Magnetic Diagram of the High-Pressure Stabilized Multiferroic Perovskites of the BiFe_1-ySc_yO₃ Series

Abstract

Magnetic properties of the high-pressure stabilized perovskite BiFe_1-ySc_yO₃ phases (0.1 ≤ y ≤ 0.9) have been studied by means of magnetization measurements and neutron diffraction. The metastable perovskites of this series undergo irreversible polymorphic transformations upon annealing, the phenomenon referred to as conversion polymorphism. It has been found that the solid solutions with y ≥ 0.70 exhibit no long-range magnetic ordering regardless of their polymorph modification, while those with y ≤ 0.60 are all antiferromagnets. Depending on the scandium content, temperature and structural distortions, three types of the antiferromagnetic orderings, involving collinear, canted and cycloidal spin arrangements, have been revealed in the phases obtained via conversion polymorphism and the corresponding magnetic phase diagram has been suggested.

1. Introduction

Chemical modifications of one of the most known type-I multiferroics, bismuth ferrite [1], make it possible to tune the properties of the parent composition (e.g., to adjust the ranges of the antiferromagnetic and the ferroelectric phase transitions) and to obtain new crystalline and magnetic structures [2,3]. While bulk single-phase perovskites with considerable and even complete isovalent substitutions of bismuth in BiFeO₃ can be easy produced using the conventional ceramic routes [4,5,6], the formation of the compositions with more than 15 mol.% substitution of iron (except for Mn³⁺ substituted BiFeO₃ where up to 30 mol.% can be achieved [7]) requires a high-pressure synthesis.

Using the high-pressure synthesis, the Bi(Fe,B³⁺)O₃ solid solutions with the Fe³⁺-to-B³⁺ substitution of 50 mol.% and above (B³⁺ = Mn [8,9], Co [10,11,12], Cr [13,14], Ga [15,16]) have been obtained. The composition-driven sequences of the crystalline phases and their structural distortions as well as the feasibility of their application as materials with desired functionalities (in particular, as multiferroics and piezoelectrics) were studied [10,17]. Even though the magnetic behavior of particular compositions of those solid solution systems were investigated in great detail, only a few magnetic phase diagrams have been published so far [8,16].

We have recently prepared and characterized the entire series of the high-pressure stabilized perovskite solid solutions of the BiFeO₃-BiScO₃ system [18,19]. BiScO₃ was chosen as the end member since substitution of Fe³⁺ by a considerably bigger Sc³⁺ was expected to induce new crystal structures and thereby new combinations of multiferroic properties. Indeed, three structure fields with the rhombohedral R3c symmetry and the √2a_p × √2a_p × 2√3a_p superstructure (where a_p is the pseudocubic perovskite unit-cell parameter), the orthorhombic Pnma symmetry (√2a_p × 4a_p × 2√2a_p) and the monoclinic C2/c one (√6a_p × √2a_p × √6a_p) were revealed [18]. Moreover, in the BiFe_1-ySc_yO₃ series, the phenomenon of annealed-stimulated irreversible transformations between metastable perovskite phases (conversion polymorphism) was observed. We have revealed at least three compositional ranges in this solid solution system, where the post-synthesis annealing results in the formation of different perovskite polymorphs with new combinations of structural distortions and magnetic orders [19].

While the as-prepared (non-annealed) phase of the BiFe_0.50Sc_0.50O₃ composition is the antipolar Pnma, the conversion-stabilized modification is a polar orthorhombic with the Ima2 symmetry (2a_p × √2a_p × √2a_p) [20]. Both the polar and the antipolar polymorphs of BiFe_0.5Sc_0.5O₃ are G-type antiferromagnets (AFM) with a weak ferromagnetic (FM) component below the Néel temperature, T_N ~220 K. Although the type of the magnetic order and the T_N value are the same for both polymorphs, magnetization of the polar polymorph below T_N was found to be 2–5 times smaller than that of the antipolar one [20]. Besides, the BiFe_0.50Sc_0.50O₃ polymorphs were shown to be also rather different in terms of effects caused by their magnetic anisotropy [21,22]. In the range of the BiFe_1-ySc_yO₃ compositions with y close to 0.30, annealing of the as-prepared antipolar Pnma phase leads to irreversible transformation into a rhombohedral R3c polymorph with a very unusual collinear magnetic ground state [19]. The monoclinic C2/c phase of the as-prepared BiFe_1-ySc_yO₃ compositions with y ≥ 0.7 irreversibly transforms into a new antiferroelectric-like phase with the orthorhombic Pnma symmetry and the 2√2a_p × 4a_p × √2a_p superstructure [19]. Hereafter, the modifications with the superstructures √2a_p × 4a_p × 2√2a_p and 2√2a_p × 4a_p × √2a_p are denoted as Pnma (I) and Pnma (II), respectively. It should be stressed here that the different unit cell choice in these modifications implies essentially different distortions involved.

