Search

The availability of large high-quality single crystals is an important prerequisite for many studies in solid-state research. The optical floating-zone technique is an elegant method to grow such crystals, offering potential to prepare samples that may be hardly accessible with other techniques. As elaborated in this report, examples include single crystals with intentional compositional gradients, deliberate off-stoichiometry, or complex metallurgy. For the cubic chiral magnets Mn$_{1-x}$Fe$_{x}$Si and Fe$_{1-x}$Co$_{x}$Si, we prepared single crystals in which the composition was varied during growth from $x = 0 - 0.15$ and from $x = 0.1 - 0.3$, respectively. Such samples allowed us to efficiently study the evolution of the magnetic properties as a function of composition, as demonstrated by means of neutron scattering. For the archetypical chiral magnet MnSi and the itinerant antiferromagnet CrB$_{2}$, we grew single crystals with varying initial manganese (0.99 to 1.04) and boron (1.95 to 2.1) content. Measurements of the low-temperature properties addressed the correlation between magnetic transition temperature and sample quality. Furthermore, we prepared single crystals of the diborides ErB$_{2}$, MnB$_{2}$, and VB$_{2}$. In addition to high vapor pressures, these materials suffer from peritectic formation, potential decomposition, and high melting temperature, respectively., Comment: 16 pages, 13 figures

Silicon waste (SW), a byproduct from the photovoltaic industry, can be a prospective and environmentally friendly source for silicon in the field of thermoelectric (TE) materials. While thermoelectricity is not as sensitive toward impurities as other semiconductor applications, the impurities within the SW still impede the enhancement of the thermoelectric figure of merit, zT. Besides, the high thermal conductivity of silicon limits its applications as a TE material. In this work, we employ traditionally metallurgical methods in industry reducing the impurities in SW to an extremely low level in an environmentally friendly and economical way, and then the thermal conductivity of purified silicon is greatly reduced due to the implementation of multiscale phonon scattering without degrading the power factor seriously. Benefiting from these strategies, from 323 to 1123 K, for the sample made from purified silicon waste, the average zT, relevant for engineering application, is increased to 0.32, higher than that of the state-of-the-art n-type Ge-free bulk silicon materials made from commercially available silicon, but the total cost of our samples is negligible.

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.

Solid state single crystal growth (SSCG) is a crystal growth technique where crystals are grown from a polycrystalline matrix. Here, we present single crystals of the iron pnictide LaFeAsO grown via SSCG using NaAs as a liquid phase to aid crystallization. The size of the as-grown crystals are up to 2 × 3 × 0.4 mm 3 . Typical for this method, but very uncommon for crystals of the pnictide superconductors and especially for the oxypnictides, the crystals show pronounced facets caused by considerable growth in c direction. The crystals were characterized regarding their composition, structure, magnetic, and thermodynamic properties. This sets the stage for further measurements for which single crystals are crucial such as any c axis and reciprocal space dependent measurements.

The TSFZ growth of Nd-substituted LSCO (La1.81-xNdxSr0.19CuO4, x = 0 ÷ 0.4) single crystals has been explored for the first time. Optimal growth parameters have been determined: oxygen pressure 5.5–6.5 bar; CuO-poor solvent (66 mol.% CuO) and growth rate 0.6 mm/hour. As a result, high quality single crystals with composition La1.81-xSr0.19NdxCuO4 (x = 0, 0.2, 0.3, 0.4) have been successfully grown and characterized by optical polarized microscopy, x-ray diffraction and energy dispersive x-ray spectroscopy analysis. Substitution of La3+ with smaller Nd3+ ions leads to a gradual decrease of lattice parameters as well as of the superconducting transition temperature from 29.5 K for x = 0 to 14.6 K for x = 0.4.

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.

Single crystals of the double perovskite Sr2NiWO6 were synthesized via SrCl2 flux growth using high quality, phase-pure polycrystalline Sr2NiWO6 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. Sr2NiWO6 orders antiferromagnetically at TN=54 K as revealed by magnetization and specific heat data.

