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Spinel oxides are an ideal setting to explore the interplay between configurational entropy, site selectivity, and magnetism in high entropy oxides. In this work we characterize the magnetic properties of the spinel (Cr,Mn,Fe,Co,Ni)$_3$O$_4$ and study the evolution of its magnetism as a function of non-magnetic gallium substitution. Across the range of compositions studied here, from 0% to 40% Ga, magnetic susceptibility and powder neutron diffraction measurements show that ferrimagnetic order is robust in the spinel HEO. However, we also find that the ferrimagnetic order is highly tunable, with the ordering temperature, saturated and sublattice moments, and magnetic hardness all varying significantly as a function of Ga concentration. Through x-ray absorption and magnetic circular dichroism, we are able to correlate this magnetic tunability with strong site selectivity between the various cations and the tetrahedral and octahedral sites in the spinel structure. In particular, we find that while Ni and Cr are largely unaffected by the substitution with Ga, the occupancies of Mn, Co, and Fe are each significantly redistributed. Ga substitution also requires an overall reduction in the transition metal valence, and this is entirely accommodated by Mn. Finally, we show that while site selectivity has an overall suppressing effect on the configurational entropy, over a certain range of compositions, Ga substitution yields a striking increase in the configurational entropy and may confer additional stabilization. Spinel oxides can be tuned seamlessly from the low-entropy to the high-entropy regime, making this an ideal platform for entropy engineering., 11 pages, 5 figures, 1 tables

We report the high-pressure synthesis and a comprehensive study of NaCo2(SeO3)2(OH) sawtooth chain structure using bulk magnetic properties and neutron scattering.

Magnetic polar materials feature an astonishing range of physical properties, such as magnetoelectric coupling, chiral spin textures, and related new spin topology physics. This is primarily attributable to their lack of space inversion symmetry in conjunction with unpaired electrons, potentially facilitating an asymmetric Dzyaloshinskii-Moriya (DM) exchange interaction supported by spin-orbital and electron-lattice coupling. However, engineering the appropriate ensemble of coupled degrees of freedom necessary for enhanced DM exchange has remained elusive for polar magnets. Here, we study how spin and orbital components influence the capability of promoting the magnetic interaction by studying two magnetic polar materials, α-Cu(IO3)2 (2D) and Mn(IO3)2 (6S), and connecting their electronic and magnetic properties with their structures. The chemically controlled low-temperature synthesis of these complexes resulted in pure polycrystalline samples, providing a viable pathway to prepare bulk forms of transition-metal iodates. Rietveld refinements of the powder synchrotron X-ray diffraction data reveal that these materials exhibit different crystal structures but crystallize in the same polar and chiral P21 space group, giving rise to an electric polarization along the b-axis direction. The presence and absence of an evident phase transition to a possible topologically distinct state observed in α-Cu(IO3)2 and Mn(IO3)2, respectively, imply the important role of spin-orbit coupling. Neutron diffraction experiments reveal helpful insights into the magnetic ground state of these materials. While the long-wavelength incommensurability of α-Cu(IO3)2 is in harmony with sizable asymmetric DM interaction and low dimensionality of the electronic structure, the commensurate stripe AFM ground state of Mn(IO3)2 is attributed to negligible DM exchange and isotropic orbital overlapping. The work demonstrates connections between combined spin and orbital effects, magnetic coupling dimensionality, and DM exchange, providing a worthwhile approach for tuning asymmetric interaction, which promotes evolution of topologically distinct spin phases.

Competition among exchange interactions is able to induce novel spin correlations on a bipartite lattice without geometrical frustration. A prototype example is the spiral spin liquid, which is a correlated paramagnetic state characterized by sub-dimensional degenerate propagation vectors. Here, using spectral graph theory, we show that spiral spin liquids on a bipartite lattice can be approximated by a further-neighbor model on the corresponding line-graph lattice that is non-bipartite, thus broadening the space of candidate materials that may support the spiral spin liquid phases. As illustrations, we examine neutron scattering experiments performed on two spinel compounds, ZnCr$_2$Se$_4$ and CuInCr$_4$Se$_8$, to demonstrate the feasibility of this new approach and expose its possible limitations in experimental realizations., Comment: 20 pages, 18 figures

Magnetic skyrmions, which are topologically distinct magnetic spin textures, are gaining increased attention for their unique physical properties and potential applications in spintronic devices. Here we present a design strategy for skyrmion host candidates based on combinations of magnetic spin, asymmetric building units having stereo-active lone-pair electrons, and polar lattice symmetry. To demonstrate the viability of the proposed rational design principles, we successfully synthesized Fe(IO3)3 polycrystalline sample and single crystals by using a new simplified low-temperature pathway, which is experimentally feasible for extending materials growth of transition metal iodates. Single crystal X-ray and powder synchrotron X-ray diffraction measurements demonstrated that Fe(IO3)3 crystallizes in the polar chiral hexagonal lattice with space group P63. The combined structural features of the macroscopic electric polarization along the c axis stemming from the coalignment of the stereo-active lone-pairs of the IO3 trigonal pyramid and the magnetic Fe3+cation residing on the three-fold rotation axis were selected to promote asymmetric exchange coupling. We find evidence of a predicted skyrmion phase at 14 K to 16 K and 2.5 T to 3.2 T driven by Dzyaloshinskii Moriya (DM) interaction, a conclusion supported by the appreciable DM exchange and the zero-field spiral antiferromagnetic ground state of Fe(IO3)3 deduced from neutron diffraction experiments. The associated magnetic modulation wavelength of the putative skyrmions is expected to be short, approximately 18 nm, comparable to the period of the DM-driven incommensurate order. This work links stereo-active lone-pair electron effects to enhanced DM interaction, demonstrating a new approach for chemical guidelines in the search for skyrmionic states of matter.

