Multiferroic materials simultaneously possess magnetic and ferroelectric orders which co-exist and couple with each other [1]. They are able to put the electrical, magnetic and optical properties together and suitable to design new multi-functional electronic information storage elements. Multiferroic materials have become one of the most active areas in the field of materials science. Hexagonal RMnO₃ (R = Y, Ho-Lu) materials occupy a very important position in the single-phase multiferroic materials. They have novel properties, indicating the potential in the material research and applications [2]. Therefore hexagonal manganites YMnO₃ have attracted widespread attention in recent years. However, because YMnO₃ exhibits the coupling of antiferromagnetism and ferroelectricity, such multiferroic is not very sensitive to the applied external electromagnetic fields [3]; thereby it is the purpose of many researchers to increase the ferromagnetic property of YMnO₃.

As an effective research tool, ion doping in A site (i.e. Y site) or B site (i.e. Mn site) of YMnO₃ is often used to change the ferromagnetic property of YMnO₃ [4]. Some research teams have selected kinds of ions replacing the Y³⁺ to modulate the antiferromagnetic order of YMnO₃, such as Lu³⁺, Sr²⁺. It will make partial antiferromagnetic order convert to ferromagnetic properties in the compounds where Y³⁺ was replaced by Lu³⁺ or Sr²⁺ [5] [6]. Transition metal ions replacing the Mn³⁺ in the B site of YMnO₃ is another way to modulate the antiferromagnetic order. Several current experiments have successfully synthesized samples in which the Mn³⁺ in the B site of YMnO₃ is replaced by kinds of ions, such as Fe³⁺, Al³⁺, Cu²⁺, Ti³⁺, etc. [7]-[10]. Y. J. Yoo et al., who have synthesized polycrystalline Cr-doped YMnO₃ with hexagonal structure and space group P6₃ cm, found that the magnetic transition temperature increased as the concentration of Cr increased [11]. Indeed, it will make the ferromagnetic property of YMnO₃ increase significantly by the substitution of the Mn³⁺ with transition metal ions in the YMnO₃ samples. However, the reason of the change of magnetism is still not explained clearly. Compared to previous studies about YMnO₃ samples, this paper will explain the change of magnetism when the Fe³⁺ replaces the Mn³⁺ in the B site of YMnO₃ samples according to XAS of O K edge and Mn L edge.

Polycrystalline YMn_1−xFe_xO₃ (0 < x < 0.1) samples were prepared by a standard solid-state reaction. The analytical pure Y₂O₃, MnO₂ and Fe₂O₃ were weighed according to stoichiometric proportion. The mixed powder was put into an agate mortar milling 5 hours with petroleum ether. The milled powder was transferred into a corundum crucible in a tube furnace. The powder was sintered 2 h at 1100˚C then heated to 1370˚C, maintaining 24 hours. Taking out the powder and milling for 2 hours, we can obtain the YMn_1−xFe_xO₃ (0 < x < 0.1) samples. The crystal structures of the samples were examined by X-ray diffraction (XRD) with Cu Kα radiation (Rigaku Smart Lab3, Japan). The magnetic properties of YMn_1−xFe_xO₃ were measured by SQUID-VSM (Quantum Design, USA). In order to observe the change of Y-O, Mn-O hybrid states, the X-ray absorption spectroscopy (XAS) of O K edge and Mn L edge of YMn_1−xFe_xO₃ (0 < x < 0.1) samples were measured utilizing total electron yield (TEY) mode in photoemission spectroscopy experiment station of Beijing Synchrotron Radiation Facility, Chinese Academy of Sciences.

XRD patterns of powder samples YMn_1−xFe_xO₃ (0 < x < 0.1) at room temperature are measured as shown in Figure 1. XRD patterns of YMn_1−xFe_xO₃ illustrate that all samples are in single phase with hexagonal lattice structure, space group P6₃ cm. It illustrates that Fe³⁺ ion replaces the lattice position of Mn³⁺ ion and doesn’t change the lattice structures of YMn_1−xFe_xO₃ samples, with the incorporation of Fe³⁺ ion. Because Fe³⁺ ionic radius (0.49 Å) is smaller than the Mn³⁺ ionic radius (0.58 Å), the lattice structures of YMn_1−xFe_xO₃ samples have a slight contraction. This change can be found from the diffraction peaks of YMn_1−xFe_xO₃ samples. The diffraction peak (112) of YMn_0.95Fe_0.05O₃ shifts toward the higher angle with respect to that of YMnO₃ (as inset of Figure 1). The contraction of lattice structure will lead to the change of Y-O, Mn-O hybrid states and Y, Mn ligand structure, which will affect the bond lengths of Y-O and Mn-O. These changes will affect the magnetic order of YMn_1−xFe_xO₃ (0 < x < 0.1) samples.

