A large anomalous Hall effect is observed in the triangular antiferromagnet Mn3Ge arising from an intrinsic electronic Berry phase.

A detailed analysis of the crystal structure in RETiO3 with RE = La, Nd, Sm, Gd, and Y reveals an intrinsic coupling between orbital degrees of freedom and the lattice which cannot be fully attributed to the structural deformation arising from bond-length mismatch. The TiO6 octahedra in this series are all irregular with the shape of the distortion depending on the RE ionic radius. These octahedron distortions vary more strongly with temperature than the tilt and rotation angles. Around the Ti magnetic ordering all compounds exhibit strong anomalies in the thermal-expansion coefficients, these anomalies exhibit opposite signs for the antiferromagnetic and ferromagnetic compounds. Furthermore the strongest effects are observed in the materials close to the magnetic cross-over from antiferromagnetic to ferromagnetic order.

Developing efficient and low‐cost electrocatalysts for the oxygen evolution reaction (OER) is of paramount importance to many chemical and energy transformation technologies. The diversity and flexibility of metal oxides offer numerous degrees of freedom for enhancing catalytic activity by tailoring their physicochemical properties, but the active site of current metal oxides for OER is still limited to either metal ions or lattice oxygen. Here, a new complex oxide with unique hexagonal structure consisting of one honeycomb‐like network, Ba4Sr4(Co0.8Fe0.2)4O15 (hex‐BSCF), is reported, demonstrating ultrahigh OER activity because both the tetrahedral Co ions and the octahedral oxygen ions on the surface are active, as confirmed by combined X‐ray absorption spectroscopy analysis and theoretical calculations. The bulk hex‐BSCF material synthesized by the facile and scalable sol–gel method achieves 10 mA cm−2 at a low overpotential of only 340 mV (and small Tafel slope of 47 mV dec−1) in 0.1 m KOH, surpassing most metal oxides ever reported for OER, while maintaining excellent durability. This study opens up a new avenue to dramatically enhancing catalytic activity of metal oxides for other applications through rational design of structures with multiple active sites.

Combining infrared reflectivity, transport, susceptibility and several diffraction techniques, we find compelling evidence that CaCrO3 is a rare case of a metallic and antiferromagnetic transitionmetal oxide with a three-dimensional electronic structure. LSDA calculations correctly describe the metallic behavior as well as the anisotropic magnetic ordering pattern of C type: The high Cr valence state induces via sizeable pd hybridization remarkably strong next-nearest neighbor interactions stabilizing this ordering. The subtle balance of magnetic interactions gives rise to magneto-elastic coupling, explaining pronounced structural anomalies observed at the magnetic ordering transition.Strongly correlated electron systems including the wide class of transition-metal oxides exhibit a quite general relation between magnetic order and electrical conductivity [1]: ferromagnetism typically coexists with metallic conductivity, whereas insulators usually exhibit antiferromagnetism. It is always a challenge to understand exceptions from this rule. The rare observations of ferromagnetism in insulating transition-metal oxides most often are due to a particular type of orbital ordering [2]. The few examples of antiferromagnetic (AFM) metals, e.g., (La/Sr) 3 Mn 2 O 7 [3] or Ca 3 Ru 2 O 7 [4], are characterized by reduced electronic and structural dimensionality, and the antiferromagnetic order corresponds to a stacking of ferromagnetic (FM) layers. Here we report the discovery of a three-dimensional transition-metal oxide with metallic conductivity, antiferromagnetic exchange interactions, and C-type antiferromagnetic order: the perovskite CaCrO 3 .Perovskites containing Cr 4+ (CaCrO 3 , SrCrO 3 , and PbCrO 3 ) were already studied previously [5,6,7,8,9,10], but neither the details of the crystal structure nor the nature of the magnetic ordering are known. Only very recently evidence for C-type AFM order was reported in multi-phase samples of SrCrO 3 [10]. Regarding the conductivity, the existing data are controversial. In Refs. [7,9] CaCrO 3 was claimed to be metallic, but more recently insulating behavior has been reported [5]. A similar controversy persists also for SrCrO 3 , which should definitely be more metallic than CaCrO 3 due to the less distorted crystal structure, but metallic behavior was observed in Ref.[5] only under pressure. These controversies most likely are connected with the difficulty to prepare high-quality stoichiometric materials and with the lack of large single crystals.CaCrO 3 exhibits an orthorhombic GdFeO 3 -type perovskite structure and early magnetization measurements indicate a magnetic transition at 90 K [8], which is confirmed in our samples. Two electrons occupy the Cr 3d shell (S=1), rendering the material electronically similar to insulating RVO 3 [11] (also 3d 2 ) and to metallic (Ca/Sr)RuO 3 (4d 2 ) [12]. CaCrO 3 shows an unusually high transition-metal valence, Cr 4+ , which may lead to a small or even negative charge-transfer gap [13,14], i.e., holes in the O band. In CrO 2 with ruti...

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron scattering experiments. There is no indication for static magnetic ordering but inelastic correlations appear at the incommensurate wave vector (0.5 ± δ, 0.5 ∓ δ, 0) with δ ∼0.07 slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.PACS numbers: 74.25. Ha,74.25.Jb,78.70.Nx,75.10.Lp Superconductivity in the FeAs-based materials [1] appears to be closely related to magnetism as the superconducting state emerges out of an antiferromagnetic phase by doping [1][2][3][4] or by application of pressure [5]. The only FeAs-based exception to this behavior has been found in LiFeAs, which is an ambient-pressure superconductor with a high T C of ∼17 K without any doping [6][7][8]. LiFeAs exhibits the same FeAs layers as the other materials but FeAs 4 tetrahedrons are quite distorted [8] suggesting a different occupation of orbital bands. Indeed ARPES studies on LiFeAs find an electronic band structure different from that in LaOFeAs or BaFe 2 As 2 type compounds [9]. The Fermi-surface nesting, which is proposed to drive the spin-density wave (SDW) order in the other FeAs parent compounds, is absent in LiFeAs [9] suggesting that this magnetic instability is less relevant. The main cause for the suppression of the nesting consists in the hole pocket around the zone center which is shallow in LiFeAs [10]. In consequence, there is more density of states near the Fermi level which might favor a ferromagnetic instability. Using a three-band model Brydon et al.[10] find this ferromagnetic instability to dominate and discuss the implication for the superconducting order parameter proposing LiFeAs to be a spin-triplet superconductor with odd symmetry. However, other theoretical analyzes of the electronic band-structure still find an antiferromagnetic instability which more closely resembles those observed in the other FeAs-based materials [11].Inelastic neutron scattering (INS) experiments revealed magnetic order and magnetic excitations in many FeAs-based families [2,[12][13][14]. Strong magnetic correlations persist far beyond the ordered state, and, most importantly, the opening of the superconducting gap results in a pronounced redistribution of spectral weight [13][14][15], which is frequently interpreted in terms of a resonance mode. Recently a powder INS experiment on superconducting LiFeAs reported magnetic excitations to be rather similar to those observed in the previously studied materials [16] but with a spin gap even in the normal-conducting phase. Magnetic excitations observed in a recent single-crystal INS study on nonsuperconducting Li deficient Li 1−x FeAs (x∼0.06) were described by spin-waves associated with commensurate antiferromagnetism, again with a large temperature independent spin gap of 13 meV [17]. We have performed INS experiments on superconducting sing...