Palladium-based ferroelectrics and multiferroics: Theory and experiment

Abstract

Palladium normally does not easily substitute for Ti or Zr in perovskite oxides. Moreover, Pd is not normally magnetic (but becomes ferromagnetic under applied uniaxial stress or electric fields). Despite these two great obstacles, we have succeeded in fabricating lead zirconate titanate with 30% Pd substitution. For 20:80 Zr:Ti the ceramics are generally single-phase perovskite (>99%), but sometimes exhibit 1% PbPdO2, which is magnetic below T=90K. The resulting material is multiferroic (ferroelectric-ferromagnet) at room temperature. The processing is slightly unusual (>8 hrs in high-energy ball-milling in Zr balls), and the density functional theory provided shows that it occurs because of Pd +4 in the oversized Pb +2 site; if all Pd +4 were to go into the Ti +4 perovskite B-site, no magnetism would result.

Lead palladium titanate: A room-temperature multiferroic

“…This observation reveals that conductivity is mainly created by PbPdO 2 impurity phase (larger weight-fraction in 30% doped sample, see Figure S3, Supplemental Material [25]). This is not surprising as PbPdO 2 is reported to have a metallic-like conductivity at 90 K -300 K. [17] Kumari et al [14] have shown that PZT (PbZr 1-x Ti x O 3 ) is ferroelectric and magnetoelectric multiferroic at room temperature over a wide range of Zr/Ti ratios, using the positive-up negative-down (PUND) method and quantitative measurements of magnetoelectric tensor components, so we do not show such data here. We do find that leakage current is a problem for 30% Pd at T = 292K and suggest that such studies will be much more precise with thin films rather than our bulk ceramic specimens.…”

“…This is remarkable because Pd atom itself is non-magnetic, but when it replaces lead (Pb) and titanium (Ti) ions, magnetism is observed as predicted from the density functional theory (DFT) studies [14] (discussed in section 1.5).…”

“…Kumari et al [14] showed that 30% Pd doped lead titanate zirconate Pb(Zr,Ti)O 3 (PZT) displays room-temperature weak ferromagnetism, strong ferroelectricity, and strong ME coupling. However their work was not as conclusive as the present study, due to multiple unresolved secondary phases present in the samples and the rather complicated electronic structure of PZT.…”

“…The seminal work of Kumari et al [14] is important but differs from the present in several important ways: (1) The main impurity phase PbPdO 2 could not be seen in XRD because its strong Bragg reflections are coincidentally under the main PZT peaks; (2) The magnetic properties measured did not reveal the magnetic PbPdO 2 contribution; (3) The SEM studies did not isolate any minor phases, including PbPdO 2 ; (4) Activation energies for conduction, important for device applications were unmeasured; and (5) No change in T c with Pd% could be measured due to the dependence of Curie temperatures on Ti/Zr ratio.…”

“…Ferromagnetism arising from doping PbTiO 3 with Pd was predicted by DFT calculations of Paudel and Tsymbal (Kumari et al [14]) and more generally for Pd in earlier work by Sun, Burton, and Tsymbal [12]. [18] and Pd 3 Pb [19] were used for refinements.…”