Magnetic and dielectric properties of Sr1-xNdxFe12-yAlyO19 co-doped hexaferrites

In forthcoming decades Radiation Absorbing Materials (RAM) are supposed to play a crucial role in wireless network communication for reduction of harmful Electromagnetic Interferences (EMI) and for protecting of living... [ view full abstract ]

In forthcoming decades Radiation Absorbing Materials (RAM) are supposed to play a crucial role in wireless network communication for reduction of harmful Electromagnetic Interferences (EMI) and for protecting of living organisms from electromagnetic smog causing unsafe biological and neurological effects. Promising candidates for inexpensive, safe and rare earth free RAM materials are hexaferrites, including SrFe₁₂O₁₉ (SFO) M-hexaferrites.

In this report we present co-doping with Nd and Al ions as an effective method for tailoring of both SFO hexaferrite properties and nanostructures. The doped hexaferrites were obtained using various methods, like the co-precipitation (Sr_1-xNd_xFe₁₂O₁₉), hydrothermal and microwave assisted hydrothermal method (Sr_0.95Nd_0.05Fe_12-yAl_yO₁₉).

Single phase solid solution with P6₃/mmc space group and Z=2 was confirmed for all samples by X-ray diffraction and the lattice parameters were found unaffected by Al³⁺ doping. It was confirmed that the proper amount of Nd dopant reduced content of the parasitic Sr₇Fe₁₀O₂₂ phase, modified the grain morphology and maximized the coercivity field due to more effective domain wall pinning. On the other hand, Al and Nd co-doping of SFO hexaferrites influenced mainly the magnetization whereas the coercivity was modified only slightly. The changes in magnetization mainly resulted from the substitution of Fe³⁺ ions in the 4f₂, 12k and 2a Wyckoff positions by Al³⁺ ions.

Substitution of Fe³⁺ with non-magnetic Al³⁺ ions influenced also electric properties creating two types of electric dipoles due to lowering the energy of the 4e positions of ferric Fe²⁺ ions in the trigonal bipyramid. Low-level doping broke the local symmetry in the 4f₂ Wyckoff positions which produced local maximum in dielectric relaxation, whereas higher doping above y>0.84 caused the symmetry breaking in the 12k positions and the second maximum in relaxation appeared.