Studies on Structural, Morphological, Local Electrical, Optical and Magnetic Properties of Iron Site Manganese Substituted Yttrium Orthoferrite

Yttrium orthoferrite (YFeO 3 ) is of considerable interest for its potential application in magnetic eld sensors and magneto optical data storage devices. Doping is one of the effective approaches to tune the compound properties. And it is important to determine the doping sites of the dopants to better understanding the related mechanism. In this work, Manganese (Mn) doped YFeO 3 , i.e., YFM x O powders with 0 ≤ x ≤ 0.1 were synthesized by hydrothermal method to study the inuences of doping on their structural, morphological, local electrical, optical and magnetic properties. The experimental results show that Mn dopants occupy Iron (Fe) sites and that all these samples exhibit an orthorhombic structure with space group Pnma. Rened structure parameters are presented. Morphology images show the shape evolution from layered to multilayered with increasing Mn content. The Fe and Y K-edge local structure studies indicate that the valency of Fe and Y is mainly found in trivalent state, which also indicate that substitution of Mn ions not only affects the nearest neighbor atomic shell of Fe but also affects the nearest neighbor’s local structure of Y atoms. IR spectra reveal the characteristic vibrations of the obtained YFM x O samples. From the magnetic study, it is observed that the substitution of Y ions by Mn ions changes the magnetic property of YFeO 3 from ferromagnetic to paramagnetic. Our results show that the addition of Mn exhibits an evident inuent on the local structural and magnetic properties.


Introduction
The second-generation multiferroic materials, i.e., rare earth orthoferrites (RFeO 3 , R = rare-earth ions or Y), have been extensively studied for decades owing to their multiferroic, spin-switching, and magnetooptical properties [1][2][3]. Some other outstanding features of these materials, such as high domain wall velocity and the existence of Bloch lines [4], have promising applications in sensors, information storage, and spintronics, etc [5,6]. In most cases, RFeO 3 is crystallized by the corner-linked FeO 6 octahedral forming a three-dimensional network in a centrosymmetric Pbnm (or Pnma) unit cell [7]. That is to say, the unit cell consists of four molecules with the R 3+ cations located on the center and the Fe 3+ ions are nearly octahedrally coordinated to six O 2− ions [8]. Unlike the rst generation multiferroics, RFeO 3 not only combines antiferromagnetic and ferroelectric orders but also show magnetoelectric coupling effects [9][10][11].
With the rise of RFeO 3 materials in recent years, as a member of them, yttrium orthoferrite (YFeO 3 , short for YFO) has attracted much attention from the research community because of its magnetic, physical, and chemical properties due to the ionic and electronic defects as well as structure distortions [12]. YFO crystallizes in the perovskite structure with the Pnma (D 2h 16 ) space group [13]. Despite the centrosymmetric nature, this material with its low Curie temperature T C ~ 256 o C and high Neel temperature T N ~ 370 o C, can exhibit both ferroelectric and antiferromagnetic behaviors [14,15]. Among all the RFeO 3 materials, YFO has been most thoroughly investigated. Doping with different types of ions is a very powerful way to modify, and enhance the properties of the compound. The substitution of manganese (Mn) for iron (Fe) is most interesting because the electron con guration of Mn 3+ and Fe 3+ ions are quite different and effective magnetic moments are different for both these ions. Kwanghee et al. [16] studied the absence of ferroelectricity and the origin of depolarization currents in YFe 0. 8 [17,18]. Dielectric relaxation, electric modulus, ac conductivity, structural and magnetic properties of Mn-doped YFO have been investigated by Zhang et al. [19,20]. Bipul et al. [21,22] reported the magnetic, dielectric properties, and exchange bias in Mn-doped YFO. Cao et al. [23] attempted to improve the conductivity of YFO by substituting Mn for Fe.
