Crystal Structures, Superconducting Properties, and the Coloring Problem in ReAlSi and ReGaSi.

The only nonsuperconducting rhenium-silicon binary compound, ReSi1.75, was heavily p-doped with Ga and Al into ReGaSi and ReAlSi in an attempt to evoke superconductivity. They were synthesized and their crystal structures were studied by both X-ray and neutron diffraction. Si and Ga/Al atoms are ordered into alternating layers, which was rationalized with the "coloring problem" study via first-principles calculations. ReGaSi cannot be further p-doped with more Ga, but ReAlSi can be doped with more Al to ReAl1.2Si0.8, in which Si and Al atoms are not ordered but randomly distributed on the same sites. The superconductivity measurements over these compounds demonstrate that the ordered ReAlSi and ReGaSi are not bulk superconductors. However, ReAl1.2Si0.8 becomes bulk superconductor with Tc = ∼3.5 K, which has been confirmed by magnetism, resistivity, and specific heat measurements.

ß-Mn-type Co(8+x)Zn(12-x) as a defect cubic Laves phase: site preferences, magnetism, and electronic structure.

The results of crystallographic analysis, magnetic characterization, and theoretical assessment of ß-Mn-type Co-Zn intermetallics prepared using high-temperature methods are presented. These ß-Mn Co-Zn phases crystallize in the space group P4(1)32 [Pearson symbol cP20; a = 6.3555(7)-6.3220(7)], and their stoichiometry may be expressed as Co(8+x)Zn(12-x) [1.7(2) < x < 2.2(2)]. According to a combination of single-crystal X-ray diffraction, neutron powder diffraction, and scanning electron microscopy, atomic site occupancies establish clear preferences for Co atoms in the 8c sites and Zn atoms in the 12d sites, with all additional Co atoms replacing some Zn atoms, a result that can be rationalized by electronic structure calculations. Magnetic measurements and neutron powder diffraction of an equimolar Co:Zn sample confirm ferromagnetism in this phase with a Curie temperature of ∼420 K. Neutron powder diffraction and electronic structure calculations using the local spin density approximation indicate that the spontaneous magnetization of this phase arises exclusively from local moments at the Co atoms. Inspection of the atomic arrangements of Co(8+x)Zn(12-x) reveals that the ß-Mn aristotype may be derived from an ordered defect, cubic Laves phase (MgCu2-type) structure. Structural optimization procedures using the Vienna ab initio simulation package (VASP) and starting from the undistorted, defect Laves phase structure achieved energy minimization at the observed ß-Mn structure type, a result that offers greater insight into the ß-Mn structure type and establishes a closer relationship with the corresponding α-Mn structure (cI58).

NiSi: A New Venue for Antiferromagnetic Spintronics.

Envisaging antiferromagnetic spintronics pivots on two key criteria of high transition temperature and tuning of underlying magnetic order using straightforward application of magnetic field or electric current. Here, it is shown that NiSi metal can provide suitable new platform in this quest. First, the study unveils high-temperature antiferromagnetism in single-crystal NiSi with Néel temperature, TN ⩾ 700 K. Antiferromagnetic order in NiSi is accompanied by non-centrosymmetric magnetic character with small ferromagnetic component in the a-c plane. Second, it is found that NiSi manifests distinct magnetic and electronic hysteresis responses to field applications due to the disparity in two moment directions. While magnetic hysteresis is characterized by one-step switching between ferromagnetic states of uncompensated moment, electronic behavior is ascribed to metamagnetic switching phenomena between non-collinear spin configurations. Importantly, the switching behaviors persist to high temperature. The properties underscore the importance of NiSi in the pursuit of antiferromagnetic spintronics.

Magnetic structures and excitations in sawtooth olivine chalcogenides Mn2SiX4 (X = S, Se).

The Mn lattice in olivine chalcogenide Mn2SiX4 (X = S, Se) compounds forms a sawtooth, which is of special interest in magnetism owing to the possibility of realizing flat bands in magnon spectra, a key component in magnonics. In this work, we investigate the Mn2SiX4 olivines using magnetic susceptibility, and X-ray and neutron diffraction. We have determined the average and local crystal structures of Mn2SiS4 and Mn2SiSe4 using synchrotron X-ray, neutron diffraction, and X-ray total scattering data followed by Rietveld and pair distribution function analyses. It is found from the pair distribution function analysis that the Mn triangle that constitutes the sawtooth is isosceles in Mn2SiS4 and Mn2SiSe4. The temperature evolution of magnetic susceptibility of Mn2SiS4 and Mn2SiSe4 shows anomalies below 83 K and 70 K, respectively, associated with magnetic ordering. From the neutron powder diffraction measurements the magnetic space groups of Mn2SiS4 and Mn2SiSe4 are found to be Pnma and Pnm'a', respectively. We find that the Mn spins adopt a ferromagnetic alignment on the sawtooth in both Mn2SiS4 and Mn2SiSe4 but along different crystallographic directions for the S and the Se compounds. From the temperature evolution of Mn magnetic moments obtained from refining neutron diffraction data, the transition temperatures are accurately determined as TN(S) = 83(2) K and TN(Se) = 70.0(5) K. Broad diffuse magnetic peaks are observed in both the compounds, and are prominently seen close to TN, suggesting the presence of a short-range magnetic order. The magnetic excitations studied using inelastic neutron scattering reveal a magnon excitation with an energy corresponding to approximately 4.5 meV in both S and Se compounds. Spin correlations are observed to persist up to 125 K much above the ordering temperature and we suggest the possibility of short-range spin correlations responsible for this.

