Influence of Doping on the Magnetic Properties and Local Microstructures in Fe-Doped YMnO 3

Polycrystalline YMn1−xFexO3 (0 < x < 0.1) samples are synthesized by solid-state reaction method and characterized by X-ray diffraction. The X-ray diffraction patterns indicate that YMn1−xFexO3 compounds maintain hexagonal structure with space group of p63 cm. Ferromagnetism of YMn1−xFexO3 increases with increasing doping concentration of Fe3+, attributed to the suppression of the frustration and the change of the Mn-O bond length certificated by XAS analysis.


Introduction
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 RMnO 3 (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 YMnO 3 have attracted widespread attention in recent years.However, because YMnO 3 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 YMnO 3 .
As an effective research tool, ion doping in A site (i.e.Y site) or B site (i.e.Mn site) of YMnO 3 is often used to change the ferromagnetic property of YMnO 3 [4].Some research teams have selected kinds of ions replacing the Y 3+ to modulate the antiferromagnetic order of YMnO 3 , such as Lu 3+ , Sr 2+ .It will make partial antiferromagnetic order convert to ferromagnetic properties in the compounds where Y 3+ was replaced by Lu 3+ or Sr 2+ [5] [6].Transition metal ions replacing the Mn 3+ in the B site of YMnO 3 is another way to modulate the antiferromagnetic order.Several current experiments have successfully synthesized samples in which the Mn 3+ in the B site of YMnO 3 is replaced by kinds of ions, such as Fe 3+ , Al 3+ , Cu 2+ , Ti 3+ , etc. [7]- [10].Y. J. Yoo et al., who have synthesized polycrystalline Cr-doped YMnO 3 with hexagonal structure and space group P6 3 cm, found that the magnetic transition temperature increased as the concentration of Cr increased [11].Indeed, it will make the ferromagnetic property of YMnO 3 increase significantly by the substitution of the Mn 3+ with transition metal ions in the YMnO 3 samples.However, the reason of the change of magnetism is still not explained clearly.Compared to previous studies about YMnO 3 samples, this paper will explain the change of magnetism when the Fe 3+ replaces the Mn 3+ in the B site of YMnO 3 samples according to XAS of O K edge and Mn L edge.

Experimental Details
Polycrystalline YMn 1−x Fe x O 3 (0 < x < 0.1) samples were prepared by a standard solid-state reaction.The analytical pure Y 2 O 3 , MnO 2 and Fe 2 O 3 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−x Fe x O 3 (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−x Fe x O 3 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−x Fe x O 3 (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.

Results and Discussion
XRD patterns of powder samples YMn 1−x Fe x O 3 (0 < x < 0.1) at room temperature are measured as shown in Figure 1.XRD patterns of YMn 1−x Fe x O 3 illustrate that all samples are in single phase with hexagonal lattice structure, space group P6 3 cm.It illustrates that Fe 3+ ion replaces the lattice position of Mn 3+ ion and doesn't change the lattice structures of YMn 1−x Fe x O 3 samples, with the incorporation of Fe 3+ ion.Because Fe 3+ ionic radius (0.49 Å) is smaller than the Mn 3+ ionic radius (0.58 Å), the lattice structures of YMn 1−x Fe x O 3 samples have a slight contraction.This change can be found from the diffraction peaks of YMn 1−x Fe x O 3 samples.The diffraction peak (112) of YMn 0.95 Fe 0.05 O 3 shifts toward the higher angle with respect to that of YMnO 3 (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−x Fe x O 3 (0 < x < 0.1) samples.In order to observe the magnetism change of YMn 1−x Fe x O 3 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−x Fe x O 3 samples are significantly enhanced, which is attributed to the incorporation of Fe 3+ ion.Since Mn trimer arrangement exists the magnetic frustration effect (shown in Figure 2(b)), the magnetic frustration effect is relieved when Fe 3+ ions partially replace Mn 3+ ions in the B-site of crystal lattice.And Fe 3+ ion having five 3d electrons will enhance the magnetism of YMn 1−x Fe x O 3 samples.In addition, due to Fe 3+ ions doping, the lattice structures of YMn 1−x Fe x O 3 samples shrink slightly, causing the magnetic exchange interaction to be enhanced.
Figure 3 shows the XAS of O K edge of YMn 1−x Fe x O 3 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.Mn 3+ ion is surrounded by 5 Oxygen atoms, forming bipyramid structure MnO 5 , as shown in Figure 4(a).
The ferroelectric transition temperature T C of YMnO 3 is about 900 K.When paraelectric phase is transformed to ferroelectric phase for YMnO 3 , the bipyramid MnO 5 will be tilted, as shown in Figure 4(b) [13].The ferroe-  Because O 2p-Mn 3d orbital hybridization is changed, the electronic orbital of Mn 3d presents a more complex structure in YMn 1−x Fe x O 3 (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 L 3 absorption spectra peak are significantly enhanced in YMn 0.95 Fe 0.05 O 3 and YMn 0.92 Fe 0.08 O 3 samples compared to that of YMnO 3 sample.This shows that there are more empty electronic states in the low energy states (such as e 1g , e 2g ), due to Fe 3+ 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 MnO 5 .samples slightly shrink.Based on O K edge and Mn L edge XAS absorption spectra of YMn 1−x Fe x O 3 (0 < x < 0.1) samples, it can be obtained that distortion occurs on the surrounded ligand structures of Y 3+ and Mn 3+ and that the orbital hybridization of Y-O and Mn-O are enhanced as Fe 3+ ions doped, explaining the magnetic enhancement of YMn 1−x Fe x O 3 (0 < x < 0.1) samples.