High-Frequency Magnetic Properties of Ca-Substituted Co2Z and Co2W Barium Hexaferrite Composites

A series of Ca-substituted Z-type hexaferrites having nominal composition Ba
_3-x
Ca
_x
Co
₂
Fe
₂₄
O
₄₁
(x = 0-1.5) was studied for their high-frequency magnetic properties, including measurement of the permeability spectra and magnetic loss over a frequency range of 0.1-10 GHz. Analyses of crystal composition and microstructure reveal that hexaferrite composites consist of the Z-type phase and increasing amounts of the W-type phase were obtained as the Ca content was increased. Correspondingly, the permeability μ' displays a near linear decrease with x varying from 0 to 1.5, from ~8 to ~4.1 at 0.1 GHz. The relationship between permeability and variations in intrinsic magnetic parameters, i.e., saturation magnetization M
_s
and coercivity H
_c
, follows the Globus model. Compared with the undoped Z-type hexaferrite, the sample with x = 0.2 has lower magnetic loss over a broad frequency range, and higher levels of Ca substitutions (x ≥ 1) lead to low magnetic loss at high frequencies. The dual-phase composite of Co
₂
Z and Co
₂
W in proper ratio is an effective technique to improve the homogeneity of grain and microstructure, and thus reduce the high-frequency magnetic loss of the sintering hexaferrites.