Structural, Magnetic and Optical Properties of Gd and Co Co-Doped YFeO3 Nanopowders

YFeO3, YFe0.95Co0.05O3, Y0.95Gd0.05FeO3 and Y1−xGdxFe0.95Co0.05O3 (x = 0.0, 0.05, 0.10, 0.15 and 0.20) nanopowders were successfully fabricated via a low-temperature solid-state reaction technique. Results obtained using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectra indicate that YFeO3 nanopowders with Gd3+ and Co3+ ions co-doping at Y and Fe-sites were fabricated at 800 °C in sizes below 50 nm, and a distorted structure was obtained. Magnetic hysteresis loop analyses illustrate that ferromagnetic behavior of YFeO3 nanopowders can be enhanced with the addition of Gd and Co. Whereas the maximum and remnant magnetization of the powders were found to be about 5.24 and 2.6 emu/g, respectively, the optical band gap was around 2.4 eV, proving that co-doped YFeO3 nanopowders have a strong capability to absorb visible light. Because both magnetic and optical properties of these materials are greatly improved with the addition of Gd and Co, one can expect the scope of their potential application in the magnetic and optical fields to increase.


Introduction
As one of the cutting edge multiferroic materials, AFeO 3 (A = La, Y and Sc) materials have been the focus of industry research because of their couple orderings of ferroelectricity and anti-ferromagnetism. As a result, they have great potential application in data storage, information exchange, and 5G mobile phone systems [1]. YFeO 3 is one of the most promising applications of rare earth AFeO 3 materials [2]. YFeO 3 has been reported to feature molecular ferroelectricity at low temperatures (10-40 K), good dielectric and magnetic properties [3][4][5][6], and is also becoming one of the most widely investigated multiferroic materials. Moreover, with a narrow optical gap (1.9-2.6 eV), its potential application as an optical material, especially decomposing organics should be considered for further study [7]. However, pristine YFeO 3 is not easy to prepare as evident by problems such as the introduction of secondary phases and the hopping of charge between Fe 2+ and Fe 3+ [8][9][10], while its low magnetism characteristics [11] are possibly its main shortcoming. Studies show that an effective and efficient way [11][12][13] to overcome these problems, is by means of doping. Some scientists substituted Y by divalent and trivalent ions [14,15] and Fe by trivalent and high-valence ions [16]. While the use of Gd doping is rarely reported, one study showed that with Gd doping on the Y site, significant enhancement of magnetization was achieved. In addition, use of other-element doping on the Y site has the potential for improvement of the material's optical properties [15]. For Fe-site ion doping, the main improvement focus was the reduction of leakage current and the enhancement of magnetic Figure 1a presents the XRD patterns of the tested (0 ≤ x ≤ 0.2) samples calcined at 800 • C. The pattern for the pristine YFeO 3 nanopowders suggests the presence of the obvious orthorhombic YFeO 3 pattern and that no minor impurity peaks were present. This outcome shows that the synthesis reaction for the orthorhombic structure was completed and the purity was high. The doped nanopowders indicate the same characteristic shape as that of the pristine YFeO 3 patterns with a slight shift in the peak position and phase. For Gd5, some peaks for the hexagonal YFeO 3 structure emerged. For Co5, peaks for Y 2 O 3 appeared, showing that some minor impurity was introduced. This is a normal situation involving the case where instability of YFeO 3 exists and which is due to the radius difference. However, this effect totally disappears for the Co-Gd co-doped sample. Figure 1b shows the YFeO 3 peaks at 2θ~33 • , shifting toward a higher 2θ angle with substitution of Co into YFeO 3 . The figure also shows a shift towards a lower 2θ angle with further substitution of Gd into YFeO 3 , and while keeping almost the same position of Gd and Co-doped samples due to Gd 3+ (Gd 3+ 0.938 Å) possessed a larger ionic radii compared with Y 3+ (Y 3+ 0.9 Å), and Co 3+ (Co 3+ 0.61 Å) possessed a smaller one compared with Fe 3+ (Fe 3+ 0.645 Å) [15,20]. Thus, it is evident that Gd 3+ and Co 3+ replaced the Y 3+ and Fe 3+ ions in YFeO 3 , respectively. The XRD pattern shows the highest peak for the Co-doped particles and its 2θ angle location is nearly identical to that of pure YFO. The stress introduced by the change in radius was reduced so that impurities disappeared.    