Magnetic investigations on six compounds with the general formula (Ca,Y)Ba(Co,Fe,Al,Zn) 4O 7 and the structures of YBaCoFeZn 2O 7 and YBaCo 2FeZnO 7

Abstract

YBaCoFeZn 2O 7 and YBaCo 2FeZnO 7 are isostructural (YBaCo 4O 7 type, P 6 3 m c , a = 6.3 , c = 10.3 Å ). Four of the compounds, CaBaCo 2FeZnO 7, CaBaFe 2Zn 2O 7, YBaCoFeZn 2O 7, and YBaCo 2FeZnO 7, exhibit magnetic transitions in the range 6–65 K. There are indications of transitions for YBaFeZn 3O 7 and CaBaFeZn 2AlO 7, but these must be below 3 K. The AC magnetic susceptibilities ( χ) are frequency dependent: higher frequency increases the transition temperature. A loss in the magnetic energy is seen as peaks in the imaginary part of χ on the low temperature side of the real-part transition. The changes in T f with frequency, for both real and imaginary part of χ, are modelled using Arrhenius-, Vogel–Fulcher-, and power-law-calculations. The modelling indicates that the compounds are similar to metallic spin-glasses. Magnetisation up to 8 T did not magnetically saturate any of the samples. CaBaCo 2FeZnO 7 and CaBaFe 2Zn 2O 7 exhibit no clear difference between zero-field cooled (ZFC) and field-cooled (FC) magnetisation as well as no thermoremanent magnetisation (TRM) in the relaxation measurements, suggesting that these two compounds are more similar to disordered anti-ferromagnets. For YBaCoFeZn 2O 7 and YBaCo 2FeZnO 7, ZFC data have lower magnetisation than FC data and TRM is present, although full relaxation is reached within 10 2–10 3 seconds; these could be disordered anti-ferromagnets with spin-glass features. Double-exchange (DE) between Co 2+ and Co 3+ is the suggested mechanism responsible for the notable difference in properties between CaBaCo 2FeZnO 7 and YBaCo 2FeZnO 7, where the former has a significant DE and the latter not.