Ba-doping effects on structural, magnetic and vibrational properties of disordered La2NiMnO6
Introduction
Rare-earth manganites with double perovskite structure RE2MeMnO6 (Me = Ni,Co) have recently attracted a lot of attention due to their magnetic and electric properties [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. In these compounds, the ferromagnetism stems from 180° ferromagnetic super-exchange interactions in consistency with the Goodenough-Kanamori rules [4], [12], [13]. La2NiMnO6 (LNMO) is an unusual case of a near room temperature ferromagnetic insulator that originates from the structural and charge ordering of Ni2+ – O–Mn4+ ions [4], [13].
B-site structural ordering in La2MeMnO6 (Me is a metal) plays a crucial role in the magnetic interactions as it promotes competing antiferromagnetic interactions when a high degree of disorder exists and, as a result, weakens the magnetization as well as induces an additional magnetic transition at low temperatures [4]. Similarly, a significant magneto-dielectric coupling of 8–20% over a wide temperature range) 150–300 K) is found in partially disordered LNMO samples [11].
Typically, a high degree of B-site ordering in perovskite ceramics can be reached by sintering the samples at high temperatures for long times [14], [15], [16], [17], [18], in order to stimulate the interionic diffusion. Recently, it has been shown that sintering La2MeMnO6 at high temperatures induces the creation of vacancies and metal volatilization [19], both of them decreasing the B-site structural order thus hindering the Me2+–O–Mn4+superexchange interaction. Another way to improve the ordering is the substitution in the A-sites to take advantage of the fact that the perovskite structure supports a broad range of different ions in its structure [20], [21], [22], [23]. For instance, Bai et al. [21], [22] doped La2CoMnO6 (LCMO) with bismuth and improved the B-site cation ordering in LCMO by substituting La by Bi in the A-site [22]. This substitution also suppressed the cluster-glass behavior in LCMO that is characteristic of a lowered Co2+/Mn4+ disorder and an improved dielectric performance [21]. Yet, the Curie temperature decreased by Bi addition. Similarly, in Sr-doped LNMO Guo et al. [23] observed a slightly decreased Curie temperature and a significantly reduced magnetic saturation, both of them due to B-site disorder induced by Sr doping. Additionally, the authors observed an exchange bias effect in all doped samples at 10 K which originates from the coupling between Ni/Mn ordered ferromagnetic regions and the antiferromagnetic clusters originated from the disorder. However, all the investigations covered doping concentrations higher than 10%.
On one hand, due to the valence difference between the alkaline earth metal ions and the lanthanum, alkaline earth metal substitutions into the A-site in LNMO can induce vacancies. Those, in turn, could induce antiferromagnetic clusters that would hinder the super-exchange interactions. On the other hand, the A-site substitutions can facilitate the B-site order [22]. Therefore, in LMNO, alkaline earth metal substitutions should trigger a competition of magnetic interactions and induce a maximal ferromagnetic response in LNMO due to the increased B-site structural ordering or valence mixing. In addition, it is expected that an increased tolerance factor in RE2NiMnO6 double perovskites would enhance the Curie temperature. See, for instance, the Curie temperature dependence with the RE ionic radius evidenced by Booth et al. [24]. Indeed, a tolerance factor increase concomitant to the unit cell volume change, reduces the octahedral tilting thus the Ni–O–Mn bond angles approach 180°. As a result, the better orbital overlap invigorates the super-exchange interaction. In this paper we show that Ba-doping improves the B-site structural ordering in disordered LNMO by producing a series of samples with Ba content between 0% and 20%. Our characterizations, comprising magnetic analyses, dielectric properties and Raman spectroscopy, echo the aforementioned competition and signal 5% Ba doping as the optimal stoichiometry.
Section snippets
Experimental methods
Polycrystalline samples of (La2−xBax)NiMnO6±δ (0% ≤ % Ba ≤ 20% in A-site molar percent) were synthesized by standard solid state method under air atmosphere using oxides as reagents. Prior to synthesis process, the La2O3 was fired at 1000 °C for 24 h to remove moisture and carbonates [25]. Stoichiometric amounts of La2O3 (Sigma Aldrich, 99.99%), NiO (Sigma Aldrich, 99.99%), MnO2 (Sigma Aldrich, >99%) and BaCO3 (Sigma Aldrich, >99%) were grounded using an agate mortar and a pestle with acetone.
Results
Fig. 1 shows the powder XRD patterns of (La2−xBax)NiMnO6−δ (0% ≤ x ≤ 20%). The refinement indicates that, at the employed synthesis conditions, the non-doped LNMO sample is described by the monoclinic P21/n (≈75.90(8)%) and hexagonal symmetries thus in agreement with Sayed et al. [19]. All samples investigated preserved the co-existence of both phases, which can be evidenced from the diffraction peak splitting around 2θ = 33° that is characteristic of the hexagonal structure.
Conclusions
In summary, we have synthesized a series of Ba-doped LNMO samples by standard solid state route. We have performed structural, magnetic and vibrational characterizations all of which signal that 5% Ba doping renders the highest B-site ordering. Complementary analyses confirm that the Ba doping was well incorporated in LNMO and triggered no changes of density or oxidation state, despite the great influence on structural aspects and magnetic properties. We have showed that the increasing Curie
Acknowledgments
Brazilian authors acknowledge the financial support from Brazilian funding agencies CNPq (Proc. Number 461621/2014), FAPEMA (Proc. Number 02121/12-PRONEM), FAPEMIG (Proc. Number PPM-00731-15) and CAPES (Proc. Number 552512/2011-7 and Scholarship Grant No. 10423-12-5). This work has been supported by the Brazilian Synchrotron Light Laboratory (LNLS) under the proposals DXAS-16869/14 and XAFS1-16869 and the Brazilian Nanotechnology Laboratory for Research in Energy and Materials (proposal
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