Antiferromagnetic spintronics is a rapidly growing field, which actively introduces new principles of magnetic storage. Despite that, most applications have been suggested for collinear ...antiferromagnets. In this study, we consider an alternative mechanism based on long-range helical order, which allows for direct manipulation of the helicity vector. As the helicity of long-range homogeneous spirals is typically fixed by the Dzyaloshinskii-Moriya interactions, bi-stable spirals (left- and right-handed) are rare. Here, we report a non-collinear room-temperature antiferromagnet in the tetragonal Heusler group. Neutron diffraction reveals a long-period helix propagating along its tetragonal axis. Ab-initio analysis suggests its pure exchange origin and explains its helical character resulting from a large basal plane magnetocrystalline anisotropy. The actual energy barrier between the left- and right-handed spirals is relatively small and might be easily overcome by magnetic pulse, suggesting Pt2MnGa as a potential candidate for non-volatile magnetic memory.
Superspace symmetry has been for many years the standard approach for the analysis of non-magnetic modulated crystals because of its robust and efficient treatment of the structural constraints ...present in incommensurate phases. For incommensurate magnetic phases, this generalized symmetry formalism can play a similar role. In this context we review from a practical viewpoint the superspace formalism particularized to magnetic incommensurate phases. We analyse in detail the relation between the description using superspace symmetry and the representation method. Important general rules on the symmetry of magnetic incommensurate modulations with a single propagation vector are derived. The power and efficiency of the method is illustrated with various examples, including some multiferroic materials. We show that the concept of superspace symmetry provides a simple, efficient and systematic way to characterize the symmetry and rationalize the structural and physical properties of incommensurate magnetic materials. This is especially relevant when the properties of incommensurate multiferroics are investigated.
We have studied the structural properties of industrial Zirconia Toughened Alumina (ZTA) composites for different compositions of zirconia and yttria at the main stages of the process. Influences on ...phases stability, lattice parameters, homogeneous and non-homogeneous strains, particles and crystallites sizes, have been considered and discussed. The structural states are explained from a very delicate balance of three competing effects: yttria-doping, strain and size effect. The first and third one have been shown to stabilize the tetragonal phase relatively to the monoclinic phase, and to destabilize the tetragonal phase relatively to the cubic phase whereas the second one is doing the opposite. In addition to this competition, the most prominent local effects have also being considered to fully understand the microscopic situation: i.e. the micro-strains and the inhomogeneous distribution of yttria which both increases the metastable coexistence of phase. At high yttria content a core–shell structure of the grains is evidenced.
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The layer-like crystal structure of franckeite from the mine of San Jose, Bolivia, exhibits a pronounced one-dimensional transversal wave-like modulation and a noncommensurate layer match in two ...dimensions. It consists of alternating pseudohexagonal (H) layers and pseudotetragonal (Q) slabs and forms a homologous pair with cylindrite, which has thinner Q slabs. The Q slabs in franckeite are four atomic layers thick. The two components have their own lattices and a common modulation. The Q slab of the refined franckeite structure, Pb21.74Sn9.34Fe3.95Sb8.08S56.87, is an MS layer (M = Pb2+, Sn2+, Sb3+) four atomic planes thick, with a = 5.805(8), b = 5.856(16) Å, and the layer-stacking vector c = 17.338(5) Å. The lattice angles are α = 94.97(2)°, β = 88.45(2)°, γ = 89.94(2)°; the modulation vector q = -0.00129(8) aw + 0.128436(10) bw - 0.0299(3) cw. The H layer is a single-octahedron MS2 layer (M = Sn4+, Fe2+) with a = 3.665(8), b = 6.2575(16), c = 17.419(5) ̊, α = 95.25(2)°, 2B = 95.45(2)°, γ = 89.97(2)°; the modulation vector is q = -0.00087(8) aw + 0.13725(16) bw - 0.0314(4) cw. The a and b vectors of both subsystems are parallel; the c vectors diverge. (3 + 2)D superspace refinement was performed in the superspace group CT, using 7397 observed reflections. It resulted in the overall R(obs) value equal to 0.094. The Q slabs are composed of two tightly bonded double layers, separated by an interspace hosting nonbonding electron pairs. Average composition of cations on the outer surface was refined as Pb0.74(Sn,Sb)0.26, whereas that of cations, which are adjacent to the interspace with lone electron pairs, with a configuration analogous to that observed in orthorhombic SnS, corresponds to (Sn,Sb)0.73Pb0.27. Iron is dispersed over the octahedral Sn4+ sites in the H layer. Transversal modulation of the Q slab is achieved by local variations in the Pb:(Sn,Sb) ratios at its surface and interior. Its purpose is to reestablish a one-dimensional commensurate contact along 010 between the curved Q and H surfaces to the greatest extent possible. Layer-stacking disorder and divergence of the Q and H stacking directions and the divergence between modulation wave-front and these stacking directions are typical for the composite structures of franckeite and cylindrite. Because of the increased rigidity of the Q component, franckeite usually forms masses of curved crystals rather than cylindrical aggregates. The existence of this family depends critically on the radius ratios of the cations involved, especially those involving (Pb2+, Sn2+) and Sn4+. Their replacement by a Pb2+:Bi3+ combination leads to misfit layer structures of a very different type, typified by cannizzarite.