A temperature–composition phase diagram of the BiFe_1-ySc_yO₃ series plotted based on the data of the in situ temperature powder diffraction studies demonstrates a sequence of the perovskite-type structures different in respect of combinations of the oxygen octahedral tiltings [19]. It should be pointed out that the octahedral tilting not only determines the structure distortion (symmetry) but also has an effect on the long-range magnetic order in perovskites. In particular, it has been recently shown that the component of the Dzyaloshinskii–Moriya vector, which couples the G-type AFM and orthogonal FM modes, is imposed by anti-phase octahedral tiltings [20,23].

Here we report the compositional behavior of magnetic properties of the BiFe_1-ySc_yO₃ solid solutions (0.1 ≤ y ≤ 0.9) before and after annealing that results in the irreversible polymorphic transformations. The obtained magnetic diagram is considered in comparison with the temperature–composition phase diagram of the BiFeO₃-BiScO₃ system.

2. Materials and Methods

Ceramics of the BiFe_1-ySc_yO₃ series (0.1 ≤ y ≤ 0.9) were synthesized under high-pressure from the precursor prepared via either a solid-state reaction or a sol-gel combustion route. Details of the precursor preparation and the high-pressure synthesis can be found in References [18] and [21]. Annealing of the obtained ceramic samples was done in air at 870 K for 3 h.

Phase analysis of the samples before and after annealing was performed using a PANalytical X’Pert Powder X-ray diffractometer (XRD, Ni-filtered Cu Kα radiation) at room temperature. Before the XRD measurements, the samples were reduced into powders.

Neutron powder diffraction data were collected at the ISIS pulsed neutron and muon facility of the Rutherford Appleton Laboratory (UK) on the WISH diffractometer [24]. The studied samples were loaded into a cylindrical 3 mm diameter vanadium can and measured in a temperature range of 1.5–300 K. The crystal structure and the magnetic structure of the samples were refined using the FULLPROF package [25].

Magnetic properties of the ceramic samples were measured in the range of 5–300 K using a commercial Quantum Design MPMS-3 magnetometer in applied fields up to 70 kOe in both zero-field-cooled (ZFC) and field-cooled (FC) modes. High temperature (over the range of 300–700 K) measurements were done using a commercial Quantum Design MPMS-XL magnetometer equipped with oven insert.

3. Results and Discussion

Magnetic behavior of the BiFe_1-ySc_yO₃ perovskites with the relative iron content above and below 30 mol.% (which corresponds to y < 0.70 and y ≥ 0.70, respectively) were found to be essentially different.

Regardless of the structure modification, C2/c or Pnma (II), BiFe_0.3Sc_0.7O₃ exhibits no sign of a long-range magnetic ordering down to the low temperatures. It is expectable since the iron content in this compound is below the percolation threshold of 31 mol.% (y = 0.69) for perovskite structures [26], when magnetic Fe³⁺ still can form a continuous path throughout the crystal to produce a long-range magnetic ordering.

Indeed, the neutron diffraction study has revealed no additional contributions to nuclear reflections that could indicate an onset of magnetic ordering. As an example, Figure 1 demonstrates the results of the crystal structure refinement of the Pnma (II) polymorph of BiFe_0.3Sc_0.7O₃ at 1.5 K.

The diffraction data are consistent with the results of the magnetic measurements. The temperature-dependent ZFC and FC curves of BiFe_0.3Sc_0.7O₃ coincide down to about 25 K and slightly diverge below this temperature. The magnetic moment of its monoclinic C2/c modification as a function of temperature is shown in Figure 2. The Curie–Weiss fitting gives the effective magnetic moment µ_eff ~2.99 µ_B/Fe³⁺, which is close to the experimental value for the low-spin Fe³⁺ in the octahedral environment. Further decrease of the iron content in the BiFe_1-ySc_yO₃ system results in reduction of the paramagnetic response, which vanishes completely in BiScO₃ (y = 1).

