**1. Introduction**

Only a small fraction of all magnetically polarized and electrically polarized materials are ferromagnetic or ferroelectric, and even fewer, namely multiferroic materials, have both properties [1,2]. In addition, the coupling between different properties of multiferroic materials will produce new properties, such as magnetoelectric effects. These materials have great development potential in the miniaturization and multi-functionalization of devices, as well as in a wide range of applications in the fields of magnetoelectric memory [3,4], sensors [5], and drivers [6]. Multiferroic materials are some of the most valuable multifunctional materials, and they have good application prospects in the field of multiferroic devices.

Multiferroic materials include single-phase materials and composite materials. However, few single-phase multiferroic materials have been discovered at present, and their Curie temperatures are usually low. Owing to its high Curie temperature (Tc = 1103 K) and Neal temperature (TN = 647 K), single-phase BiFeO3 (BFO) exhibits ferroelectric and G-type antiferromagnetism at room temperature [7,8]. Thus, it has attracted extensive attention

**Citation:** Wang, Y.; Li, Z.; Ma, Z.; Wang, L.; Guo, X.; Liu, Y.; Yao, B.; Zhang, F.; Zhu, L. Phase Structure and Electrical Properties of Sm-Doped BiFe0.98Mn0.02O3 Thin Films. *Nanomaterials* **2022**, *12*, 108. https://doi.org/10.3390/ nano12010108

Academic Editor: Seiichi Miyazaki

Received: 7 December 2021 Accepted: 24 December 2021 Published: 30 December 2021

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from materials scholars and has become a hot topic for in-depth exploration of multiferroic materials [9–12].

In BFO, Bi ions are volatile at high temperature. To balance the charge, the valence of Fe ions may change from +3 to +2 [13]:

$$\rm{Bi}\_{\rm Bi}^{\rm{x}} + \frac{3}{2} \rm{O}\_{2} \Leftrightarrow \rm{Bi}\_{2} \rm{O}\_{3} \uparrow + \rm{V}\_{\rm Bi}^{\prime\prime\prime} + 3 \rm{h}^{\bullet} \,,\tag{1}$$

$$2\text{Fe}^{3+} + \text{O}\_{\text{O}}^{\text{x}} \Leftrightarrow 2(\text{Fe}\_{\text{Fe}^{3+}}^{2+})\prime + \text{V}\_{\text{O}}^{\bullet \bullet} + \frac{1}{2}\text{O}\_{2}.\tag{2}$$

As a result, a large number of oxygen vacancies or other defects often exist in the prepared BiFeO3 samples, which increases the leakage current density of BiFeO3 materials and adversely affects its performance [14]. There are many ways to improve the properties of BiFeO3 materials, including element doping, solid solution, formation of a heterostructure, and control of film orientation [15–18]. Among them, many researchers adopt the element doping method to improve the performance of BiFeO3 materials [19–22]. Yun et al. prepared single-phase multiferroic BiFeO3 and Ho-doped BiFeO3 films [23]. The ferroelectric property was enhanced, and the leakage current decreased significantly. The ferroelectric property reached 20.69 <sup>μ</sup>C/cm<sup>2</sup> and the leakage current density was 2.89 × <sup>10</sup>−<sup>9</sup> A/cm2, and these effects were attributed to the transformation from a rhombohedral structure to a coexisting cubic and orthosymmetric structure after Ho doping. Moreover, the fatigue properties of the films doped with Ho also improved, as evidenced by a 0.4% reduction in the value of the switchable polarization. Liu et al. grew a Bi1−xEuxFeO3 (BEFOx, x = 0, 0.03, 0.05, 0.07, 0.1) thin film on LaNiO3-coated Si substrate by the pulse laser deposition method. As the doping amount increased, the position of the A1-1 mode of the films shifted to a higher wave number in the Raman spectrum [24]. With the increase in Eu, the refractive index of the film increased, and the extinction coefficient and band gap width decreased. Yang et al. prepared a BiFe1−xZnxO3 (BFZO) film (x = 0%, 1%, 2%, 3%) and found that when x = 2%, the film reached the maximum remanent polarization intensity and the minimum correction field [25]. At the same time, under a low electric field, Zn doping can significantly reduce the leakage current of BFO films. In addition, the leakage mechanism changes from Ohmic conduction under a low electric field to F-N tunneling under a high electric field. Zhang et al. prepared high-quality BiFe1−2xZnxTixO3 (BFZTO, x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) films [26]. The authors found that the BFZTO film with x = 0.02 had uniform fine grains and high density, which can inhibit the transformation of Fe3+ to Fe2+ and, thus, greatly reduce the oxygen vacancy concentration. This film had the lowest leakage current density and the highest remanent polarization intensity. By comparing P–E hysteresis loops in different areas of BiFe0.96Zn0.02Ti0.02O3 thin films, the films have high uniformity and stable properties. Concurrently, Zn and Ti co-doping also increased the dielectric permittivity from 24.9 to 35.3 and remnant magnetization from 0.05 to 0.80 emu/cm<sup>3</sup> of BFZTO films. Liu et al. prepared Bi0.9Er0.1Fe1−xMnxO3 (BEFMxO, x = 0.00–0.03) thin films by the sol-gel method [27]. By co-doping Er and Mn, the coexistence of two phases (space groups are R3c:H and R3m:R) and the reduction of oxygen vacancy and Fe2+ concentration in BEFMxO were realized. Among all the samples, the BEFM0.02O film had the lowest oxygen vacancy concentration, the maximum remanent polarization value, and the maximum switching current. It also exhibited excellent ferroelectric stability, which means its low concentration of oxygen vacancies had less influence on the ferroelectric domains.

Kan et al. found that doping with Sm affected the phase structure of BFO samples [28,29]. Xue et al. prepared BFO films with different Sm content by the sol-gel method and found that the rhombohedral phase to pseudo-tetragonal phase transition occurs gradually with the increase in Sm [30]. Although there are many studies on the influence of element doping on BiFeO3 properties, there are few on the influence of Sm doping on the content change in the BiFeO3 thin film phase structure and thus on ferroelectric properties. In addition, the literature review revealed that for Sm doping, when the doping content is

less than 10 mol%, BFO has better properties than heavily doped [31,32]. It is necessary to further adjust the doping content. In this experiment, the doping amounts of Sm were 2 mol%, 4 mol%, and 6 mol%, in order to understand the influence of Sm doping on BSFM films. The performance changes were analyzed in detail from the aspects of oxygen vacancy content, grain size, relative content of the R3c phase and the Pnma phase. Additionally, the effects of different Sm content on the ferroelectric, dielectric, leakage, and aging properties of the thin film samples were systematically studied.
