Weak ferromagnetic response in PbZr_1−xTi_xO₃ single crystals

Abstract

For the first time, a weak ferromagnetic hysteresis loop at room temperature has been observed in PbZr_1−xTi_xO₃ (PZT) single crystals. The occurrence of these properties has been related to the presence of oxygen vacancies and/or lower oxidation state of titanium. This proves that ferroelectric and ferromagnetic properties can occur simultaneously in this “famous” piezoelectric perovskite without any magnetic dopant.

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The solid solutions of PbZr_1−xTi_xO₃ (PZT) are one of the most known piezoelectrics. Their large variety of applications have caused these materials to be intensely studied for decades. PZT ceramics are mainly used because of the difficulties to grow single crystals. Although there are some reports concerning their physical properties, there is still a deficiency of single crystals in the whole range of Ti content.^1–6 Recently, we have been working on the technology of obtaining PZT crystals with homogeneous distribution of Ti ions in the crystal volume. We have finally succeeded in obtaining single crystals with x = 0.13, and their dielectric, elastic and piezoelectric properties have been reported in ref. 6 and 7. Additionally, the technology has also supplied defected crystals, and according to theoretical predictions^8–10 such crystals may become very interesting from the point of view of appearance of magnetization in perovskite materials.

The literature on the magnetic properties of PZT solid solutions concerns mainly heterostructures,^11–17 interfaces,¹⁸ ceramics,¹⁹ thin films^20–23 or multilayers^24,25 containing PZT, but there is no information on the ferromagnetic bulk properties of single PZT crystals. Moreover, what is important too, it concerns relatively high temperatures. Based on those theoretical predictions and the above experimental studies, we decided to check whether our defective PZT single crystals with average composition x = 0.10, with possible O and Ti vacancies and with the Ti³⁺ state, may also reveal magnetic properties, and this is the subject of this letter.

PZT single crystals were grown by a top-seeded solution growth technique. Magnetic measurements were performed using a Quantum Design MPMS-XL-7AC SQUID magnetometer. Magnetization isotherms were measured at 2 K, 10 K, 20 K, 40 K, 60 K, and 300 K in external fields up to 70 kOe. The electronic structure was examined by the X-ray photoelectron spectroscopy (XPS) method with the use of a PHI 5700/660 Physical Electronics spectrometer, using an Al Kα monochromatic X-ray source with an energy of 1486.6 eV. All the photoelectron spectra were calibrated against the peaks of Au 4f_7/2 at 83.98 eV, Ag 3d_5/2 at 368.27 eV, and Cu 2p_3/2 at 932.67 eV of binding energy. The energy resolution in the XPS regime was about 0.35 eV. The test of the surfaces of the crystal was carried out at a standard take-off angle of 45°. The electronic structure of the PZT sample was tested at room temperature for the samples cleaved under UHV conditions. An electron float gun was used to compensate the surface positive charge appearing on the surface of insulating materials during X-ray irradiation. The core lines of O 1s, Zr 3d, Pb 4f, Ti 2p states and the valence band regions were recorded at pass energy 23.5 eV, and an energy step of 0.1 eV and 0.05 eV, respectively. Atomic concentration calculations were conducted from the analysis of the shape of the core lines, according to the standard procedure of Multipak (ver. 9.7.0.1). The deconvolution of photoemission lines was performed with a Doniach-Sunjic splitting function by means of the Simpeak program. The background was subtracted by the Shirley method.

Fig. 1 presents the dependence of magnetization M vs. magnetic field H for a single PZT crystal. In the low field range, i.e. below 22 000 Oe for isotherms measured at 2 K, and below 16 000 Oe for isotherms measured at 10 K, 20 K, 40 K, 60 K and 300 K, ferromagnetic behaviour was observed. Above these H values diamagnetism was dominant. In the low magnetic field region, the M(H) curve at T = 300 K (Fig. 2) clearly shows a feature of ferromagnetic hysteresis with coercive field H_c = 65 Oe, and a remnant magnetization M_r = 1.1 × 10⁻³ emu g⁻¹. Similar loops were observed for several different PZT crystals with the range of H_c = 65–100 Oe and M_r = (0.185–1.1) × 10⁻³ emu g⁻¹.

Fig. 1 Magnetization M vs. magnetic field H at 2 K, 10 K, 20 K, 40 K, 60 K, and 300 K for a single PZT crystal.

Fig. 2 Magnetization as a function of the magnetic field at 300 K for a single PZT crystal. The insets represent magnified M(H) run for the PZT crystal (red points) and diamagnetic behaviour in PbZrO₃ at room temperature.

