Tuning spin reorientation in Er_1-xY_xFeO₃ single crystal family

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Abstract

A temperature-induced spin reorientation transition between Γ₄ (G_x, A_y, F_z) and Γ₂ (F_x, C_y, G_z) has been studied in the family of Er_1-xY_xFeO₃ (x=0, 0.25, 0.5, 0.75, 1) single crystals. By doping nonmagnetic Y³⁺, we tuned the spin reorientation temperature to low temperature with increasing x. Moreover, the typical compensation point and spin flip transition of ErFeO₃ also decreases with doping, and disappears above x=0.75. We also report the Rietveld refinements and Raman spectroscopy of Er_1-xY_xFeO₃, where some Raman peaks are shifted to low frequency with increasing doping. Our results shed light on the understanding of the interaction between two magnetic sub-lattices of rare earth (R^{3+}) and iron (Fe³⁺) ions, and will also contribute to the materials design and potential applications.

Graphical abstract

Keywords

Er_1-xY_xFeO₃ / single crystal growth / magnetic measurements

1 Introduction

Rare-earth orthoferrites RFeO₃ (R = rare-earth ions and Y) have a distorted perovskite structure with a space group of Pbnm. Owing to the distorted structure and the presence of Dzyaloshinskii–Moriya interaction, they exhibit an antiferromagnetic structure, and is therefore widely used in many research areas [1]. Spin reorientation transition (SRT) is a widely studied content for RFeO₃, and White et al. had reported the mechanism of this transition in detail [1]. Allowed spin structures in this orthoferrites are Γ₁ (A_x, G_y, C_z), Γ₂ (F_x, C_y, G_z), and Γ₄ (G_x, A_y, F_z), the mutual conversion of different spin configurations is known as spin reorientation. It is worth noting that the origin of SRT is not the same for different RFeO₃, and we generally pay attention to temperature-induced SRT. For example, ErFeO₃ shows an SRT from Γ₄ to Γ₂ in the temperature range between 88 K and 98 K [2], while the temperature-induced SRT disappears in YFeO₃ due to the filled electron shells of Y ions [1]. Not only the temperature but also magnetic field can play an inductive role for SRT. We still use YFeO₃ as an example; previous studies revealed that the field-driven spin reorientation occurred at a relatively high magnetic field around 7 T [3], and the dynamical SRT of YFeO₃ have also been studied by using terahertz spectroscopy [4].

RFeO₃ has two kinds of magnetic sub-lattices of Fe³⁺ and R³⁺, resulting in three different kinds of magnetic interactions: Fe³⁺-Fe³⁺, Fe³⁺-R³⁺ and R³⁺-R³⁺ interaction [1]. The Fe³⁺ magnetic moments dominate magnetization at room temperature and rare-earth ions are paramagnetic. At low temperatures, rare-earth ions tend to be ordered because of the molecular field effect of Fe³⁺ magnetic sub-lattices [5]. For example, ErFeO₃ has an antiferromagnetic structure along a-axis and weak ferromagnetism along c-axis. The Er³⁺ gradually forms a magnetic moment that is opposite to the magnetic moment of Fe³⁺ during the cooling process. So at a certain temperature, the overall magnetization of ErFeO₃ will reach zero (Zero magnetization phenomenon), which is a major mechanism for the understanding of Er³⁺-Fe³⁺ interaction. Similar phenomenon has also been reported in NdFeO₃ [6]. Another unique property of ErFeO₃ is that an abrupt jump of magnetization has been observed both in cooling and heating processes [7], a phenomenon called spin flip, which might lead to potential applications in developing new magnetic devices. Besides ErFeO₃, other RFeO₃ materials with the above-mentioned spin flip behaviors are: SmFeO₃ [8], NdFeO₃ [6], Ho_0.5Pr_0.5FeO₃ [9] and so on. Recently, there are many studies focusing on ErFeO₃: the magnetic structures have been investigated by neutron diffraction, corresponding symmetry and refinement [10]. Previous publications reported the giant magnetic entropy and the refrigerant capacity in the ErFeO₃ single crystal. The results explained the effect of R³⁺-R³⁺ interaction near the rare-earth ordering temperature for total magnetization [2]. Furthermore, ErFeO₃ has also been studied by terahertz emission spectroscopy [11-13]. The rich magnetization behavior of ErFeO₃ has attracted intensive attention in the last few years. However, various interactions of ErFeO₃ still remain elusive.

This paper reported the study of Er_1-xY_xFeO₃ (x=0, 0.25, 0.5, 0.75, 1) single crystals. We confirm the spin reorientation between Γ₄ (G_x, A_y, F_z) and Γ₂ (F_x, C_y, G_z) and the spin flip phenomenon. By doping of nonmagnetic Y³⁺, we found that the spin reorientation temperature ranges, the compensation point as well as the spin flip temperature can be tuned to low temperatures. Moreover, the structural changes are also confirmed by using Raman Spectroscopy measurement. Our study suggests that Y³⁺ weakens the interaction between Er³⁺ and Fe³⁺ instead of introducing new magnetic structures in ErFeO₃.

