Dipolar glass-like dielectric response of nanocrystalline Sr0.95Nd0.05Fe12-xScxO19 hexaferrites

Recently reported magnetic quantum paraelectric properties in M-type hexaferrite single crystals have encouraged us to study the dielectric response of SrFe12O19 nanocrystallites down to the temperature of 10 K. As Sc-induced multiferroicity, promising for electromagnetic control, has been reported in bulk and films of hexaferrites, we also studied the size effect in dielectric response of Sr0.95Nd0.05Fe12-xScxO19 nanocrystallites with x = 0.36, 1.08, and 1.56. The nanopowders were obtained by citric method and the phase purity and the microstructure were controlled using X-ray diffraction and scanning electron microscopy. No clear evidence of quantum paraelectric behavior has been observed in temperature variation of dielectric permittivity of SrFe12O19 nanopowder. In the case of Nd-stabilized Sc-doped nanocrystallites, a low-temperature dielectric relaxation, similar to that in dipolar glasses, has been discovered. Activation energy of 62.5 meV was obtained for the lowest doping level and a modest increase in the energy was found at higher Sc concentrations. We relate the low-temperature relaxation in Sc-doped hexaferrite nanopowder to dielectric displacive polarization at the 4e Wyckoff sites modified by Sc-ions substituting the ferric ions in 4f2 and 12k positions.