Attila et al. [24] highlighted some structural, electric, and magnetic properties of YMn However, the local electronic structure of Mn-doped YFO has rarely been investigated. To address this issue, we have studied the local structure properties of Fe-site Mn-doped YFO powders, using an X-ray absorption ne structure (XAFS) spectroscopy technique. The result of absorption spectra comes from the removal of a core electron when an X-ray photon is absorbed. The emitted photoelectrons are backscattered by the neighboring atoms. As it is well known, XAFS has proven to be one of the best tools to probe the local electronic environment each absorbing atom in simple or complex systems. As a structural tool, extended X-ray absorption ne structure (EXAFS) is capable of determining atomic arrangements, which could provide a reliable structural parameters information. EXAFS can directly measure the radial displacement between an absorbing atom and its neighbors [28]. However, in some cases, the accuracy of the structural determinations obtained from EXAFS can be inconclusive, such as the well-known limitations of the tting methodology to determine coordination numbers and polyhedral environments [29]. Fortunately, X-ray absorption near edge structure (XANES) can ll these holes, which is very sensitive to the geometrical details of the absorbing atoms, e.g., formal oxidation state, speci c symmetry, coordination chemistry, and bond angles, etc [30]. In our work, both EXAFS and XANES were examined at the K-edges of Y and Fe. We have also studied the morphological, optical, and magnetic properties of the target compound.

Characterization
Phase purity and crystal structure were characterized by X-ray diffraction (XRD) on the Mac Science M18XHF22-SRA X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å). Rietveld re nement of the samples was performed using the GSAS-EXPGUI program. The scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) images were used to determine the shape, morphology, and composition of the samples using LEO1430VP equipment. XAFS spectra were recorded at the Beamline 1W2B of the Beijing Synchrotron Radiation Facility (BSRF), the Institute of High Energy Physics, China. Pellets prepared with boron nitride to optimize the thickness were used and put on tape for the detection in the whole experiment. XAFS spectra at the Fe and Y K-edges were collected in the transmission mode. Data elaboration has been performed using the ATHENA code of the IFEFFIT software package for analysis [31]. E o was de ned as the maximum of the rst derivative of the absorption edge. The atomic absorption data were a transition from energy E space to wave vector K space. A Fourier transform was performed to obtain a distribution function around the absorbing atom in radial distance R space. An isolated single shell χ(k) was obtained by back transformation of the rst shell signal from R space to K  From the enlarged spectra in Fig. 1(b), other main peaks, such as (311) and (321) also have the same shifting trend. This shift in the diffraction angle might be ascribed to the unit cell contraction or the decrease in lattice constants because the ion radius of Mn 3+ (r Mn = 0.580 Å) is smaller than that of Fe 3+ ion (r Fe = 0.645 Å). However, it is worth noting that the intensity of the diffraction peaks reduced and merged partially to form broadened peaks after the Mn concentration further increased, especially for the sample with x = 0.1 (see in Fig. 1(a) and (b)), which is attributed to the presence of distortions, and a diminution in the crystalline size [32]. From the crystallography point of view, the intensity of the peaks is usually related to the crystallinity, thus, the broadened width of the XRD peaks with the increase of Mn content indicates the decreasing crystallinity of YFM x O. The reduced crystallinity may be due to the reason that Mn 3+ favors the creation of more nucleation sites, which in turn inhibits the growth of crystal grains.
To quantify the structure in detail and determine lattice parameters of the samples, an analysis of the XRD patterns by the Rietveld re nement was done using the Pnma space group in the orthorhombic unit cell. However, it is con rmed from the XRD results that the diffraction pro les belong to the orthoferrite structure for all samples. For such reason, only the samples with x = 0 and x = 0.1 were analyzed by Rietveld re nement, as shown in Fig. 2(a)-(b). Figure 2(c) shows the common crystal structure of the YFO.