2 Experimental

We have prepared a family of Er_1-xY_xFeO₃ (x=0, 0.25, 0.5, 0.75, 1) single crystals. According to stoichiometric ratio, polycrystalline samples were synthesized by conventional solid-state reaction method using Er₂O₃ (99.9%), Y₂O₃ (99.9%) and Fe₂O₃ (99.98%) powders. Mixtures were sintered twice for sufficient reaction at 1350 °C for 17h, and then cooled to room temperature. In the sintering process, the compounds of feed and seed rods were prepared by hydrostatic press system at 70 MPa. Single crystals were grown by optical floating zone furnace (FZ-T-10000-H-VI-P-SH, Crystal Systems Corp). During this procedure, the feed and seed rods were simultaneously rotated in opposite directions at 15 rpm, the molten zone was moved upwards at a rate of 3 mm/h. Large-size single crystals of Er_1-xY_xFeO₃ family were obtained as shown in Fig. 1(a). The structure was characterized by X-ray diffraction (XRD, Rigaku, D/max2500) using Cu-Kα radiation in the angle range {10}^{\circ }\le 2\theta \le {90}^{\circ }. All of their crystallographic orientations were determined by back-reflection Laue X-ray photography with a tungsten target (with the X-ray beam of 0.5 mm in diameter). The Raman spectra were performed on a confocal Raman spectroscope using a 514 nm excitation laser (Renishaw, Invia Raman microscope). The Physics Property Measurement System (Quantum Design, PPMS-9) with a Vibrating Sample Magnetometer (VSM) option was used to measure the magnetization properties. We follow the Zero-field-cooling (ZFC), Field-cooled-cooling (FCC) and Field-cooled-warming (FCW) mode to measure the M-T curves. The ZFC and FCW curves were performed during heating process and the FCC curves correspond to the cooling process.

3 Results and discussion

3.1 Structure characterization

Figures 2(a)–(e) show the powder XRD patterns of the Er_1-xY_xFeO₃ (x=0, 0.25, 0.5, 0.75, 1) family at room temperature. By using Rietveld method in the Fullprof program, the refinements of crystal structures are shown in Figure 2. Table 1 lists the parameters observed from the Rietveld refinements for five samples. The results show that Er_1-xY_xFeO₃ has orthorhombic perovskite structure with the space group Pbnm and no impurity phases are detected for all samples. Due to the small difference in ion radius, Y doping does not lead to significant changes of the crystallographic structure of ErFeO₃. The refinement of XRD data shows high quality of our samples. Figure 1(a) presents the back-reflection Laue photograph of ErFeO₃ (along a, b and c axis respectively). Clear Laue spots also confirm the high quality of our samples. Moreover, Figs. 1(b)–(d) present XRD patterns along a, b, and c axis, respectively. Characteristic peaks for each axis can be well indexed.

3.2 Raman measurements

The Raman Spectroscopy of the RFeO₃ compounds can provide the information of the key length, bond angle, vibration mode and other [14-18]. According to the literatures [16, 19], the irreducible representation of the center of the Brillouin district for RFeO₃ can be written as: G=7A_1g+8A_1u+7B_1g+8B_1u+5B_2g+10B_2u+5B_3g+10B_3u, which contains 24 Raman-active vibrational modes: 7A_1g+7B_1g+5B_2g+5B_3g. In the wavenumbers range of 100-800 cm^-1, only 8-10 Raman peaks can be observed [15]. The vibration modes below 200 cm^-1 are mainly characterized by rare-earth ions, while above 300 cm^-1, they are related to the vibration of oxygen ions or FeO₆ octahedra. This is because the high frequency mode is mainly related to the lighter oxygen ions. Figure 3(a) shows the Raman Spectroscopy of Er_1-xY_xFeO₃ single crystals observed at room temperature with a laser of 514 nm. Under the selected test conditions, the results show that there are five possible Raman peaks with a corresponding wave number of approximately 269 cm^-1, 342 cm^-1, 430 cm^-1, 502 cm^-1 and 643 cm^-1. According to the relevant literatures [15], we obtain the model information corresponding to the different Raman peaks, which are marked in the Figure. These peaks reflect the rotation and stretching operations associated with the oxygen ions or the FeO₆ octahedra. With increasing doping, minor shift of some peaks are observed. Figures 3(b) and (c) indicate the shift of Raman peaks with wave numbers around 502 cm^-1 and 430 cm^-1, respectively. The positions of two peaks vary with the doping amount of Y³⁺ ions. With increasing doping, the peak near 502 cm^-1 and 430 cm^-1 are shifted to low frequency. This result is attributed to the two effects of the doped Y³⁺ ions: i) the enlargement of the unit cell volume and chemical bond and ii) the reduced effective mass.

