Multiferroic properties of GdFe0.9M0.1O3 (M = Ag1+, Co2+ and Cr3+) nanoparticles and evaluation of their antibacterial activity

Multifunctional nanoparticles NPs with material composition GdFe0.9M0.1O3; M = Ag, Co, and Cr have successfully been synthesized using the citrate auto-combustion technique. The single phase of the orthorhombic perovskite structure is ratified from the XRD data. The structural, magnetic, and thermoelectric power of the samples along with the results of antibacterial activities are reported in the present manuscript. The variation in the magnetization is argued in view of the strength and type of exchange interaction as well as buckling of the < BO6 > octahedron. The super exchange interaction between the Fe–O–Fe and the Cr–O–Cr and the randomness of Cr ions in the host lattice site are the main reasons behind the weak ferromagnetism obtained from GdFe0.9Cr0.1O3. Ferroelectricity and antiferromagnetism have a dissimilar origin and appear independently. The origin of antiferromagnetism is the spin canting of the B ions. However, the origin of the ferroelectric properties is the hybridization between B cations and O2− anion. The use of silver metal particles as antibacterial agents is noteworthy due to their advantages in terms of chemical stability, efficacy and long-term durability. These advantages can be extended by considering the relatively low toxicity of these particles to the human body compared to other inorganic metals.

Ag-doped orthoferrite materials exhibit mixed conduction with a significant amount of Fe 4+ and a sufficient number of oxygen vacancies leading to mixed ion conduction which can position the material as an attractive cathode material to be used in numerous applications.
In accordance with Goodenough-Kanamori theory, Cr 3+ is the best choice of substitute element to enhance the magnetization of orthoferrites samples where Fe and Cr ordered at the B site as Fe 3+ (d 5 )-O-Cr 3+ (d 3 ) would show superior magnetic property due to the super exchange interaction [24,25]. So, the second choice of transition metal is Cr to induce interesting magnetic and electrical properties.
Substitution of Fe ions with transition ions with different valance gives rise to the coexistence of mobile ions and or vacancy on the B sites. Thus, either the ferromagnetic (FM) or the antiferromagnetic (AFM) interaction can be dominant depending on the B site doping type. Substitution with ions of different ionic radii at a fixed valence modifies the cubic structure in the perovskite-based material and induces lattice distortion in the BO 6 octahedral [26]. The average size of cations in A and B sites has a noticeable effect on the degree of octahedral distortion. This indicates that the size of the cation has a major effect on the bond length B-O, and the bond angle B-O-B, as well as the hopping integral between Fe 3+ /Fe 4+ degenerate states [27,28].
In the present study, the effect of identical quantities of numerous valence elements (monovalent, divalent, and trivalent) on the physical properties of the GdFeO 3 is examined. Furthermore, the antibacterial effect of the orthoferrite GdFe 0.9 Me 0.1 O 3 NPs was screened against Gram-positive bacteria as (Staphylococcus aureus and Bacillus subtilis) and Gram-negative bacteria as (Pseudomonas aeruginosa and Escherichia coli) using the inhibition zone method.

Experimental work
Perovskites GdFe 0.9 Me 0.1 O 3 (Me Ag 1+ , Co 2+ , Cr 3+ ) were manufactured by using citrate auto-combustion technique. Complete details are published in the previous work and illustrated in Fig. 1 [29].

Antibacterial activity
The antibacterial activity of GdFe 0.9 Me 0.1 O 3 ; Me Ag, Co, Cr NPs was evaluated by their zone of inhibitions against six bacterial strains. It was studied using a modified Kirby-Bauer disk diffusion method [30]. Briefly, 100 μl of the test bacteria were grown in 10 ml of fresh media until they reached a count of approximately 108 cells/ml for bacteria [31]. 100 μl of microbial suspension was spread onto agar plates. Isolated colonies of each organism that might be playing a pathogenic role should be selected from primary agar plates and tested for susceptibility by the disk diffusion method.
Plates inoculated with Gram (+) bacteria as Staphylococcus aureus, Gram (−) bacteria as Escherichia coli, were incubated at 35-37°C for 24-48 h. After that, the diameters of the inhibition zones were measured in millimeters [32][33][34][35]. Standard disks of Ampicillin (Antibacterial agent) served as positive controls for antimicrobial activity, while filter disks were used as a negative control. Blank paper disks with a diameter of 8.0 mm were impregnated with 10 μl of tested concentration of the stock solutions. When a filter paper disk is placed on agar, the chemical will diffuse from the disk into the agar. The solubility of the chemical and its molecular size will determine the size of the area of chemical infiltration around the disk. If an organism is placed on the agar, it will not grow in the area around the disk. This area of no growth around the disk is known as a "Zone of inhibition" or "Clear zone."   Figure 2 represents the XRD pattern for the samples of the general formula GdFe 0.9 Me 0.1 O 3 (Me Ag, Co, and Cr). The single phase of the orthorhombic perovskite structure is ratified from the obtained results. This is confirmed by comparing with ICDD cards (01-074-1900). The peak position of GdFe 0.9 Me 0.1 O 3 ; M Ag, Co, and Cr are shifted to a higher angle (2θ ) compared with the main peak for GdFeO 3 . This confirms the absence of biphasic structure [36] and the decrease in cell parameters a, c compared to the parent sample. The crystallite size (L) is detected using the Williamson-Hall relation [37] and the full width at half maximum of the diffraction peak (β 1/2 ) as mentioned in the previous work [38]. The obtained data are given in Table 1.