The temperature dependences of the magnetization under zero-field-cooled (ZFC) and field-cooled (FC) modes for the applied magnetic fields μ₀H = 10 kOe and 30 kOe (Fig. 3) confirmed the isothermal measurements. The obtained room-temperature values of magnetic quantities are comparable to those for nanocrystalline PbTiO₃ ceramics,¹⁹ but lower than those for e.g. PZT 52/48 thin films deposited on the alumina substrate,²² probably because of a greater amount of Ti and thus also Ti³⁺. To ensure that the presence of ferromagnetic properties in single PZT crystals is not a spurious effect, we have studied PbZrO₃, which is one of the parent materials of these solid solutions. PbZrO₃ has been chosen since it certainly contains defects in the Pb and O sublattices (it is difficult to avoid them during a technological process), but there are no Ti vacancies or the Ti³⁺ state. The magnetic measurements have shown that PbZrO₃ is diamagnetic in the whole range of the applied magnetic field (inset of Fig. 2). Thus, we may conclude that the observed magnetism in the PZT crystals is not due to a spurious effect, and the role of titanium is crucial.

Fig. 3 Temperature dependences of magnetization in zero-field-cooled (ZFC) and field-cooled (FC) modes for the PZT crystal.

The main purpose of the XPS investigation was to check whether the Ti³⁺ state and oxygen vacancies are present, as well as magnetic contamination in the investigated PZT single crystals. The distribution of Pb, Ti, Zr, and O was examined, and huge differences in their content were noticed. However, the average concentration of the defected single PbZr_1−xTi_xO₃ crystal was estimated to be x = 0.10. It was also determined that in the crystal volume there were areas of more or less content of Pb. On the other hand, oxygen deficiency was found for all the studied areas. This means that the XPS atomic concentration analysis has evidently shown that the investigated single PZT crystal was not, as it was presupposed, stoichiometric. Moreover, the presence of different charge states of ions was detected. Due to the lack of oxygen ions in the crystal, changes in the valence states of Ti (from Ti⁴⁺ to Ti³⁺) and of Zr (from Zr⁴⁺ to Zr³⁺) were detected. As a consequence, the changes in the valence of Ti and Zr may contribute to the appearance of the Pb⁴⁺ state at the Pb²⁺ site. Furthermore, vacancies created at the Ti³⁺ position can be balanced by the occurred oxygen vacancies. Based on results reported²⁶ all observed states can be described as:

In the analysis of the ionic states in the crystal, the dynamics of the process of filling the oxygen vacancies in the UHV chamber by residual gases under vacuum conditions has to be taken into account. Breaking the crystal under vacuum produces active centers with unsaturated bonds which, as a consequence, may bind these residual gases. We cannot exclude a diffusion within the crystal volume that plays a role in this process. However, we suppose that after breaking the crystal under vacuum (Fig. 4a) one can observe saturation of additional Ti 2p states localized at 457.3 eV (Fig. 4b) which can be ascribed to the Ti³⁺ electronic state. Finally, the re-creation of Ti³⁺ states is produced through reduction by means of Ar⁺ ion treatment (Fig. 4c). The intensity of the Ti³⁺ line decreases with time (as can be seen in Fig. 4a and b), which may point to the filling of oxygen vacancies and to the change from the Ti³⁺ to the Ti⁴⁺ state. A similar scenario which has been observed in the case of reduced SrTiO₃²⁷ supports this explanation. The line within the Ti 2p multiplet, visible at about 457.3 eV, can thus be related to the Ti³⁺ species, which do not form metallic bonds of Ti.²⁸

Fig. 4 The titanium 2p core line measured at room temperature for a cleaved surface (a), after residence in the XPS chamber for about 6 hours (under vacuum conditions of about 10⁻⁹ Tr) (b), after 1 min of reduction with an Ar⁺ ion beam with an energy of 1 keV (c).