Many Heusler compounds are predicted to be half-metallic ferromagnets and thus find extensive interest as materials for spintronic applications, an example of which is the Co2Cr1–xFexAl series. So far, bulk samples of that series, in particular, Cr-rich samples, do not verify those predictions, and various studies have yielded results that are fraught with anomalies. Thin films as well do not meet expectations, neither in magnetoresistance ratio nor in degree of spin polarization and magnetic moments. Polycrystalline samples of Co2CrAl, Co2FeAl, and Co2Cr0.6Fe0.4Al were found to be phase segregated, a hurdle easily tackled by using the optical floating zone (FZ) technique. Hence, in this work, we have carried out crystal growth of Co2Cr1–xFexAl Heusler compounds, as a step toward understanding the intrinsic material properties of this series. Our results demonstrate that, although the phase segregation is eliminated in the FZ grown samples, an unexpected phase transformation via spinodal decomposition tak...

A facile synthesis approach for the synthesis of ternary intermetallic nanocrystals was demonstrated exemplarily with the Heusler compound Co2FeGa. The method, involving prefabricated multiwalled carbon nanotubes which act as a template and protective shell, results in the formation of mainly spherical, chemically stable, and crystalline nanoparticles with a well-defined diameter distribution. As an example, for novel functionalities arising from downscaling a bulk material, we observe an enhancement of the coercive field of the Co2FeGa nanocrystals by a factor of ≈30. Our work can facilitate the exploration and eventually the tuning of physical properties of ternary and other intermetallic compounds at the nanoscale.

The floating-zone growth of massive intermetallic single crystals is very often unsuccessful due to unfavorable solid–liquid interface geometry and insufficient mixing of the melt which depends on the flow in the molten zone. A tailored magnetic two-phase stirrer system with radio frequency (RF) heating has been developed which enables the control of the melt flow by a significant change of the flow field. The magnetic system was used for the crystal growth of the Heusler compound Mn3Si due to their interesting properties such an itinerant antiferromagnetic and incommensurate spin-density wave state. The grown crystals were oriented and cut to samples with the crystallographic orientations (100) and (110) normal to a plane. Measurements of the temperature dependence of magnetization and specific heat are discussed in terms of contributions of other thermodynamic phases and phase transitions.

Nd2PdSi3 single crystals were grown by a vertical floating zone method with radiation heating at a zone traveling rate of 3 mm/h. The compound exhibits congruent melting behavior at a liquidus temperature of about 1790 °C. The actual crystal composition (35.3 ± 0.5) at.% Nd, (16.2 ± 0.5) at.% Pd, and (48.5 ± 0.5) at.% Si is slightly depleted in Pd and Si with respect to the nominal stoichiometry. Therefore, the gradual accumulation of these elements in the traveling zone led to a decrease of the operating temperature during the growth process. Single crystalline samples exhibit a large anisotropy due to the crystal electric field effect and order ferromagnetically below the Curie temperature TC = 15.1 K. The [001] orientation was identified as the magnetic easy axis at low temperatures. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lu 2 PdSi 3 crystals have been grown by the floating zone method with optical heating. The congruently melting compound is a non-magnetic member of the class of R 2 PdSi 3 intermetallics (R: rare earths) with hexagonal AlB 2 -type crystallographic structure. The grown crystals are slightly Pd-depleted with respect to the nominal stoichiometry and contain a minor fraction of LuSi binary phase precipitates. Lu 2 PdSi 3 exhibits normal metallic behavior, but its electrical resistivity is highly anisotropic and deviates from the Bloch–Gruneisen law.

In this report the Co2 based Heusler compounds are discussed as potential materials for spin voltage generation. The compounds were synthesized by arcmelting and consequent annealing. Band structure calculations were performed and revealed the compounds to be half-metallic ferromagnets. Magnetometry was performed on the samples and the Curie temperatures and the magnetic moments were determined. The Seebeck coefficients were measured from low to ambient temperatures for all compounds. For selected compounds high temperature measurements up to 900 K were performed., accepted contribution o the Special Issue "Spin Caloritronics" of Solid State Communications

We present a detailed experimental study of bulk and powder samples of the ferromagnetic Heusler shape-memory alloy ${\mathrm{Ni}}_{2}\mathrm{MnGa}$, including zero-field static and dynamic $^{55}\mathrm{Mn}$ 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 $\text{high}\ensuremath{-}T$ austenitic phase down to the $\text{low}\ensuremath{-}T$ martensitic phase. In addition, a detailed investigation of the so-called $rf\ensuremath{-}\text{enhancement}$ 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.