Abstract The kagome lattice is an intriguing and rich platform for discovering, tuning and understanding the diverse phases of quantum matter, crucial for advancing modern and future electronics. ...Despite considerable efforts, accessing correlated phases at room temperature has been challenging. Using single-crystal X-ray diffraction, we discovered charge order above room temperature in La(Ru 1− x Fe x ) 3 Si 2 ( x = 0, 0.01, 0.05), where charge order related to out-of-plane Ru atom displacements appears below T CO,I ≃ 400 K. The secondary charge ordered phase emerges below T CO,II ≃ 80–170 K. Furthermore, first principles calculations reveal both the kagome flat band and the van Hove point near the Fermi energy in LaRu 3 Si 2 , driven by Ru- d z 2 orbitals. Our results identify LaRu 3 Si 2 as the kagome superconductor with the highest known charge ordering temperature, offering a promising avenue for researching room temperature quantum phases and developing related technologies.
The modulated crystal structure and modulated magnetic ordering of the multiferroic CaCuxMn7−xO12 is studied by analysing neutron and synchrotron‐radiation (SR) powder diffraction data with a model ...based on the magnetic superspace group R31′(00γ)ts. Both atomic position modulations and magnetic modulations are described with the modulation vector (0, 0, q). The magnetic ordering is a screw‐type circular helix where the magnetic moments are perpendicular to the c direction. The temperature dependence of the modulation vector length and the ordered magnetic moments of Mn3+ and Mn4+ ions is given between T = 50 K and the Néel temperature TN≃ 90 K. The atomic position modulation length Lp and the magnetic modulation length Lm fulfil the relation Lm = 2Lp at all temperatures between 50 K and TN.
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The synthetic tin-selenium member of the cylindrite structural family, with the empirical formula Sn31.52Sb6.23Fe3.12S59.12 based on electron-microprobe data, has a triclinic crystal structure ...composed of two alternating layer types, both with a pronounced one-dimensional modulation, and with a non-commensurate layer match in two dimensions. The pseudotetragonal (Q) layer is a MeSe layer two-atomic planes thick with lattice parameters a = 5.969(2) Å, b = 6.004(1) Å, and the layer-stacking vector c = 12.238(1) Å, α = 87.98(4)°, β = 83.14(3)°, and γ = 90.01(4)°. The pseudohexagonal (H) layer is a single-octahedral MeSe2 layer with a = 3.831(1) Å, b = 6.580(3) Å, c = 12.151(5) Å, α = 87.79(4)°, β = 90.59(3)°, and γ = 89.99(3)°; the a and b vectors of the two subsystems are parallel, the c vectors diverge. The transversal wave-like modulation has the wave-normal parallel to b, so that the modulation vector q is 0.0001(3) a* +0.1921(4) b* -0.0119(3) c* in terms of the pseudohexagonal subsystem. Superspace structure refinement in the superspace group X1 where X stands for non-primitive centering vectors (1/2,1/2,0,0,0), (0,0,0,0,1/2), (1/2,1/2,0,0,1/2) in five-dimensional superspace, and based on 2128 observed reflections, resulted in R1 = 0.038 for all reflections. Composition of the H layer has been modeled as Sn2404+ Fe542+ Se588, that of the Q layer as Sn3062+ Sb1083+ Se414. The cation-anion distances in the Q layer vary between 2.63 and 3.30 Å, indicating that the cations present are primarily Sn2+ (and Sb3+), whereas distances in the H layer lie between 2.665 and 2.721 Å and correspond to Sn4+ with admixture of Fe2+. The shortest cation-anion distance across the interlayer space is 3.24 Å. Relations between layer match and the modulation vector, divergence of layer stackings of the two components, and reasons for the modulation and for the pronounced disorder of the Q component, as well as the differences and similarities with levyclaudite, franckeite, and synthetic layer-misfit compounds are discussed in detail. In its structural principles, although not in numerical values, the Sn-Se cylindrite corresponds fully to the natural Pb-Sn-S cylindrite previously described.
Monoclinic and tetragonal zirconia samples were characterized by X‐ray diffraction, pycnometry, thermogravimetric analysis (TGA), Fourier transform (FT) IR and mass (MS) spectroscopies, and scanning ...and transmission electron (TEM) microscopies. The results show, for the particular case of a tetragonal zirconia sample, an X‐ray‐undetected subproduct identified as an amorphous organic phase by FTIR–ATR (attenuated total reflection) and TGA–MS. The observations by TEM allowed this amorphous phase to be localized on the surface as a shell coating the nanoparticles. Moreover, this amorphous phase was quantified by Rietveld refinement via the addition of an internal silicon standard. Because zirconia and silicon have different linear absorption coefficients, the microabsorption effect was minimized by using small particle sizes. The amorphous phase was calculated to constitute 11.4 (30)% of the initial mass before Brindley correction and 10.6 (30)% of the initial mass after Brindley correction. The closeness of these values shows that the contribution of the Brindley correction can be neglected if precautions are taken on the microabsorption effect. This work has also highlighted the importance of thoroughly characterizing commercial products, which are not necessarily pure. Indeed, the presence of impurities could become a non‐negligible parameter for physical and chemical properties studies related to commercial materials.
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The refinement of high pressure single-crystal data is difficult as the intensities of the reflections from the sample can be falsified due to effects originating from the components of the diamond ...anvil cell. A series of procedures were developed and implemented into Jana2006 to allow for an easy identification and handling of the outlying intensities in the structure determination process. The use of these procedures is described on the basis of a working example.
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