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In this work, the 55Mn spin-echo nuclear magnetic resonance technique is used to systematically analyze the complete substitutional series Co2Mn1-xFexSi in terms of the resonance frequencies of the satellite structure and the corresponding hyperfine fields of the 55Mn nuclei. The frequency spacing between satellite lines is constant within a compound, but decreases with increasing Fe concentration x, which originates in changes in the band structure. These changes are also responsible for the altered resonance frequencies observed for the same third shell environment but corresponding to a different overall Fe concentration x. All modifications in the hyperfine field upon substitution of Mn by Fe can be uniquely assigned to an entirely random distribution of Mn and Fe limited to only one (4b) out of four fcc sublattices constituting the L21 Heusler structure, demonstrating that the complete substitutional series Co2Mn1-xFexSi is crystallographically very well defined. This very high degree of order sets the stage for the observation of stable half-metallic ferromagnetism which is typically very sensitive to the structural order. [ABSTRACT FROM AUTHOR]

Orbital and spin magnetic moments of the Heusler compounds Co2FeAl and Co2Cr0.6Fe0.4Al were measured by magnetic circular dichroism in X-ray absorption (XMCD). The orbital magnetic moments per spin are quite large (0.1–0.2) compared to bulk values of Fe and Co metals, indicating a considerable spin–orbit coupling in these Heusler compounds. A strong localization of the 3d electron states might be responsible for this observation. The Co and Fe orbital to spin moment ratio shows a distinct decrease of Δr(Fe)=0.04±0.02 and Δr(Co)=0.06±0.02 with increasing external field for the ternary compound Co2FeAl, while the ratio is within error limits independent of the field for Co2Cr0.6Fe0.4Al. This is discussed in terms of a relation to magnetocrystalline anisotropies. [ABSTRACT FROM AUTHOR]

We systematically study the changes in the local atomic environments of Co in CoFe-Al alloys as a function of Al content by means of nuclear magnetic resonance. We find that a Co2FeAl Heusler type structure is formed on a local scale. The observed formation of a highly spin-polarized Heusler compound may explain the improved magnetotransport properties in CoFe-Al based current-perpendicular-to-the-plane spin-valves. [ABSTRACT FROM AUTHOR]

We report on the magnetotransport properties of Co2Cr0.6Fe0.4Al mixed with insulating Al2O3. The powder compacts show a maximum magnetoresistance of 88% at a saturation moment of 0.125 T at 295 K. Different explanations for the reversible mechanism will be discussed, such as tunneling between contiguous ferromagnetic grains, particle movement, and magnetostriction. The effect in Co2Cr0.6Fe0.4Al/Al2O3 powder compacts is the largest room temperature magnetoresistance that has been measured until now. [ABSTRACT FROM AUTHOR]

The majority of thin film studies that were devoted to Fe-oxyarsenides has focused so far on F-substituted (i.e. indirectly electron doped) LnOFeAs (Ln = La, Nd, Sm). Here we turn our attention towards Co-substituted (i.e. directly electron doped) LaOFeAs and SmOFeAs in order to investigate its growth on different substrate materials by using pulsed laser deposition (PLD). We detected dominant LnOFeAs phase formation and discuss the occurrence of minor impurity phases in the different films on different substrates. The lack of a superconducting transition in LnOFe0.85Co0.15As films on MgO(100) could be due to strain, since we observe an onset of superconductivity in SmOFe1 − xCoxAs (x = 0.07, 0.15) films on other oxide substrates. In addition, Co-substitution (i.e. within the Fe2As2 layers) and F-substitution (i.e. within the Ln2O2 layers) leading to direct and indirect electron doping respectively, appears for films deposited on CaF2 substrates. In contrast to the F-substituted but Co-free Fe-oxyarsenides, the co-doped SmO1 − xFxFe0.85Co0.15As film has experimentally accessible upper critical fields down to the lowest temperatures and may serve as an ideal test bed for further theoretical modeling of Fe-oxyarsenides. [ABSTRACT FROM AUTHOR]

Using the magnetic circular dichroism in X-ray absorption at the L2,3-edges of the 3d-transition metals, we determined the element specific ratio between orbital and spin magnetic moment of the Heusler compounds Co2CrAl, Co2Cr0.6Fe0.4Al and Co2FeAl. The orbital magnetic moment per spin is large (0.1–0.2) compared to bulk values of Fe and Co metals. [Copyright &y& Elsevier]

Single crystals of LaCuO2 have been grown for the first time using the travelling-solvent floating zone method. The crystal was grown in an Ar-atmosphere by reduction of La2Cu2O5, which was used as the feed rod composition for the growth. The grown crystal has been characterized with regard to phase purity and single crystallinity using powder X-ray diffraction, energy dispersive X-ray analysis, Laue diffraction and scanning electron microscopy. [ABSTRACT FROM AUTHOR]

We report a ferromagnetic resonance (FMR) study of a single crystal of Cr 4 PtGa 17 , the first material of a half-Heusler (HH) type with a metallic ferromagnetic breathing pyrochlore structure. The spatial distribution of the magnetic free energy density derived from the FMR data appears to be of a cubic symmetry despite the lower symmetry coordination of magnetic Cr ions. This surprising finding strongly suggests that the Cr 4 Ga 14 structural unit at the Y site of the HH general XYZ formula forms an octahedrally shaped integral magnetic cluster, likely due to strong exchange interaction between the Cr ions via covalent Cr-Ga-Cr bonds. The presence of magnetic clusters within a regular metallic lattice appears to be a new intriguing characteristics of these novel types of HH compounds whereas Cr 4 PtGa 17 , at the same time, constitutes the rare example of such metallic pyrochlore-type material. • Magnetic anisotropy of Cr 4 PtGa 17 compound is studied by ferromagnetic resonance. • Magnetic free energy density in Cr 4 PtGa 17 appears to be of a cubic symmetry. • (Cr 4 Ga 14) structural unit forms an octahedrally shaped integral magnetic cluster. • Magnetic clusters are found in a regular metallic lattice. [ABSTRACT FROM AUTHOR]