In this structure, Y 3+ is surrounded by 12 O 2− ions, and Fe 3+ is surrounded by six O 2− ions arranged in FeO 6 octahedra. Table 1  observed in Y doped BiFeO 3 [35]. The variations in the intensity of peaks and lattice parameters can be attributed to the incorporation of the dopant in the crystal [36]. Thus, it is reasonable to believe that Mn 3+ ions are introduced to the iron sites of YFO, which can also be further illustrated by EDX results.

Morphological evolution and composition
The morphology and phase structure of the pure YFO and YFM x O powders are investigated by SEM micrographs, as shown in Fig. 3(a)-(e). From these images, we can observe that the size average (ca. 10 µm) is nearly the same in all the particles with different shapes, except for the sample with x = 0.1. It is observed that the pure YFO exhibit a layered cuboid shape. When the Mn content x = 0.025, the multilayered cuboid is observed, and it is continuously layered with further doping (see Fig. 3(b)-(d)). In the hydrothermal crystallization processes of RFeO 3 , the addition of KOH could transfer R and Fe ions into amorphous hydroxides R(OH) 3 and Fe(OH) 3 for a very short time [37]. The formation of RFeO 3 can be described by the chemical reactions, as follows: R 3+ + OH − = R(OH) 3 (s); Fe 3+ + OH − = Fe(OH) 3 (s). The transition metal or rare-earth hydroxides usually form layered structures with ions inserted between the layers of metal hydroxide [38], which is in good agreement with the results observed by SEM. In contrast, when the Mn content reaches x = 0.1, the grain morphology changes to larger agglomerates shape with a remarkably reduced grain size. The larger agglomerates are composed of smaller particles. From the ionic radii point of view, smaller Mn ions can enter the Fe-site of YFO, which maintains the charge balance in the system. After Mn doping, mass transportation becomes weaker and the grain growth is inhibited. A similar reduction of particle size can be found in our previous work [39]. compound. As seen in Table 2 in Fig. 4, for all powder samples, the absorption edge energies were found at 7127 eV (less than 0.5 eV error) which is close to the absorption edge energy of reference Fe 2 O 3 at ca. samples. The pre-edge peak is usually related to quadrupole transition from 1 s core state to 3d empty state, which is expected to be very weak for a Fe cation in an octahedral environment [41]. It is well known that the pre-edge peak is a ngerprint of the octahedral coordination of Fe. Our spectra show almost no pre-edge shift as a function of x but their intensity is changing with x. In the enlarged XANES spectra in Fig. 4(a), it can be seen that, in the beginning, the intensity of the pre-edge peak is slightly increased. For 0.025 < x < 0.1, the intensity of the pre-edge peak begins to decrease and it is largest for the x = 0.1 sample. It is worth noting that for the compounds with x = 0.1 the pre-peak increases its intensity with x, which possibly indicates a decrease of the symmetry of the Fe environment. A similar phenomenon has been observed in the other perovskite ABO 3 system [42,43]. The increasing intensity in the pre-edge peak indicates the enhancement of the 1 s-3d electric dipole-forbidden transition while decreasing the intensity caused by the 1 s-4p dipole-allowed transition. These transitions have caused by Mn substitution, indicating a distortion of FeO 6 octahedron. The two post-edge peaks are attributed to the transfer of 2p electrons in the oxygen 2p band to the Fe 3d orbital by a shakedown process [44]. The intensity of these two post-edge peaks rst increases then decreases when x = 0.1, as shown in Fig. 4(b). This indicates that the 3d-4p transition and charge transfer from the O 2p-Fe 3d is enhanced with both low and high doping contents of Mn due to the loss of inversion octahedral symmetry of the oxygens around the Fe atoms [45]. These evolutions indicate that the local geometry and structure of Fe have changed.