Figure 2. These demonstrate that the particle sizes of the test nanopowders were homogeneous with some minor agglomeration. The particle sizes of the test samples of YFO, Co5, Gd5, Co5Gd5, Co5Gd10, Co5Gd15 and Co5Gd20 were approximately 150, 120, 95, 80, 75, 45 and 40 nm, respectively. From the results obtained, it is apparent that the Co and Gd substitution decreased the grain size, while particle size decreased significantly with Co and Gd co-doping. As is well known, whenever the diffusion rate is low, rare earth ions can inhibit grain growth of RFeO3 [11]. In addition, Gd 3+ ions (radius of 0.983 Å) are larger than Y 3+ ions (radius of 0.9 Å). The mismatch of ion sizes introduces defects in the lattice, leading to particle refinement. In addition, Co 3+ can suppress hopping of Fe 2+ and Fe 3+ , depriving the oxygen vacancy and subsequently preventing growth of the grains [21]. Some agglomeration of particles appears in Co and Gd codoping samples, and may be attributed to the large surface area to volume ratio of the nanoparticles. The latter is to be expected whenever the low-temperature solid-state reaction method is used. Similar effects of grain refinement induced by rare earth and magnetic element co-doping has been reported in other references [14,17,21,22].   Figure 2. These demonstrate that the particle sizes of the test nanopowders were homogeneous with some minor agglomeration. The particle sizes of the test samples of YFO, Co5, Gd5, Co5Gd5, Co5Gd10, Co5Gd15 and Co5Gd20 were approximately 150, 120, 95, 80, 75, 45 and 40 nm, respectively. From the results obtained, it is apparent that the Co and Gd substitution decreased the grain size, while particle size decreased significantly with Co and Gd co-doping. As is well known, whenever the diffusion rate is low, rare earth ions can inhibit grain growth of RFeO 3 [11]. In addition, Gd 3+ ions (radius of 0.983 Å) are larger than Y 3+ ions (radius of 0.9 Å). The mismatch of ion sizes introduces defects in the lattice, leading to particle refinement. In addition, Co 3+ can suppress hopping of Fe 2+ and Fe 3+ , depriving the oxygen vacancy and subsequently preventing growth of the grains [21]. Some agglomeration of particles appears in Co and Gd co-doping samples, and may be attributed to the large surface area to volume ratio of the nanoparticles. The latter is to be expected whenever the low-temperature solid-state reaction method is used. Similar effects of grain refinement induced by rare earth and magnetic element co-doping has been reported in other references [14,17,21,22].  Figure 3 shows the Raman patterns obtained from the YFeO3 powders. As is well known, for the case involving the Pnma structure of YFeO3, only modes of A1g, B1g, B2g, and B3g are active. Furthermore, one can say with a high degree of certainty that the first and second A1g modes are associated with Y-O bonds, and the A1g and Bg modes are related to Fe-O bonds at higher frequencies [23]. The 221 cm −1 A1g mode is represented as an FeO6 octahedral structure [9,10,24]. The vibrating modes of Co5 were similar to those of pure YFO, while the intensity was reduced slightly and the  Figure 3 shows the Raman patterns obtained from the YFeO 3 powders. As is well known, for the case involving the Pnma structure of YFeO 3 , only modes of A 1g , B 1g , B 2g , and B 3g are active. Furthermore, one can say with a high degree of certainty that the first and second A 1g modes are associated with Y-O bonds, and the A 1g and B g modes are related to Fe-O bonds at higher frequencies [23]. The 221 cm −1 A 1g mode is represented as an FeO 6 octahedral structure [9,10,24]. The vibrating modes of Co5 were similar to those of pure YFO, while the intensity was reduced slightly and the peak at 610 cm −1 increased as expected owing to the minor change of Fe-O bonds resulting from the doping of the Co element. For the case involving substitution of Gd 3+ for Y 3+ , the parameters of lattice (grain size) increased, leading to a shift in the Raman bands to a lower wave-number. Raman bands were found to broaden with increase of Gd 3+ and Co 3+ (in YFeO 3 ) because of the disorder introduced by two different cations. The Gd-O and Y-O bonds, with their inherent different strengths, can influence their vibration frequencies. In general, one can state that these results are consistent with the XRD patterns. Thus, it can be concluded that Co and Gd co-doping has a substantial effect on the modification of the YFeO 3 structure.   