The local spin susceptibility in superconducting LiFeAs was studied by polarized neutron diffraction as a function of temperature. In the superconducting phase the spin susceptibility is clearly suppressed and it can be well described by the Yosida function suggesting a singlet pairing to occur at low temperature. The spin susceptibility in the normal state and its suppression in the superconducting phase are fully comparable to observations in Co-doped BaFe2As2. [ABSTRACT FROM AUTHOR]

Abstract: Nonequilibrium quasiparticle relaxation dynamics is reported in superconducting Ca(Fe0.944Co0.056)2As2 single crystals by measuring transient reflectivity changes using femtosecond time-resolved pump-probe spectroscopy. Large changes in the temperature-dependent differential reflectivity values in the vicinity of the spin density wave (T SDW) and superconducting (T SC) transition temperatures of the sample have been inferred to have charge gap opening at those temperatures. We have estimated the zero-temperature charge gap value in the superconducting state to be ∼1.8k B T SC and an electron–phonon coupling constant λ of ∼0.1 in the normal state that signifies the weak coupling in iron pnictides. From the peculiar temperature-dependence of the quasiparticle dynamics in the intermediate temperature region between T SC and T SDW we infer a temperature scale where the charge gap associated with the spin ordered phase is maximum and closes on either side while approaching the two phase transition temperatures. [Copyright &y& Elsevier]

The anisotropy of the magnetic excitations in BaFe2As2 was studied by polarized inelastic neutron scattering, which allows one to separate the components of the magnetic response. Despite the in-plane orientation of the static ordered moment, we find the in-plane polarized magnons to exhibit a larger gap than the out-of-plane polarized ones, indicating very strong single-ion anisotropy within the layers. It costs more energy to rotate a spin within the orthorhombic a-b plane than to rotate it perpendicular to the FeAs layers. [ABSTRACT FROM AUTHOR]

Despite the simple crystal structure, LiFeAs having nontrivial band structure and anisotropy of superconducting properties seems the most exotic compound among iron-based superconducting pnictides. Nonetheless, the available experimental studies are not so numerous due to LiFeAs chemical instability even in presence of a trace amount of water vapors or oxygen. Here, using the Andreev spectroscopy of break junctions in LiFeAs single crystals, we unambiguously show a coexistence of three superconducting condensates below critical Tc = 17 K. The temperature dependences of the order parameters and the anisotropy of the middle and the large gaps, as well as the excess Andreev current, and zero-bias conductance are directly measured. We estimate the partial band contributions to the total conductance. Additionally, considered is an influence of the band structure topology near the Fermi level on the dynamic conductance in the Andreev regime. [ABSTRACT FROM AUTHOR]

Despite the simple crystal structure, LiFeAs having nontrivial band structure and anisotropy of superconducting properties seems the most exotic compound among iron-based superconducting pnictides. Nonetheless, the available experimental studies are not so numerous due to LiFeAs chemical instability even in presence of a trace amount of water vapors or oxygen. Here, using the Andreev spectroscopy of break junctions in LiFeAs single crystals, we unambiguously show a coexistence of three superconducting condensates below critical Tc = 17 K. The temperature dependences of the order parameters and the anisotropy of the middle and the large gaps, as well as the excess Andreev current, and zero-bias conductance are directly measured. We estimate the partial band contributions to the total conductance. Additionally, considered is an influence of the band structure topology near the Fermi level on the dynamic conductance in the Andreev regime. [ABSTRACT FROM AUTHOR]

We present thermal expansion studies on single-crystalline LaFe 1 - x Co x AsO with doping levels of x = 0, 0.026, and 0.034, and on BaFe 2 As 2 . Anomalies in the thermal expansion coefficient α enable reading off the structural and magnetic transition temperatures T S and T N . The evolution of the orthorhombic distortion at T S is illustrated by length changes of the a - and b -axes and the experimentally determined orthorhombicity δ. The results are compared to BaFe 2 As 2. Finally, the electronic phase diagram is discussed. [ABSTRACT FROM AUTHOR]

Highlights • Confirmation of twin structure in the orthorhombic BaFe 2 As 2 bulk single crystals. • Absence of orthorhombic domains in a fully strained 20 nm BaFe 2 As 2 thin film. • Absence of orthorhombic domains in a partially strained 60 nm BaFe 2 As 2 thin film. Abstract The ternary iron arsenide compound BaFe 2 As 2 exhibits a structural phase transition from tetragonal to orthorhombic at a temperature of about 140 K. The twin lamellae arising below this transition temperature were studied in undoped single crystalline bulk and epitaxial thin film samples using electron backscatter diffraction in a scanning electron microscope equipped with a helium cryostat. Applying this technique on bulk single crystals a characteristic twin lamella size in the range of 0.1 μm up to a few μm was observed. In contrast, in epitaxially strained thin films the phase transition is not observed at temperatures above 19 K. [ABSTRACT FROM AUTHOR]