In addition to the XANES data above, further analysis is carried out using the EXAFS. The Fourier transformation of the EXAFS is also shown. EXAFS features could provide useful information on both the short-range and the long-range orders (i.e., in the rst shell and higher shell than the second). Figure 5 shows the variation of the observed k 3 -weighted EXAFS oscillation of the YFM x O powders. The error noise is observed above ca. 9 Å −1 . Oscillations are still visible above ca. 12 Å −1 , being less intense at the higher k, and show clear evolution as a function of Mn concentration, as given in Fig. 5(a). This phenomenon may be related to the less symmetric environment around Fe cations. The changes in the local structure could be better revealed in the Fourier transforms of the EXAFS oscillations providing real space information. The Fourier Transforms of the k 3 -weighted EXAFS spectra of the YFM x O samples are shown in Fig. 6 and the detailed coordination distances are listed in Table 3 in Fig. 6. The rst and the second neighbor distributions in distance are easier to separate from the other shells in the Fourier transform. There are some characteristics peaks in spectra: (1) There are two strong amplitude peaks between 1 Å and 4 Å, with the rst peak located at 1.54 Å, which is corresponding to the Fe-O coordinate peak due to the rst oxygen coordination sphere of Fe ions. (2) The second strong peak is located at 3.28 Å, which is known as the Fe-Fe/Mn peak caused by the second rate nearby metal ions. (3) The small intensity of other peaks is not yet clear. They are probably due to the multiple scattering processes in the rst coordination shell. Compared to the pure YFO, there is almost no shift of peak position (see Table 3 in Fig. 6) but the intensity of the Fe-O peak is decreased with the Mn content increases, as shown in Fig. 6(a). Moreover, with a close look at the Fe-O peak, we can observe that the intensity of this peak rst decreases (x = 0.025) then slightly increases (x = 0.05) and then decreases again when x beyond 0.05. The reduction of the Fe-O peak intensity represents the loss of short-range order in the system. The intensity of the Fe-Fe/Mn peak also has the same trend (see Fig. 6(b)), which is due to the change of the local structure from Fe-Fe into Fe-Fe/Mn. (less than 0.5 eV error). These can be used simply as a ngerprint of phases and valence states, from which it can be seen that the edge positions of Mn-doped samples are quite similar to that of the standard sample of Y 2 O 3 . Thus, this result indicates that the Y ions in our samples are in the 3 + valence state. All samples show no pre-edge peaks, which is associated with the 1 s to 4d transition of Y. More speci cally, this transition is partially allowed for the distortion of octahedral, only when p orbitals are mixed with d orbitals. The fact that this transition is not observed indicates a small distortion of the octahedral symmetry. From the examination of Table 4 in Fig. 7, it is clear that with Mn concentration increasing, there is no evident shift of the absorption edges in the whole series, but their intensity shows some difference. The two main peaks can be observed for all samples, which could be due to the transition from 1 s state to 5p state [46]. For better clarity, the enlarged post-edge peak is shown in Fig. 7(a), from which it can be seen that the intensity of the rst strong post-edge peak decreases as Mn content increases. As for the second strong post-edge peak, highest intensity can be found for x = 0.025 and x = 0.075 samples and lowest for x = 0.1 sample. The evolution of the post-edge peak intensity indicates that the local structure of Y has changed. Although Mn is doped in the Fe-site, the Y-site atom is also affected by the substitution.