Figure 4 shows the magnetic properties of the nanopowders at room temperature. Values of maximum magnetization (Mm), remnant magnetization (Mr), and the coercive field (Hc) of the test samples are listed in Table 2. As is commonly known, YFeO3 is antiferromagnetic and features weak magnetic properties. In the case of pristine YFeO3, the hysteresis loop is typical weak magnetization in antiferromagnetic type. The magnetization parameters for the pristine YFeO3 sample Mm, Mr, and Hc had magnitudes of 3.50 emu/g, 0.89 emu/g, and 161 Oe. For the case involving the doped samples, a large open region was seen at the center of the hysteresis loops suggesting ferromagnetic behavior. Even when exposed to a 60 kOe outer magnetic field, the loops were not saturated. For Co5, Mm, Mr were slightly improved and Hc was reduced. Their corresponding values were about 4.56 emu/g, 0.95 emu/g and 120 Oe, respectively. For the case of co-doping of Co and Gd, the net magnetization decreased slightly at first, and then began to increase, reaching a high value of 5.24 emu/g for the Co5Gd20 particles. These values are summarized in Table 2. The value of Mr reached a maximum (Mr = 1.66 emu/g) for the Co5Gd5 nanoparticles. The reasons for observed improvement of the magnetization may be summarized as follows: (1) The effect of nanoparticles. Uncompensated surface spins of Fe 3+ ions are created when particle size is small. This situation leads to a strong magnetic enhancement [25,26].
(2) The distorted structure is affected by the doping effect. A FeO6 octahedron structure consists of one Fe 3+ ion and six O 2− ions, while each Fe 3+ magnetic moment is not precisely parallel to the neighboring ones, forming a small angle [25,26], which causes weak ferromagnetism in the   Table 2. As is commonly known, YFeO 3 is antiferromagnetic and features weak magnetic properties. In the case of pristine YFeO 3 , the hysteresis loop is typical weak magnetization in antiferromagnetic type. The magnetization parameters for the pristine YFeO 3 sample M m , M r, and H c had magnitudes of 3.50 emu/g, 0.89 emu/g, and 161 Oe. For the case involving the doped samples, a large open region was seen at the center of the hysteresis loops suggesting ferromagnetic behavior. Even when exposed to a 60 kOe outer magnetic field, the loops were not saturated. For Co5, M m , M r were slightly improved and H c was reduced. Their corresponding values were about 4.56 emu/g, 0.95 emu/g and 120 Oe, respectively. For the case of co-doping of Co and Gd, the net magnetization decreased slightly at first, and then began to increase, reaching a high value of 5.24 emu/g for the Co5Gd20 particles. These values are summarized in Table 2. The value of M r reached a maximum (M r = 1.66 emu/g) for the Co5Gd5 nanoparticles. The reasons for observed improvement of the magnetization may be summarized as follows: (1) The effect of nanoparticles. Uncompensated surface spins of Fe 3+ ions are created when particle size is small. This situation leads to a strong magnetic enhancement [25,26].
(2) The distorted structure is affected by the doping effect. A FeO 6 octahedron structure consists of one Fe 3+ ion and six O 2− ions, while each Fe 3+ magnetic moment is not precisely parallel to the neighboring ones, forming a small angle [25,26], which causes weak ferromagnetism in the antiferromagnetic YFeO 3 . Substituting an Fe 3+ ion with a larger radius Co 3+ ion will reduce the canted angle of the FeO 6 octahedra and release the distortion, and combined with a refined powder size, a higher magnetization is achieved. [27].
(3) The Gd 3+ ion with a large magnetic moment (µ eff = 8.0 µ B ) is magnetically active. When Y 3+ is replaced by Gd 3+ , Y-O-Gd chains instead of Gd-O-Gd chains are formed, further improving the magnetization of the Co and Gd co-doped nanopowders [15].