Among the large class of Heusler compounds, Co2CrAl is predicted to be 100 % spin polarized and is hence, a potential candidate for application in spintronics. So far, the predicted properties have not been experimentally realized which may be attributed to the phase segregated nature of samples. This phase segregation is avoided using floating zone growth. However, the grown sample was found to have undergone phase transformation via spinodal decomposition at low temperatures. In the present work, thermal annealing has been done on the spinodally decomposed samples and its effect on microstructure, crystallographic structure and magnetic properties has been studied. Annealing experiments were done and analyzed in order to understand the extent of the solid state miscibility gap. With regards to the phase diagram, the two-phase regime was found to extend till 1000 {\deg}C. Even at 1250 {\deg}C, we are still inside the immiscibility region. The thermodynamic miscibility gap was thus found to exist until high temperatures and alternate routes might be required to obtain a single phase sample with the desired high spin polarization and Curie temperature in the Co-Cr-Al system., Comment: 10 pages, 5 figures, 24 references, Proceedings of 17th International Conference on Crystal Growth and Epitaxy (ICCGE-17)

LiMn 0.95 Ni 0.05 PO 4 single crystals have been grown for the first time by the travelling-solvent floating-zone method at low argon pressure. The grown sample exhibits large single crystalline grains as revealed by means of polarization microscopy, X-ray Laue back scattering, and single-crystal X-ray diffraction. The composition of the crystal was determined by Energy-dispersive X-ray (EDX) spectroscopy. LiMn 0.95 Ni 0.05 PO 4 orders in an orthorhombic olivine-like structure as expected and phase purity was confirmed by powder X-ray diffraction. An oriented cuboid with size of 2.4×2.5×2.7 mm 3 along a , b , and c crystalline directions, respectively, was used for anisotropic magnetic measurements.

We present properties of single crystals of the iron pnictide superconductor RbFe2As2 grown using a self-flux method. A bulk superconducting transition at Tc ∼2.6 K was consistently observed in resistivity, magnetic susceptibility, and specific heat measurements. While the resistivity behaves like a conventional metal at low T, the magnetic susceptibility shows an unusual broad maximum around 80 K in the normal state. A large Sommerfeld coefficient, γ0=127mJ/mol K2, was observed seemingly a common feature of heavily hole-doped “122” compounds. In the superconducting state, we analyzed our specific heat data based on the s- and d-wave two-gap models. The presence of the small gap with 2Δ1(0)/kBTc

The quaternary Heusler compound ${\text{Co}}_{2}{\text{Mn}}_{1\ensuremath{-}x}{\text{Ti}}_{x}\text{Sn}$ with $x=0$, 0.2, 0.4, 0.5, 0.6, 0.8, and 1 shows a phase separation into the two Heusler compounds, ${\text{Co}}_{2}\text{MnSn}$ and ${\text{Co}}_{2}\text{TiSn}$. Only at the edges of the composition range a slight admixture of Mn and Ti to the respective other phase is observed. This phase separation leads to a distinct microstructure which can be altered by the composition of the material. Pronounced changes in the magnetic and electronic properties take place with varying composition. Two magnetic transitions occur which indicate different Curie temperatures for both phases. The reduction in the thermal lattice conductivity is of particular interest for an optimization of Heusler compounds for thermoelectric applications. In the field of spintronics the use of superlattices composed of ${\text{Co}}_{2}\text{MnSn}$ and ${\text{Co}}_{2}\text{TiSn}$ without any interlayer diffusion is suggested.

