Rare Earth-Doped BiFeO3 Thin Films: Relationship between Structural and Magnetic Properties
Rare Earth- (RE-) doped BiFeO3 (BFO) thin films were grown on LaAlO3 substrates by using pulsed laser deposition technique. All of BFO films doped with 10% of RE show a single phase of rhombohedral structure. The saturated magnetization in the Ho- and Sm-doped films is much larger than those reported in literature and was observed at a quite low field as of 0.2 T. As for Pr- and Nd-doped BFO films, Fe2+ amount is not dominant; thus, ferromagnetism is not favored. As the RE concentration goes up to 20%, all compounds have drastically gone through a structural transition. The RE-doped BFO films have changed from rhombohedral to either pure orthorhombic phase (for Ho, Sm), or a mixed phase of orthorhombic and tetragonal (for Pr, Nd), or pure tetragonal (for Eu). We observed magnetic properties of RE-doped BFO films have significantly changed. While 20% Ho/Sm-doped BFO films have ferromagnetism degraded in comparison with the 10% doping case, the 20% Pr/Nd-doped BFO thin films, whose structure is a mixed phase, have magnetic ordering improved due to the fact that the Fe2+ amount has become greater. It seems that one can control the magnetic properties of BFO films by using appropriate RE dopants and concentrations.
BiFeO3 (BFO) is a promising multiferroic material due to its high ferroelectric and antiferromagnetic ordering temperatures. Generally, BiFeO3 has G-type antiferromagnetic due to the local spin ordering of Fe3+ that basically forms a cycloidal spiral spin structure. In order to make BFO be room temperature ferromagnetic to be used in industrial applications, people have tried to suppress the spiral magnetic ordering by applying a very high magnetic field, or making them nanostructured, or by replacing Bi3+ or Fe3+ by other ions of comparable ionic sizes [1, 2]. Dimension reduction is likely to be a good way to improve the magnetization in BFO [3, 4]. Some groups have reported about the increase of magnetization in the bulk, thin films, and nanoparticles of BFO, by substituting either on the Bi-site by trivalent rare earth and divalent ions or on the Fe-site by transition metal ions [5, 6]. Some groups had showed that the magnetic moment of the nanoparticles could be increased three times by substituting Bi by Sr, or Ho. However, the reported saturated magnetization is found only at very high fields as of 5 T  and 6 T , respectively.
Our study focuses on tailoring the magnetic properties of BiFeO3 by choosing the appropriate RE ion for substitution and selecting the appropriate concentration, in order to obtain the largest magnetization possible at room temperature, at relatively low fields. Understanding the structure dependence of magnetic properties of RE-doped BFO films is of utmost importance in order to tune the magnetism of this type of materials to match up the requirements of applications.
ceramic targets (where RE = Ho, Sm, Pr, Nd, and Eu; , 0.1, and 0.2) were prepared by a sol-gel method. Rare earth- (RE-) doped BiFeO3 (RBFO) thin films have been fabricated by pulsed laser deposition (PLD) technique (excimer KrF laser with nm; the repetition rate was 13 Hz and the energy density was 2.1 J/cm2), with typical thicknesses as of 200 nm. All the films were grown on (001) LaAlO3 (LAO) substrates. During deposition, the substrate temperature was kept at 700°C and the oxygen partial pressure () was 1.4 × 10−3 Torr. After deposition, the sample was kept in the chamber at 500°C with the same oxygen partial pressure as during deposition for 30 min and then finally cooled down slowly to room temperature.
The structural analysis was done by high resolution X-ray diffraction (HRXRD) with Cu Kα radiation. The magnetic measurements were performed by a Quantum Design Superconducting Quantum Interference Device (SQUID) system under magnetic field () from 0 up to 0.5 T under a range of temperatures () from 350 K down to 5 K.
The surface chemistry and composition of RE-doped BFO thin films were characterized by X-ray photoelectron spectroscopy (XPS, KRATOS, and AXIS-HSi). XPS measurements were performed with an Mg/Al X-ray source. The energy calibrations were made against the C 1s peak and the Shirley background subtraction was used.