Magnetic polar materials feature an astonishing range of physical properties, such as magnetoelectric coupling, chiral spin textures, and related new spin topology physics. This is primarily attributable to their lack of space inversion symmetry in conjunction with unpaired electrons, potentially facilitating an asymmetric Dzyaloshinskii-Moriya (DM) exchange interaction supported by spin-orbital and electron-lattice coupling. However, engineering the appropriate ensemble of coupled degrees of freedom necessary for enhanced DM exchange has remained elusive for polar magnets. Here, we study how spin and orbital components influence the capability of promoting the magnetic interaction by studying two magnetic polar materials, α-Cu(IO

Super Invar (SIV), i.e., zero thermal expansion of metallic materials underpinned by magnetic ordering, is of great practical merit for a wide range of high precision engineering. However, the relatively narrow temperature window of SIV in most materials restricts its potential applications in many critical fields. Here, we demonstrate the controlled design of thermal expansion in a family of ${R}_{2}{(\mathrm{Fe},\mathrm{Co})}_{17}$ materials ($R=\text{rare}$ Earth). We find that adjusting the Fe-Co content tunes the thermal expansion behavior and its optimization leads to a record-wide SIV with good cyclic stability from 3--461 K, almost twice the range of currently known SIV. In situ neutron diffraction, M\"ossbauer spectra and first-principles calculations reveal the $3d$ bonding state transition of the Fe-sublattice favors extra lattice stress upon magnetic ordering. On the other hand, Co content induces a dramatic enhancement of the internal molecular field, which can be manipulated to achieve ``ultrawide'' SIV over broad temperature, composition and magnetic field windows. These findings pave the way for exploiting thermal-expansion-control engineering and related functional materials.

The (Li(1–x)Fe(x)OH)FeSe superconductor has been suspected of exhibiting long-range magnetic ordering due to Fe substitution in the LiOH layer. However, no direct observation such as magnetic reflection from neutron diffraction has been reported. Here, we use a chemical design strategy to manipulate the doping level of transition metals in the LiOH layer to tune the magnetic properties of the (Li(1–x–y)Fe(x)Mn(y)OD)FeSe system. We find Mn doping exclusively replaces Li in the hydroxide layer resulting in enhanced magnetization in the (Li(0.876)Fe(0.062)Mn(0.062)OD)FeSe superconductor without significantly altering the superconducting behavior as resolved by magnetic susceptibility and electrical/thermal transport measurements. As a result, long-range magnetic ordering was observed below 12 K with neutron diffraction measurements. This work has implications for the design of magnetic superconductors for the fundamental understanding of superconductivity and magnetism in the iron chalcogenide system as well as exploitation as functional materials for next-generation devices.

The oxygen storage capacity, structure and thermodynamic stability were studied for hexagonal Y0.7Tb0.3MnO3+δ in oxygen and air to assess its applicability for oxygen separation from air by a temperature-swing adsorption process. We show that large amounts of oxygen excess can be reversibly incorporated into and extracted from small particle-size samples of Y0.7Tb0.3MnO3+δ prepared by sol–gel synthesis for fixed oxygen partial pressures. The hyperstoichiometric material, with δ ≥ 0.45 prepared in oxygen or high-pressure oxygen atmospheres, assumes a new hexagonal structure with interstitial oxygen defects near the nominally five coordinated Mn site as determined by neutron diffraction and supported by scanning and transmission electron microscopy. Thermogravimetric measurements demonstrate reversible intercalation of oxygen in a pure O2 atmosphere at around 300 °C, but more importantly, also in air over a remarkably narrow temperature range of ∼20 °C, albeit producing smaller oxygen amounts δ ≤ 0.25. Comparison of samples' properties obtained by the sol–gel and solid-state synthesis methods confirms enhanced oxygen storage capacity and oxygen exchange kinetics for the small particle-size samples which exhibit larger specific surface areas. Sequential temperature-swing absorption and long-term annealing experiments demonstrate the high practical potential of Y0.7Tb0.3MnO3 compounds for the industrial production of oxygen enriched gases by utilization of waste heat at 250–350 °C.

