Ferromagnetic Mn3+ Sawtooth Chains in A2(Mn2O)(SeO3)3 (A = K, Rb) Quaternary Selenites.

Two quaternary manganese selenites, A2(Mn2O)(SeO3)3 (A = K, Rb), have been synthesized by hydrothermal reactions. They both crystallize in a complex triclinic (P-1) structure built of Jahn-Teller (JT) distorted Mn3+O4+2 octahedra, connected into nearly isosceles [Mn3O14] triangles, themselves arranged into so-called "sawtooth (ST) chains". The K and Rb compounds show subtle variations in the orientations of the MnO4 planes inside the elementary triangles. The ST chains are structurally and magnetically isolated by SeO3 groups and alkali cations. In the ST chains, predominant ferromagnetic interactions were calculated and verified experimentally, which finally order antiferromagnetically between the chains around TN ≈ 22 K. The spin exchanges calculated by DFT + U and fitted by Monte Carlo simulations allow for the quantification of an effective "overall" model. The specific role of the µ3-O bridge on the ferromagnetic (FM) exchanges is discussed, together with spin reorientations observed in the ordered state. Magnetocrystalline anisotropy along the [110] direction stabilized by ∼50 meV per Mn by spin-orbit coupling (SOC) was found by DFT + U + SOC.

Layered Monophosphate Tungsten Bronzes [Ba(PO4 )2 ]Wm O3m-3 : 2D Metals with Locked Charge-Density-Wave Instabilities.

Phosphate tungsten and molybenum bronzes represent an outstanding class of materials displaying textbook examples of charge-density-wave (CDW) physics among other fundamental properties. Here we report on the existence of a novel structural branch with the general formula [Ba(PO4 )2 ][Wm O3m-3 ] (m=3, 4 and 5) denominated 'layered monophosphate tungsten bronzes' (L-MPTB). It results from thick [Ba(PO4 )2 ]4- spacer layers disrupting the cationic metal-oxide 2D units and enforcing an overall trigonal structure. Their symmetries are preserved down to 1.8 K and the compounds show metallic behaviour with no clear anomaly as a function of temperature. However, their electronic structure displays the characteristic Fermi surface of previous bronzes derived from 5d W states with hidden nesting properties. By analogy with previous bronzes, such a Fermi surface should result into CDW order. Evidence of CDW order was only indirectly observed in the low-temperature specific heat, giving an exotic context at the crossover between stable 2D metals and CDW order.

Multivalued Memory via Freezing of Super-Hard Magnetic Domains in a Quasi 2D-Magnet.

The design of high-density non-volatile memories is a long-standing dream, limited by conventional storage "0" or "1" bits. An alternative paradigm exists in which regions within candidate materials can be magnetized to intermediate values between the saturation limits. In principle, this paves the way to multivalued bits, vastly increasing storage density. Single-molecule magnets, are good examples offering transitions between intramolecular quantum levels, but require ultra-low temperatures and limited relaxation time between magnetization states. It is showed here that the quasi 2D-Ising compound BaFe2 (PO4 )2 overcomes these limitations. The combination of giant magneto-crystalline anisotropy, strong ferromagnetic exchange, and strong intrinsic pinning creates remarkably narrow magnetic domain walls, collectively freezing under Tf ≈15 K. This results in a transition from a soft to a super-hard magnet (coercive force > 14 T). Any magnetization can then be printed and robustly protected from external fields with an energy barrier >9T at 2 K.