferrites with the compositions Sr_1−xLa_xFe_12−xMg_xO₁₉ (x = 0, 0.05, 0.1, 0.15), synthesized via a conventional solid-state reaction route. X-ray diffraction confirmed the formation of single-phase M-type hexaferrites up to x = 0.15. The Sr₀.₉La₀.₁Fe₁₁.₉Mg₀.₁O₁₉ composition (x = 0.1) composition exhibited the best magnetic performance, achieving a saturation magnetization, M_S = 71.9 emu/g, remanent magnetization, 4πM_r = 2280 G, and coercivity, H_C = 3549 Oe. These improvements were attributed to the synergistic effect of La³⁺ and Mg²⁺ substitution, which enhanced magnetocrystalline anisotropy and refined grain morphology. Further enhancement was obtained by adding sintering additives (0.8 wt% SiO₂, 0.5 wt% CaO, and 0.7 wt% SrCO₃), resulting in an H_C of 4489 Oe and a 4πM_r of 2285 G. The results demonstrate that La–Mg co-substitution, combined with optimized sintering conditions, provides a cobalt-free strategy for developing high-performance, environmentally benign M-type ferrite magnets for advanced motor and electronic applications.

W-type hexaferrites with compositions Ba_0.5Sr_0.5Zn_2-xMe_xFe₁₆O₂₇ (Me = Fe, Ni, Co, Cu, Mn; x = 1) and Ba_0.5Sr_0.5Zn_2−xMn_xFe₁₆O₂₇ (x = 0–2.0) were synthesized via solid-state reaction and optimized using a two-step calcination process to obtain single-phase or nearly single-phase structures. Their electromagnetic (EM) wave absorption properties were investigated by fabricating composites with 10 wt% epoxy and measuring the complex permittivity and permeability across two frequency bands: 0.1–18 GHz and 26.5–40 GHz. Reflection loss (RL) was calculated and visualized as two-dimensional (2D) maps with respect to frequency and sample thickness. In the 0.1–18 GHz range, only the Co-substituted sample exhibited strong ferromagnetic resonance (FMR) and broadband absorption, achieving a minimum RL of −41.5 dB at 4.84 GHz and a −10 dB bandwidth of 11.8 GHz. In contrast, the other Ba_0.5Sr_0.5Zn_2-xMe_xFe₁₆O₂₇ samples (Me = Fe, Mn, Ni, Cu) showed no significant absorption in this range due to the absence of FMR. However, all these samples clearly exhibited FMR characteristics and distinct absorption peaks in the 26.5–40 GHz range, particularly the Mn-substituted series, which demonstrated RL values below −10 dB over the 32.0–40 GHz range with absorber thicknesses below 1 mm. The FMR frequency varied depending on the substitution type and amount. In the Mn-substituted series, the FMR frequency was lowest at x = 1.0 and increased as x deviated from this composition. This study confirms the potential of Co-free W-type hexaferrites as efficient, cost-effective, and broadband EM wave absorbers in the 26.5–40 GHz range.