Co 2 CrAl is predicted as a promising candidate for half-metallic ferromagnetism. Unfortunately, synthesis of the stoichiometric composition is a challenge as the samples prepared so far using various techniques suffer from formation of secondary phases and consequently anomalous physical properties. Off-stoichiometric compounds have been explored as a possible approach to avoid the secondary phase precipitation. The Co 2 Cr 0.8 Al 1.2 , Co 2 Cr 0.6 Al 1.4 and Co 2 Cr 0.4 Al 1.6 compositions were synthesized using Floating Zone (FZ) technique as they melt incongruently. The secondary phase fraction was reduced as Cr was partially substituted by Al. FZ-grown Co 2 Cr 0.4 Al 1.6 was obtained as a single phase, signifying that the composition is out of the two-phase region. Furthermore, combining the data with an earlier work on annealing of spinodally decomposed samples, valuable insight into the extent of immiscibility was obtained. [ABSTRACT FROM AUTHOR]

Although Mn-Ga alloys are discussed as novel permanent magnetic materials, there are several drawbacks, including the high price of Ga and the yet too small saturation magnetization to name a few. We used several guidelines to motivate the replacement of Ga by Sn to tackle these issues and explored the phase formation, structural and magnetic properties of a Mn 2.6 Ga 1− x Sn x alloy series. The structure of Sn-rich samples is dominated by the corresponding hexagonal and orthorhombic phases, while the structure of Ga-rich samples shows additional contributions from the tetragonal DO 22 structure. However, formation of the tetragonal phase happens only upon an appropriate annealing procedure. In contrast to the discussion in the literature, our experiments do not hint on the tetragonal DO 22 structure being metastable, but rather indicate the DO 22 structure being an equilibrium phase. In general, the experimentally found phase fractions impact on the magnetic properties and hard magnetic properties are downgraded very rapidly even if only a small amount of Ga is substituted by Sn. This indicates that the DO 22 phase found only in the Ga-rich samples is indeed the one which is most relevant for realization of high magnetic anisotropy. [ABSTRACT FROM AUTHOR]

The impact of synthesis conditions, post-preparation heating procedure, aging and influence of pressure on the superconducting properties of FeSe 0.4 Te 0.6 crystals is reported. Two FeSe 0.4 Te 0.6 single crystals were used in the study, prepared from stoichiometric melt but cooled down with very different cooling rates, and investigated using magnetic bulk and electrical-resistivity methods. The fast-cooled crystal contains large inclusions of F e 3 Se 2.1 Te 1.8 and exhibits bulk superconductivity in its as-prepared state, while the other is homogeneous and shows only traces of superconductivity. AC susceptibility measurements under hydrostatic pressure show that the superconducting transition temperature of the inhomogeneous crystal increases from 12.3 K at ambient pressure to T sc = 17.9 K at 9 kbar. On the other hand, neither pressure nor mechanically-induced stress is sufficient to induce superconductivity in the homogeneous crystal. However, an additional heat treatment at 673 K followed by fast cooling down and/or long-term aging at ambient conditions leads to the appearance of bulk superconductivity also in the latter sample. This sample remains homogeneous on a scale down to few μm but shows an additional magnetic phase transition around 130 K suggesting that it must be inhomogeneous. For comparison also F e 3 Se 2.1 Te 1.8 polycrystals have been prepared and their magnetic properties have been studied. It appears that this phase is not superconducting by itself. It is concluded that nano-scale inhomogeneities that appear in the F e S e x Te 1 − x system due to a spinodal decomposition in the solid state are necessary for bulk superconductivity, possibly due to minor changes in the crystal structure and microstructure. Macroscopic inclusions quenched by fast cooling from high temperatures lead obviously to strain and hence variations in the lattice constants, an effect that is further supported by application of pressure/stress. [ABSTRACT FROM AUTHOR]

The detailed high field electron spin resonance (HF-ESR) study of three 3D oxalate-based coordination polymers with antiferromagnetic ordering below 13 K, supported by crystallographic, photoluminescence (PL), elemental (EDX) and X-band ESR analysis, is presented. Two of the investigated compounds are described by the same chemical formula {[Cu(bpy) 3 ][Mn 2 (C 2 O 4 ) 3 ]·H 2 O} n , both exhibit PL but have different colors: green–blue and red–pink (CuMn2-Green and CuMn2-Red, respectively) while {[Co(bpy) 3 ][Mn 2 (C 2 O 4 ) 3 ]·H 2 O} n is yellow (CoMn2-Yellow) without PL. Despite different colors of CuMn2-Green and CuMn2-Red, X-ray diffraction, EDX, as well as X-band ESR, could not reveal any difference between these compounds. Therefore, detailed temperature dependent multifrequency HF-ESR experiments were performed. Owing to very high resolution of HF-ESR, two different Mn(II) centers in CuMn2-Green and CoMn2-Yellow compounds could be resolved, while in CuMn2-Red only one Mn(II) center is detected. A possible difference in the ligand coordination of Mn(II) ions in the oxalate-networks of CuMn2-Green and CuMn2-Red, that gives rise to slightly different g -factors, is discussed with respect to the different colors of these compounds. [ABSTRACT FROM AUTHOR]

We present a microscopic investigation of frequently observed impurity-induced states in stoichiometric LiFeAs using low-temperature scanning tunneling microscopy and spectroscopy (STM/STS). Our data reveal seven distinct well-defined defects which are discernible in topographic measurements. Depending on their local topographic symmetry, we are able to assign five defect types to specific lattice sites at the Li, Fe, and As positions. The most prominent result is that two different defect types have a remarkably different impact on the superconducting state. A specific and quite abundant Fe-defect with [ABSTRACT FROM AUTHOR]