X-ray analysis (XRD)
The comparison of the lattice parameters (a, b and c), the unit cell volume and the tolerance factor for the investigated samples are given in the table. The data reveal that the Ag substituted sample has the smallest unit cell volume compared to Co, and Cr, in spite of the fact that the Ag + has the greatest ionic radii (1.28 Å) [39]. The decrease in the cell volume is due to the increase in Fe 4+ content, as the size of Fe 4+ (0.585 Å) is smaller compared to the size of Fe 3+ (0.645 Å). The valence exchange between Fe 4+ /Fe 3+ leads to the formation of oxygen vacancy that has an effect on the unit cell volume. This behavior agrees well with the obtained data for Ca doping PbTiO 3 [40].
The specific surface area (S) of the grain is calculated using the following equation [41] and the obtained data are given in the table.

S
grain surface area grain mass Additionally, the theoretical density (D x ) is calculated from the following equation [42]: where Z is the number of molecules per unit cell, M is the molecular weight, N A is the Avogadro's number and V is the volume of the unit cell, and the obtained data are given in the table.  Table 1 Lattice parameters, volume of the unit cell (V ), theoretical density (D x ), crystallite size (L), the tolerance factor (T ), dislocation density (δ) and specific surface area (S) for the investigated samples It is revealed that the addition of B cations (Co 2+ , Ag 1+ , and Cr 3+ ) with different valances increases D x compared to the parent sample. This increase can be attributed to that the ionic radii of Co (0.745 Å), and Ag (1.28 Å) are higher than the ionic radius of Fe 3+ (0.645 Å) ion. On the other side, Cr substitution induces a higher oxidation state [43,44]. This may induce defects that affect the magnetic properties of the prepared sample as will be discussed later [45].
As shown from table, GdFe 0.9 Ag 0.1 O 3 -doped sample has the largest density compared to GdFe 0.9 Co 0.1 O 3 and GdFe 0.9 Cr 0.1 O 3 substituted samples. This is due to the high atomic weight (103.5) and high density (10.49 g/cm 3 ) of Ag compared to Cr and Co-doped samples. In fact, the increment rate of the molecular mass is more than that of the lattice parameter.
The dislocation density (δ) [46,47] has also been determined using the following equation: The tolerance factor (T ) is a geometrical factor characterizing the size mismatch which takes place when a site cation is very small that is accommodated into the BO 6 octahedra of the three-dimensional networks. As the "T " decreases, the bond angle (θ ) decreases, and orthorhombic cell distortion increases. For GdFe 0.9 Ag 0.1 O 3 , there is an increase in the orthorhombic distortion as a result of doping Ag at the expense of Fe because of the mismatch between the ionic radii of Ag + and Fe 3+ (1.28 Å and 0.645 Å, respectively). The variation in the ionic radii leads to a change in the "T," as well as in the bond angle θ Fe-O-Fe . This is a measure of the tilting of the octahedral and is also directly linked to orthorhombic cell distortion [48].
On the other hand, when adding Cr by 10% at the expense of Fe in GdFe 0.9 Cr 0.1 O 3 , an increase in the "T " compared with the parent sample is observed. The reduced distortion can be attributed to the variance in the ionic radii between Cr (0.615 Å) and Fe (0.645 Å) considering the oxidation state 3 + and octahedral coordination for both atoms [49].