The magnetic behavior of the BiFe_1-ySc_yO₃ perovskites with the relative iron content of 60 mol.% and above (y < 0.7) was revealed to be typical for antiferromagnets. Dilution of the iron magnetic subsystem by scandium leads to the near-linear decrease of the Néel temperature, T_N, from about 635 K in the parent bismuth ferrite (y = 0) down to about 150 K in the composition with y = 0.60 (Figure 3). The presence of the long-range antiferromagnetic ordering has also been confirmed by the neutron diffraction in the compositions with y = 0.3 [19], y = 0.4 (see Figure 4) and y = 0.5 [20]. The T_N values were found to be practically the same for the non-annealed and annealed perovskites, independent on their crystal structure modification. The magnetization of the all studied perovskites is far from the saturation up to the highest applied field of 70 kOe.

In the BiFe_1-ySc_yO₃ solid solutions with the composition in the range of 0.20 ≤ y ≤ 0.60, a weak FM component was detected. Figure 5 shows the static magnetic moment of a non-annealed BiFe_0.70Sc_0.30O₃ sample, which is the orthorhombic Pnma (I) [19], as functions of temperature and field. It is seen from Figure 5b that the orthorhombic polymorph of BiFe_0.70Sc_0.30O₃ demonstrates weak ferromagnetism down to the low temperatures: its remnant magnetization and the coercive field are non-zero.

Annealing of the as-prepared metastable BiFe_1-ySc_yO₃ phases (0.20 ≤ y ≤ 0.60) leads to the decrease of their magnetic moment in the magnetically ordered state, the remnant magnetization, and the coercive field. In other words, the annealing results in a reduction of the weak ferromagnetism and the magnetic anisotropy. It is known that annealing of the high-pressure synthesized ceramics is accompanied by a release of the mechanical stress induced by the quenching from high temperature and pressure down to ambient conditions [27]. In the samples of the BiFe_1-ySc_yO₃ system with 0.20 ≤ y ≤ 0.60, a gradual stress release leads to the irreversible polymorphic transformations. [19].

Upon annealing, BiFe_0.70Sc_0.30O₃ transforms into a rhombohedral R3c polymorph, which is isostructural to the undoped bismuth ferrite. BiFeO₃ is known to be AFM with an incommensurate cycloidal magnetic structure below T_N ~ 635 K [1]. The neutron diffraction study [19] revealed that the magnetic structure of the rhombohedral modification of BiFe_0.70Sc_0.30O₃ just below T_N ~ 375 K is incommensurately modulated in the same manner as that in BiFeO₃. However, while the cycloidal structure in bismuth ferrite remains upon cooling, the magnetic structure of BiFe_0.70Sc_0.30O₃ below the transition temperature T_m ~ 230 K becomes a collinear G-type AFM with the spins polarized along the threefold axis [19]. This structure allows no spin-canting and is the unique example of a BiFeO₃-derived composition, in which the collinear magnetic structure does not activate the anti-symmetric exchange. In contrast with BiFe_0.70Sc_0.30O₃, the rhombohedral phase of BiFe_0.60Sc_0.40O₃ perovskite adopts the collinear AFM structure just below T_N ~ 300 K and this type of magnetic ordering remains stable over the whole temperature range (see Figure 4). The irreversible transformation of the BiFe_0.60Sc_0.40O₃ composition into the R3c polymorph upon annealing was not complete and the annealed species contained 30% of the polar Ima2 phase. The magnetic structure of the polar phase was found to be canted AFM similar to the Ima2 phase of BiFe_0.50Sc_0.50O₃ [20].

Variations of the static magnetic moment of the rhombohedral polymorph of BiFe_0.70Sc_0.30O₃ with temperature and the applied field are shown in Figure 6. The material demonstrates a usual AFM behaviour with a magnetization linearly dependent on the magnetic field (Figure 6b). One can notice two distinct regions in the temperature dependence of the magnetic moment below T_N (Figure 6a). (1) Between T_N and T_m, the moment grows fast enough on cooling and a small difference between the ZFC and the FC curves is observed. (2) Below T_m, the moment changes slower and there is the evident near-constant difference between the ZFC and FC curves. Such a behavior is associated with a highly anisotropic state of the material below T_m. It should be also noticed that both magnetic transitions observed in the annealed BiFe_0.70Sc_0.30O₃ at T_N and T_m are both reversible with no visible temperature hysteresis.