In this article, we review our angle- and time-resolved photoemission studies (ARPES and trARPES) on various ferropnictides. In the ARPES studies, we focus first on the band structure as a function of control parameters. We find near optimally 'doped' compounds a Lifshitz transition of hole/electron pocket vanishing type. Second, we investigated the inelastic scattering rates as a function of the control parameter. In contrast to the heavily discussed quantum critical scenario, we find no enhancement of the scattering rate near optimally 'doping.' Correlation effects which show up by the non-Fermi-liquid behavior of the scattering rates, together with the Lifshitz transition offer a new explanation for the strange normal state properties and suggests an interpolating superconducting state between BCS and BE condensation. Adding femtosecond time resolution to ARPES provides complementary information on electron and lattice dynamics. We report on the response of the chemical potential by a collective periodic variation coupled to coherent optical phonons in combination with incoherent electron and phonon dynamics described by a three temperature heat bath model. We quantify electron phonon coupling in terms of [ABSTRACT FROM AUTHOR]

Large single crystals of La 8 Cu 7 O 19 have been grown using the travelling-solvent floating zone method. A rather high oxygen pressure of 9 bar in the growth chamber and a slow growth speed of 0.5 mm/h were among the most important parameters in stabilizing the growth of this incongruently melting compound. Interestingly, a novel growth scenario has been witnessed. The crystal structure of the grown La 8 Cu 7 O 19 crystal has been analyzed using single crystal diffractometry to extract important structural parameters of this compound. We find that La 8 Cu 7 O 19 crystallizes in a monoclinic structure with space group C 2 / c and has the lattice parameters a ≈ 13.83 Å , b ≈ 3.75 Å , c ≈ 34.59 Å , and β ≈ 99.33 ° , in good agreement with the data obtained on polycrystalline samples in the literature. The magnetization shows a highly anisotropic behavior, and an anomaly at T ≈103 K. [ABSTRACT FROM AUTHOR]

Materials with a 5d4 electronic configuration are generally considered to have a nonmagnetic ground state (J = 0). Interestingly, Sr2YIrO6 (Ir5+ having 5d4 electronic configuration) was recently reported to exhibit long-range magnetic order at low temperature and the distorted IrO6 octahedra were discussed to cause the magnetism in this material. Hence, a comparison of structurally distorted Sr2YIrO6 with cubic Ba2YIrO6 may shed light on the source of magnetism in such Ir5+ materials with 5d4 configuration. Besides, Ir5+ materials having 5d4 are also interesting in the context of recently predicted excitonic types of magnetism. Here we report a single-crystal-based analysis of the structural, magnetic, and thermodynamic properties of Ba2YIrO6. We observe that in Ba2YIrO6 for temperatures down to 0.4 K, long-range magnetic order is absent but at the same time correlated magnetic moments are present. We show that these moments are absent in fully relativistic ab initio band-structure calculations; hence, their origin is presently unclear. [ABSTRACT FROM AUTHOR]

Chemical and structural phase compositions of two single-crystalline samples prepared with different cooling rates from stoichiometric FeTe 0.6 Se 0.4 melts were studied. Both types of samples were investigated in a very comprehensive way using magnetic and electrical transport measurements combined with X-ray, neutron and electron backscatter diffraction. We show that slowly cooled samples are homogeneous on a microscopic scale with only a small excess of iron. Those slowly cooled samples do not exhibit bulk superconductivity down to 1.8 K. In contrast, fast-cooled samples are superconducting below about 14 K but are composed of several chemical phases: they consist of a matrix preserving the crystal structure of slow-cooled samples, and of core-shell structured dendritic inclusions (about 20–30 vol%). These have different crystal structures and chemical compositions and order magnetically at temperatures far above the superconducting transition temperature of the inhomogeneous samples. These structural and chemical inhomogeneities seem to play a vital role in the superconducting properties of this and similar iron-based systems as they lead to internal stress and act in a similar way as the application of the external pressure that reportedly increase the superconducting transition temperature in many iron pnictides and chalcogenides. We argue that a phase pure, homogeneous and stress-free FeTe 0.6 Se 0.4 is non-superconducting. [ABSTRACT FROM AUTHOR]

Angle-resolved photoemission spectroscopy is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictide systems. We find the scattering rate for any given band to depend linearly on energy but to be independent of the control parameter. We demonstrate that the linear energy dependence gives rise to a weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. The resulting small effective Fermi energy favors a BCS [J. Bardeen et al., Phys. Rev. 108, 1175 (1957)] -Bose-Einstein [S. N. Bose, Z. Phys. 26, 178 (1924)] crossover state in the superconducting phase. [ABSTRACT FROM AUTHOR]

Single crystals of the double perovskite Sr 2 NiWO 6 were synthesized via SrCl 2 flux growth using high quality, phase-pure polycrystalline Sr 2 NiWO 6 as precursor material. This high quality precursor enabled us to grow large and phase pure crystals with sizes up to 1 mm ×1 mm in the basal plane and octahedral morphology. We measured the temperature dependence of the magnetization along the c -axis and along the ab plane. The analysis of the data allows a precise determination of the effective magnetic moment and the Curie–Weiss temperature. Sr 2 NiWO 6 orders antiferromagnetically at T N =54 K as revealed by magnetization and specific heat data. [ABSTRACT FROM AUTHOR]

The superconducting properties of LaFeAsO1-xFx under conditions of optimal electron doping are investigated upon the application of external pressure up to ∼23 kbar. Measurements of muon-spin spectroscopy and dc magnetometry evidence a clear mutual independence between the critical temperature Tc and the low-temperature saturation value for the ratio ns/m* (superfluid density over effective band mass of Cooper pairs). Remarkably, a dramatic increase of ∼30% is reported for ns/m* at the maximum pressure value while Tc is substantially unaffected in the whole accessed experimental window. We argue and demonstrate that the explanation for the observed results must take the effect of nonmagnetic impurities on multiband superconductivity into account. In particular, the unique possibility to modify the ratio between intraband and interband scattering rates by acting on structural parameters while keeping the amount of chemical disorder constant is a striking result of our proposed model. [ABSTRACT FROM AUTHOR]

