Double perovskite Y₂NiMnO₆ nanowires: high temperature ferromagnetic–ferroelectric multiferroic

Abstract

This work demonstrates the unusual room temperature ferromagnetism and ferroelectricity of the double perovskite multiferroic Y₂NiMnO₆ nanowires, fabricated using a facile solvothermal route. The physics behind the co-existence and coupling of ferroelectricity and ferromagnetism in Y₂NiMnO₆ nanowires at high temperature is the key focus of this article. The studies indicate that the large concentration of surface spin and the surface charge polarization associated with the surface electrons of the unique one dimensional high aspect ratio nanowires are responsible for the ferromagnetism and ferroelectricity, respectively at room temperature. It is believed that the ferromagnetism and ferroelectricity are mutually coupled with each other and they have a similar origin.

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The coexistence of ferroelectric and magnetic dipoles in multiferroic and magnetoelectric materials holds immense promise for potential applications in sensors, data storage and novel spintronics devices.^1–3 However, there are few multiferroic materials showing the coexistence of ferromagnetic and ferroelectric properties in the same phase. Therefore, the search for novel multiferroics, where these two phenomena are mutually coupled, is of significant scientific and technological interest.^4,5 In type II multiferroic materials, the coupling between the electrical and magnetic ordering coexists and they have the same origin, where ferroelectricity is caused by an unusual magnetic order. However, the compounds or semiconductors having perovskite structure have been found to exhibit spontaneous magnetic and electrical ordering in the same phase, where both the polarization and magnetization can be switched by an applied electric and magnetic field, respectively.^5–7 Generally, multiferroicity has been found to exist in perovskites at low temperature, where the polarization values are low too.^6,7 Hence, the quest for high temperature multiferroics is still very challenging. Among various approaches adopted, doping of metal ions and fabrication of various nanostructures help to improve the coexistence of the magnetic and electrical properties of the perovskites at room temperature.^8,9 Recently, the multiferroic properties of different double perovskites have attracted considerable attention.^10,11 Among them bulk Y₂NiMnO₆ have been found having low Curie temperature and E-type antiferromagnetic (AFM) ordering.¹²

In this work, we have fabricated and studied the multiferroic property of the Y₂NiMnO₆ nanowires (NWs), prepared by solvothermal route, for the first time, to the best of our knowledge. The unique one dimensional (1D) morphology and high surface area of the NWs provide ample opportunity to tune the multiferroic property of the Y₂NiMnO₆ compound. Although the Curie temperature of the Y₂NiMnO₆ NWs is ∼50 K, the NWs are found to exhibit unexpected weak ferromagnetism and ferroelectric ordering at room temperature. Here, we have explored the fundamental physics behind the mutual coexistence of ferromagnetism and ferroelectricity at room temperature. Comparing the results of the bulk Y₂NiMnO₆ with that of the Y₂NiMnO₆ NWs we demonstrate that the surface spin and the surface charge polarization are responsible for the unusual high temperature ferromagnetism and ferroelectricity of the NWs, respectively. It is found that the spontaneous ferromagnetism and ferroelectricity are mutually coupled and they have the same origin.

Single-phase polycrystalline Y₂NiMnO₆ NWs were fabricated by solvothermal route. Here, Y(NO₃)₃·6H₂O, Ni(NO₃)₃·6H₂O and C₄H₆MnO₄·4H₂O were dissolved in equal weights in 40 ml DI water according to their stoichiometric ratios. A 5 M 10 ml solution of NaOH was added drop by drop to the above solution to adjust the pH of the solution to 12. After the addition of NaOH the precipitation of mixed Y(OH)₃, Ni(OH)₂ and Mn(OH)₂ took place. The mixture was stirred for 30 minutes and then transferred it into a Teflon-lined stainless steel autoclave. The autoclave was tightly sealed and then heated at 200 °C for 24 hours. The product thus obtained was washed successively in DI water, acetone and ethanol and dried at 70 °C for another 24 hours.

The bulk powder of polycrystalline Y₂NiMnO₆ was prepared by the sol–gel method. High purity Y(NO₃)₃·6H₂O, Ni(NO₃)₃·6H₂O and C₄H₆MnO₄·4H₂O were dissolved in DI water in equal weights according to their stoichiometric ratios, followed by constant stirring of the mixture for about 30 minutes at room temperature. The citric acid was added to the solution with a 2 : 1 molar ratio with respect to metal ions under constant stirring. Afterwards, the solution was heated at 150 °C until a thick solution was formed and then the thick mixture was calcined at 1000 °C for 12 hours to prepare the powder sample.

