Investigation of the role of morphology on the magnetic properties of Ca₂Mn₃O₈ materials

Abstract

CaMnO exhibits a complex layered structure comprised of MnO layers separated by Ca ions. In contrast with the more traditional triangular delafossite layered materials the MnO layers additionally exhibit an ordered vacancy, which forms a 'bow-tie' like arrangement of the Mn ions. We report a comprehensive study of the magnetic properties of a series of CaMnO materials with different morphologies. EXAFS and XANES analysis confirm no differences in either manganese environment or oxidation state between materials. Apparent differences in magnetic order from SQUID magnetometry can be rationalised by uncompensated surface spins arising as a result of changes to the surface to volume ratio between morphologies. Furthermore, these data suggest these materials are potentially frustrated in nature, due to the triangular connectivity of Mn spins, with a simple 'spin-up/spin-down' (↑↓) antiferromagnetic model unable to explain the data collected.

Magnetic ordering in a frustrated bow-tie lattice

“…24 An effective magnetic moment, m eff , was calculated at 300 K to be 6.47 m B per formula unit consistent with that expected for Mn 4+ (m calc = 6.7 m B per formula unit) in Ca 2 Mn 3 O 8 consistent with our previous study. 17 The Weiss constant, y CW , was determined to have a value of À150.5 K. The level of frustration in the material can be determined using the frustration index, f, as given by;…”

“…1). 17 We demonstrated that it could be expected that Ca 2 Mn 3 O 8 materials would exhibit more complex antiferromagnetic spin order than a simple mk spin arrangement. 17 We also noted that the quality of materials produced by these methods limited any detailed magnetic studies.…”

“…17 We also noted that the quality of materials produced by these methods limited any detailed magnetic studies. 17 The magnetic behaviour of the isostructural material, Mn 2 Mn 3 O 8 (Mn 5 O 8 ) has also recently been reported by Kishore et al 18 They proposed four different magnetic spin arrangements and generated simulated patterns which they then compared to powder neutron diffraction data collected at 9 K. 18 Subsequently, they differentiate between their assumed models using ab initio total energy calculations which suggested a model where both the MnO 6 layers and Mn 2+ interlayer ions are coupled antiferromagnetically with all the spins located around the manganese 'bow-tie' coupled ferromagnetically to be the lowest energy. 18 We note, however, that these materials were synthesised using a wet chemical route (similar to our previous studies on Ca 2 Mn 3 O 8…”

Phase stability of the layered oxide, Ca2Mn3O8; probing interlayer shearing at high pressure

“…28 However, the understanding of the structure-property relationships in Ca 2 Mn 3 O 8 has largely been limited by the low crystallinity of the as synthesised materials. 29 Previously, we have demonstrated that synthetic route can have a large effect on the morphology observed. 29 Solid-state methods give large spherical particles comprised of small crystallites, sol-gel methods produce nano-sized particles and hydrothermal methods yield flake-like geometries with large surface areas.…”

“…29 Previously, we have demonstrated that synthetic route can have a large effect on the morphology observed. 29 Solid-state methods give large spherical particles comprised of small crystallites, sol-gel methods produce nano-sized particles and hydrothermal methods yield flake-like geometries with large surface areas. 29 However, all synthetic routes led to materials with low crystallinity making further analysis difficult.…”