CuAl2O4 is a ternary oxide spinel with Cu2+ ions ($$s=1/2$$ s = 1 / 2 ) primarily populating the A-site diamond sublattice. The compound is reported to display evidence of spin glass behavior but possess a non-frozen magnetic ground state below the transition temperature. On the other hand, the spinel CuGa2O4 displays spin glass behavior at ~ 2.5 K with Cu2+ ions more readily tending to the B-site pyrochlore sublattice. Therefore, we investigate the magnetic and structural properties of the solid solution CuAl2(1-x)Ga2xO4 examining the evolution of the magnetic behavior as Al3+ is replaced with a much larger Ga3+ ion. Our results show that the Cu2+ ions tend to migrate from tetrahedral to octahedral sites as the Ga3+ ion concentration increases, resulting in a concomitant change in the glassy magnetic properties of the solution. Results indicate glassy behavior for much of the solution with a general trend towards decreasing magnetic frustration as the Cu2+ ion shifts to the B-site. However, the $$x=0.1$$ x = 0.1 and 0.2 members of the system do not show glassy behavior down to our measurement limit (1.9 K) suggesting a delayed spin glass transition. We suggest that these two members are additional candidates for investigation to access highly frustrated exotic quantum states.

Super Invar (SIV), i.e., zero thermal expansion of metallic materials underpinned by magnetic ordering, is of great practical merit for a wide range of high precision engineering. However, the relatively narrow temperature window of SIV in most materials restricts its potential applications in many critical fields. Here, we demonstrate the controlled design of thermal expansion in a family of R_{2}(Fe,Co)_{17} materials (R=rare Earth). We find that adjusting the Fe-Co content tunes the thermal expansion behavior and its optimization leads to a record-wide SIV with good cyclic stability from 3-461 K, almost twice the range of currently known SIV. In situ neutron diffraction, Mössbauer spectra and first-principles calculations reveal the 3d bonding state transition of the Fe-sublattice favors extra lattice stress upon magnetic ordering. On the other hand, Co content induces a dramatic enhancement of the internal molecular field, which can be manipulated to achieve "ultrawide" SIV over broad temperature, composition and magnetic field windows. These findings pave the way for exploiting thermal-expansion-control engineering and related functional materials.

We report the synthesis and physical properties studies of quasi-one-dimensional (quais-1D) iron chalcogenide ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{4}$ which shares the ${\mathrm{FeSe}}_{4}$ tetrahedra building motif commonly seen in the iron chalcogenide superconductors. A high-quality polycrystalline sample was achieved by solid-state reaction method and characterized by x-ray diffraction, electrical resistivity, magnetic susceptibility, and neutron diffraction measurements. ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{4}$ is a narrow gap semiconductor that magnetically orders at $\ensuremath{\sim}310$ K. Both neutron powder diffraction results and isothermal $M\ensuremath{-}H$ loops suggest a canted antiferromagnetic structure where Fe sublattices are antiferromagnetically ordered along the $c$-axis quasi-1D chain direction, resulting in a net ferromagnetic moment in the perpendicular direction along the $a$ axis with tilted angle of $18.{7}^{\ensuremath{\circ}}$ towards the $b$ axis.

Recently $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ was predicted to be a magnetic Weyl semimetal with a lone pair of Weyl nodes generated by $A$-type antiferromagnetism and protected by a rotational symmetry. However, it was soon discovered that the actual magnetic structure broke the rotational symmetry and internal pressure was later suggested as a route to stabilize the desired magnetic state. In this work we test this prediction by synthesizing a series of ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ single crystals and studying their structural, magnetic, and transport properties via both experimental techniques and first-principles calculations. We find that small concentrations of Ba ($\ensuremath{\sim}3%--10%$) lead to a small out-of-plane canting of the Eu moment. However, for higher concentrations this effect is suppressed and a nearly in-plane model is recovered. Studying the transport properties we find that all compositions show evidence of an anomalous Hall effect dominated by the intrinsic mechanism as well as large negative magnetoresistances in the longitudinal channel. A nonmonotonic evolution of the transport properties is seen across the series which correlates to the proposed canting suggesting canting may enhance the topological effects. Careful density functional theory calculations using an all-electron approach revise prior predictions finding a purely ferromagnetic ground state with in-plane moments for both the $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{Eu}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ compounds, corroborating our experimental findings. This work suggests that Ba substitution can tune the magnetic properties in unexpected ways which correlate to changes in measures of topological properties, encouraging future work to locate the ideal Ba concentration for Eu moment canting.

The I-Mn-V antiferromagnet, NaMnBi, develops a very large positive magnetoresistance (MR) up to 10,000% at 2 K and 9 T in crystals showing a semiconductor-to-metal transition (SMT). In the absence of an SMT, a modest (20%) MR is achieved. Here, we show that upon cooling below the magnetic transition, a spatial modulation appears giving rise to new Bragg peaks due to charge and defect ordering in a checkerboard pattern, with two kinds of modulation vectors, $q_1$=($\frac23$, 0, 1) and $q_2$=($\frac23, \frac13, \frac12$). This constitutes a superlattice transition ($T_s$) that lowers the symmetry from the high temperature centrosymmetric P4/nmm to the non-centrosymmetric P$\overline4$m2. In crystals with a large MR, a close to room temperature $T_s$ is observed with $q_1$ appearing first, followed by $q_2$. In crystals with low MR however, $T_s$ is much lower and only $q_1$ is observed. The charge modulation and spin fluctuations may both contribute to the enhancement of MR.