Co2Mn1-xFexSi Heusler alloys crystallize in the L21 structure. This structure type requires random distribution of Mn and Fe in case of the mixed alloys. The spin echo nuclear magnetic resonance (NMR) technique probes the direct local environments of the active atoms and is thus able to resolve next neighboring shells providing a unique tool to verify the random distribution of Mn and Fe in Co2Mn1-xFexSi. Exemplarily, the half-metallic quaternary Heusler alloy Co2Mn0.5Fe0.5Si was investigated by means of NMR. The 55Mn NMR measurements unambiguously demonstrate the random distribution of Mn and Fe on the 4b Wyckoff position in Co2Mn0.5Fe0.5Si.

Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics.Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1 − xMnxSi. The local ordering is studied by 59Co nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the differences in the electronic structure at the Fermi energy and in the heat diffusion of thin films and bulk crystals.

Co2FeAl0.5Si0.5 films with a surface roughness of 0.12 nm have been grown epitaxially on lattice-matched MgAl2O4 (001) substrates by off-axis sputtering. X-ray diffraction shows pronounced Laue oscillations, rocking curves as narrow as 0.0043°, and clear Co2FeAl0.5Si0.5 (111) peaks indicating L21 ordering. Magnetic characterizations show a clear magnetocrystalline anisotropy comprising cubic and epitaxy-induced uniaxial terms. Nuclear magnetic resonance measurements reveal L21 order of 81% in the Co2FeAl0.5Si0.5 films. Magnetotransport measurements show a distinct separation of anisotropic magnetoresistance and ordinary magnetoresistance. These results demonstrate the state-of-the-art crystalline quality and magnetic uniformity of the Co2FeAl0.5Si0.5 films.

Lu2PdSi3 represents a non-magnetic member of the class of R2PdSi3 intermetallic compounds (R = rare earths) with hexagonal AlB2-type crystallographic structure. The electrical resistivity of single crystalline Lu2PdSi3 is highly anisotropic and reflects metallic behaviour. Its temperature dependence follows the Bloch–Gruneisen law for c-axis orientation [001]. For [100] orientation in the perpendicular plane the observed deviation from the Bloch–Gruneisen law can be explained by the Ioffe–Regel criterion. The relatively low Debye temperature ΘD of the order of 200 K fairly well agrees with that derived from specific heat data of Lu2PdSi3. Notably, Lu2PdSi3 allows extracting the magnetic entropy contribution of isostructural R2PdSi3 compounds.

The thermoelectric properties of a Heusler compound with NiZr0.5Hf0.5Sn composition were studied. A comparison of the properties of a single crystal and a polycrystal was carried out by measurements of the electrical conductivity, Seebeck coefficient, and thermal conductivity. The transport properties are directly compared to the valence band electronic structure measured by photoelectron spectroscopy. The single crystal shows a higher figure of merit (ZT = 0.1) at room temperature which originates from the high electrical conductivity that is mediated by “in-gap” states observed by photoemission spectroscopy.

Mn2NiGa is reported to be a shape memory material with a martensite–austenite phase transition. Temperature dependent measurements of the transport and magnetic properties reveal the martensitic transition close to room temperature with a thermal hysteresis of about 27 K. The electronic structure of the valence band in both phases was studied by hard x-ray photoelectron spectroscopy. The results clearly indicate that strong changes in the electronic structure appear at the phase transition.

Recent spin echo nuclear magnetic resonance (NMR) revealed that the half-metallic Co2Mn1-xFexSi Heusler alloys exhibit the L21 structure with random distribution of Mn and Fe on the 4b Wyckoff position. This random distribution is most prominent in the third coordination shell of the 55Mn nuclei, as this shell is the first magnetically active shell, leading to a line splitting in the 55Mn NMR spectra. A comparison, as presented in this paper, of all 55Mn NMR satellite lines shows that the resonance frequencies corresponding to the same number of Fe atoms in the third shell of 55Mn shift to slightly different resonance frequencies with increasing overall Fe content x. This shift is related to the constructive contribution of higher shells to the hyperfine field. These higher shells affect also the satellite linewidth, leading to a parabolic behavior with increasing Fe concentration x. This parabolic behavior is modified by the frequency spacing related to the replacement of Mn by Fe.