On the “alpha-phase” of Ca2−xSrxMnO4 and extending the chemistry of Sr7−yCayMn4O15 to y>1

Synthesis and Characterization of Magnetoelectric Ba7Mn4O15

“…In the case of Sr 7 Mn 4 O 15 , the strings of Mn 2 O 9 dimers were predicted to result in strong magnetic exchange interactions in the bc plane, but weak interactions along a . Previous reports of magnetic susceptibility experiments on Sr 7 Mn 4 O 15 describe a broad maximum centered around 75–90 K , with a small upward tail below around 20 K. The FC and ZFC curves diverge from one another at temperatures below this maximum. This behavior has been explained with two different mechanisms: Vente et al proposed that it resulted from spin glass-like behavior producing clusters of antiferromagnetically ordered spins, which crystallize into true antiferromagnetic order below ∼75 K, whereas we have previously suggested that it might represent a weak FM ordering component, arising from the local symmetry-breaking associated with the aforementioned disorder of the Sr and O sites. , …”

“…These fits are highlighted in bold in Table . Refinements in which the irreps were also allowed to refine in a were also investigated; we find that constraining the irreps along the c and b directions does not negatively impact the quality of the fit to the data, in line with literature predictions of the magnetic structure of Sr 7 Mn 4 O 15 . , While the components of the magnetic moments were not constrained to be equal along the b and c lattice directions, they consistently refined to approximately equal values as shown in Figure .…”

“…A powder neutron diffraction pattern for Sr 7 Mn 4 O 15 is shown in Figure , with the magnetic peaks observed at 1.5 K inset. The most plausible fit to the magnetic peaks is achieved using a model containing only the m Γ 2 – irrep along the b direction to describe the magnetic moments, consistent with previously published models. , The spin configuration refined against the data and transforming as the m Γ 2 – irrep is shown in Figure . This produces a calculated magnetic moment of 2.2 μ B consistent with the magnitude observed for Ba 7 Mn 4 O 15 , giving us further confidence in our proposed spin-wave solutions.…”

“…In Sr 7 Mn 4 O 15 , these dimer units result in strong antiferromagnetic (AFM) interactions between the neighboring Mn 4+ sites, a broad peak in the DC magnetic susceptibility and divergence between the field-cooled cooling (FC) and zero-field-cooled (ZFC) warming curves. , In addition to the broad maximum, we have previously observed divergences between the FC and ZFC susceptibility results for the series Sr 7 Mn 4 O 15 to Sr 3.5 Ca 3.5 Mn 4 O 15 below 175 K, leading us to suggest that the behavior could be explained by weak ferromagnetism (wFM) in which the AFM spins are slightly canted. We propose that the space group in which this is allowed, P 2 1 , would also allow local displacements of the oxide anions to produce FE ordering, resulting in a magnetoelectric ground state for the phase . In general, the low symmetry of this structural type means that a large number of ways exist in which the magnetic exchange interactions may break inversion symmetry and hence allow for the magnetoelectric effect.…”

First-principles investigation of the magnetoelectric properties of Ba7Mn4O15

“…The Sr 7 Mn 4 O 15 analogue, proposed by Kriegel et al [11,12], crystallises in a monoclinic phase containing face-sharing Mn 2 O 9 octahedral dimer motifs. In 2017, Craddock and Senn have assigned a canted antiferromagnetic ground state to Sr 7 Mn 4 O 15 which is attributed to the presence of a weak ferromagnetic ordering component [13]. However, recent symmetry-adapted refinement against neutron powder diffraction (NPD) data by Clarke et al shows that the Mn ions inside the octahedral dimers exhibit strong antiferromagnetic coupling with the spins aligned along the monoclinic b-direction and there is no weak ferromagnetic component which can give rise to a multiferroic ground state [10].…”