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Lanthanum orthoferrite, LaFeO3, is one of the most common perovskite-type oxides having an orthorhombic perovskite structure (space group Pbnm), where the distortion from the ideal cubic structure occurs to form the tilting of the FeO6 octahedra. LaFeO3 has much practical interest for electroceramic applications due to their attractive mixed conductivity displaying ionic and electronic defects [1, 2]. The mixed ionic-electronic conductivity of LaFeO3 exhibits a linear response to oxygen pressure and provides oxygen sensor applications [3]. The excellent sensitivity and selectivity towards various toxic gases such as CO and NOx are observed as well [4]. Moreover, LaFeO3 nanoparticles exhibited good photocatalytic properties such as water decomposition and dye degradation under visible light irradiation [5, 6]. These properties are enhanced by the homogeneity and high surface area of the fabricated LaFeO3 particles. Fine particles with diameter of less than 100 nm are potentially required for these purposes.

Besides, the orthoferrites are known to be prototype materials for magnetic bubble devices because of their large magnetic anisotropy with small magnetization [7]. LaFeO3 is an interesting model system of orthoferrite antiferromagnets showing a weak ferromagnetism. The Néel temperature, TN, of LaFeO3 is 738 K, which is the highest temperature in the orthoferrite family [8]. The magnetic moments of Fe3+ ions are aligned antiferromagnetically along the orthorhombic a-axis. But they are slightly canted with respect to one another due to the presence of Dzyaloshinskii-Moriya interaction. A weak ferromagnetic component parallel to the c-axis appears. The magnetization of LaFeO3 bulk crystals is considerably small, 0.044 B/Fe [8]. However, magnetic structures of small particles are often different

© 2012 Fujii et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

from those of bulk ones. For instance, antiferromagnetic nanoparticles exhibit increasing net magnetization due to the presence of uncompensated surface spins [9, 10]. If the ferromagnetic behavior is promoted in LaFeO3, it should provide facile handling of their applications by using magnetic field. Magnetic properties of well-defined LaFeO3 nanoparticles are worthy to investigate.

It is well known that the wet-chemical methods offer large advantages for low-temperature oxide formation with high surface area, small particle size, and exact cation-stoichiometry. Several methods such as co-precipitation technique [11, 12], polymerized complex method [13], combustion synthesis [14], and sol-gel technique [15] were reported to prepare LaFeO3 nanoparticles. For instance the formation of a single phase of LaFeO3 with the perovskite structure was observed at lower calcination temperatures of 300°C in [11, 12]. This temperature was much lower than that of conventional solid state reaction method. Recently we have successfully prepared LaFeO3 nanoparticles by using the new chemical synthesis method, so-called "hot soap method" [16, 17]. It showed high controllability over the formation of nanoparticles with narrow size distribution, which was performed in the presence of surfactant molecules at high temperatures. The hot soap method is based on the thermal decomposition of reaction precursors of organometallic compounds in polyol solvent. But the presence of surfactant molecules in the solution prevents aggregation of precursors during growth. It was widely applied to prepare nanoparticles of compound semiconductors [18] and metallic alloys [19]. However there were few reports on preparing oxide nanoparticles [20].

In this paper we describe the details of our synthesis procedure of LaFeO3 nanoparticles by using the hot soap method. The magnetic properties of the resultant particles were also discussed as a function of the particle sizes.