Fig. 1(a) shows the field emission scanning electron microscope (FESEM) image of the as prepared Y₂NiMnO₆ NWs. The diameter of the NWs is found to be quite uniform (∼200 nm) in nature. Transmission electron microscope (TEM) image of a single Y₂NiMnO₆ NW, shown in Fig. 1(b), again clarify the uniformity in the diameter of the NW. The representative high resolution TEM (HRTEM) micrograph of the NW, as shown in Fig. 1(c), demonstrates the polycrystalline nature of the NW with dissimilar orientation of the different crystallographic planes having different lattice spacing. The lattice spacing between the (012) and (112) crystalline planes has been measured as 0.31 and 0.26 nm, respectively. The selective area electron diffraction (SAED) pattern taken from the area covered under the HRTEM study also clarifies the polycrystalline nature of the NWs (see Fig. 1(d)). Furthermore, the X-ray diffraction (XRD) has also been conducted to probe the crystallographic nature of the NWs. The XRD pattern (Fig. 1(e)) indicates the single phase polycrystalline monoclinic structure of the Y₂NiMnO₆NWs with P2₁/n space group.^6,13 Fig. 1(f)–(j) represent the energy filtered TEM (EFTEM) micrographs of a single Y₂NiMnO₆ NW, where the colour mapping of different elements present in Y₂NiMnO₆ NW shows an uniform distribution in the body of the NW, which again confirm the successful synthesis of the NWs.

Fig. 1 (a) SEM, (b) TEM and (c) HRTEM micrographs and (d) SAED pattern of the as prepared Y₂NiMnO₆ NWs. (e) XRD pattern and (f–j) EFTEM micrographs of the Y₂NiMnO₆ NWs.

The chemical composition of the as prepared Y₂NiMnO₆ NWs has also been studied by X-ray photoelectron spectroscopy (XPS). In the as prepared NWs the Y 3d_5/2 and Y 3d_3/2 doublet peaks appears at 156.5 and 158.5 eV, respectively, which indicate the +3 oxidation state of Y in Y₂NiMnO₆ (ref. 14) as shown in Fig. 2(a). Fig. 2(b) shows the XPS spectrum for the Ni, where the peaks of Ni 2p_3/2 and Ni 2p_1/2 situated at 855.5 and 873.4 eV, respectively, represent the divalent (+2) state for Ni in Y₂NiMnO₆.^6,7 The broad peak around 862 eV is a satellite peak related to Ni 2p_3/2, again indicating +2 oxidation state of Ni in the compound.¹⁴ In the XPS spectrum of Mn (Fig. 2(c)) the positions of the characteristics peaks of Mn 2p_3/2 and Mn 2p_1/2 have been found at 642.2 and 653.4 eV, respectively, correspond to the tetravalent (+4), oxidation state of Mn in the NWs. The XPS spectrum of O 1s core level, presented in Fig. 2(d), shows a sharp Gaussian peak centered on 529.7 eV, can be assigned to the −2 oxidation state of oxygen in the Y₂NiMnO₆ NWs.¹⁵

Fig. 2 XPS spectra of (a) Y 3d, (b) Ni 2p, (c) Mn 2p and (d) O 1s core levels of the Y₂NiMnO₆NHs.

Y₂NiMnO₆ NWs have been found to exhibit interesting magnetic properties. The variation of magnetization as a function of temperature under both zero field cooled (ZFC) and field cooled (FC) measured at 1 kOe of applied field are shown in Fig. 3(a) for the NWs and bulk Y₂NiMnO₆. The sharp change of the FC and ZFC curves at a particular temperature indicates the ferromagnetic transition. The ferromagnetic Curie temperature (T_c) calculated for bulk and NWs of Y₂NiMnO₆ are 91 and 50 K, respectively. The T_c for the NWs has been found to be considerably lower than that of the bulk Y₂NiMnO₆. However, both the NWs and bulk Y₂NiMnO₆ exhibit ferromagnetism (FM) at 4 K, as shown in Fig. 3(b). The ferromagnetism in the Y₂NiMnO₆ bulk and NWs samples indicates strong coupling among the dipoles at temperature below T_c. Fig. 3(c) shows the magnetization versus magnetic-field (M–H) hysteresis curves for theY₂NiMnO₆ NWs at different temperatures, below and well above T_c. It is evident that the Y₂NiMnO₆ NWs exhibit weak FM above T_c. The saturation magnetism is found to decrease with increase in temperature as expected. Fig. 3(d) compares the M–H curve of the bulk and NWs of Y₂NiMnO₆ at 300 K. It is evident that at 300 K, the bulk sample exhibits strong paramagnetic behavior whereas the NWs still show weak ferromagnetic signature.