We report a combined experimental study of magnetic properties of a single crystal of the frustrated diamond lattice antiferromagnet CoAl2O4 with Co2+ electron spin resonance, 27Al nuclear magnetic resonance, and muon spin rotation/relaxation techniques. With our local probes, we show that the frustration of spin interactions and the Co/Al site disorder strongly affect the spin dynamics. The experimental results evidence inhomogeneous and slow magnetic fluctuations and the occurrence of short-range electron spin correlations far above a characteristic temperature T* = 8 K at which the spin system turns into in a quasistatic state. Our data indicate that this spin order is likely short range and unconventional with spin fluctuations persistent even at T ⪡ T*. The results of three spectroscopy techniques highlight a nontrivial role of structural disorder for the magnetism of a frustrated diamond spin lattice at the proximity to the critical point. [ABSTRACT FROM AUTHOR]

We present a detailed experimental study of bulk and powder samples of the ferromagnetic Heusler shape-memory alloy Ni2MnGa, including zero-field static and dynamic 55Mn NMR experiments, x-ray powder diffraction and magnetization experiments. The NMR spectra give direct access to the sequence of structural phase transitions in this compound, from the high-T austenitic phase down to the low-T martensitic phase. In addition, a detailed investigation of the so-called rf-enhancement factor delivers the local magnetic stiffness and restoring fields for each separate structural environment, thus, differentiating signals coming from austenitic and martensitic components. In this way we can also resolve differences in the local spin moments of the two phases of the order of 0.08 Bohr magnetons, and reveal precursor phenomena of the martensitic transformation well inside the parent austenitic phase. [ABSTRACT FROM AUTHOR]

A new representative of the RAuIn series was discovered in the Sc-Au-In system. ScAuIn crystallizes in the hexagonal HfRhSn-type structure (space group P -62 c) with a ​= ​0.75667(2), c ​= ​0.71873(2) nm). Magnetization measurements revealed that ScAuIn is a Pauli paramagnet with magnetic susceptibility of ∼6 ​× ​10−5 emu/mol. The Sommerfeld coefficient γ of 2.8 ​mJ ​K−2 ​mol−1 derived from the specific heat measurements is comparable with those obtained from the modeled density of states at the Fermi level N (E F). Ab initio calculations predict metallic conductivity and show strong chemical bonding within the [InAu] sublattice. The structural stability of the RAuIn with ZrNiAl and HfRhSn structure types is discussed based on crystal chemistry analysis and DFT modeling. Field dependence of the magnetization of ScAuIn at different temperatures and calculated isosurface of the charge density. [Display omitted] • A new representative of the RAuIn series was discovered in the Sc-Au-In system. • ScAuIn crystallizes in the hexagonal HfRhSn-type structure. • ScAuIn is a Pauli paramagnet. • Ab initio calculations predict metallic conductivity of ScAuIn. [ABSTRACT FROM AUTHOR]

Type, degree, and evolution of structural order are important aspects for understanding and controlling the properties of highly spin-polarized Heusler compounds, in particular, with respect to the optimal film growth procedure. In this work, we compare the structural order and the local magnetic properties revealed by nuclear magnetic resonance (NMR) spectroscopy with the macroscopic properties of thin Co2FeAl0.5Si0.5 Heusler films with varying thickness. A detailed analysis of the measured NMR spectra presented in this paper enables us to find a very high degree of L21 type ordering up to 81% concomitantly with excess Fe of 8%-13% at the expense of A1 and Si. We show that the formation of certain types of order depends not only on the thermodynamic phase diagrams as in bulk samples, but also that the kinetic control may contribute to the phase formation in thin films. It is an exciting finding that Co2FeAl0.5Si0.5 can form an almost ideal L21 structure in films, though with a considerable amount of Fe-Al/Si off stoichiometry. Moreover, the very good quality of the films as demonstrated by our NMR study suggests that the technique of off-axis sputtering used to grow the films sets the stage for the optimized performance of Co2FeAl0.5Si0.5 in spintronic devices. [ABSTRACT FROM AUTHOR]

We have studied the magnetic ordering in Ba1-xNaxFe2As2 with 0.25 ⩽ x ⩽ 0.4 by unpolarized and polarized neutron diffraction using single crystals. Unlike most FeAs-based compounds that magnetically order, Na-doped BaFe2As2 exhibits two successive magnetic transitions: For x = 0.35, upon cooling, magnetic order occurs at ∼ 70 K with in-plane magnetic moments being arranged as in pure or Ni-, Co-, or K-doped BaFe2As2 samples. At a temperature of ∼ 46 K a second phase transition occurs, which the single-crystal neutron-diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in Ba065Na0.35Fe2As2 point along the c direction. The two nearly degenerate magnetic states document orbital degeneracy to persist in the superconducting phase. [ABSTRACT FROM AUTHOR]

We present a detailed local probe study of the magnetic order in the oxychalcogenide La2O2Fe2OSe2 utilizing 57Fe Mossbauer, l39La NMR, and muon-spin relaxation spectroscopy. This system can be regarded as an insulating reference system of the Fe arsenide and chalcogenide superconductors. From the combination of the local probe techniques we identify a noncollinear magnetic structure similar to Sr2F2Fe2OS2. The analysis of the magnetic order parameter yields an ordering temperature TN = 90.1 K and a critical exponent of β = 0.133, which is close to the two-dimensional Ising universality class as reported in the related oxychalcogenide family. [ABSTRACT FROM AUTHOR]