Molecular magnets based on heavy transition metals have recently attracted significant interest in the quest for novel magnetic properties. For systems with an odd number of valence electrons per molecule, high or low molecular spin states are typically expected in the double exchange or quasimolecular orbital limits, respectively. In this work, we use bulk characterization, muon spin relaxation, neutron diffraction, and inelastic neutron scattering to identify a rare intermediate spin-3/2 per dimer state in the 6H-perovskite ${\mathrm{Ba}}_{3}{\mathrm{LaRu}}_{2}{\mathrm{O}}_{9}$ that cannot be understood in a double exchange or quasimolecular orbital picture and instead arises from orbital-selective Mott insulating behavior at the molecular level. Our measurements are also indicative of collinear stripe magnetic order below ${T}_{N}=26(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for these molecular spin-3/2 degrees-of-freedom, which is consistent with expectations for an ideal triangular lattice with significant in-plane next nearest-neighbor exchange. Finally, we present neutron diffraction and Raman scattering data under applied pressure that reveal low-lying structural and spin state transitions at modest pressures $P\phantom{\rule{4pt}{0ex}}\ensuremath{\le}$ 1 GPa, which highlights the delicate balance between competing energy scales in this system.

The $({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{OH})\mathrm{FeSe}$ superconductor has been suspected of exhibiting long-range magnetic ordering due to Fe substitution in the LiOH layer. However, no direct observation such as magnetic reflection from neutron diffraction has been reported. Here, we use a chemical design strategy to manipulate the doping level of transition metals in the LiOH layer to tune the magnetic properties of the $({\mathrm{Li}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Fe}}_{x}{\mathrm{Mn}}_{y}\mathrm{OD})\mathrm{FeSe}$ system. We find Mn doping exclusively replaces Li in the hydroxide layer resulting in enhanced magnetization in the $({\mathrm{Li}}_{0.876}{\mathrm{Fe}}_{0.062}{\mathrm{Mn}}_{0.062}\mathrm{OD})\mathrm{FeSe}$ superconductor without significantly altering the superconducting behavior as resolved by magnetic susceptibility and electrical/thermal transport measurements. As a result, long-range magnetic ordering was observed below 12 K with neutron diffraction measurements. This work has implications for the design of magnetic superconductors for the fundamental understanding of superconductivity and magnetism in the iron chalcogenide system as well as exploitation as functional materials for next-generation devices.

Recently, a local-Ising-type magnetic order was inferred from neutron diffraction of the antiferromagnetic ${\mathrm{Er}}_{2}{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$ (pg-ErGO) with an applied magnetic field. Here, we use neutron spectroscopy to investigate the energetics of pg-ErGO and the isostructural ${\mathrm{Yb}}_{2}{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$ (pg-YbGO) to evaluate the adequacy of the local-Ising description. To begin, we generate a model of the magnetic structure of pg-YbGO using neutron diffraction and find a net ferromagnetic moment. While pg-ErGO and pg-YbGO have highly symmetric crystal structures ($P{4}_{1}{2}_{1}2$ tetragonal space group 92) with only one trivalent rare-earth magnetic site, the point symmetry of the rare-earth site is low with only a single symmetry element (point group ${\mathrm{C}}_{1}$). For both compounds, the energy scale of the first excited state is large compared to the magnetic ordering temperature, suggesting Ising character. The ground-state Kramer's doublet of both compounds is dominated by the maximal ${m}_{\mathrm{J}}$ component. However, the low point group symmetry of the rare-earth site leads to finite mixing of all other ${m}_{\mathrm{J}}$'s, which suggests potential deviations from Ising behavior. Moreover, quasielastic scattering is observed deep in the ordered state of pg-ErGO and pg-YbGO that may be due to non-Ising behavior. The dominant magnetic interaction in both compounds is found to be magnetostatic by considering the magnetic excitations in the ordered state. From consideration of these data, the pg-YbGO is more Ising-like than pg-ErGO. Also, quantum multicritical points are anticipated with applied magnetic field in both compounds.

Cubic spinel GeNi2O4 exhibits intriguing magnetic properties with two successive antiferromagnetic phase transitions (TN1 12.1 and TN2 11.4 K) with the absence of any structural transition. We have performed detailed heat capacity and magnetic measurements in different crystallographic orientations. A new magnetic phase in presence of magnetic field (H > 4 T) along the [111] direction is revealed, which is not observed when the magnetic field is applied along the [100] and [110] directions. High field neutron powder diffraction measurements confirm such a change in magnetic phase, which could be ascribed to a spin reorientation in the presence of magnetic field. A strong magnetic anisotropy and competing magnetic interactions play a crucial role on the complex magnetic behavior in this cubic system., Accepted in PRB

Bulk cobalt ferrite, being a hard ferrite, shows high magnetization, high resistivity and high coercivity. If thin films of cobalt ferrite can be deposited at a low enough temperature and if its coercivity can be reduced, cobalt ferrite will make a very good candidate for use as a magnetic core of an integrated inductor in RF-CMOS ICs. Though polycrystalline and epitaxial thin films of cobalt ferrite have been made by various techniques, there are no reports of thin films of superparamagnetic cobalt ferrite. In this work, nanocrystalline cobalt ferrite thin films, which are superparamagnetic as deposited, have been prepared in the solution medium at ∼190 °C, using microwave irradiation. The as-prepared films have a saturation magnetization (MS) of 401 emu/cc and coercivity (HC) of 19 Oe at room temperature for a crystallite size of 2 nm. The cobalt ferrite powder obtained as a by-product during the same process has MS of 50 emu/g and HC of 5 Oe at room temperature, making it superparamagnetic. The as-prepared films were annealed in air at 300 °C for 5 min and 10 min. Annealing for 10 min results in an increase in crystallite size to 36 nm, MS increases from 401 emu/cc to 545 emu/cc, and HC increases from 19 Oe to 860 Oe. The change in magnetic properties can be directly associated with change in the crystallite size and degree of crystallographic inversion, as determined by neutron diffraction and deduced from X-ray photoelectron spectroscopy.