In order to observe the magnetism change of YMn_1−xFe_xO₃ samples, field cooled (FC) temperature dependent magnetization (M-T) curves were measured from 30 K to 300 K with cooling field of 5000 Oe as shown in Figure 2. The magnetism of YMn_1−xFe_xO₃ samples are significantly enhanced, which is attributed to the incorporation of Fe³⁺ ion. Since Mn trimer arrangement exists the magnetic frustration effect (shown in Figure 2(b)), the magnetic frustration effect is relieved when Fe³⁺ ions partially replace Mn³⁺ ions in the B-site of crystal lattice. And Fe³⁺ ion having five 3d electrons will enhance the magnetism of YMn_1−xFe_xO₃ samples. In addition, due to Fe³⁺ ions doping, the lattice structures of YMn_1−xFe_xO₃ samples shrink slightly, causing the magnetic exchange interaction to be enhanced.

Figure 3 shows the XAS of O K edge of YMn_1−xFe_xO₃ samples which illustrate the hybrid states between O 2p and Mn 3d, Y 4d, Mn 4sp/Y 5sp. The absorption spectra of the O 2p-Mn 3d hybrid states can be refined to four peaks, namely a _1g ↑, e _1g ↓, e _2g ↓, a _1g ↓ [12]. O 2p-Y 4d electron orbitals also have a strong hybridization as shown by the XAS of O K edge. Mn³⁺ ion is surrounded by 5 Oxygen atoms, forming bipyramid structure MnO₅, as shown in Figure 4(a).

The ferroelectric transition temperature T_C of YMnO₃ is about 900 K. When paraelectric phase is transformed to ferroelectric phase for YMnO₃, the bipyramid MnO₅ will be tilted, as shown in Figure 4(b) [13]. The ferroe-

lectric order of YMnO₃ is induced by the inclined bipyramid MnO₅ which leads to the orbital hybridization of O 2p-Y 4d enhanced. As shown in Figure 3, due to Fe³⁺ ion doped, the intensity of e _1g ↓ and e _2g ↓ peaks are enhanced which indicates the orbital hybridization of O 2p-Mn 3d is enhanced. The change in the intensity of e _1g ↓ and e _2g ↓ peaks also shows that the structural distortion of MnO₅ has been minorly changed. The structural distortion of MnO₅ will also affect the coordination environment of Y³⁺ ion, resulting in the change of O 2p-Y 4d orbital hybridization. The absorption spectra of O 2p-Y 4d are enhanced in intensity, as shown in Figure 3, consistent with our discussion.

Because O 2p-Mn 3d orbital hybridization is changed, the electronic orbital of Mn 3d presents a more complex structure in YMn_1−xFe_xO₃ (0 < x < 0.1) samples. The electronic orbital of Mn 3d splits into e _1g , e _2g , a _1g (as shown in Figure 4(c)) [12]. As shown in Figure 5, the lower energy segments of Mn 3d L₃ absorption spectra peak are significantly enhanced in YMn_0.95Fe_0.05O₃ and YMn_0.92Fe_0.08O₃ samples compared to that of YMnO₃ sample. This shows that there are more empty electronic states in the low energy states (such as e _1g , e _2g ), due to Fe³⁺ doping, which is consistent with the situation of O K edge absorption spectra. The electronic orbital of Mn 3d is closely related to magnetic exchange interaction and lattice distortion of MnO₅.

Polycrystalline YMn_1−xFe_xO₃ (0 < x < 0.1) samples were prepared by a standard solid-state reaction. The lattice structures of hexagonal YMn_1−xFe_xO₃ (0 < x < 0.1) samples are unchanged with Fe³⁺ doping. The magnetic properties of YMn_1−xFe_xO₃ samples are significantly enhanced, and can be attributed to doping Fe³⁺ ion in YMnO₃. According to XRD patterns, it can be obtained that the lattice structures of YMn_1−xFe_xO₃ (0 < x < 0.1)

samples slightly shrink. Based on O K edge and Mn L edge XAS absorption spectra of YMn_1−xFe_xO₃ (0 < x < 0.1) samples, it can be obtained that distortion occurs on the surrounded ligand structures of Y³⁺ and Mn³⁺ and that the orbital hybridization of Y-O and Mn-O are enhanced as Fe³⁺ ions doped, explaining the magnetic enhancement of YMn_1−xFe_xO₃ (0 < x < 0.1) samples.

This work is supported by the National Natural Science Foundation of China (U1232133). We are in debt to photoemission spectroscopy experiment station of Beijing Synchrotron Radiation Facility for their help in measuring the XAS spectra.

Xiaopeng Ge,Jiaou Wang,Kurash Ibrahim,Wenbo Yang,Xueguang Dong,Qi Li, (2015) Influence of Doping on the Magnetic Properties and Local Microstructures in Fe-Doped YMnO₃. Journal of Applied Mathematics and Physics,03,262-266. doi: 10.4236/jamp.2015.32038