We present experimental XMLD spectra measured on epitaxial (001)-oriented thin Co2FeSi films, which are rich in features and depend sensitively on the degree of atomic order and interdiffusion from capping layers. Al- and Cr-capped films with different degrees of atomic order were prepared by DC magnetron sputtering by varying the deposition temperatures. The local structural properties of the film samples were additionally investigated by nuclear magnetic resonance (NMR) measurements. The XMLD spectra of the different samples show clear and uniform trends at the L3,2 edges. The Al-capped samples show similar behavior as previous measured XMLD spectra of Co2FeSi0.6Al0.4. Thus, we assume that during deposition Al atoms are being implanted into the subsurface of Co2FeSi. Such an interdiffusion is not observed for the corresponding Cr-capped films, which makes Cr the material of choice for capping Co2FeSi films. We report stronger XMLD intensities at the L3,2 Co and Fe egdes for films with a higher saturation magnetization. Additionally, we compare the spectra with ab initio predictions and obtain a reasonably good agreement. Furthermore, we were able to detect an XMCD signal at the Si L-edge, indicating the presence of a magnetic moment at the Si atoms. [ABSTRACT FROM AUTHOR]

We present an inelastic neutron scattering study on single-crystalline LiFeAs devoted to the characterization of the incommensurate antiferromagnetic fluctuations at Q=(0.5±δ,0.5∓δ,ql). Time-of-flight measurements show the presence of these magnetic fluctuations up to an energy transfer of 60 meV, while polarized neutrons in combination with a longitudinal polarization analysis on a triple-axis spectrometer prove the pure magnetic origin of this signal. The normalization of the magnetic scattering to an absolute scale yields that magnetic fluctuations in LiFeAs are by a factor of 8 weaker than the resonance signal in nearly optimally Co-doped BaFe2As2, although a factor of 2 is recovered due to the split peaks owing to the incommensurability. The longitudinal polarization analysis indicates weak spin-space anisotropy with a slightly stronger out-of-plane component between 6 and 12 meV. Furthermore, our data suggest a fine structure of the magnetic signal most likely arising from superposing nesting vectors. [ABSTRACT FROM AUTHOR]

I(V) characteristIcs and their first derivatives of ScS and ScN-type (S is superconductor, c is constrIction, N is normal metal) point contacts (PCs) based on Ba1-xNaxFe2As2 (x = 0.25 and 0.35) were studied. ScS-type PCs with S = Nb,Ta, and Pb show Josephson-like resistively shunted I(V) curves with mIcrowave induced Shapiro steps whIch satisfy relation 2 eV = ħω . The IcRN product (Ic is critIcal current, RN is normal state PC resistance) in these PCs is found to be up to 1.2 mV. All this data with the observed dependence of the Ic on the mIcrowave power of ScS PCs with Pb counter electrode indIcates the presence of the singlet s-wave type pairing in Ba1-xNaxFe2As2. From the dV/dI(V) curves of ScN-type PCs demonstrating Andreev-reflection like features, the superconducting gap ∆ ratio 2∆/kBTc = 3.6 ± 1 for the compound with x = 0.35 was evaluated. Analysis of these dV/dI(V) at high biases V, that is well above ∆, testifies transition to the thermal regime in PCs with a voltage increase. [ABSTRACT FROM AUTHOR]

We present 75As nuclear magnetic resonance measurements on single crystals of BaFe2As2, BaFe1.8Co0.2As2, and BaFe1.82Cu0.18As2. While only Co doping induces bulk superconductivity on a broad doping range, the spin fluctuations probed by the nuclear spin-lattice relaxation rate (T1T)-1 are identical for both dopings down to Tc. Below this temperature, (T1T)-1 of the Cu-doped sample continues to rise, proving that (a) there is a quantum critical point below the superconducting dome, and (b) adding electrons does not affect the spin fluctuations. Consequently, we analyze the Knight shift data in terms of a two-component scenario, with one hyperfine coupling to an itinerant degree of freedom and the other to Fe moments. [ABSTRACT FROM AUTHOR]

We report an experimental and theoretical investigation of the electron-boson interaction in KFe2 As2 by point-contact (PC) spectroscopy, model, and ab initio local density approximation-based calculations for the standard electron-phonon Eliashberg function. The PC spectrum viz. the second derivative of the I -V characteristic of representative PC exhibits a pronounced maximum at about 20 meV and surprisingly a featureless behavior at lower and higher energies. We discuss phonon and nonphonon (excitonic) mechanisms for the origin of this peak. Analysis of the underlying source of this peak may be important for the understanding of serious puzzles of superconductivity in this type of compound. [ABSTRACT FROM AUTHOR]

