Direct atomic scale determination of magnetic ion partition in a room temperature multiferroic material

The five-layer Aurivillius phase Bi6TixFeyMnzO18 system is a rare example of a single-phase room temperature multiferroic material. To optimise its properties and exploit it for future memory storage applications, it is necessary to understand the origin of the room temperature magnetisation. In this work we use high resolution scanning transmission electron microscopy, EDX and EELS to discover how closely-packed Ti/Mn/Fe cations of similar atomic number are arranged, both within the perfect structure and within defect regions. Direct evidence for partitioning of the magnetic cations (Mn and Fe) to the central three of the five perovskite (PK) layers is presented, which reveals a marked preference for Mn to partition to the central layer. We infer this is most probably due to elastic strain energy considerations. The observed increase (>8%) in magnetic cation content at the central PK layers engenders up to a 90% increase in potential ferromagnetic spin alignments in the central layer and this could be significant in terms of creating pathways to the long-range room temperature magnetic order observed in this distinct and intriguing material system.

the Bruker software, lowering pixel resolution and integrating neighbouring pixels to increase signal to noise. This was reasonable as the step size was small and there was an amount of oversampling so each atomic column has several pixels.
The EELS data was acquired separately to the EDX and the data was collected and processed using Gatan Digital Micrograph. To produce the maps energy windows were placed over the Ti L3 peak 455-460eV, Mn L3 peak 638-642 and Fe L3 peak 705-711 with a power-law background subtracted before each edge. RGB colour maps were generated with an 8-bit pixel-depth using the colour mix feature in Digital Micrograph. (1) Where ri and rm are the individual and mean bond lengths respectively and the sum is taken over all the bonds (in this case n = 6).
Giddings et al. 2 determined that ∆6 is 5.2 and 5.6 for the outer and inner octahedral respectively at 295K in their m=4 system. There is insufficient difference between these figures to drive cation partitioning, but at 970K (much closer to the compound synthesis temperature) the numbers are 6.17 and 0.75 respectively, providing a clear driving mechanism.
8 Energetics for the reduction of Mn 3+ and Fe 3+ at 1100 K: If we consider the two reduction reactions: and 6Fe2O3 → 4Fe3O4 +O2 We can calculate the standard enthalpy (∆Hf) and Gibbs free energy of the reaction at the synthesis temperature of 1100K (∆G) using literature thermodynamic data 3 .
For reaction ( Accordingly, it is much easier to reduce Mn 3+ than Fe 3+ at the B6TFMO synthesis temperature, and as ∆G is negative we expect the reaction to proceed forwards. If the thermodynamics is similar in the context of the Aurivillius phase environment, this means that a substantial part of the Mn in the system will be present as Mn 2+ . For iron ∆G is positive, which means that there will be no Fe 2+ .

Boltzmann equation to model the expected distribution of magnetic cations within PK layers:
The Gaussian, or Normal distribution model can be applied to model cation distribution. This is described by: where µ = value of x for which C(x) (the concentration of an ion at a point x in the structure) is a maximum and σ = standard deviation. Now, we find that: Which is a Gaussian function (see Equation (4) In the case of La 3+ , the ionic radius is 1.18 Å in 8-fold coordination by oxygen, compared with 1.11 Å in Bi 3+ . The much later paper by Zhou & Goodenough 6 indicates that in the rare earth manganate perovskites, the smaller the A-site cation, the stronger the Jahn-Teller distortion, and the lower the antiferromagnetic Nèel temperature. In this report, GdMnO3 has the optimal orbital mixing for ferromagnetic coupling. Bi 3+ is much more similar in size to Gd 3+ (1.11 Å) than La 3+ , and the Nèel temperature for GdMnO3 is <50 K, with a high JT transition temperature (1400 K). On these grounds, we might reasonably expect a significant proportion of Mn 3+ -O-Mn 3+ sequences to be ferromagnetic in character in B6TFMO.

Probability of Bonds being Potentially-Ferromagnetic Sequences
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