Very similar temperature behavior of the static magnetic moment below T_N ~ 370 K was revealed in the annealed sample with y = 0.25 (Figure 7). Following the description done for the temperature dependence of the magnetic moment of the R3c polymorph of BiFe_0.70Sc_0.30O₃ (see above), the transition between two magnetically ordered states at T_m ~ 175 K can be suggested. It is important to emphasize here that, as opposed to the BiFe_1-ySc_yO₃ composition with y = 0.30, BiFe_0.75Sc_0.25O₃ was found to exhibit no annealing-stimulated transformation [19]. The crystal structure of the latter remains the same, namely rhombohedral with the R3c symmetry over the entire heating/cooling cycle. Nevertheless, the magnetic ground states of BiFe_0.75Sc_0.25O₃ before and after annealing are different. This suggests that the phenomenon of conversion polymorphism is more general than considered previously: annealed-stimulated irreversible transformations of the magnetic structure are possible without change of the crystal structure, within the same structural polymorph.

The same type of the magnetic transformation was then found in other annealed BiFe_1-ySc_yO₃ perovskites with a lower content of scandium, namely those corresponded to y = 0.10 and 0.20 (with T_m ~ 105 and 185 K, respectively).

In addition to the magnetic phase transition at T_m, an anomaly in the temperature dependence of the magnetic moment, which indicates another magnetic transformation (at T_a), was observed in both as-prepared as well as annealed BiFe_1-ySc_yO₃ samples with scandium content in the range of 0.10 ≤ y ≤ 0.25. The suggested magnetic transition at T_a is strongly hysteretic, which implies that it is likely to be of the 1st order (Figure 7).

Based on the obtained data, a magnetic phase diagram of the BiFe_1-ySc_yO₃ system has been plotted (Figure 8).

There are at least three types of magnetically ordered structures in the system and all three are AFM. Two of those structures are the incommensurate cycloid and the collinear one as detected in the rhombohedral polymorph of BiFe_0.70Sc_0.30O₃ above and below T_m, respectively. A question about a possible magnetic transition at T_a in this polymorph remains open: whether it is very close to that at T_N or does not exist. The exact nature of the suggested AFM structures and transition between them in the range of 0.10 ≤ y ≤ 0.30 certainly requires a particular study. Neutron diffraction experiments are planned and their results will be published elsewhere.

4. Conclusions

The static magnetic moment of the as-prepared and the annealed BiFe_1-ySc_yO₃ solid solutions with the relative scandium content y in the range of 0 < y < 1 have been studied. Annealing of the as-prepared monoclinic C2/c perovskite polymorph in the solutions with y ≥ 0.70 results in the irreversible transformation (conversion polymorphism) to the antipolar orthorhombic Pnma (II) modification; nevertheless, the magnetic behaviors of both polymorphs are essentially similar: they are paramagnetic down to the low temperatures with no sign of magnetic ordering. The BiFe_1-ySc_yO₃ perovskites with y ≤ 0.60 are all antiferromagnets with a near-linear dependence of T_N on y. Although these perovskites in the range of 0.30 ≤ y ≤ 0.55 exhibit the annealing-stimulated polymorphism, their T_N values are practically independent on the type of polymorph (before and after the annealing). In the perovskites with the composition in the range of 0.20 ≤ y ≤ 0.60, a weak ferromagnetic contribution to the AFM state was detected. The annealing was found to lead to a reduction of this contribution.

In the BiFe_1-ySc_yO₃ solid solution with y = 0.30, the conversion from the as-prepared antipolar orthorhombic Pnma (I) phase to the polar rhombohedral R3c polymorph results in the appearance of a reversible phase transition (at T_m < T_N) from the BiFeO₃-like incommensurate cycloidal magnetic structure to a collinear G-type AFM with the spins polarized along the threefold axis. The solid solutions with a lower scandium content, 0.10 ≤ y ≤ 0.25, exhibit no conversion polymorphism. Nevertheless, the annealed perovskites from this compositional range demonstrate the magnetic transition, which is very similar to that observed in the solid solution with y = 0.30 at T_m. Moreover, the indications of another magnetic transition (at T_a, where T_m < T_a < T_N) were observed in these solid solutions. The magnetic phase diagram suggests the presence of at least three different AFM structures in the BiFe_1-ySc_yO₃ system.