We report the charge doping of ${\mathrm{KCr}}_{3}{\mathrm{As}}_{3}$ via H intercalation. We show that the previously reported ethanol bath deintercalation of ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ to ${\mathrm{KCr}}_{3}{\mathrm{As}}_{3}$ has a secondary effect whereby H from the bath enters the quasi-one-dimensional ${\mathrm{Cr}}_{6}{\mathrm{As}}_{6}$ chains. Furthermore, we find that---contrary to previous interpretations---the difference between nonsuperconducting as-grown ${\mathrm{KCr}}_{3}{\mathrm{As}}_{3}$ samples and superconducting hydrothermally annealed samples is not a change in crystallinity but due to charge doping, with the latter treatment increasing the H concentration in the CrAs tubes, effectively electron doping the 133 compound. These results suggest another stoichiometry ${\mathrm{KH}}_{x}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$, that superconductivity arises from a suppressed magnetic order via a tunable parameter and paves the way for a charge-doped phase diagram in these materials.

Frustrated magnets provide an exciting playground for the study of exotic magnetic properties. Here, the authors use a combination of neutron scattering and muon spin rotation to determine crystal electric fields, spin anisotropy, and the magnetic structure of Er${}_{3}$Ga${}_{5}$O${}_{12}$, as well as the fingerprints of a dynamical ground state. These findings shows an excellent model system for studying the complexities of metamagnetism.

YFexCr(1-x)O3 (0 ≤ x ≤ 1) compounds prepared by autocombustion were studied with X-ray and neutron powder diffraction, Mossbauer spectroscopy and ab-initio electronic structure calculations to try to understand the interplay of magnetic order and structural parameters as a function of doping in this multiferroic material. All samples exhibit the orthorhombic nuclear structure (space group symmetry Pnma) and the cell parameters vary continuously as a function of doping. In the neutron powder diffraction patterns it was observed that in addition to the Pnma nuclear structure peaks reflections are seen in all samples which arise from magnetic order. These peaks can be indexed using the nuclear unit cell indicating an ordering vector of k = (0,0,0). Using representational analysis and Rietveld refinements the magnetic structure was determined to be Pn’ma’ and the mean magnetic moment per transition metal atom was observed to decrease continuously from Fe to Cr. For compounds with x > 0.25 a weak ferromagnetic contribution is also present which is due to a small canting of the moments out of the basal plane. The Mossbauer spectra corroborate the NPD results with all samples showing single magnetic sextets at 4.2 K. The energy gap and the main direct interband transition of the endmember samples were obtained by optical absorption measurements. These optical data were analyzed with the help of DFT band structure calculation using the GGA-PBE + U approximation.

We present the synthesis and magnetic characterization of a polycrystalline sample of the 6H-perovskite Ba

We study the relationship between antiferromagnetic order and the electronic properties of ${\mathrm{KCuMnS}}_{2}$ with the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure. We propose two magnetic structures for ${\mathrm{KCuMnS}}_{2}$ with the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure. Powder samples of ${\mathrm{KCuMnS}}_{2}$ and ${\mathrm{KLiMnS}}_{2}$ were prepared for structural studies and magnetization measurements. In both compounds, the ${\mathrm{Mn}}^{2+}$ site is alloyed by either ${\mathrm{Cu}}^{+}$ or ${\mathrm{Li}}^{+}$. We also prepared single crystals of ${\mathrm{KCuMnS}}_{2}$ for x-ray and neutron-diffraction studies and resistivity measurements. We relate these properties to the electronic structure calculated with density functional theory. Neutron-diffraction studies reveal that ${\mathrm{KCuMnS}}_{2}$ exhibits long-range magnetic ordering with a N\'eel temperature near 160 K and a moment of 0.92(2) ${\ensuremath{\mu}}_{B}/{\mathrm{Mn}}^{2+}$ at 6 K. In contrast, ${\mathrm{KLiMnS}}_{2}$ never exhibits long-range magnetic ordering down to 3.5 K. Both sulfides never display a crystallographic phase transition from our temperature-dependent x-ray and neutron-diffraction studies. We discuss the magnetic phases in detail and how they relate to isostructural phases such as iron-based superconductors and related chalcogenides. Electrical resistance measurements indicate that while ${\mathrm{KCuMnS}}_{2}$ is semiconducting, there is an anomaly around the N\'eel temperature, which indicates that long-range magnetism influences its electronic structure.

