1 Introduction

Recently, the perovskite-type transition metal oxides (TMOs) have been proposed as promising ABO3 perovskite-type materials for the coexisting states of multiple coupled such as superconductivity and magnetism. Lanthanum iron oxide (LaFeO3) is an ABO3 perovskite oxide having an orthorhombic structure at room temperature and an antiferromagnetic ordering below 740 K [1,2,3,4]. It has gained a considerable attention because of its wide range of applications in chemical sensors, nonvolatile magnetic memory devices, and ultra-sensitive magnetic read heads of modern hard disk drives, electromechanical devices, solid electrolyte, solid fuel cell, actuators, fixed resistor, transducers, etc. [5,6,7,8,9,10].

It is well known that the undoped LaFeO3 is an antiferromagnetic (AFM) material. The AFM spin order in LaFeO3 is a rare magnetic property due to the finite size effect. However, the decrease in particle size of LaFeO3 has been expected to develop the magnetic property. For example, the ferromagnetism (FM) of LaFeO3 with a particle size of 10–50 nm has been reported [11]. LaFeO3 is a canted G-type AFM structure with a high Neel temperature (NT) of 750 K [12]. Orthoferrites are the weak ferromagnetic materials with interesting magneto-optical properties [13, 14]. It has been credited to the presence of uncompensated surface spin, which is called the ferromagnetic (FM) shell [15, 16]. Besides, the magnetization of LaFeO3 samples increased with decreasing particle size [9, 10] and the change in Curie temperature (CT) for finite size effect has also been studied. It has been examined that both the doping and the preparation method could help in decreasing the particle size which results in the improvement of the magnetization.

Further, the exchange on A and/or B site is also pragmatic to improve the electrical properties. Various types of exchange have been studied, for example, La0.8Sr0.2Fe1−xCuxO3 [17] La1/3Sr2/3FeO3d [18], LaZnFeO3 [19], La0.5Al0.5FeO3 [20], La1−xCxFeO3 [21], and LaZnxFe1−xO3 [22, 23]. These show the interesting properties of electrical materials such as high electrical conductivity, high dielectric constant, low dielectric loss, ferroelectricity or piezoelectricity, thermal stability, etc. Therefore, the substitution of divalent or trivalent ions into the La or Fe sublattices has been specifically investigated for achieving these properties [24,25,26, 34,35,36,37,38,39,40,41,42,43,44] .

The main objective of the present work is to study the effect of substituting Fe ions with Ti ions on the structural and magnetic properties of the compositions of the LaFe1−xTixO3 system with perovskite orthorhombic structure. At present, no studies are available on the magnetic properties of substitution on B site by hydrothermal method of Ti ions. We believe that the structural and magnetic properties that are explored in this paper are useful for various applications as Fe and Ti are present in LaFe1−xTixO3 perovskite systems.

Several methods such as coprecipitation technique [27], combustion synthesis [28], and sol-gel technique [29] were reported to prepare LaFeO3 nanoparticles. In this work, the effect of various doping concentration of titania is investigated in the preparation of LaFeO3 (LaFe1−xTixO3, x = 0, 0.2, 0.4, and 0.6) using hydrothermal synthesis. The prepared sample has been characterized for exploring various structural and magnetic properties.

2 Experimental Procedure

Various doping concentrations of LaFe1−xTixO3 (x = 0, 0.2, 0.4, and 0.6) are prepared by hydrothermal method with the aqueous solutions of La(NO3)2 ⋅ 6H2O (99.6% purity, Sigma-Aldrich), Fe(NO3)2 ⋅ 9H2O (99.6% purity, Sigma-Aldrich), and titanium(IV) isopropoxide (99.6% purity, Sigma-Aldrich) mixtures in alkaline medium. The solutions of La(NO3)2 ⋅ 6H2O, Fe(NO3)2 ⋅ 9H2O, and titanium(IV) isopropoxide in their stoichiometry ratio (1 g of La(NO3)2 ⋅ 6H2O in 50 ml), (0.6g of Fe(NO3)2 ⋅ 9H2O in 50 ml), (0.2 g of titanium(IV) isopropoxide in 50 ml) were dissolved in double-distilled water with a constant stirring at 600 rpm. Then, NaOH solution is added and the pH = 10.0 is maintained. The prepared mixed solvent was transferred into a 250-ml Teflon-lined stainless autoclave. The autoclave was sealed and preserved at 120 ∘C for 24 h. After the hydrothermal reaction time, the autoclave was taken out and the autoclave was cooled to room temperature (RT) naturally. The samples obtained were washed four to five times with double-distilled water. A general flow chart of synthesis process is shown in Fig. 1. The sample was dried in a hot-air oven at 100 ∘C overnight (12 h) to remove dampness. The dried powder was mixed homogeneously in an agate mortar and pestle for 2 h. This homogenous mixture was sintered at 600 ∘C for 4 h in a hot-air furnace. After sintering, the sample is again ground for 3 h using the agate mortar, and the resulting powder is analysed using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM).

