Study On The Magnetocaloric Effect And Critical Behavior Of Spinel Structure Compound Mg_xZn_1-xFe₂O₄（x=0.1-0.5）

In recent years,energy and environmental protection issues have attracted more and more attention,which has increased the requirements for raw materials and devices with relevant characteristics such as safety and pollution-free,high energy efficiency,and miniaturization.In order to find suitable green materials,traditional MgZn ferrites have excellent characteristics such as easy preparation,low cost,high performance,and wide transformation temperature range,which have attracted widespread attention.In this paper,a series of spinel compound MgxZn1-xFe2O4(x=0.1-0.5)experimental samples were prepared by the standard solid phase method at the optimum sintering temperature of 1573K,and their microstructure,magnetic properties,magnetocaloric effect and critical behavior were investigated.The research will fully analyze the potential value of the sample,the specific experimental results are as follows:1.The microstructure and magnetic properties of MgxZn1-xFe2O4(x=0.1-0.5)were studied.XRD and FT-IR spectroscopy confirmed that all samples have a spinel structure,with a gradually decreasing lattice constant and volume.The SEM,EDX and XPS spectra were used to characterize the microstructure,ion concentration and element valence of the sample.For the obtained sample,as the amount of Mg2+ions doping increases,the competition between antiferromagnetic(AFM)and ferromagnetic(FM)causes the transition temperature to increase from 19 K to 383 K,making it have a broader application prospect.Under the proper doping amount,the saturation magnetization(MS)and coercivity(HC)of all samples were ameliorated at ultra-low temperature and normal temperature.Experimental results show,with the continuous doping of Mg2+ions,the values of Ms and HC continue to increase,the maximum value of MS can reach 43.6 emu/g(at 300 K)and 96.8 emu/g(at 5 K),and the maximum value of HC can reach 38.5 Oe(at 300 K)and 99.6 Oe(at 5 K)in the series,which has just reached the use requirements of inductors,transformers and other magnetic components.2.Studied on the magnetocaloric effect of Mg0.35Zn0.65Fe2O4.The XRD data refined by Rietveld technology confirms that the sample had a pure phase spinel structure.We conducted a magnetic study on the sample using SQUID and indicated that the sample’ ferromagnetic curie temperature Tc=295K,the paramagnetic curie temperature TP=330K,and the saturation magnetization can reach 125.41emu/g at an ultra-low temperature of 5K.We tested the initial magnetization curve near the phase transition temperature(Tc)from ferromagnetic(FM)to paramagnetic states(PM)and produced an Arrott plot that confirmed the presence of a secondary phase transition.The sample’ maximum magnetic entropy change |ΔSMmax| was 1.642 J/kg K and relative cooling power RCP was 185.2 J/kg in the external magnetic field H=50 kOe.3.The critical behavior of Mg0.35Zn0.65Fe2O4 was systematically studied.The critical parameter β,γ,δ,and TP were calculated using a Modified Arrott plot(MAP),Kouvel-Fisher(KF)plot and critical isotherm(CI)methods to study the type of magnetic order in the ferromagnetic state that corresponded to the magnetic state equation and mean field model.By analyzing the critical parameters,we obtained the ferromagnetic exchange integral constant J(r)～r-4.8498,which was between the 3D Heisenberg model and mean field models and very close to the mean field model,confirming that the long-range and short-range ferromagnetic orderly interactions coexisted with interaction phases and the long-range ferromagnetic orderly interaction dominates.