ORIGINAL PAPER Dielectric permittivity of nickel ferrites at microwave frequencies 1 MHz to 1.8 GHz Jumiah Hassan & Farm Mooi Yen & Mansor Hashim & Zulkifly Abbas & Zaidan Abdul Wahab & W. Mohd. Daud W. Yusoff & Azmi Zakaria Received: 4 September 2006 / Accepted: 25 April 2007 / Published online: 26 June 2007 # Springer-Verlag 2007 Abstract NiFe 2 O 4 prepared via the sol–gel technique were pre-sintered at 900 °C and synthesized at different sintering temperatures from 1,000 °C to 1,200 °C at 100 °C intervals. The samples were characterized for microwave dielectric properties. These samples were measured using Agilent Impedance/Material Analyzer at frequencies 1 MHz to 1.8 GHz. Results showed a decrease in the dielectric constant and loss factor with frequency except at the turning point, around 150 MHz, where the loss factor showed a gradual increase. However, both the dielectric constant and loss factor increase with increasing sintering temperature. The grain size and density also increased with increasing sintering temperature, but the porosity and grain boundary density showed a decrease. Keywords Dielectric constant . Loss factor . Grain boundary . Porosity . Sintering . Dielectric permittivity . Nickel ferrites Introduction Ferrites are ceramic materials, dark grey or black in appearance, composed of oxides containing ferric ions as the main constituent [1]. The general formula of the spinel ferrite is MeFe 2 O 4 , where Me represents one or more than one of the divalent transition metals, Mn, Fe, Co, Ni, Cu, Zn, and Cd. These ceramic materials were developed for a wide range of application. Metal oxides in general have high dielectric constants, which made them good candidates for the manufacture of very small high-value capacitors. The application of magnetic oxides at microwave frequencies resulted from the aligned magnetic moments of the electron spins within the crystal lattice, which may be made to process at the frequency that depends on the strength of the static internal magnetic field. An incident circularly polarized electromagnetic wave will stimulate processional resonance only if the rotation of the polariza- tion is in the right sense with respect to the direction of the static fields. This gives rise to non-reciprocal devices in which the transmission properties in one direction are quite different from those in the other. A variety of microwave devices such as waveguide and strip line isolators, switches, circulators, modulators, and limiters have been developed. Dielectric permittivity or relative complex permittivity can be expressed as " * ¼ " 0 À j" ″ , where ε′ is the dielectric Ionics (2007) 13:219–222 DOI 10.1007/s11581-007-0094-y This paper was presented at the International Conference on Solid State Science and Technology 2006, Kuala Terengganu, Malaysia, Sept. 4–6, 2006. J. Hassan (*) : F. M. Yen : M. Hashim : Z. Abbas : Z. Abdul Wahab : W. M. D. W. Yusoff : A. Zakaria Department of Physics, Faculty of Science, Universiti Putra Malaysia, UPM, 43400 Serdang, Selangor, Malaysia e-mail: jumiah@fsas.upm.edu.my F. M. Yen e-mail: mooiyen@hotmail.com M. Hashim e-mail: mansor@fsas.upm.edu.my Z. Abbas e-mail: za@fsas.upm.edu.my Z. Abdul Wahab e-mail: zaidan@fsas.upm.edu.my W. M. D. W. Yusoff e-mail: wmdaud@fsas.upm.edu.my A. Zakaria e-mail: azmizak@fsas.upm.edu.my