The effects of hydrostatic pressure and partial Na substitution on the normal-state properties and the superconducting transition temperature (Tc) of K1-xNaxFe2As2 single crystals were investigated. It was found that a partial Na substitution leads to a deviation from the standard T² Fermi-liquid behavior in the temperature dependence of the normal-state resistivity. It was demonstrated that non-Fermi-liquid like behavior of the resistivity for K1-xNaxFe2As2 and some KFe2As2 samples can be explained by a disorder effect in the multiband system with rather different quasiparticle effective masses. Concerning the superconducting state our data support the presence of a shallow minimum around 2 GPa in the pressure dependence of Tc for stoichiometric KFe2As2. The analysis of Tc in K1-xNaxFe2As2 at pressures below 1.5 GPa showed that the reduction of Tc with Na substitution follows the Abrikosov-Gor'kov law with the critical temperature Tc0 of the clean system (without pair breaking), which linearly depends on the pressure. Our observations also suggest that Tc of K1-xNa Fe2As2 is nearly independent of the lattice compression produced by the Na substitution. Further, we theoretically analyzed the behavior of the band structure under pressure within the generalized gradient approximation (GGA). A qualitative agreement between the calculated and the recently measured--in de Haas-van Alphen experiments [T. Terashima et al., Phys. Rev. B 89, 134520 (2014)]--pressure dependencies of the Fermi-surface cross sections has been found. These calculations also indicate that the observed minimum around 2 GPa in the pressure dependence of Tc may occur without a change of the pairing symmetry. [ABSTRACT FROM AUTHOR]

We report the low-temperature properties of 11B-enriched single-crystal CrB2 as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, pxx, Hall effect, pxy, and specific heat, C, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap Δ ≈ 220 K below TN = 88 K. This observation is corroborated by the absence of a Curie dependence in the magnetization for T → 0 reported in the literature. Comparison of C with dpxn/dT, where we infer lattice contributions from measurements of VB2, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature Tsf ≈ 257 K in the paramagnetic state, followed by a pronounced second-order mean-field transition at TN, and unusual excitations around ≈TN/ 2. The pronounced anisotropy of pxx above TN is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the Neel temperatures, f = - Θcw/TN ≈ 8.5, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to 14 T, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies CrB2 as a weak itinerant antiferromagnet par excellence with strong geometric frustration. [ABSTRACT FROM AUTHOR]

A commercially available calorimeter has been used to investigate the specific heat of a high-quality K $$_{0.71}$$ Na $$_{0.29}$$ Fe $$_2$$ As $$_2$$ single crystal. The addenda heat capacity of the calorimeter is determined in the temperature range $$0.02 \, \mathrm{K} \le T \le 0.54 \, \mathrm{K}$$ . The data of the K $$_{0.71}$$ Na $$_{0.29}$$ Fe $$_2$$ As $$_2$$ crystal imply the presence of a large $$T^2$$ contribution to the specific heat which gives evidence of $$d$$ -wave order parameter symmetry in the superconducting state. To improve the measurements, a novel design for a calorimeter with a paramagnetic temperature sensor is presented. It promises a temperature resolution of $$\Delta T \approx 0.1 \, \mathrm{\mu K}$$ and an addenda heat capacity less than $$200 \, \mathrm{pJ/K}$$ at $$ T < 100 \, \mathrm{mK}$$ . [ABSTRACT FROM AUTHOR]

We report a low-temperature specific heat study of high-quality single crystals of the heavily hole-doped superconductor Ca0.32Na0.68Fe2As2. This compound exhibits bulk superconductivity with a transition temperature Tc ≈ 34 K, which is evident from the magnetization, transport, and specific heat measurements. The zero-field data manifest a significant electronic specific heat in the normal state with a Sommerfeld coefficient γ Ri 53 mJ/mol K2. Using a multiband Eliashberg analysis, we demonstrate that the dependence of the zero-field specific heat in the superconducting state is well described by a three-band model with an unconventional s± pairing symmetry and gap magnitudes Δ1 of approximately 2.35, 7.48, and -7.50 meV. Our analysis indicates a non-negligible attractive intraband coupling, which contributes significantly to the relatively high value of Tc. The Fermi surface averaged repulsive and attractive coupling strengths are of comparable size and outside the strong coupling limit frequently adopted for describing high-Tc- iron pnictide superconductors. We further infer a total mass renormalization of the order of five, including the effects of correlations and electron-boson interactions. [ABSTRACT FROM AUTHOR]

We present a study of the derivatives of current-voltage I(V) characteristics of point contacts (PCs) based on Ba1−xNaxFe2As2 (x=0.25) in the normal and superconducting states. The detailed analysis of dV/dI(V) data (also given in Appendix A) shows that the thermal regime, when the temperature increases with voltage at a rate of about 1.8 K/mV, is realized in the investigated PCs at least at high biases V above the superconducting (SC) gap Δ. In this case, the specific resistivity ρ(T) in the PC core is responsible for a peculiar dV/dI(V) behavior, while a pronounced asymmetry of dV/dI(V) is caused by the large value of thermopower in this material. A reproducible minima detected in dV/dI(V) at low biases in the range ±(6–9) mV, well below the SC critical temperature Tc, could be connected with the manifestation of the SC gap Δ. An evaluation of these Andreev-reflection-like structures on dV/dI(V) points out to the preferred value of 2Δ/kBTc≈6. The expected second gap features in dV/dI(V) are hard to resolve unambiguously, likely due to impurity scattering, spatial inhomogeneity, and transition to the mentioned thermal regime as the bias further increases. Suggestions are made how to separate the spectroscopic features in dV/dI(V) from those caused by the thermal regime. [ABSTRACT FROM AUTHOR]

Using the angle-resolved photoemission spectroscopy (ARPES) with resolution of all three components of electron momentum and electronic states symmetry, we explicate the electronic structure of hole-doped BaFe2As2, and show that widely discussed nesting and dimensionality of Fermi surface (FS) sheets have no immediate relation to the superconducting pairing in iron-based superconductors. Alternatively a clear correlation between the orbital character of the electronic states and their propensity to superconductivity is observed: The magnitude of the superconducting gap maximizes at 10 meV exclusively for iron 3dxz,yz orbitals, while for others drops to 3 meV. Presented results imply that the relation between superconducting and magnetostructural transitions goes beyond simple competition for FS, and demonstrate importance of orbital physics in iron superconductors. [ABSTRACT FROM AUTHOR]