In summary, the three reasons noted above serve to bring about the enhancement of magnetization. Others who conducted similar research made similar findings. Khalifa et al. [16] reported that M m and M r were about 0.8 emu/g and 0.1 emu/g for Ti-doped YFeO 3 nanoparticles prepared using an improved sol-gel technique. Shi et al. [17] synthesized YFe 0.5 Cr 0.5 O 3 nanoparticles by the sol-gel method, and showed that M m and M r were approximately 2.0 and 0.5 emu/g, respectively. As reported by Shi, the maximum M m and M r values for Y 0.95 Ho 0.05 Fe 0.5 Cr 0.5 O 3 nanoparticles prepared using a low-temperature citric acid assisted sol-gel technology were about 4.5 and 1.2 emu/g, respectively [17]. Yuan et al. [15] synthesized Y 0.9 Gd 0.1 FeO 3 using a solid-state reaction method, and obtained values of M m , M r , and H c of 2.5 emu/g, 1.0 emu/g and 30,000 Oe, respectively. Still, it is worth noting that the property values in the present study are comparable with, or better than the results reported by others. antiferromagnetic YFeO3. Substituting an Fe 3+ ion with a larger radius Co 3+ ion will reduce the canted angle of the FeO6 octahedra and release the distortion, and combined with a refined powder size, a higher magnetization is achieved. [27].
(3) The Gd 3+ ion with a large magnetic moment (μeff = 8.0 μB) is magnetically active. When Y 3+ is replaced by Gd 3+ , Y-O-Gd chains instead of Gd-O-Gd chains are formed, further improving the magnetization of the Co and Gd co-doped nanopowders [15].
In summary, the three reasons noted above serve to bring about the enhancement of magnetization. Others who conducted similar research made similar findings. Khalifa et al. [16] reported that Mm and Mr were about 0.8 emu/g and 0.1 emu/g for Ti-doped YFeO3 nanoparticles prepared using an improved sol-gel technique. Shi et al. [17] synthesized YFe0.5Cr0.5O3 nanoparticles by the sol-gel method, and showed that Mm and Mr were approximately 2.0 and 0.5 emu/g, respectively. As reported by Shi, the maximum Mm and Mr values for Y0.95Ho0.05Fe0.5Cr0.5O3 nanoparticles prepared using a low-temperature citric acid assisted sol-gel technology were about 4.5 and 1.2 emu/g, respectively [17]. Yuan et al. [15] synthesized Y0.9Gd0.1FeO3 using a solid-state reaction method, and obtained values of Mm, Mr, and Hc of 2.5 emu/g, 1.0 emu/g and 30,000 Oe, respectively. Still, it is worth noting that the property values in the present study are comparable with, or better than the results reported by others.    YFeO3 possesses a narrow optical band gap (1.6-2.4 eV) and has been used in light-electric energy conversion applications. From the UV-visible absorption spectra (Figure 5), the optical energy band gap (Eg) of the seven test samples can be calculated using a Tauc function expressed in Equation (1) [28]: The Eg values shown in Figure 5 illustrate strong visible light absorption, indicating their promising decomposition application (See Table 2). YFO has the maximum Eg value (2.42 eV). It is decreased to 2.15 eV for Gd20Co5 nanoparticles. Clearly, the energy band gap becomes smaller with refined particle size of the YFeO3 powders. Thus, the reduction of the energy gap with co-doping is ascribed to the smaller particle size and lattice distortion reduction generated by the addition of Co and Gd. Further, according to Reference [23], reduced particle size leads to a narrow energy gap. Nonuniform microstrains caused by lattice distortions can impact energy levels, affecting the energy band gap [28,29]. YFeO3 possesses the band gap of 2p O, and 4d Y atoms. All these states are partially   YFeO 3 possesses a narrow optical band gap (1.6-2.4 eV) and has been used in light-electric energy conversion applications. From the UV-visible absorption spectra (Figure 5), the optical energy band gap (E g ) of the seven test samples can be calculated using a Tauc function expressed in Equation (1) [28]: The E g values shown in Figure 5 illustrate strong visible light absorption, indicating their promising decomposition application (See Table 2). YFO has the maximum E g value (2.42 eV). It is decreased to 2.15 eV for Gd20Co5 nanoparticles. Clearly, the energy band gap becomes smaller with refined particle size of the YFeO 3 powders. Thus, the reduction of the energy gap with co-doping is ascribed to the smaller particle size and lattice distortion reduction generated by the addition of Co and Gd. Further, according to Reference [23], reduced particle size leads to a narrow energy gap. Nonuniform microstrains caused by lattice distortions can impact energy levels, affecting the energy band gap [28,29]. YFeO 3 possesses the band gap of 2p O, and 4d Y atoms. All these states are partially filled [28].