Fig. 1
figure 1

Flowchart for synthesis process LFTO nanoparticles using hydrothermal method

Full size image

2.1 Measurements Techniques

The purity and crystallinity of the samples were characterized by XRD using Bruker, D8 Advance model diffractometer. Measurements were taken in the 2𝜃 range of 15–80∘ , using Cu-Kα1 radiation at a scan rate of 2∘ /min. The average crystallite size was determined based on Scherrer’s formula from the line broadening of the peak. Microstructure and morphology were analyzed using a Carl Zeiss SUPRA-555 FESEM. The scanning electron microscope was fitted with an energy-dispersive X-ray spectrometer (EDX) for the determination of the different elements present in the sample. The morphologies of samples were investigated by TEM using a Philips (model CM200) instrument. Room-temperature magnetic measurement of the samples was carried out with VSM. The parameters like saturation magnetization (Ms), coercive force (Hc), and remanent magnetization (Mr) were evaluated.

3 Results and Discussion

3.1 XRD Structural Analysis

LaFe1−xTixO3 (LFTO) nanoparticles exhibited the orthorhombic symmetry and crystallized with the well-known orthorhombic perovskite structure at 600 ∘C/4 h. In Fig. 2, the XRD data from LFTO are shown. All the reflection planes are in good agreement with the standard JCPDS (No. 82-1958) of orthorhombic phase. Little secondary phases (preceded by an asterisk) corresponding to La2TiO5 are detected for the increased titanium concentrations (x = 0, 0.2, 0.4, and 0.6). The average crystallite size (D) of the sample is evaluated for the intense peak positions using Scherrer’s formula [30].

$$ D=\frac{k\lambda }{\beta \cos \theta } $$
(1)
Fig. 2
figure 2

XRD pattern of LaFeO3 and LaFe1−xTixO3 (x = 0, 0.2, 0.4, and 0.6)

Full size image

Here, k is a constant and is approximately equal to 0.9 for a spherical symmetry and λ is X-ray wavelength of Cu-Kα1 = 1.5418 Å, where 𝜃 is diffraction angle and β is full-width half maxima (FWHM).

As the calcination temperature was from 600 ∘C/4 h, the FWHM values of diffraction curve decrease sharply and hence the particle sizes also increased (Table 1). That is, the average crystallite size of Ti content increases and found to vary from 5 ± 1 to 11 ± 1 nm [30]. By increasing the dopant level, Dx exhibits unsystematic trend due to the presence of secondary phases. This behavior is qualified due to fading crystal growth or an increase of elastic strain (å = â/4tan𝜃) by Ti addition.

Table 1 Structural parameters of LaFe1−xTix O3 (x = 0.2, 0.4, and 0.6)
Full size table

3.2 Surface Morphology

To well understand the micro-structural features of LaFeO3 and LaFe1−xTixO3 (x = 0–0.2) samples, FESEM was performed. The FESEM showed that the microstructure consisted of submicron-sized particle. However, the rest of the compositions are observed to be almost spherical in shape. Figure 3 shows the FESEM photographs wherein the particle size around 31–39 nm grains were found. FESEM images of the local morphology of LaFeO3 and LaTixFe1−x03 images show good nanocrystalline grain.

Fig. 3
figure 3

SEM with EDAX images of LaFeO3 and LaFe1−xTixO3 (x = 0 and 0.2)

Full size image

EDX in combination with FESEM showed that the synthesized samples have a homogeneous composition. The peaks of La, Fe, and O were perceived for all samples. We also perceived peaks of Ti for the samples with x = 0.2, 0.4, and 0.6. It is found that the improved elements of Ti atomic replies to the reduction of that of Fe atomic as well as the increase of Ti content. It designates that the substitution of Ti ions into Fe site is comprehensive.

