Magnetic Properties Of LaFeO_3-? Nanoparticles Prepared By Sol-Gel Method

LaFeO_3-? is one of the few multiferroic materials with the ferromagnetic and ferroelectric ordering at the room temperature.Because of the unique magnetoelectric coupling effect,LaFeO_3-? has potential applications in information storage,processing,magnetic sensors and electronics.The emergence of ferromagnetism in perovskite La FeO3-? nanoparticles has inspired scholars to study the effect of various dopants on the magnetic properties of LaFeO_3-? nanoparticles.However,research on the magnetic properties of alkali element doping LaFeO_3-? nanoparticles is still rare,and the ferromagnetic origin LaFeO_3-? is still ambiguous.As the radius of Na+ is close to that of La³⁺,Na is chosen as doping element from the view of lattice stability.The La_1-xNa_xFeO_3-? powder samples with different doping concentration?x= 0,0.1,0.2 and 0.3?were prepared by citric sol-gel method.XRD,SEM,EDS and XPS were employed to characterize and the surface composition and the morphology,which are combined with the magnetic result measured by SQUID,the influence of Na doping upon the magnetic properties of LaFeO_3-?nanoparticles is analyzed in detail.Nowadays,there are many kinds of methods to synthesize LaFeO_3-?nanoparticles.Because the preparation process of the sol gel method is easier tobe manaed,and the cost for raw materials is lower,the citric acid sol-gel method is commonly used by researchers.By conducting literature reviews and analyzing,experimental data,we find that the physical properties of LaFeO3 nanoparticles by the citric sol-gel method have the close relationship to synthesis process,so the influence of EG or PEG and pre-calcination process on the magnetic properties of LaFe O3-? nanoparticles are investiated.LaFeO_3-?nanoparticles are synthesized using EG or PEG as the raw material with or without pre-calcination by citric sol-gel method.According to XRD,SEM and XPS results,the origin of ferromagnetism is further explored.We have previously studied the effect of the reductive ethanol gas on the electrical property of LaFe O3-? nanoparticles,and have found that the ethanol can change the electric resistance mainly through the influence on the transformation between Fe³⁺and Fe⁴⁺.Since the double exchange interaction of Fe³⁺-O2--Fe⁴⁺ between neighboring Fe³⁺ and Fe⁴⁺ ions might be responsible for the weak FM behavior of LaFeO_3-? nanoparticles,we study the effect of the ethanol gas on the magnetic properties of La FeO3 nanoparticles which were previously prepared using PEG as raw material with additional pre-calcination at400 oC.The conclusions of our results are as follows:1.There is a limit for the Na substitution at La site in LaFeO_3-? lattice,above which La1-x Nax FeO3-? samples can produce Na2 O.In the present case,the substitution limit can be estimated to be 20-30%.The substitution of Na ions at La sites could facilitate the formation of Fe⁴⁺ or induce oxygen vacancy,in order to keep the material to have the charge neutrality,which would enhance ferromagnetism by the double exchange?DE?interaction of Fe³⁺-O2--Fe⁴⁺between neighboring Fe³⁺ and Fe⁴⁺ ions.Our work demonstrates that the proper substitution of Na+ions for La ions in LaFeO_3-? crystals is an effective way toachieve ferromagnetism and magnetization of LaFeO_3-? nanoparticles at the room temperature.2.When the LaFeO_3-? nanoparticles are prepared by the sol-gel method,the particles prepared using PEG as the raw material with additional pre-calcination at 400oC?PEG400+600?are the smallest and the magnetization is the largest.The uncompensated spins of the Fe moment on the surface of the nanoparticles induce the ferromagnetism of the LaFeO_3-?,at the same time,the Fe³⁺-O2--Fe⁴⁺double exchange between the neighboring Fe³⁺ and Fe⁴⁺ ions strengthen the FM shell and weaken the AFM core.The Fe⁴⁺ ion in the sample can provide holes for the compound,increasing the conductivity and reducing the resistance of the sample.In addition,Fe⁴⁺ ions can enhance the ferromagnetism of the sample by double exchange interaction.3.The electrical and magnetic properties of LaFeO_3-? nanoparticles are affected by the adsorption of oxygen on the surface.The introduction of the ethanol can increase the resistance of the sample while reducing the ferromagnetism simultaneously.