BaNdFeO₄ revisited: true phase composition, magnetic structure evolution, and broadband microwave absorption in an...

Nominal BaNdFeO₄ has previously been regarded as a single-phase rare-earth ferrite; however, its true phase constitution and intrinsic functional properties remain insufficiently clarified. In this work, we revisit the nominal BaNdFeO₄ composition and demonstrate that it does not form as a single-phase compound, but rather as an intrinsic Ba-Nd-Fe-O self-composite predominantly consisting of Ba₆Nd₂Fe₄O₁₅ and BaNd₂Fe₂O₇ with comparable weight fractions. Combined X-ray diffraction, neutron powder diffraction, and morphological characterization, and energy-dispersive X-ray spectroscopy analyses provide consistent evidence for this revised phase composition. Neutron diffraction further reveals a previously unreported magnetic structure evolution in BaNd₂Fe₂O₇, in which an in-plane Fe-sublattice antiferromagnetic state with k ₁ = (0, 0, 0) is established at room temperature and progressively rotates toward the c axis near 20 K with the onset of Nd magnetic ordering. Meanwhile, Ba₆Nd₂Fe₄O₁₅ develops long-range collinear antiferromagnetic ordering of both Fe and Nd ions below T _N ≈ 18 K with a magnetic propagation vector k ₂ = (1/2, 0, 0). The intrinsic coexistence of these structurally and magnetically distinct phases produces an interface-rich and defect-prone microstructure that promotes dielectric-dominant broadband microwave absorption. Reflection-loss analysis reveals thickness-tunable absorption governed by effective impedance matching and quarter-wavelength interference, yielding a maximum effective absorption bandwidth of approximately 5.3 GHz at a thickness of ∼5.0 mm. These findings underscore the potential of this Ba-Nd-Fe-O intrinsic self-composite as a promising chemically stable oxide for broadband electromagnetic wave attenuation.