3.3 TEM Analysis

The structure and morphology of LaFe1−xTixO3 nanoparticles were investigated by TEM with the selected area electron diffraction (SAED) pattern, as shown in Fig. 4. The TEM images showed the nanoparticles with a size of approximately 21–50 nm. It can be seen from Fig. 4 that the even particle size of the samples decreases with increasing the Ti doping content, which is reliable with the XRD results. The SAED patterns of the sample presented the spotty ring patterns which imply a creation of polycrystalline structure. These rings were indexed as the LaFeO3 perovskite phase with random orientation, which is in agreement with the XRD results [27].

Fig. 4
figure 4

TEM Photographs of LaFeO3 and LaFe1−xTixO3 (x = 0.2)

Full size image

3.4 VSM Analysis

The room temperature hysteresis loops for LaFeO3 samples (x = 0, 0.2, 0.4, and 0.6) are shown in Fig. 5. The substitution of nonmagnetic ion titanium, which has a different B site occupancy results in the decrease of the exchange interaction B sites. Hence, by varying the Ti content, it is possible to vary magnetic properties of the nanoparticles. LaFeO3 had been proven to be an antiferromagnet with a Neel temperature of 750 K. Generally, La3+ is nonmagnetic since all the electrons were paired. The magnetic moments of Fe are the source of magnetic properties. Therefore, the FM behavior in the LaFeO3 nanoparticles is because of the spin-canted Fe magnetic moments due to the disordered surface spins [31]. The VSM measurements for LaFe1−xTixO3 nanoparticles measured at RT for various values of magnetic field (H) are shown in Fig. 5. The Ms, Mr), squareness, and Hc values are listed in Table 2. It can be seen from the table that the above-mentioned physical parameters increase when doping concentration is increased. We emphasize that the coercivity is found to be maximum for pure LaFeO3. Then, it decreases when the doping concentration is increased from 0.2 to 0.4. However, it increases when the doping concentration is 0.6.

Fig. 5
figure 5

M–H curves at ± 15 kOe measured at RT of the LaFeO3 and LaFe1−xTixO3 nanoparticles

Full size image
Table 2 Magnetic properties of Ti-doped LaFeO3
Full size table

All the samples exhibit a weak FM behavior and an increase of magnetization (M) with increased content of Ti. From the magnetization curve, coexistence of antiferromagnetism is observed, i.e., at the high field, the M–H curve does not saturate. Enhanced magnetization is observed in the case of Ti4+ doping which is due to the uncompensated spin of Ti4+ in the Fe3+ site.

The observed change in the magnetic properties, i.e., magnetic moment of undoped LaFeO3, is approximately zero; as a result, it increases with Ti doping along with an increase in magnetization (Fig. 5). This could be understood from the canted-antiferromagnetic (AFM) behavior of these materials. It is known that LaFeO3 exhibits G-type AFM behaviors [30,31,32,33]. The canting of the Fe spins at small angles is due to exchange coupling, which, in turn, results in a small net magnetic moment (Table 2). With increase in Fe4+ content due to Ti doping, net magnetic moment increases. In addition, the presence of oxygen vacancies disturb antiparallel spin ordering in the Fe3+–O–Fe3+ and Fe4+–O2 −Fe4+ by super-exchange interaction. This leads to an increase of magnetization; the maximum value of magnetization shows magnetic memory device applications. The effect of the magnetization curve confirms that the TC of the sample (with x = 0.4) is stable even at higher temperature. We strongly believe that this is foremost and would be useful for the magnetic memory devices in future.

4 Conclusion

In summary, nanoparticles of LaFeO3 and LaFe1−xTixO3 were prepared using the hydrothermal method. XRD pattern confirms the phase and the presence of Ti4+ in the lattice. Structural classification showed that the phase of orthorhombic structure (LaFeO3). The average crystallite size calculated from the Scherrer’s formula suggested the formation of nanoparticles, and this was further confirmed by the TEM images. The weak ferromagnetic behavior was observed from hysteresis loop. We found that the coercivity is found to be maximum for pure LaFeO3. Then, it decreases when the doping concentration is increased from 0.2 to 0.4. However, it increases when the doping concentration turns 0.6.