Site-substitution in GdMnO3 : effects on structural, electronic and magnetic properties

Abstract

We report on detailed structural, electronic and magnetic studies of GdMn1−xCrxO3 for Cr doping levels 0 ≤ x ≤ 1. In the solid solutions, the Jahn-Teller distortion associated with Mn 3+ ions gives rise to major changes in the bc-plane sub-lattice and also the effective orbital ordering in the ab-plane, which persist up to the compositions x ∼ 0.35. These distinct features in the lattice and orbital degrees of freedom are also correlated with bc-plane anisotropy of the local Gd environment. A gradual evolution of electronic states with doping is also clearly seen in O K-edge x-ray absorption spectra. Evidence of magnetization reversal in field-cooledcooling mode for x ≥ 0.35 coinciding the Jahn-Teller crossover, suggests a close correlation between magnetic interaction and structural distortion. These observations indicate a strong entanglement between lattice, spin, electronic and orbital degrees of freedom. The nonmonotonic variation of remnant magnetization can be explained by doping induced modification of magnetic interactions. Density functional theory calculations are consistent with a layer-by-layer type doping with ferromagnetic (antiferomagnetic) coupling between Mn (Cr) ions for intermediate compound (x = 0.5), which is distinct from that observed for the end members GMnO3 and GdCrO3.

Organic-inorganic hybrid lead halides as absorbers in perovskite solar cells: a debate on ferroelectricity

“…Further, due to the breaking of inversion symmetry from the helicoidal/spiral spin order, ferroelectricity arises and an inverse Dzyloshinskii-Moriya interaction is the underlying mechanism for the generation of electric polarization [50]. Besides this, the exchange striction mechanism (best known in Ca 3 CoMnO 6 ) [51,52] and metal-ligand orbital hybridization mechanisms (example: Ba 2 CoGe 2 O 7 ) [53] are also responsible for magnetically-driven ferroelectricity. Apart from these proper and improper ferroelectrics, CdTiO 3 is a unique system, in which ferroelectricity is driven by a phase transition from the centrosymmetric (Pbnm) to a non-centrosymmetric structure (Pna2 1 ) via displacement of Ti and O ions, even though the overall orthorhombic symmetry is maintained [54].…”

“…NMR studies revealed the number of disordered states of MA + was reduced from 24 in the cubic phase to eight in the tetragonal phase and frozen (only one degree of freedom) in the orthorhombic phase [76,77]. Although there is broad agreement on the structural transitions, curiously enough, there is a huge debate on the space group of this system, with both polar and nonpolar space groups frequently reported (table 1) [34,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57]. It is interesting to note that all three phases have been reported to have both polar and non-polar space groups for hybrid perovskites.…”

“…The presence of organic moieties, which have a permanent dipole moment in each formula unit of hybrid perovskites raises the possibility of making the crystal structure polar and we know that polar structure is a precondition for a material to be ferroelectric. Both polar and non-polar space groups in all three phases have been reported frequently, as tabulated in table 1 [34,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57]. This has made for a serious lack of any agreement on such a fundamental topic and led researchers to observe SHG because only polar crystals with non-centrosymmetric point groups can generate a light signal with twice the frequency of the incident light.…”