An oxygen vacancy is one of the most common and important point defects in perovskite oxides. The authors of a report²⁹ showed that this defect causes redistribution of charge, relative displacements of ions and defect levels within the band gap, with respect to pure crystals. The influence of oxygen deficiency on the energy gap in the studied materials is shown in Fig. 5. In the case of the PZT crystal, a weak intensity of photoemission was detected, just below the Fermi level. In the parent materials PbZrO₃ and PbTiO₃ no photoemission was detected. A similar increase of photoemission in the energy gap, as a consequence of doping, was observed for Fe-doped SrTiO₃.²⁷ In the case of the SrTiO₃ bulk crystals, the main contribution of titanium states to the valence band was observed in the range 3–8 eV,^27,30,31 while the contribution in the 1–2 eV energy range was attributed to the Ti³⁺ state.³²

Fig. 5 Comparison of the valence band for the PZT single crystal cleaved and cleaned in Ar⁺ ions, as well as for pure PbZrO₃ and PbTiO₃. The most interesting change was found in the energy gap where small photoemission was observed for the PZT crystal cleaved under UHV conditions in contrast to the lack of signal for pure PbZrO₃ and PbTiO₃ materials. An increase of photoemission was clearly observed after Ar⁺ ion treatment for the PZT crystal, which can be related to the Ti³⁺ state.

According to the calculations in ref. 29, in the case of PbZrO₃, the oxygen vacancy affects other atoms more than Zr. It is in contrast to PbTiO₃, in which Ti is the most sensitive element. Based on that, we may assume that in the investigated crystal the oxygen defects affect the Ti ions first of all changing their valence or creating titanium vacancies. This may be the main reason why weak ferromagnetism has been detected in PZT, and not in PbZrO₃. On the one hand, the calculated energies of surface defect formation point to an increase in the oxygen vacancy concentration at the surface compared to the bulk. On the other hand, the properties of oxygen vacancy depend on the chemical nature of the A and B atoms in ABO₃ perovskites. That is why it could be the deep donor state in PbZrO₃, but shallow in PbTiO₃ and SrTiO₃. The authors of ref. 29 have also reported that charge redistribution, relative displacements of ions, and defect levels within the band gap, caused by defect oxygen vacancies, have different values for the bulk and for the surface of oxide perovskites. Surface defects are more spatially delocalized and more energetically shallow than in the bulk, and thus the oxygen vacancy segregation towards the surfaces is possible.²⁹ This predicted mechanism may explain the differences between the values of magnetic quantities obtained at room temperature for the investigated single PZT crystal and PZT thin films (see e.g.ref. 22).

In relation to all the results described above, one has to keep in mind that, according to theoretical prediction reported in ref. 8, titanium vacancies may generate ferromagnetism at high temperatures, i.e. calculations made for BaTiO₃ showed that Ti vacancy, with a charge equal to −3, can survive even at 1300 K. Moreover, it was predicted that all vacancy species in BaTiO₃ are able to induce ferromagnetism, and the magnitude of the optimal magnetic moment depends on their charge states. The highest values were estimated for the Ti defect with a charge equal to −3, −2, −1, or even 0, but that with the charge of −4 was not magnetic. Similarly, the dependence of the occurrence of magnetism on the charge state of oxygen vacancy was predicted; only neutral oxygen vacancy was ferromagnetic. All these defects may occur in a real material and if so there are strong interactions between them. The question remains when (i.e. at which concentration of those defects) these interactions are strong enough to generate the ferromagnetic response in the bulk perovskites. It is worth comparing that the magnetic moment of the single Ti³⁺ center, originating from oxygen vacancies, is estimated at about 0.05 μ_B,³³ while the magnetic dopant of Fe maximally 2.2 μ_B per atom.³³ This value is sufficient to create the multiferroic properties of perovskites by doping with iron [e.g.ref. 34]. Moreover, ferromagnetism at room temperature was observed in the bulk Nb-doped SrTiO₃ single crystals, in which substitution of some Ti⁴⁺ ions by Nb⁺⁵ resulted in the Ti³⁺ state with unpaired electrons.³³ As the defect-forming energy was estimated to have a lower value for bulk PbTiO₃ and PbZrO₃ than for bulk SrTiO₃,²⁹ this might explain why the Ti³⁺ state occurred without this type of substitution in our PZT. This showed that the concentration of defects inducing magnetism was sufficient to generate ferromagnetic properties in this single crystal without purposely introducing magnetic dopants. Therefore, the room-temperature ferromagnetism found in defected PZT crystals is comprehensible, especially due to the presence of the Ti³⁺ state and oxygen vacancies.

In summary, the results show that single PZT crystals “modified“ by defects created during the crystal growth process exhibit multiferroic behavior without doping with magnetic ions. This is the first report describing the magnetic properties of bulk PZT, and not as before, of various composites with solid solutions of lead titanate and lead zirconate. This makes PZT a potential multiferroic and makes it even more attractive from the point of view of application than ever before.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to sincerely thank Stanislav Kamba from the Czech Academy of Sciences in Prague for fruitful discussions on the magnetic properties of oxide perovskites. This work was supported by the National Science Centre, Poland [grant number 2016/21/B/ST3/02242].

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