Fig. 3 (a) Variation of magnetization as a function of temperature under ZFC and FC condition for the Y₂NiMnO₆ NWs and bulk samples measured in magnetic field of 1 KOe. Magnetization versus magnetic field hysteresis loops for the (b) Y₂NiMnO₆ NWs and bulk samples at 4 K, (c) Y₂NiMnO₆ NWs at 4, 50, 80, 100, 200 and 300 K and (d) Y₂NiMnO₆ NWs and bulk samples at 300 K.

Y₂NiMnO₆ is demonstrated as an E-type antiferromagnetic material with ↑↑↓↓ spin structure because of the superexchange interaction between Ni²⁺ and Mn⁴⁺.^6,12 From the XPS experiment we also have evidenced the presence of Ni²⁺ and Mn⁴⁺ in Y₂NiMnO₆. However, very interestingly the magnetic measurements indicate definite ferromagnetic characteristics of the Y₂NiMnO₆ NWs and bulk samples with specific T_c. It is reported that the ↑↑↓↓ spin structure in Y₂NiMnO₆ only exists at zero magnetic field and this AFM spin order can be destroyed with the applied magnetic fields.⁶ Therefore, we propose that, below T_c, the AFM (↑↑↓↓) spin arrangement of Y₂NiMnO₆ have been transformed into ferromagnetic ordering under the applied magnetic field and this is the reason behind the FM of NWs and bulk structure of Y₂NiMnO₆ below T_c. However, most surprisingly, Y₂NiMnO₆ NWs show weak FM above T_c, whereas the bulk Y₂NiMnO₆ structure is found to exhibit purely paramagnetic behavior beyond T_c. This type of unexpected room temperature FM is reported for nanostructures of different antiferromagnetic oxides.^8,9,16 The RTFM for such oxides could have different origins as demonstrated in different reports. However, in present study the unusual RTFM in Y₂NiMnO₆ NWs is attributed to the surface spins, which are expected to dominate because of their lower coordination and uncompensated exchange couplings in nanostructures.^17–20 Moreover, in 1D nanostructures like NWs the concentration of surface electron is significantly higher because of the large surface area and high aspect ratio of the NWs. Now, an applied magnetic field can orient the surface spins and the interaction among the uncompensated surface spins leads to the ferromagnetic ordering in the NWs.¹⁹ Therefore, the spins of the surface electron or spins of the space charge are responsible for the high temperature FM in Y₂NiMnO₆ NWs whereas such spin coupling is feeble for the bulk Y₂NiMnO₆ sample because of its low surface area and hence the bulk Y₂NiMnO₆ exhibits paramagnetism at high temperature (above T_c).

More clear understanding about the origin of unusual high temperature ferromagnetic behavior of Y₂NiMnO₆ NWs can be obtained from the study of the ferroelectric property of the sample. Recently, it is reported that the magnetic order can build up weak ferroelectricity and vice versa.^4,21 Studies indicate that the magnetic order and electric polarization are likely to be mutually coupled and seem to have similar origin.³ Hence, study on the ferroelectric property of the Y₂NiMnO₆ NWs could be meaningful in order to understand the unusual RTFM of the NWs. The ferroelectric P–E loop of the Y₂NiMnO₆ NWs has been measured by dispersing the NWs assembly on FTO glass substrate and by connecting with the wire through conducting Ag paste as shown in Fig. 4(a). The same measurement has been performed on the bulk Y₂NiMnO₆ sample by preparing a pellet using the powder of Y₂NiMnO₆. Fig. 4(b) and (c) display the P–E loops of the Y₂NiMnO₆ NWs and bulk samples recorded at 80 and 300 K. It is evident from Fig. 4(b) that at 80 K both the Y₂NiMnO₆ NWs and bulk samples exhibit stable ferroelectric property (measured at a applied bias of 1 V), where the bulk sample shows strong ferroelectric signature compare to the NWs. On the other hand, very interestingly, at 300 K, the Y₂NiMnO₆ NWs exhibit strong and stable ferroelectric polarization compared with the bulk sample, which shows a loosy type feeble ferroelectric characteristics (Fig. 4(c)). The RT (300 K) P–E hysteresis loops of the Y₂NiMnO₆ NWs measured at different applied voltages also found to be stable and repeatable as shown in Fig. 4(d).

