Study On The Magnetic Properties Of A-site Doped LaFeO₃ Materials Prepared By Citric Acid Sol Gel Method

Lanthanum ferrite LaFeO3, which have a high Neel temperature (TN≈740 K), is a metal oxides with the structure of perovskite (ABO3). Because the simultaneous presence of the antiferromagnetic and ferroelectric orderings, the LaFeO3 is one of the multiferroic materials. LaFeO3 is unique mixed materials with special crystal structure, unique electromagnetic properties, higher electron ion conductivity, excellent catalysis and gas sensitive activity, suitable thermal expansion coefficient and better chemical stability etc. These advantages make it widely used in the fields of electricity, magnetic sensors, industrial catalysis, gas sensing materials, information storage and spin electronic devices etc. It can be used as a versatile material, which has important application value. Usually the LaFeO3 materials has a G-type antiferromagnetic structure at room temperature, it’s because of the superexchange interaction occurs between iron ions of an ideal FeO6 octahedron structure and its surrounding six iron ions by oxygen ion, magnetic moment of iron ion is an antiparallel arrangement. The magnetic properties of the LaFeO3 materials can be controlled by adjusting the size of the A position ion and its valence state, and the doping modification can be realized, and the value of the sample in the practical application can be improved.Usually the A-site and B-site element of LaFeO3 substituted modified and its formula were La1-xAxFeO3 (a=Ca, Sr, Mg, Ba,Ce or other rare earth elements),LaFe1-xBxO3(b= Mn, Co, Ni, Cr or other transition metal elements),respectively.In this paper, the bivalent nonmagnetic ion Mg and Ba were selected to dope LaFeO3 materials in the A-site, result in the sample produce ion radius effects and valence state effects. Such as Fe-O bond length and the Fe-O-Fe bond angle changes, mixed valence state of Fe ions generate, resulting in La vacancies and oxygen defects etc, which leads to the distortion of crystal lattice. As a result, the spin structure is tilted and appear remanent magnetization, the samples exhibit ferromagnetism.In addition, the magnetic properties of samples were improved by studying different calcination temperature and calcination time. In this paper, all experimental methods are used in the citric acid sol gel method, which has the advantages of accurate control, simple synthesis process, the product is nanometer size, and good dispersion etc. The crystal structure, chemical bond and functional group, morphology and magnetic properties of the synthesized samples were characterized by XRD, IR, SEM and VSM.The main research results of this paper are as follows:1.A-site Mg2+ doping LaFeO3, the XRD results show that the introduction of Mg2+ does not change the LaFeO3 perovskite orthorhombic structure (a=0.55669 nm, b=0.78547 nm, c=0.55530 nm), space group is Pnma (62), all of the main diffraction peak are characteristic peaks of LaFeO3, the average grain size of the samples were less than 31 nm. We can obtain pure sample La0.85Mg0.15FeO3 in the environment of the calcination temperature below 600 ℃, the grain size increases with the increase of calcination temperature. FT-IR spectroscopy confirmed the antisymmetric stretching vibration near the wavenumber 583 cm-1 is caused by Fe-O-Fe bond vibration in the FeO6 octahedra. VSM measurements show that Mg doping can significantly improve saturated magnetization and remanent magnetization of the samples. The saturated magnetization of La0.85Mg0.15FeO3 compared to undoped LaFeO3 sample improve 5.91 emu/g. And the remanent magnetization improve 1.93 emu/g.2. A-site Ba2+ doping LaFeO3. XRD results show that when the Ba2+ doping concentration x<0.05, does not change the LaFeO3 perovskite orthorhombic structure with space group is Pnma (62). All samples are pure phase,when x≥0.10,structure of the sample from the orthorhombic to cubic (a=b=c=0.39260 nm) change, space group is pm3m (221).The average grain size of the samples decreases gradually with the increase of the doping amount of Ba2+, when x=0.25,the average grain size of samples reaches the minimum, is about 15.2 nm. Sample La0.95Ba0.05FeO3 after different calcination temperature and time is pure phase. FT-IR spectroscopy confirmed the antisymmetric stretching vibration between the wavenumber 500 cm-1-600 cm-1 is caused by Fe-O-Fe bond vibration in the FeO6 octahedra. And with the increase of Ba2+ concentration appear blue shift phenomenon. VSM measurements show that when x<0.05, the coercivity of the samples occurred increased mutation. The coercivity of La0.95Ba0.05FeO3 compared to updoped LaFeO3 sample improve 6120 Oe. The coercivity of sample La0.95Ba0.05FeO3 increases with the increase of calcination temperature.3. A-site Mg2+ and Ba2+ double-doping LaFeO3. XRD results show that when the Ba2+ doping concentration x≥0.08, we obtain pure phase perovskite type LaFeO3 orthorhombic structure, space group is Pnma(62). FT-IR spectroscopy confirmed antisymmetric stretching vibration of the Fe-O-Fe bond near the 436 cm-1. SEM showed that the uniform distribution of the sample particle size, with clear boundaries, the dual doping improve the samples for the particle morphology and dispersion. VSM measurements show that Mg2+ and Ba2+ double-doping can make the magnetic parameters (saturation magnetization, remanent magnetization and coercivity) of samples were obviously changes.The last work is a brief summary for this paper and outlook the development prospect of LaFeO3 perovskite materials.