Ferromagnetism and optical properties of La1 − x Al x FeO3 nanopowders

La1 − x Al x FeO3 (x = 0.0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5) nanopowders were prepared by polymerization complex method. All prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and UV-vis spectrophotometry (UV-vis). The magnetic properties were investigated using a vibrating sample magnetometer (VSM). The X-ray results of all samples show the formation of an orthorhombic phase with the second phase of α-Fe2O3 in doped samples. The crystallite sizes of nanoparticles decreased with increasing Al content, and they are found to be in the range of 58.45 ± 5.90 to 15.58 ± 4.64 nm. SEM and TEM images show the agglomeration of nanoparticles with average particle size in the range of 60 to 75 nm. The FT-IR spectra confirm the presence of metal oxygen bonds of O-Fe-O and Fe-O in the FeO6 octahedra. The UV-vis spectra show strong absorption peaks at approximately 285 nm, and the calculated optical band gaps are found to be in the range of 2.05 to 2.09 eV with increasing Al content. The M-H loop of the pure sample is antiferromagnetic, whereas those of the doped samples tend to be ferromagnetic with increasing Al content. The magnetization, remanent magnetization, and coercive field of the Al-doped sample with x = 0.5 are enhanced to 1.665 emu/g, 0.623 emu/g, and 4,087.0 Oe, respectively.

Thus, we propose in this research the synthesis of La 1 − x Al x FeO 3 (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5) nanopowders using a simple polymerization complex method. The magnetic and optical properties of the products were studied. The magnetization, coercive field, and remanent magnetization are measured, and they are expected to be enhanced due to the substitution of smallradius ions of Al on the La site. The calcined powders were characterized using an X-ray diffractometer (XRD; XRD-6100, Shimadzu, Kyoto, Japan) with CuKα 1 radiation (λ = 1.5405 Å). The morphologies of the synthesized products were observed using a scanning electron microscope (SEM; 1450VP, LEO, Hurley, UK) and a transmission electron microscope (TEM; Tecnai G2 20, FEI, Hillsboro, OR, USA). The components of the powders were analyzed by energy-dispersive X-ray spectroscopy (EDX; Tecnai G2 20, FEI). Fourier transform infrared spectroscopy (FT-IR; Spectrum One FT-IR, Perkin Elmer Instrument, Waltham, MA, USA) was employed to investigate functional groups in all samples. The optical properties were studied by ultraviolet-visible spectroscopy (UV-vis; UV-3101PC, Shimadzu). The magnetizations of all samples were measured using a vibrating sample magnetometer (VSM; VersaLab™ Cryogen-free, Quantum Design, San Diego, CA, USA).  Table 1. The lattice parameters a, b, and c of the doped samples decreased with the increase of Al content due to the replacement of the larger La 3+ ion (radius approximately 1.36 Å) by a smaller Al 3+ ion (radius approximately 0.535 Å) [22], as summarized in Table 1. The significant change in the decrease of lattice parameters with increasing Al content is confirmed by the shift of the diffraction peaks to a higher diffraction angle. On the other hand, Al 3+ ions can be substituted on B sites of Fe 3+ ions because the ionic radius of Al 3+ is close to that of the Fe 3+ ion (radius approximately 0.78 Å), resulting in the formation of the impurity phase of α-Fe 2 O 3 .

SEM analysis
The SEM micrographs of La 1 − x Al x FeO 3 (x = 0.0, 0.1, 0.3, and 0.5) nanopowders are shown in Figure 2. In Figure 2a, the powders are almost irregularly nanoagglomerated with a mean size of approximately 60 to 75 nm. In Figure 2b,c,d, agglomeration of nanoparticles with a size larger than 100 nm and grain growth can be observed in doped samples. Moreover, the SEM images reveal a uniform grain size distribution and homogeneous nanostructure.   Figure 3a,b,c,d shows bright-field TEM images with the corresponding selected area electron diffraction (SAED) patterns and EDX spectra of La 1 − x Al x FeO 3 (x = 0.0, 0.1, 0.3, and 0.5) nanopowders. It is obvious in Figure 3a1, b1,c1,d1 that the particulates consist of the agglomeration of numerous nanocrystallite particles of irregular shape, corresponding to the SEM observation in Figure 2.

TEM analysis
The average particle size is estimated and found to be approximately 60 to 75 nm. The SAED patterns in Figure 3a2,b2,c2,d2 show ring patterns, indicating that all doped samples are polycrystalline. Each SAED pattern can be indexed to a certain crystalline plane which is found to be consistent with that of the XRD results in Figure 1. The EDX spectra of these samples are shown in Figure 3a3,b3,c3,d3. The EDX results clearly show that all samples contain La, Fe, Al, and O with higher intensity peaks of Al in samples of high Al content. The Cu peaks that appeared come from the copper grid.   [14,41,43,47,50].
In Figure 5, broad absorption peaks are observed in all samples at approximately 285 nm with the infinitesimal redshifted to approximately 290 nm. From the plot of (αhν) 2 vs. hν in Figure 6a,b,c,d, the optical band gaps (E g ) of the samples can be determined by extrapolating the slope to the zero value of (αhν) 2 , and the obtained values are summarized in Table 2. It is found that the optical band gaps do not significantly vary with increasing Al content. As can be seen in Figure 7a, the magnetization curve of the pure sample is very narrow, indicating the antiferromagnetic behavior of the sample, while those of the doped samples show larger loops of ferromagnetic behavior with higher magnetization according to higher Al content (Figure 7b,c,d,e,f,g). In addition, the values of coercive field (H c ), magnetization (M), and remanent magnetization (M r ) are enhanced with increasing Al content, as summarized in Table 2. In general, it is well known that pure LaFeO 3 exhibits antiferromagnetic behavior. This behavior is due to the anti-alignment of the magnetic moments of the Fe 3+ ions. However, LaFeO 3 can behave ferromagnetically due to the small crystallite size. The decrease of crystallite size can increase the uncompensated spins at the surface [60,61]. In our work, it is evident in Table 1 that the crystallite size of La 1 − x Al x-FeO 3 decreases for higher Al content, resulting in the enhancement of ferromagnetism with higher M value. In addition, the second phase of α-Fe 2 O 3 detected in the   XRD measurements may also be attributed to the ferromagnetism in La 1 − x Al x FeO 3 . Figure 8 shows the temperature-dependent magnetization of La 0.5 Al 0.5 FeO 3 nanopowder investigated by field-cooled (FC) measurement in the temperature range of 50 to 390 K. The M decreases as the temperature increases because of the thermal fluctuations causing the randomization of polarization direction. It is clearly seen in Figure 8 that the zero value of magnetization cannot be observed in the temperature range of measurement, implying that the Curie temperature (T c ) is above 400 K.

Conclusions
In summary, La

Competing interests
The authors declare that they have no competing interests.
Authors' contributions YJ designed and carried out all the experiments and data analysis and participated in preparing the draft of the manuscript. SH co-supervised the research and gave discussion. ES, the project coordinator, supervised the   research, designed the experiment, participated in preparing the draft of the manuscript, and revised the manuscript. All authors read and approved the final manuscript.