The recent discoveries of proximate quantum spin-liquid compounds and their potential application in quantum computing informs the search for new candidate materials for quantum spin-ice and spin-liquid physics. While the majority of such work has centered on members of the pyrochlore family due to their inherently frustrated linked tetrahedral structure, the rare-earth pyrogermanates also show promise for possible frustrated magnetic behavior. With the familiar stoichiometry ${R}_{2}{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$, these compounds generally have tetragonal symmetry with a rare-earth sublattice built of a spiral of alternating edge and corner-sharing rare-earth site triangles. Studies on ${\mathrm{Dy}}_{2}{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Ho}}_{2}{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$ have shown tunable low temperature antiferromagnetic order, a high frustration index, and spin-ice-like dynamics. Here we use neutron diffraction to study magnetic order in ${\mathrm{Er}}_{2}{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$ (space group $P{4}_{1}{2}_{1}2$) and find the lowest yet Ne\'el temperature in the pyrogermanates of 1.15 K. Using neutron powder diffraction, we find the magnetic structure to order with $k=(0,0,0)$ ordering vector, magnetic space group symmetry $P{4}_{1}^{{}^{\ensuremath{'}}}{2}_{1}{2}^{{}^{\ensuremath{'}}}$, and a refined Er moment of $m=8.1\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ near the expected value for the ${\mathrm{Er}}^{3+}$ free ion. Provocatively, the magnetic structure exhibits similar ``local Ising'' behavior to that seen in the pyrocholres where the Er moment points up or down along the short Er-Er bond. Upon applying a magnetic field, we find a first-order metamagnetic transition at $\ensuremath{\sim}0.35\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ to a lower symmetry $P{2}_{1}^{{}^{\ensuremath{'}}}{2}_{1}^{{}^{\ensuremath{'}}}2$ structure. This magnetic transition involves an inversion of Er moments aligned antiparallel to the applied field describing a class I spin-flip-type transition, indicating a strong local anisotropy at the Er site---reminiscent of that seen in the spin-ice pyrochlores.

Recent reported very high thermal conductivities in the cubic boron arsenide (BAs) and boron phosphide (BP) crystals could potentially provide a revolutionary solution in the thermal management of high power density devices. To fully facilitate such application, compatible coefficient of thermal expansion (CTE) between the heat spreader and device substrate, in order to minimize the thermal stress, need to be considered. Here we report our experimental CTE studies of BAs and BP in the temperature range from 100K to 1150K, through a combination of X-ray single crystal diffraction and neutron powder diffraction. We demonstrated the room temperature CTE, 3.6 $\pm$ 0.15 $\times$ 10E-6 /K for BAs and 3.2 $\pm$ 0.2 $\times$ 10E-6 /K for BP, are more compatible with most of the semiconductors including Si and GaAs, in comparison with diamond, and thus could be better candidates for the future heat spreader materials in power electronic devices., Comment: 13 pages, 3 figures, accepted by applied physics letters

Author(s): Frandsen, BA; Taddei, KM; Bugaris, DE; Stadel, R; Yi, M; Acharya, A; Osborn, R; Rosenkranz, S; Chmaissem, O; Birgeneau, RJ | Abstract: We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system Sr1-xNaxFe2As2. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels (x=0.48, near optimal superconductivity) and high temperatures (above room temperature for x=0, decreasing to 150 K for x=0.48), with a typical length scale of 1-3 nm. This work highlights the ubiquity of nematic fluctuations in a representative iron-based superconductor and provides important details about the evolution of these fluctuations across the phase diagram.

We present density functional theory and neutron total scattering studies on quasi-one-dimensional superconducting ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ revealing a frustrated structural instability. Our first principles calculations find a significant phonon instability, which, under energy minimization, corresponds to a frustrated orthorhombic distortion. In neutron diffraction studies we find large atomic displacement parameters with anomalous temperature dependencies, which result from highly localized orthorhombic distortions of the CrAs sublattice and coupled K displacements. These results suggest a more complex phase diagram than previously assumed for ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ with subtle interplays of structure, electron-phonon, and magnetic interactions.

Sr2Cr3As2O2 is composed of alternating square-lattice CrO2 and Cr2As2 stacking layers, where CrO2 is isostructural to the CuO2 building-block of cuprate high-Tc superconductors and Cr2As2 to Fe2As2 of Fe-based superconductors. Current interest in this material is raised by theoretic prediction of possible superconductivity. In this neutron powder diffraction study, we discovered that magnetic moments of Cr(II) ions in the Cr2As2 sublattice develop a C-type antiferromagnetic structure below 590 K, and the moments of Cr(I) in the CrO2 sublattice form the La2CuO4 -like antiferromagnetic order below 291 K. The staggered magnetic moment 2.19(4){\mu} B /Cr(II) in the more itinerant Cr2As2 layer is smaller than 3.10(6){\mu}_B/Cr(I) in the more localized CrO2 layer. Different from previous expectation, a spin-flop transition of the Cr(II) magnetic order observed at 291 K indicates a strong coupling between the CrO2 and Cr2As2 magnetic subsystems.

The suite of neutron powder diffractometers at Oak Ridge National Laboratory (ORNL) utilizes the distinct characteristics of the Spallation Neutron Source and High Flux Isotope Reactor to enable the measurements of powder samples over an unparalleled regime at a single laboratory. Full refinements over large Q ranges, total scattering methods, fast measurements under changing conditions, and a wide array of sample environments are available. This article provides a brief overview of each powder instrument at ORNL and details the complementarity across the suite. Future directions for the powder suite, including upgrades and new instruments, are also discussed.

