Dielectric response of Sr0.95Nd0.05Fe12-xScxO19 hexaferrites nanoceramics as dependent on crystal and microstructure and ceramic heterogeneity

Abstract

M-hexaferrite nanoceramics is of importance for advanced applications in which the dielectric response plays an essential role. Intrinsic dielectric response of M-hexaferrites related to its unusual crystal structure (ferrimagnetic unit cell contains two electric dipoles, switchable along the c-axis) is in homogeneous single crystals of quantum paraelectric type. The response becomes more complex when the hexagonal ferrites are functionalized by doping for various applications. We studied dielectric response of Sr0.95Nd0.05Fe12-xScxO19 (0 ≤ x ≤ 1.56) nanoceramics consisting of 50–100 nm thick single domain crystallites (for x ≥ 0.36). Ferric ions in oxygen octahedra substituted by Sc3+ change the collinear magnetism into a conical magnetic structure below the temperature of Tcone and results in dielectric polarization of Dzyaloshinskii-Moriya type. The electric dipole of spin origin were found by us to contribute to the dielectric response (both free nanocrystallites and in form of pellets) of polar glass-like response below 250 K. Moreover, the scandium substituted expanded octahedron coupled to acoustic phonons may modify the electric dipoles at the nearest trigonal bipyramid. The modified electric dipoles may either interact with other dipoles or contribute to the dielectric response of Debye type relaxation. In the case of Nd3+ stabilized structure we found an additional dielectric relaxation (above 330 K) due to charge oxygen vacancies created to compensate the difference valence of Sr2+ and Nd3+. In the case of nanoceramics the dielectric response contains also the contribution from the heterogeneous ceramic structure and its microstructure. According to Maxwell – Wagner – Koops model the high permittivity decreases with increasing frequency due to space charge polarization piled up at the poorly conducting grain boundaries. At higher frequencies impurity controlled electron hopping between the Fe4-Fe4 and Fe5-Fe5 Wyckoff positions is predominant. The third extrinsic contribution due to microstructure is related to size effect on magnetic conical structure resulting in a downward shift of glass transition anomaly.