Nanoscale-driven structural changes and associated superparamagnetism in magnetically diluted Ni–Zn ferrites

Abstract

The investigation under study deals with the combustion synthesis of nanocrystalline Mg substituted nickel zinc ferrites i.e., Mg_xNi_0.6−xZn_0.4Fe₂O₄ (x = 0.0 to 0.6) using malic acid dihydrazide as a novel fuel and the effect of magnesium substitution on the structural and magnetic properties. The nanocrystalline monophasic nature of “as prepared” Mg_xNi_0.6−xZn_0.4Fe₂O₄ (x = 0.0 to 0.6) has been confirmed from XRD and Raman spectroscopic studies and the crystallite sizes were found to be in the range of 19–25 nm which nearly matches with TEM. The RT and low temperature magnetization studies indicate a continuous decrease in magnetization with increase in Mg substitution. In contrast, the hysteretic behaviour diminished and the emergence of superparamagnetism is observed in all the samples as the Mg content increases. The decrease in M_S with increase in Mg concentration is attributed to the replacement of magnetic Ni²⁺ ions with non-magnetic Mg²⁺ ions and also to the enhancement of the Y–K angle by cation redistribution. The variation of AC susceptibility with temperature exhibits broad maxima, indicating a distribution of particle sizes in the sample with the existence of single domain and superparamagnetic type domain structures. The susceptibility decreases continuously beyond x = 0.4 until 0.6, and the samples exhibit only superparamagnetism. The AC susceptibility studies also show a decrease in Curie temperature (T_C) with an increase in x for Mg_xNi_0.6−xZn_0.4Fe₂O₄ (x = 0.0 to 0.6) resulting from weakening of A–B interaction. The ZFC–FC studies along with the Mössbauer studies were also corroborated by the proposed existence of dominant superparamagnetism with an increase in Mg concentration.