3. Results and Discussion
Concerning structure, for the case of 10% of RE substituted for Bi in BFO, there is no significant change in comparison to the pristine BiFeO3 . Our laser ablated RE0.1Bi0.9O3 films are single phase rhombohedral R3c. Doping with different RE elements only causes some shift of the BFO peak positions, indicating some change in lattice parameters. The out-of-plane parameters are 3.925, 3.925, 3.9329, 3.933, and 4.585 Ǻ for Sm, Ho, Nd, Pr, and Eu doping, respectively. One can notice here that the Eu doping distorts the structure the most (see details in Table 1), if we compare with parameters of the undoped BiFeO3 .
When the RE doping concentration increases up to 20%, a structural transition has occurred. When doping Ho and Sm up to 20%, the structure has become single phase orthorhombic, while for Pr and Nd, it has turned to be a mixture of orthorhombic and tetragonal phases. For Eu doping case, it is single phase tetragonal. A representative feature of X-ray patterns for RE-doped BFO films is shown in Figure 1. In order to make it easier to have an overall picture, we list the structure types as well as parameters for all of thin films in Table 1. It is known that changing dopant concentration can lead to a structural transition, as reported in [5, 6, 9]. Normally for concentration of RE greater than 0.2, the orthorhombic phase is likely favored [5, 9, 10].
Magnetization versus magnetic field taken at 300 K for RE0.1Bi0.9O3 film was reported in . Our new result for Eu-doped BFO films (shown in Figure 2) reveals a strong ferromagnetic behavior that is quite similar to those of Sm-doped and Ho-doped BFO films. Previously, some ferromagnetic ordering in Ho-doped BFO was reported, however, with a much smaller magnitude, and was obtained at much larger field (as of 6 T) , while ours has a larger , but at a much lower field (as of 0.2 T), as seen in Figure 3(a). While other groups reported paramagnetic behavior for Eu-doped BFO ceramics , as seen in Figure 2, our Eu-doped BFO film is room temperature ferromagnetic at 0.2 T. This should be quite advantageous for applications. Comparing to Ho-, Sm-, and Eu-doped BFO films, then the Pr- and Nd-doped BFO films show much smaller magnetic moments. As seen in Figure 3(b), the typical M-H curves of Pr-doped BFO (10%) taken at room temperature are almost linear, showing a paramagnetic behavior. In Figures 3(a) and 3(b), one can also see the obvious difference in magnetic properties for samples of 10% and 20% doping of RE. As for Ho, when doping 10%, because the Fe2+ : Fe3+ ratio is 50% : 50% (see Table 1), the ferromagnetic phase is more favored. When doping 20% of Ho, the Fe2+ amount is decreased (45.6%, shown in Table 1); then, the ferromagnetic phase is a bit degraded. One can see from Figure 3(a) that the M-H curve of the Ho0.2Bi0.8FeO3 film shows that it has a different slope in comparison with that of the Ho0.1Bi0.9FeO3 film. A very similar feature is found for Sm and Eu doping cases. This remark is consistent with our comments earlier about structural transition. Changing from rhombohedral to orthorhombic phases would be the main reason for the difference in observed magnetic behaviors. This remark is in accordance with previous reports about RE-doped BFO ceramics, or powders [5, 9].
As mentioned earlier, as for doping Pr and Nd case, when the concentration is up to 20%, the structure is a mixed phase of orthorhombic and tetragonal. Then, as a consequence, the samples have changed from paramagnetic to be a mixture of paramagnetic and ferromagnetic phases (data for Pr case is shown in Figure 3(b)). The inset of Figure 3(b) shows a clear hysteresis loop confirming the existence of the ferromagnetic phase. However, from the shape of the M-H curve, one can see that it is accumulated by two phases, paramagnetic and ferromagnetic. Even though the 20% Nd-doped BFO film has a much weaker magnetic moment (see Figure 3(c)), the behavior is quite similar to the Pr doping case. One can see from Table 1 that, in Pr (then Nd) case, when doping 10%, the Fe2+ amount was less than 50%, so that ferromagnetic ordering was not favored. However, when doping up to 20%, then Fe2+ has become 51.8% for Pr doping case and 48.5% for Nd doping case. Thus, in the Pr case, it helps to improve the ferromagnetism, while for Nd doping, it also made Fe3+ less dominant.