Designing Magnetism in High Entropy Oxides.

In magnetic systems, spin and exchange disorder can provide access to quantum criticality, frustration, and spin dynamics, but broad tunability of these responses and a deeper understanding of strong limit disorder are lacking. Here, it is demonstrated that high entropy oxides present a previously unexplored route to designing materials in which the presence of strong local compositional disorder may be exploited to generate tunable magnetic behaviors-from macroscopically ordered states to frustration-driven dynamic spin interactions. Single-crystal La(Cr0.2 Mn0.2 Fe0.2 Co0.2 Ni0.2 )O3 films are used as a model system hosting a magnetic sublattice with a high degree of microstate disorder in the form of site-to-site spin and exchange type inhomogeneity. A classical Heisenberg model simplified to represent the highest probability microstates well describes how compositionally disordered systems can paradoxically host magnetic uniformity and demonstrates a path toward continuous control over ordering types and critical temperatures. Model-predicted materials are synthesized and found to possess an incipient quantum critical point when magnetic ordering types are designed to be in direct competition, this leads to highly controllable exchange bias behaviors previously accessible only in intentionally designed bilayer heterojunctions.

Magnetic structure, excitations and short-range order in honeycomb Na2Ni2TeO6.

Na2Ni2TeO6has a layered hexagonal structure with a honeycomb lattice constituted by Ni2+and a chiral charge distribution of Na+that resides between the Ni layers. In the present work, the antiferromagnetic (AFM) transition temperature of Na2Ni2TeO6is confirmed atTN≈ 27 K, and further, it is found to be robust up to 8 T magnetic field and 1.2 GPa external pressure; and, without any frequency-dependence. Slight deviations from nominal Na-content (up to 5%) does not seem to influence the magnetic transition temperature,TN. Isothermal magnetization curves remain almost linear up to 13 T. Our analysis of neutron diffraction data shows that the magnetic structure of Na2Ni2TeO6is faithfully described by a model consisting of two phases described by the commensurate wave vectorsk→c,0.500and0.500.5, with an additional short-range order component incorporated in to the latter phase. Consequently, a zig-zag long-range ordered magnetic phase of Ni2+results in the compound, mixed with a short-range ordered phase, which is supported by our specific heat data. Theoretical computations based on density functional theory predict predominantly in-plane magnetic exchange interactions that conform to aJ1-J2-J3model with a strongJ3term. The computationally predicted parameters lead to a reliable estimate forTNand the experimentally observed zig-zag magnetic structure. A spin wave excitation in Na2Ni2TeO6atE≈ 5 meV atT= 5 K is mapped out through inelastic neutron scattering experiments, which is reproduced by linear spin wave theory calculations using theJvalues from our computations. Our specific heat data and inelastic neutron scattering data strongly indicate the presence of short-range spin correlations, atT>TN, stemming from incipient AFM clusters.

Development and Validation of a Practical Instrument for Injury Prevention: The Occupational Safety and Health Monitoring and Assessment Tool (OSH-MAT).

BACKGROUND: The Occupational Safety and Health Monitoring and Assessment Tool (OSH-MAT) is a practical instrument that is currently used in the German woodworking and metalworking industries to monitor safety conditions at workplaces. The 12-item scoring system has three subscales rating technical, organizational, and personnel-related conditions in a company. Each item has a rating value ranging from 1 to 9, with higher values indicating higher standard of safety conditions. METHODS: The reliability of this instrument was evaluated in a cross-sectional survey among 128 companies and its validity among 30,514 companies. The inter-rater reliability of the instrument was examined independently and simultaneously by two well-trained safety engineers. Agreement between the double ratings was quantified by the intraclass correlation coefficient and absolute agreement of the rating values. The content validity of the OSH-MAT was evaluated by quantifying the association between OSH-MAT values and 5-year average injury rates by Poisson regression analysis adjusted for the size of the companies and industrial sectors. The construct validity of OSH-MAT was examined by principle component factor analysis. RESULTS: Our analysis indicated good to very good inter-rater reliability (intraclass correlation coefficient = 0.64-0.74) of OSH-MAT values with an absolute agreement of between 72% and 81%. Factor analysis identified three component subscales that met exactly the structure theory of this instrument. The Poisson regression analysis demonstrated a statistically significant exposure-response relationship between OSH-MAT values and the 5-year average injury rates. CONCLUSION: These analyses indicate that OSH-MAT is a valid and reliable instrument that can be used effectively to monitor safety conditions at workplaces.

Quantum Magnetic Properties in Perovskite with Anderson Localized Artificial Spin-1/2.

Quantum magnetic properties in a geometrically frustrated lattice of spin-1/2 magnet, such as quantum spin liquid or solid and the associated spin fractionalization, are considered key in developing a new phase of matter. The feasibility of observing the quantum magnetic properties, usually found in geometrically frustrated lattice of spin-1/2 magnet, in a perovskite material with controlled disorder is demonstrated. It is found that the controlled chemical disorder, due to the chemical substitution of Ru ions by Co-ions, in a simple perovskite CaRuO3 creates a random prototype configuration of artificial spin-1/2 that forms dimer pairs between the nearest and further away ions. The localization of the Co impurity in the Ru matrix is analyzed using the Anderson localization formulation. The dimers of artificial spin-1/2, due to the localization of Co impurities, exhibit singlet-to-triplet excitation at low temperature without any ordered spin correlation. The localized gapped excitation evolves into a gapless quasi-continuum as dimer pairs break and create freely fluctuating fractionalized spins at high temperature. Together, these properties hint at a new quantum magnetic state with strong resemblance to the resonance valence bond system.