Incommensurate magnetic structure, Fe/Cu chemical disorder, and magnetic interactions in the high-temperature multiferroicYBaCuFeO5

Abstract

Motivated by the recent observations of incommensurate magnetic order and electric polarization in YBaCuFeO 5 up to temperatures T N2 as high as 230 K [B. Kundys et al., Appl. Phys. Lett. 94, 072506 (2009); Y. Kawamura et al., J. Phys. Soc. Jpn 79, 073705 (2010)], we report here for the first time a model for the incommensurate magnetic structure of this material, which we complement with ab initio calculations of the magnetic exchange parameters. Using neutron powder diffraction, we show that the appearance of polarization below T N2 is accompanied by the replacement of the high-temperature collinear magnetic order by a circular inclined spiral with propagation vector k i = (1/2,1/2,1/2 ± q). Moreover, we find that the polarization approximately scales with the modulus of the magnetic modulation vector q down to the lowest temperature investigated (∼3 K). Further, we observe occupational Fe/Cu disorder in the FeO 5 -CuO 5 bipyramids, although a preferential occupation of such units by Fe-Cu pairs is supported by the observed magnetic order and by density functional calculations. We calculate exchange coupling constants for different Fe/Cu distributions and show that, for those containing Fe-Cu dimers, the resulting magnetic order is compatible with the experimentally observed collinear magnetic structure [k c = (1/2,1/2,1/2), T N2 > T > T N1 = 440 K]. Based on these results, we discuss possible origins for the incommensurate modulation and its coupling with ferroelectricity. M. MORIN et al. PHYSICAL REVIEW B 91, 064408 (2015) J ⊥ (meV) J (meV) J O (meV) J NNN (meV) (a) J 1,2 = 134.5 J 1,3 = 10.6 J 5,3 = −1.6 J 1,5 = −0.05 J 7,8 = 8.7 J 5,7 = 2.8 J 1+c,5 = −0.01 (b) J 1,2 = 129.9

Tuning magnetic spirals beyond room temperature with chemical disorder

“…Since Cu and Fe are usually disordered in this material, this implies that the bow-ties are preferentially occupied by FM Cu–Fe ‘dimers' randomly distributed in the structure. The lower ground state energy of Fe–Cu distributions containing disordered FM Cu–Fe dimers has been confirmed by density functional theory (DFT) calculations21.…”

“…1b). In the ab plane all possible nearest-neighbour J ab interactions (Cu–Cu, Cu–Fe and Fe–Fe) are AFM and very strong (up to 130 meV), whereas weaker, up to two orders of magnitude smaller AFM ( J c1 ) and ferromagnetic (FM) J c2 couplings alternate along the c axis21. The only FM coupling in the structure corresponds to ions occupying the bow-ties ( J c2 , Fig.…”

“…1a21. It consists of two perovskite units where the A-site cations Ba 2+ and Y 3+ are ordered in layers.…”

“…YBaCuFeO 5 undergoes two magnetic transitions at temperatures T N1 and T N2 (refs 21, 23, 24, 25, 26, 27, 28). The high temperature collinear antiferromagnetic (AFM) phase ( T N1 > T > T N2 ) is characterized by the propagation vector k c =(½ ½ ½) (Fig.…”

“…In the following, we address this question by investigating the layered perovskite YBaCuFeO 5 , one of the few oxides with a spin-spiral phase above 200 K (refs 5, 20, 21). As for CuO22, spontaneous electric polarization was reported to exist at zero magnetic field in the spiral phase212324, which is stable over a temperature range more than 10 times larger than in cupric oxide.…”