An example of scan of the Fe 2p line from XPS data for RE0.2Bi0.8FeO3 films is shown in Figure 4. One can see that the peak of Fe 2p could be observed at 711 eV for Fe3+ and at 709.5 eV for Fe2+. From the shape of the peaks, it is shown that both Fe2+ and Fe3+ exist in all doping cases (Ho, Sm, Nd, Pr, and Eu). The oxidation states of Fe in our films are listed in Table 1. One can see that, for the 10% doping of Ho and Sm, Fe2+ : Fe3+ ratio is about 50% : 50%; for Eu doping, it is similar; however, it is only about 40% : 60% for the 10% doping of Pr and Nd (refer also to ). This is in agreement with a previous report on BFO films made by sol-gel method . The coexistence of Fe2+ and Fe3+ is supposed to be in favor of the ferromagnetic ordering in BFO films [11, 12]. When the amount of Fe3+ is more favored, the ferromagnetism gets weaker due to the fact that the Fe3+-Fe3+ interaction is in favor of antiferromagnetic ordering . This explains why, from the corresponding SQUID data shown in Figures 3(b) and 3(c) for the 10% doping case, we see that the ferromagnetic phase is not favored in Pr- and Nd-doped BFO films.
When RE concentration is 20%, the Fe2+ : Fe3+ ratio has changed as a result of structural transition. It is obvious from Table 1 that, for Pr- and Nd-doped BFO, there is a significant increase of Fe2+ amount, leading to a favor of the ferromagnetic ordering. On the other hand, as for Ho, Sm, and Eu doping cases, Fe2+ seems to be decreased, relating to the weakening of ferromagnetic phase. It is obvious that once the structure has gone through a transition, it changes the Fe2+ : Fe3+ ratio, and as a consequence, the magnetic properties have changed. It appears that one can use all these factors to completely manipulate the magnetic properties of the BFO system. Our experimental findings were further completed by performing density functional theory calculations to explore the effect of RE doping in BFO for the considered three phases. Details could be seen in .
All the RE0.1Bi0.9FeO3 films show a single phase of rhombohedral structure. The saturated magnetization for Ho-, Sm-, and Eu-doped BFO films is much larger than those reported elsewhere and was observed at a much lower field as of 0.2 T. The observed ferromagnetism in our RE-doped BFO films was likely due to the coexistence of Fe2+ and Fe3+. When the RE concentration is increased up to 20%, there is a big change in structure: while Ho-, Sm-, and Eu-doped BFO films have become orthorhombic (and as a result, the Fe2+ amount is reduced), the Pr- and Nd-doped BFO films have turned to be a mixture of orthorhombic and tetragonal phases, leading to Fe2+ amount increased, thus, favoring the ferromagnetic phase. Our results show that there is a strong relationship between structural and magnetic properties in RE-doped BFO films. It is found that, by choosing correctly the type and concentration of dopant, one can tailor the magnetism of BiFeO3 films with expectation to satisfy the needs of industrial applications. It should be important to guide the materials search.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank Project 3348-20120033 of National Research Foundation of Korea for its financial support. The work of N. T. Huong is financially supported by the International Exchange Fellowship for the Academic Year 2013-2014 of Korea Foundation for Advanced Studies and by the “BK 21 Plus” Program of Department of Physics and Astronomy, SNU for 2014-2015. The authors greatly thank TY Kim for re-providing some data of the 10% doping case. The authors are grateful for the help of the SNU National Center for Inter-University Research Facilities on XRD measurements and SNU Center for Materials Analysis on XPS measurements.
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