Along the main edge, the XAFS spectra show a typical oscillation, which is caused by the scattering process of the electron wave near the nearest neighboring atoms. After the standard data processing, these waves yield of the χ(k)k 3 function and its Fourier transformation data could provide valuable information about the coordination number, nearest neighbor distances, and the coordination geometry, etc. Among them, the radial distribution function is generated by backscattering along the R axis of the atom at different distances. This indicates the distance between the atom level and the central absorption atom. The analyzable k range of the EXFAS data is from 0 to 14 Å −1 and the spectra are weighted by k to amplify the oscillations at high k. The Y K-edge k 3 -weighted EXAFS curves of the YFM x O samples are given in Fig. 8. All the oscillations show similar patterns in the higher and lower k (see Fig. 8(a)), which indicates that the EXAFS functions of the Y atoms seem unchanged in all samples. The (2) The second shell with an R of 2.54 Å, corresponding to the Y-Y peak, which can be explained by the scattering of oxygen anions from the next nearest neighboring Y atomic shell. The low second peak is a common feature in the case of EXAFS, which is often explained as the presence of high levels of disorder in the materials. (3) The other small peaks are probably due to a large number of multiple scattering in the rst shell. More details are shown in Table 5 in Fig. 9, where the peak positions of YFM x O samples remain almost unchanged but the intensities of which are affected by Mn substitution. Compared to pure YFO, the intensity of the Y-O peak is increased then largely decreases for x = 0.1 sample. More speci cally, the intensity of Y-O peak rst increases with x = 0.025 then a little decrease when x = 0.05 and then increases again for x = 0.075, as shown in Fig. 9(a). Unlike the Y-O peak, the intensity of the Y-Y peak decreases from x = 0.025 to x = 0.1, as shown in Fig. 9(b). Although the changes in intensity can be explained by various reasons, such as the Debye-Waller factor, coordination number, and amplitude reduction factor, etc, the main reason for this reduction in intensity is most likely originated from the decrease in coordination number. These changes above indicated that the substitution of Mn ions not only affects the nearest neighbor atomic shell of Fe but also affects the nearest neighbor's local structure of Y.

Magnetic property
As is commonly known, the magnetic properties in rare-earth orthoferrites originated from the super- through O 2− ions at 180 o C. The GKA (Goodenough, Kanamori, and Anderson) rules were used to predict the super-exchange interaction and the anti-ferromagnetic nature of YFO [50]. However, due to the Dzyaloshinski-Moriya antisymmetric exchange mechanism, each Fe 3+ magnetic moment is not being exactly anti-parallel to the moments of the rest of the Fe 3+ ions. This leads to the occurrence of weak ferromagnetism in YFO [51]. The schematic representation of the magnetic structure of YFO is presented in Fig. 11(a) and (b), respectively. In this structure, the anti-ferromagnetism is along the a-axis with antiparallel of Fe 3+ spin with a small canted angle along the b-axis, as shown in Fig. 11(a). Figure 11(b) shows the weak ferromagnetism along the c-axis, the canted angle for all Fe 3+ spin is parallel arrangement [52]. Figure 12 shows  Table 6 in Fig. 12 for comparison. It is striking to note that all the samples behave as signi cant hysteresis loops, indicating the ferromagnetic properties of them. When an external eld is up to 60 KOe, the magnetization has not reached saturation. In this structure, Y 3+ is a diamagnetic cation and has a zero magnetic moment, thus, the observed ferromagnetism in our samples could be due to the spin canted of Fe 3+ as the source of the magnetic moments [53]. It means that the Fe 3+ spins are not completely anti-parallel, but in reality, they may be canted. It is evident from  [54] and Y 1 − x Gd x FeO 3 [55]. The higher Mn concentration, the weaker ferromagnetic property, which is consistent with the change of the hysteretic loop.

Conclusions
In this work, the effect of Fe-site Mn 3+ doping with 0 ≤ x ≤ 0.1 concentration on structural, morphological, local electrical, optical and magnetic properties of YFeO 3 powders synthesized by using the hydrothermal method has been investigated. In the XRD patterns, the sharp and well-de ned peaks show that all samples have an orthorhombic structure with space group Pnma. By using Rietveld tting of the XRD pro le we could con rm the orthorhombic crystalline structure of YFeO 3 . Re ned structure of x = 0 and x = 0.1 samples also reveal the decreasing parameters with increasing Mn content, which was considered to be the smaller ionic radii of Mn 3+ compared with that of Fe 3+ . As shown by SEM images, with increasing dopant concentration, the layered shape changes to a multilayered shape and a large reduction of particle size observed when x = 0.1 with larger agglomerates shape. XAFS spectroscopy, including XANES and EXAFS has been used to investigate and obtain the structural information around Fe and Y atoms in YFM x O samples. However, the spectra have a similar shape for the whole series. The Declarations