We present density functional theory and neutron total scattering studies on quasi-one-dimensional superconducting K_{2}Cr_{3}As_{3} revealing a frustrated structural instability. Our first principles calculations find a significant phonon instability, which, under energy minimization, corresponds to a frustrated orthorhombic distortion. In neutron diffraction studies we find large atomic displacement parameters with anomalous temperature dependencies, which result from highly localized orthorhombic distortions of the CrAs sublattice and coupled K displacements. These results suggest a more complex phase diagram than previously assumed for K_{2}Cr_{3}As_{3} with subtle interplays of structure, electron-phonon, and magnetic interactions.

Quasi-one-dimensional ${A}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ (with $A=\text{K}$, Cs, Rb) is an intriguing new family of superconductors which exhibit many similar features to the cuprate and iron-based unconventional superconductor families. Yet, in contrast to these systems, no charge or magnetic ordering has been observed which could provide the electronic correlations presumed necessary for an unconventional superconducting pairing mechanism---an absence which defies predictions of first-principles models. We report the results of neutron scattering experiments on polycrystalline ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ (${T}_{c}\ensuremath{\sim}7\phantom{\rule{0.28em}{0ex}}\text{K}$) which probed the low-temperature dynamics near ${T}_{c}$. Neutron diffraction data evidence a subtle response of the nuclear lattice to the onset of superconductivity while inelastic scattering reveals a highly dispersive column of intensity at the commensurate wave vector $q=\left(00\frac{1}{2}\right)$ which loses intensity beneath ${T}_{c}$---indicative of short-range magnetic fluctuations. Using linear spin-wave theory, we model the observed scattering and suggest a possible structure to the short-range magnetic order. These observations suggest that ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ is in close proximity to a magnetic instability and that the incipient magnetic order both couples strongly to the lattice and competes with superconductivity, in direct analogy with the iron-based superconductors.

We report on temperature-dependent pair distribution function measurements of Sr_{1-x}Na_{x}Fe_{2}As_{2}, an iron-based superconductor system that contains a magnetic phase with reentrant tetragonal symmetry, known as the magnetic C_{4} phase. Quantitative refinements indicate that the instantaneous local structure in the C_{4} phase comprises fluctuating orthorhombic regions with a length scale of ∼2 nm, despite the tetragonal symmetry of the average static structure. Additionally, local orthorhombic fluctuations exist on a similar length scale at temperatures well into the paramagnetic tetragonal phase. These results highlight the exceptionally large nematic susceptibility of iron-based superconductors and have significant implications for the magnetic C_{4} phase and the neighboring C_{2} and superconducting phases.

Since its discovery in 2014, the magnetic tetragonal ${C}_{4}$ phase has been identified in a growing number of hole-doped 122 Fe-based superconducting compounds. Exhibiting a unique double-Q magnetic structure and a strong competition with both superconducting and magnetic order parameters, the ${C}_{4}$ phase and the conditions of its formation are of significant interest to understanding the fundamental mechanisms in these materials. Particularly, separating the importance of direct changes to the relative size of hole and electron pockets at the Fermi surface (achieved via charge doping) from the role of structural changes due to differences of ionic radii of dopants is useful to determine the underlying parameter which causes the ${C}_{4}$ instability. Here, we report the discovery of the ${C}_{4}$ phase in a fourth member of the hole-doped 122 materials ${\mathrm{Ca}}_{\text{1-x}}{\mathrm{Na}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}\phantom{\rule{4pt}{0ex}}(0.20\ensuremath{\le}x\ensuremath{\le}0.50)$ as determined from neutron and x-ray powder diffraction studies. The maximum of the ${C}_{4}$ dome is observed at $x=0.44$ with a reentrant temperature ${T}_{r}=52$ K and an extent of $\mathrm{\ensuremath{\Delta}}x\ensuremath{\sim}0.07$ in composition. It is observed that for a range of compositions within the ${C}_{4}$ dome $(0.40\ensuremath{\le}x\ensuremath{\le}0.42$), there is a second reentrance $({T}_{{r}_{2}}\phantom{\rule{4pt}{0ex}}l\phantom{\rule{4pt}{0ex}}{T}_{r})$ where the antiferromagnetic ${C}_{2}$ phase is recovered---a feature previously only seen in ${\mathrm{Ba}}_{\text{1-x}}{\mathrm{K}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}$. A phase diagram is presented for ${\mathrm{Ca}}_{\text{1-x}}{\mathrm{Na}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}$ and compared to the other $\mathrm{Na}$-doped 122's---${A}_{\text{1-x}}{\mathrm{Na}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}$ with $A$ = Ba, Sr, and Ca. The structural parameters for these three systems are compared and the importance of the ``chemical pressure'' due to changing the $A$-site ion ($A$ = Ba, Sr, Ca) is discussed.

