The role of Zn and Ru substitution on the structural, magnetic and dielectric properties of ferrite cuprospinels

A comprehensive study on the structural, magnetic and dielectric behavior of ferrite cuprospinels with site specific substitution of non-magnetic Zn and weakly magnetic Ru ions has been presented. Such substitution leads to two classes of stable crystal symmetries, (i) tetragonal (I41/amd): CuFe1.95Ru0.05O4 and Cu0.95Zn0.05Fe1.95Ru0.05O4 and (ii) cubic (Fd-3m):Cu0.60Zn0.40Fe1.95Ru0.05O4 and Cu0.05Zn0.95Fe1.95Ru0.05O4 bulk polycrystals. The former set of compounds undergo significant Jahn-Teller (JT) distortion due to high Cu content and such distortion gets lifted with the substitution of non-magnetic Zn ions. Ferrimagnetic (FiM) behavior is predominant in all these compounds except the highest Zn (95 at.%) content cubic system with diluted substitution of Ru (2.5 at.%) which exhibits antiferromagnetic ordering below the Neel temperature (T-N similar to 10.9 K). We employed Neel's expression for FiM systems applicable for the two sublattice model to extract the dominant exchange interactions ((JA)A, J(AB) and J(BB)) between the A-site and B-site cations. Subsequently the high temperature (300 K <= T <= 900 K) inverse paramagnetic susceptibility data has been fitted to the above expression which yields dominant J(AA)/k(B) similar to 391 K with very high asymptotic Curie temperature for the tetragonally distorted dilute Zn substituted cuprospinel, while the Zn-rich cubic system exhibits J(AA)/k(B) similar to 300 K. On the other hand, the tetragonally distorted spinel devoid of Zn exhibits dominant JBB/kB similar to 229 K with negligible JAA/kB and moderate J(AB)/k(B) (65 K) signifying that the JT active ions cause the onset of magnetic frustration and enhanced ordering temperatures. All the investigated systems exhibit significant magneto-crystalline anisotropy with a maximum anisotropy field H-K similar to 6 kOe and maximum anisotropy constant (K-1 similar to 10.88x10(5) erg cm(-3)). The broadband dielectric spectroscopy studies reveal significant enhancement of mid-frequency dielectric constant is an element of(R)(f, T) with plateau-like feature over a wide temperature and frequency window, together with low-loss as a result of the incorporation of Ru ions into the tetragonally distorted spinel lattice. The ac resistivity rho(f, T) studies reveal that the mode of conduction follows Mott's variable-range hopping (VRH) model with a linear logarithmic variation of rho(ac) versus T-0.25. The temperature dependence of the VRH activation energy of charge carriers follows a linear behavior which indicates the presence of localized polaron in all the investigated systems.