3.3 Magnetization measurements

The ErFeO₃ has been reported to show rich magnetization behaviors. In order to carefully study the effect of rare-earth ion doping on the spin reorientation phase transition and other phenomena, we choose ErFeO₃ as the parent phase. Figure 4(a) displays the temperature dependent magnetization (M-T) for an ErFeO₃ single crystal with an applied field of H=50 Oe. Consistent with other results [2], the spin reorientation transition from Γ₄ (G_x, A_y, F_z) to Γ₂ (F_x, C_y, G_z) state has been observed, from M-T curves, in the temperature range of 88 K \le T_{SR}\le 98 K. The compensation point at T_{comp}=46 K, where the magnetization goes through zero, is also observed along a-axis. In addition, both ZFC and FCC curves in the a-direction show a characteristic feature of spin flip, which appears at 66 K (for ZFC) and 26 K (for FCC). The Y³⁺ only has a filled electron shell, so it does not show magnetic behaviors [13]. Y³⁺ doping will not introduce new magnetic structures to ErFeO₃. Based on this, a family of Er_1-xY_xFeO₃ single crystals is discussed below, the M-T curves along a-axis of Er_1-xY_xFeO₃ are shown in Fig. 5. The M-T curves show that, with the increase of Y³⁺ component in Er_1-xY_xFeO₃, a downward trend of the spin reorientation temperature range is observed. When x is increased to 1, there is no SRT in YFeO₃. Against the magnetization compensation phenomenon of ErFeO₃ we can find that, above x=0.75, there is no compensation point. Comparing the ZFC and FCC curves of a-axis of Er_1-xY_xFeO₃ family, it can be seen that as the doping amount of Y³⁺ increases, the spin flip temperature move to low temperature.

Now we focus on the cooling process (FCC curves). Previous studies reported the magnetizations of Er and Fe sublattices are antiparallel to each other. Since Y³⁺ is a non-magnetic ion, doping does not change the type of spin reorientation of ErFeO₃ but can regulate the temperature range at which this phase transition occurs. Similarly, here we mainly consider the interaction between Er³⁺ and Fe³⁺ at the spin flip temperature. When the temperature is much higher than the spin reorientation temperature, the overall magnetization is mainly contributed by the iron ions and the magnetic moments of Er³⁺ and Fe³⁺ are arranged in antiparallel direction [1]. With decreasing temperature, the contribution of the Er³⁺ magnetic moment gradually increases, and then undergoes a spin reorientation transition [see Figs. 4(b)–(d)]. After this, Fe³⁺ magnetic moments and Er³⁺ magnetic moments (which are antiparallel to Fe³⁺) compensate each other (Magnetization compensation phenomenon). When cooling the sample below 30 K, the energy of the system formed by the two sublattices cannot compete with the energy of the external magnetic field, so the magnetic moments of the two sublattices are reversed together. As a result, the magnetization changes from a negative value to a positive (spin flip phenomenon) [see Figs. 4(e–f)].

The FCC curves of each sample in Er_1-xY_xFeO₃ family are obviously inconsistent. For x=0.25 and 0.5, the samples show a second magnetic jump at 14 K (for x=0.25) and 7 K (for x=0.5). Y³⁺ does not contribute to the magnetic moment of the rare earth ion sublattice. Its addition weakens the original interaction of the system, so that the system will experience a longer period of cooling for the appearance of spin flip phenomenon. For x=0.75 it is observed that the spin reorientation and the spin flip do not exist. However, there is a sharp increase in the FCC curve at 13 K. The magnetization behavior at this time is completely dominated by Er³⁺ ordering. Moreover, the magnetization behavior of the heating process (ZFC curves) also presents the same phenomenon.

4 Conclusions

This paper focuses on the spin reorientation transition and the spin flip characteristics of Er_1-xY_xFeO₃ family. We successfully prepared five high quality single crystals and conducted the Rietveld refinements and Raman Spectroscopy measurement to evidence the structural changes. Spectroscopy results show that there are five possible Raman peaks and some of them are shifted to low frequency with the increasing doping. By doping of nonmagnetic Y³⁺, we found that the spin reorientation temperature range, the compensation point and the spin flip temperature are tuned to low temperatures. We have found that the type of spin reorientation of ErFeO₃ does not change in Er_1-xY_xFeO₃ family. To summarize, Y³⁺ do not introduces new magnetic structures in ErFeO₃ and weakens the interaction between Er³⁺ and Fe³⁺.

CHANGE HISTORY

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