Fig. 4 (a) The photographs for the P–E measurement set up for the Y₂NiMnO₆ NWs. (b) Variation of the P–E hysteresis loops for the Y₂NiMnO₆ NWs and bulk at (b) 80 K and (c) 300 K (room temperature). (d) Variation of the room temperature (300 K) P–E hysteresis loops with applied voltage for the Y₂NiMnO₆ NWs. Variation of the (e) saturation polarization and remnant polarization and (f) coercive field with applied voltage in Y₂NiMnO₆ NWs.

This study reveals that at temperature of 80 K (which is below the T_c for Y₂NiMnO₆ bulk samples ∼91 K but above the T_c for Y₂NiMnO₆ NWs ∼50K) both the Y₂NiMnO₆ bulk and NWs sample exhibit stable ferromagnetism and ferroelectricity, whereas, only the Y₂NiMnO₆ NWs possess a net electric polarization and ferromagnetism above T_c (also at 300 K). The bulk Y₂NiMnO₆ sample exhibits stable paramagnetism and feeble ferroelectricity at a temperature of 300 K. Based on this results it is evident that the electric polarization is intimately coupled to the ferromagnetic ordering. It is found that whenever ferromagnetism exists the dielectric polarization exists and if there is no ferromagnetism there is no ferroelectric polarization too. Hence, this coexistence of ferromagnetism and ferroelectricity is most expectedly having the similar origin.^6,22 Here, we expect that the space charges associated with the large surface area of the Y₂NiMnO₆ NWs, which are responsible for the ferromagnetic order in the NWs under applied magnetic field at 300 K (room temperature), can also induce electric polarization and tune the P–E hysteresis loops and field distributions in NWs.^23,24 However, for bulk Y₂NiMnO₆ sample the coexistence of the ferromagnetism and ferroelectricity at a temperature below its T_c has already been reported in details based on the braking of the spatial inversion symmetry.^6,7

However, the coexistence of ferromagnetism and ferroelectricity in the same phase of different compound materials are known, where the spontaneous magnetization can be switched by an applied external magnetic field and the electrical polarization can be triggered by an applied electric field too.^5,25 In most of the magnetic ferroelectric materials the coexistence of the ferromagnetism and ferroelectricity below Curie temperature are demonstrated based on the braking of the spatial inversion symmetry.^5,12 However, in our work the unusual high temperature FM and ferroelectricity in Y₂NiMnO₆ NWs must have different origin and that could be ascribed to the effects of space charge. The steady variation of the remnant polarization, saturation polarization and coercive field with applied external bias clearly demonstrate the stable ferroelectric characteristics of the Y₂NiMnO₆ NWs at room temperature, 300 K (Fig. 4(d)–(f)).

In summary, single phase, high crystalline double perovskite Y₂NiMnO₆ NWs have been successfully fabricated by solvothermal route. Y₂NiMnO₆ NWs exhibit unexpected weak ferromagnetism and ferroelectricity at room temperature though the Curie temperature of the NWs is 50 K, which is much lower than that of the bulk Y₂NiMnO₆ (∼91 K). The coexistence of the ferromagnetic and ferroelectric ordering in Y₂NiMnO₆ NWs at high temperature is ascribed to the large concentration of surface spin and surface polarization of the electron, respectively, associated with the large surface area of the one dimensional NWs. The study indicates that for Y₂NiMnO₆ NWs, at room temperature, ferroelectricity and ferromagnetism are mutually coupled with each other and have similar origin too.

Acknowledgements

Author Gobinda Gopal Khan is thankful to the Department of Science and Technology (DST), Government of India, for providing research support through the ‘INSPIRE Faculty Award’ (IFA12-ENG-09).

Notes and references

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