Complex Magnetic Ordering in the Oxide Selenide Sr2Fe3Se2O3

Noncollinear Magnetic Structures in the Chiral Antiperovskite β-Fe2SeO

We present the magnetic properties of the chiral, polar, and possibly magnetoelectric antiperovskite β-Fe2SeO as derived from magnetization and specific-heat measurements as well as from powder neutron diffraction and Mössbauer experiments. Our macroscopic data unambiguously reveal two magnetic phase transitions at TN1 ≈ 103 K and TN2 ≈ 78 K, while Rietveld analysis of neutron powder diffraction data reveals a noncollinear antiferromagnetic structure featuring magnetic moments in the a-b plane of the trigonal structure and a ferromagnetic moment along c. The latter is allowed by symmetry between TN1 and TN2, weakly visible in the magnetization data yet unresolvable microscopically. While the intermediate phase can be expressed in the trigonal magnetic space group P31, the magnetic ground state is modulated by a propagation vector q = (1/2 1/2 0) resulting in triclinic symmetry and an even more complex low-temperature spin arrangement which is also reflected in the Mössbauer hyperfine patterns indicating additional splitting of Fe sites below TN2. The complex noncollinear spin arrangements suggest interesting magnetoelectric properties of this polar magnet.

Single phase charge ordered stoichiometric CaFe3O5 with commensurate and incommensurate trimeron ordering

Mixed-valent transition metal compounds display complex structural, electronic and magnetic properties which can often be exquisitely tuned. Here the charge-ordered state of stoichiometric CaFe₃O₅ is probed using neutron powder diffraction, Monte Carlo simulation and symmetry analysis. Magnetic ordering is dominated by the formation of ferromagnetic Fe³⁺-Fe²⁺-Fe³⁺ trimers which are evident above the magnetic ordering transition. Between T_N= 289 K and 281 K an incommensurate magnetically ordered phase develops due to magnetic frustration, but a spin Jahn-Teller distortion lifts the frustration and enables the magnetic ordering to lock in to a charge-ordered commensurate state at lower temperatures. Stoichiometric CaFe₃O₅ exhibits single phase behaviour throughout and avoids the phase separation into two distinct crystallographic phases with different magnetic structures and Fe valence distributions reported recently, which likely occurs due to partial Fe²⁺ for Ca²⁺ substitution. This underlines the sensitivity of the magnetism and chemistry of these mixed-valent systems to composition.

Synthesis, Crystal Structure, and Physical Properties of Layered LnCrSe2O (Ln = Ce-Nd)

The layered oxyselenides with the formula LnCrSe₂O (Ln = Ce-Nd) were synthesized via molten salt methods. The isostructural compounds crystallize in the monoclinic space group of C2/m. The crystal structures feature _∞²[CrSe₂O]^3- motifs stacked along the a axis, which are separated by Ln³⁺ ions. The _∞²[CrSe₂O]^3- layers are composed of [Cr1Se₆]^9- and [Cr2Se₄O₂]^9- octahedra via corner and edge sharing. Powder X-ray diffraction results confirm the phase purities of the as-synthesized compounds. LnCrSe₂O (Ln = Ce-Nd) show typical antiferromagnetic ordering with T_N = 125, 120, and 118 K, respectively. Heat capacity measurement for NdCrSe₂O indicates that the Debye temperature is 278.4 K. Similar metal-to-semiconductor phase transitions were observed for LnCrSe₂O (Ln = Ce-Nd) plates with transition temperatures of 115, 109, and 95 K, respectively. NdCrSe₂O also possesses a magnetoresistance effect at low temperature (<25 K) with a significant positive magnetoresistance ∼ 16% at 2 K and 1 T.