The Gd atom is with the 4f state which is also not fully filled orbits. Thus f-d hybridization of the 4d Y and 3d Fe atoms causes light absorption. In addition, the same d states of Fe and Co overlap with the 2p states O atom, causing a narrow energy gap [15,30]. Moreover, Gd 3+ and Co 3+ ions partially substituting Y 3+ and Fe 3+ ions refine the as-synthesized nanoparticles size and increase chemical pressure, thereby resulting in a smaller value of E g [30]. Zhang et al. [28] obtained E g values of 1.94, 2.43 and 2.30 eV, respectively, for the hexagonal, orthorhombic, and YFeO 3 containing a mixture of the two phases. Shen et al. [31] reported that the E g value obtained from a first-principles calculation for YFeO 3 ceramics was 2.58 eV. Wu et al. [23] prepared YFeO 3 in which, hexagonal and orthorhombic phases co-existed; this material had a band gap of 2.41 eV. For YFeO 3 ceramics prepared by the conventional solid-state method, the energy band gap was found to be 2.58 eV [7]. Liu et al. [32] measured the optical properties of hexagonal-YFeO 3 /α-Fe 2 O 3 heterojunction composite nanowire and obtained E g values of approximately 2.15 eV. Once again, it is worth noting that our E g values compare favorably with these results. Therefore, one can reasonably conclude that the YFeO 3 nanopowders used in this study can be used in the decomposition of organic compounds. filled [28]. The Gd atom is with the 4f state which is also not fully filled orbits. Thus f-d hybridization of the 4d Y and 3d Fe atoms causes light absorption. In addition, the same d states of Fe and Co overlap with the 2p states O atom, causing a narrow energy gap [15,30]. Moreover, Gd 3+ and Co 3+ ions partially substituting Y 3+ and Fe 3+ ions refine the as-synthesized nanoparticles size and increase chemical pressure, thereby resulting in a smaller value of Eg [30]. Zhang et al. [28] obtained Eg values of 1.94, 2.43 and 2.30 eV, respectively, for the hexagonal, orthorhombic, and YFeO3 containing a mixture of the two phases. Shen et al. [31] reported that the Eg value obtained from a first-principles calculation for YFeO3 ceramics was 2.58 eV. Wu et al. [23] prepared YFeO3 in which, hexagonal and orthorhombic phases co-existed; this material had a band gap of 2.41 eV. For YFeO3 ceramics prepared by the conventional solid-state method, the energy band gap was found to be 2.58 eV [7]. Liu

Conclusions
Co and Gd co-doped YFeO3 nanopowders were fabricated using a low-temperature solid-state reaction method. Data obtained using XRD and Raman analyses show that, with Gd and Co substitution, YFeO3 nanoparticles exhibit a distortion refined structure. The particle size for pristine YFeO3 nanoparticles is about 100-200 nm, and those for Co5, Gd5, Co5Gd5, Co5Gd10, Co5Gd15 and Co5Gd20 are approximately 120, 95, 80, 75, 45 and 40 nm, respectively. The maximum magnetization and remnant magnetization for the co-doped YFeO3 powders are about 5.49 and 2.20 emu/g, respectively, when exposed to a magnetic field of 60 kOe. The energy band gap of YFeO3 nanopowders was reduced from 2.41 to 2.23 using the co-doping method, thereby indicating their potential in decomposition applications. Because the co-doping method used in the study was found to be easy to control for fabricating the YFeO3 nanopowder samples, it is proposed that the method be adopted for use in the applied magnetic and optical fields.

Conclusions
Co and Gd co-doped YFeO 3 nanopowders were fabricated using a low-temperature solid-state reaction method. Data obtained using XRD and Raman analyses show that, with Gd and Co substitution, YFeO 3 nanoparticles exhibit a distortion refined structure. The particle size for pristine YFeO 3 nanoparticles is about 100-200 nm, and those for Co5, Gd5, Co5Gd5, Co5Gd10, Co5Gd15 and Co5Gd20 are approximately 120, 95, 80, 75, 45 and 40 nm, respectively. The maximum magnetization and remnant magnetization for the co-doped YFeO 3 powders are about 5.49 and 2.20 emu/g, respectively, when exposed to a magnetic field of 60 kOe. The energy band gap of YFeO 3 nanopowders was reduced from 2.41 to 2.23 using the co-doping method, thereby indicating their potential in decomposition applications. Because the co-doping method used in the study was found to be easy to control for fabricating the YFeO 3 nanopowder samples, it is proposed that the method be adopted for use in the applied magnetic and optical fields.

Conflicts of Interest:
The authors declare no conflict of interest.