Fourier transform infrared spectroscopy (FTIR)
FTIR spectra of Multiferroic samples GdFe 0.9 Me 0.1 O 3 (M Ag, Co, and Cr) are displayed in Fig. 3. The detected bands in the range from~420 to 435 cm −1 are recognized Fe-O or Co-O bending vibrations of FeO 6 . These bands are sensitive to Fe-O-Fe bond angle [50]. In the present case, this band does not seem due to the technical limitations of the device. The presence of antisymmetric stretching vibrations Fe-O or Co-O bonds in octahedral is ratified via the presence of ν 1 mode around 556 cm −1 . The two observed bands are the most significant issues in the perovskite oxides [51]. The ν 3 mode is detected around 1446 cm −1 , which corresponds to symmetric carboxylate stretches (COO-), while C-H bend mode (ν 2 ) appears at 850 cm −1 . The symmetric stretching vibrational band of carboxyl root is represented by a small band (ν 4 ) at about 2345 cm −1 [52]. Additionally, ν 5 mode represents the symmetric and asymmetric stretching vibration of water molecules and is detected at 3445 cm −1 [53]. The other detected bands with a small intensity appear around 2400 cm −1 which is due to O-H (carboxylic acids) [54].
The observed peaks are the main features in the perovskite oxides [55] regardless of the doping of transition metals either Ag, Co, or Cr which does not result in any significant shift of the IR data.
It has been observed that the broadness of the bands is greater for GdFe 0.9 Ag 0.1 O 3 . This change may be due to a change in the tolerance factor value which decreases the distortion of the structure.

High-resolution transmission electron microscope (HRTEM)
High-resolution transmission electron microscopy (HRTEM) is utilized to scrutinize the morphology and crystalline structure of the investigated NPs. Figure 4a-c displays the HRTEM images of the investigated samples. The agglomeration of the nanoparticles with uniform size and orthorhombic shape is observed. This reveals that the prepared samples exhibit fine nano-structure of the orthoferrites. Page 5 of 11 443

Energy-dispersive X-ray analysis (EDAX)
The energy-dispersive X-ray analysis (EDAX) of GdFe 0.9 Me 0.1 O 3 ; (Me Co, and Ag) is shown in Fig. 5a, b. The relative elemental concentrations of Gd, Fe, Co, Ag, and O in GdFe 1 − x M x O 3 are determined with the help of EDAX spectra. The EDAX spectra clearly reveal that the samples are without any impurity and also the relative concentrations of Gd, Co, Ag, and Fe are close to the expected concentration.

Magnetic properties
The magnetization hysteresis loops of the orthoferrites samples are shown in Fig. 6a, b. Generally, the substitution in the B site by monovalent and divalent is expected to convert a fraction of Fe 3+ to Fe 4+ . Though, Fe 4+ is difficult to form at ambient pressure. Therefore, the imbalance of charge introduced by Ag 1+ , and Co 2+ cations have to be compensated by oxygen deficiency [56]. The systematic change in the saturation magnetization with different ionic radii/valence of the cation possibly indicates that the oxygen deficiency alone may not cause the enhanced magnetization. It may be related to the progressive suppression of spiral spin structure and/or increase in the canting angle of antiferromagnetically coupled layers due to tilting of FeO 6-δ octahedra. Thus, it is observed that the magnetization is unsaturated even at a very high field and the coercivity of all samples is found to be quite low, i.e., in the range of 100-700 Oe.
The weak ferromagnetism is detected for GdFe 0.9 Cr 0.1 O 3 NPs. This behavior is due to different issues. The uncompensated canted spins from the surface, as well as the internal canted spin, are the main issue. The second issue is the interaction of Cr-O-Cr and the Cr 3+ (d 3 ) dilutes the AFM (Fe-O-Fe (d 5 )) matrix leading to reduced magnetization as discussed by Copogna et al. [57]. The third is due to the cluster or impurity phase coexisting in samples [58].   Table 2 shows the remnant magnetization (M r ), saturation magnetization (M s ), and coercivity (H c ), squareness for the investigated samples. It is clear that the decrease in H C for Cr can be attributed to the dilution of long-range Fe-O-Fe interaction, which indicates that Cr acts as an antiferromagnetic ion in the investigated samples. A similar signature has been detected for Cr-substituted LaFeO 3 .
The "M S " values for GdFe 0.9 Cr 0.1 O 3 and GdFe 0.9 Ag 0.1 O 3 decrease compared to the pure samples as given in the table. This decrease is due to diluting the magnetization in the B site by increasing accommodation of the less magnetic moment Cr 3+ (3.87 μB), and Ag 1+ (3 μB) instead of Fe 3+ (5 μB) ions. Moreover, this causes the weakening of the direct exchange magnetic interaction between the crystal sublattices [59][60][61].
As shown from the table, the squareness values are less than 0.5 which suggests that the particles are not in a predominant single domain state despite their small size. When R < 0.5, the particles interact by magnetostatic interaction according to Z.L. Wang [62,63]. In the present case, the calculated values assure that a magnetostatic interaction is possible with an expected spin coupling between the layers of the nanoparticles.