The authors present a careful phase diagram determination of the hole-doped Sr${}_{1-x}$Na${}_{x}$Fe${}_{2}$As${}_{2}$ iron-based superconductors, with special attention paid to the region around $x$=0.36, where they recently found the existence of a magnetic $C$4 phase. This phase has already proven instrumental in settling longstanding questions in these materials. In contrast to the two previous hole-doped Ba-based materials exhibiting this phase, here they find the $C$4 phase to be more robust, existing over a larger range of compositions. This should prove useful for future studies.

Irradiation with 4 MeV protons was used to systematically introduce defects in single crystals of the iron-arsenide superconductor BaFe2(As1-xPx)2, x = 0.33. The effect of disorder on the low-temperature behavior of the London penetration depth λ(T) and transition temperature Tc was investigated. In nearly optimally doped samples with Tc ~ 29 K, signatures of a superconducting gap with nodes were observed. Contrary to previous reports on electron-irradiated crystals, we do not see a disorder-driven lifting of accidental nodes, and we observe that proton-induced defects are weaker pair breakers than electron-induced defects. Lastly, we attribute our findings to anisotropic electron scattering caused by proton irradiation defects.

We report a study of the ${\mathrm{Ca}}_{0.73}{\mathrm{La}}_{0.27}{\mathrm{FeAs}}_{2}$ single crystals. We unravel a monoclinic to triclinic phase transition at 58 K, and a paramagnetic to stripe antiferromagnetic phase transition at 54 K, below which spins order ${45}^{\ensuremath{\circ}}$ away from the stripe direction. Furthermore, we demonstrate this material is substantially structurally untwinned at ambient pressure with the formation of spin rotation walls (S walls). Finally, in addition to the central-hole and corner-electron Fermi pockets usually appearing in Fe pnictide superconductors, angle-resolved photoemission measurements resolve a fermiology where an extra electron pocket of mainly As chain character exists at the Brillouin zone edge.

Combined neutron and x-ray diffraction experiments demonstrate the formation of a low-temperature minority magnetic tetragonal phase in ${\mathrm{Ba}}_{0.76}{\mathrm{K}}_{0.24}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ in addition to the majority magnetic, orthorhombic phase. The coincident enhancement in the magnetic ($\frac{1}{2} \frac{1}{2}$ 1) peaks shows that this minority phase is of the same type that was observed in ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($0.24\ensuremath{\le}x\ensuremath{\le}0.28$), in which the magnetic moments reorient along the $c$ axis. This is evidence that the tetragonal magnetic phase is a universal feature of the hole-doped iron-based superconductors. The observations suggest that in this regime the energy levels of the ${C}_{2}$ and ${C}_{4}$ symmetric magnetic phases are very close.

By simultaneously displaying magnetism and superconductivity in a single phase, the iron-based superconductors provide a model system for the study of magnetism's role in superconductivity. The class of intercalated iron selenide superconductors is unique among these in having the additional property of phase separation and coexistence of two distinct phases---one majority phase with iron vacancy ordering and strong antiferromagnetism, and the other a poorly understood minority microscopic phase with a contested structure. Adding to the intrigue, the majority phase has never been found to show superconductivity on its own while the minority phase has never been successfully synthesized separate from the majority phase. In order to better understand this minority phase, a series of high-quality $\mathrm{C}{\mathrm{s}}_{x}\mathrm{F}{\mathrm{e}}_{2\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{2}$ single crystals with $(0.8\ensuremath{\le}x\ensuremath{\le}1;0\ensuremath{\le}y\ensuremath{\le}0.3)$ were grown and studied. Neutron and x-ray powder diffraction performed on ground crystals show that the average $I4/mmm$ structure of the minority phase is distinctly different from the high-temperature $I4/mmm$ parent structure. Moreover, single-crystal diffraction reveals the presence of discrete superlattice reflections that remove the degeneracy of the Cs sites in both the majority and minority phases and reduce their structural symmetries from body centered to primitive. Group theoretical analysis in conjunction with structural modeling shows that the observed superlattice reflections originate from three-dimensional Cs vacancy ordering. This model predicts a $25%$ vacancy of the Cs site in the minority phase which is consistent with the site's refined occupancy. Magnetization measurements performed in tandem with neutron single-crystal diffraction provide evidence that the minority phase is the host of superconductivity. Our results also reveal a superconducting dome in which the superconducting transition temperature varies as a function of the nominal valence of iron.

Elucidating the nature of the magnetic ground state of iron-based superconductors is of paramount importance in unveiling the mechanism behind their high temperature superconductivity. Until recently, it was thought that superconductivity emerges only from an orthorhombic antiferromagnetic stripe phase, which can in principle be described in terms of either localized or itinerant spins. However, we recently reported that tetragonal symmetry is restored inside the magnetically ordered state of a hole-doped BaFe2As2. This observation was interpreted as indirect evidence of a new double-Q magnetic structure, but alternative models of orbital order could not be ruled out. Here, we present Mossbauer data that show unambiguously that half of the iron sites in this tetragonal phase are non-magnetic, establishing conclusively the existence of a novel magnetic ground state with a non-uniform magnetization that is inconsistent with localized spins. We show that this state is naturally explained as the interference between two spin-density waves, demonstrating the itinerant character of the magnetism of these materials and the primary role played by magnetic over orbital degrees of freedom., Comment: 7 pages, 3 figures