We present a systematic study of the electronic specific-heat jump (ΔCel) at the superconducting transition temperature Tc of K1-xNaxFe2As2. Both Tc and ΔCel monotonously decrease with increasing x. The specific heat jump scales approximately with a power law, ΔCel∝Tβc, with β≈2 determined by the impurity scattering rate, in contrast to most iron-pnictide superconductors, where the remarkable Bud'ko-Ni-Canfield (BNC) scaling ΔCel∝T3 has been found. Both the T dependence of Cel(T) in the superconducting state and the nearly quadratic scaling of ΔCel at Tc are well described by the Eliashberg theory for a two-band d-wave superconductor with weak pair breaking due to nonmagnetic impurities. The disorder induced by the Na substitution significantly suppresses the small gaps, leading to gapless states in the slightly disordered superconductor, which results in a large observed residual Sommerfeld coefficient in the superconducting state for x > 0. [ABSTRACT FROM AUTHOR]

We report the angular dependence of three distinct de Haas-van Aiphen (dHvA) frequencies of the torque magnetization in the itinerant antiferromagnet CrB2 at temperatures down to 0.3 K and magnetic fields up to 14 T. Comparison with the Fermi surface calculations considering an incommensurate cycloidal magnetic order suggests that two of the observed dHvA oscillations arise from electronlike Fermi surface sheets formed by bands with strong B-px,y character. The third orbit could correspond to a Cr-d derived Fermi surface sheet. The measured effective masses of these Fermi surface sheets display strong enhancements of a factor of about 2 over the calculated band masses which can be attributed to electron-phonon coupling and electronic correlations. Signatures of further heavy d-electron bands that are predicted by the calculations are not observed in the temperature and field range studied. In the view that the B-p bands are at the heart of conventional high-temperature superconductivity in the isostructural MgB2, we consider possible implications of our findings for nonmagnetic CrB2 and an interplay of itinerant antiferromagnetism with superconductivity. [ABSTRACT FROM AUTHOR]

The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO1-xFx was studied over a broad doping, temperature, and magnetic field range (x = 0-0.15, T ⩽ 480 K, μ0H ⩽ 30 T) by means of 75As nuclear magnetic resonance. An enhanced spin-lattice relaxation rate divided by temperature (TiT)-1 in underdoped superconducting samples (x = 0.045, 0.05, and 0.075) suggests the presence of antiferromagnetic spin fluctuations, which are strongly reduced in optimally doped (x = 0.10) and completely absent in overdoped (x = 0.15) samples. In contrast to previous analysis, Curie-Weiss fits are shown to be insufficient to describe the data over the whole temperature range. Instead, a Bloembergen-Purcell-Pound (BPP) model is used to describe the occurrence of a peak in (T, T)-1 clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition. [ABSTRACT FROM AUTHOR]

Co2MnSi films were recently reported to show a significant increase in current-perpendicular-to-plane-giant-magnetoresistance upon annealing. Here, nuclear magnetic resonance was used to study the impact of annealing on the structure of such films. Below 550 °C, no long-range L21-order is observed, while annealing above 550 °C leads to the formation of the ideal L21 configuration, however, with a distinct degree of off-stoichiometry. Further evidence from restoring field measurements hints that interdiffusion may account for the drop in magnetoresistance observed for samples annealed above 600 °C. These results show that optimizing films for spintronics involves the identification of the best annealing temperature, high enough for long-range order to emerge, but low enough to maintain smooth interfaces. [ABSTRACT FROM AUTHOR]

Co2FeSi, a Heusler alloy with the highest magnetic moment per unit cell and the highest Curie temperature, has largely been described theoretically as a half-metal. This conclusion, however, disagrees with point contact Andreev reflection (PCAR) spectroscopy measurements, which give much lower values of spin polarization, P. Here, we present the spin polarization measurements of Co2FeSi by the PCAR technique, along with a thorough computational exploration, within the DFT and a GGA + U approach, of the Coulomb exchange U parameters for Co and Fe atoms, taking into account spin-orbit coupling. We find that the orbital contribution (mo) to the total magnetic moment (mT) is significant, since it is at least 3 times greater than the experimental uncertainty of mT. The account of mo radically affects the acceptable values of U. Specifically, we find no values of U that would simultaneously satisfy the experimental values of the magnetic moment and result in the half-metallicity of Co2FeSi. On the other hand, the ranges of U that we report as acceptable are compatible with spin polarization measurements (ours and the ones found in the literature), which all are within approximately the 40-60 % range. Thus, based on reconciling experimental and computational results, we conclude that (a) spin-orbit coupling cannot be neglected in calculating Co2FeSi magnetic properties, and (b) Co2FeSi Heusler alloy is not half-metallic. We believe that our approach can be applied to other Heusler alloys such as Co2FeAl. [ABSTRACT FROM AUTHOR]

From the measurement and analysis of the specific heat of high-quality K1-xNaxFe2As2 single crystals we establish the presence of large T2 contributions with coefficients asc ≈30 mJ/mol K3 at low T for both x = 0 and x =0.1. Together with the observed √B behavior of the specific heat in the superconducting state both findings give evidence of d-wave superconductivity on almost all Fermi-surface sheets with an average gap amplitude of Δ0 in the range of 0.4-0.8 meV. The derived Δ0 and observed Tc agree well with the values calculated within Eliashberg theory, adopting a spin-fluctuation mediated pairing in the intermediate coupling regime. [ABSTRACT FROM AUTHOR]