Ferroelectric properties
The origin of the ferroelectric properties is the covalent bond between the B cations and O anion. The B ions shift from the center of < BO 6 > octahedra toward the three oxygens to form a strong covalent bond. Figure 7a-d illustrates the room temperature P-E hysteresis loops of GdFe 0.9 Me 0.1 O 3 (Me Ag, Co, Cr).
All samples exhibit clear hysteric behavior indicating ferroelectric nature. It is observed that the P-E loops are unsaturated. This can be attributed to the pinning of the domain walls due to the presence of the oxygen vacancies. The values of the ferroelectric parameters like remnant polarization (P r ), saturation polarization (P s ), and coercive electric field (E c ) are given in Table 3.
At ambient temperature, and increasing electric field, the P s , P r are found to be maximum for GdFe 0.9 Co 0.1 O 3 . The detected increase in P r polarization for GdFe 0.9 Co 0.1 O 3 may be due to both, reduction in leakage current and the small crystallite size [64]. As shown from the figure, the loops are slightly shifted around the origin. This abnormality can be clarified by the existence of an internal electric field originating from the off-centering and intrinsic movement of bound electrons inside the samples. The increase in the coercive field for GdFe 0.9 Co 0.1 O 3 can be attributed to dipole friction that arises owing to the domain orientation [65].
Finally, one can conclude that the investigated samples have antiferromagnetic and ferroelectric behaviors simultaneously. The origin of antiferromagnetism is the spin canting of the B ions, while the origin of the ferroelectric properties is the hybridization between B cations and O 2− anion. Ferroelectricity and ferromagnetism have a dissimilar origin and appear independently of one another.

Thermoelectric power
The valence of the cations, the spin of the charge carriers, the symmetry of the structure, and the strengths of the crystal field are the main issues that affect the thermoelectric power of samples. Figure 8a-d shows the temperature dependence of the Seebeck coefficient (dV /dT ) of the GdFe 0.9 Me 0.1 O 3 (Me Co, Cr and Ag) nanoparticles. In the present case, the value of the (dV /dT ) changes from − Ve to + Ve depending on the types of dopants. The majority carriers of the GdFeO 3 (x 0) are electrons over the temperature range of 300-600 K. Figure 8b shows the increase in the Seebeck coefficient with increasing temperature up to T C 453 K. This observation can be attributed to the formation of Co 3+ high spin state configuration (t 2g 4 e g 2 ) at high temperature [66]. The spin entropy is most probably the reason for the large thermopower found in GdFe 0.9 Co 0.1 O 3 sample.
According to Mizusaki et al. [67], the sign change can be attributed to the concentration of cation defects. This means that the deviation from the stoichiometric cation composition causes an important discrepancy in the Seebeck coefficient. as good stability, biocompatibility, colloidal stability, effective treatment at lower doses, and controllable sizes [68]. However, the probable mechanisms of antimicrobial activity for the NPs can be attributed to numerous issues. The first one is interference through cell wall synthesis. The second and third are suppression through protein biosynthesis and the disturbance of primary metabolic pathways, respectively [69,70]. Generally, the activity of antibacterial of the samples is mainly related to the existence of holes/electrons generated by electro valence compensation in the samples. Furthermore, the antibacterial effect principally depends on the NPs size and their distribution [71]. Smaller nanoparticles, due to having a much larger surface area, can greatly increase the production of the reactive oxygen species which is a significant parameter essential for antibacterial applications. The average amount of inhibition zone with its standard deviation is determined in Table 4. However, the GdFe 0.9 Co 0.1 O 3 , GdFe 0.9 Cr 0.1 O 3 have weak antibacterial activity as shown in the Table and Fig. 10. These samples cannot be able to destroy the bacterial species.
The obtained data revealed that the GdFe 0.9 Ag 0.1 O 3 NPs are relatively effective than other investigated samples. This is due to the fact that the antimicrobial activity of GdFe 0.9 Ag 0.1 O 3 depends on the amount of monovalent ions released and interacts with thiol groups existing in bacterial enzymes. This destroys the bacterial species [72]. The use of silver metal particles as antibacterial agents is noteworthy due to their advantages in terms of chemical stability, resistance to temperature variations, efficacy, and longterm durability. These advantages can be extended by considering the relatively low toxicity of these particles to the human body compared to other inorganic metals [44].

Conclusion
The perovskite samples were synthesized by the citrate auto-combustion technique. The single phase of the orthorhombic perovskite structure is ratified from the X-ray data. The variation in the magnetization by Co 2+ , Cr 3+ , and Ag 1+ ion substitution depends on the strength and type of exchange interaction and/or canting angle, as well as buckling of the < BO 6 > octahedron.
The deviation from the stoichiometric cation composition causes a significant variation in the Seebeck coefficient.
The obtained data revealed that the GdFe 0.9 Ag 0.1 O 3 nanoparticles exhibited enhanced antibacterial activity compared to GdFe 0.9 Co 0.1 O 3 and GdFe 0.9 Cr 0.1 O 3 samples.
The Ag ions accompanying GdFeO 3 nanoparticles possibly lead to microorganism cytotoxicity, which increases their bio-active impact and recommended them as an effective bactericidal compound.