History dependence of the magnetic properties of single-crystal Fe1−x_{1-x}Cox_{x}Si

We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of Fe$_{1-x}$Co$_{x}$Si, $0.20 \leq x \leq 0.50$. We determine the magnetic phase diagrams for all major crystallographic directions and cooling histories. After zero-field cooling, the phase diagrams resemble that of the archetypal stoichiometric cubic chiral magnet MnSi. Besides the helical and conical state, we observe a pocket of skyrmion lattice phase just below the helimagnetic ordering temperature. At the phase boundaries between these states evidence for slow dynamics is observed. When the sample is cooled in small magnetic fields, the phase pocket of skyrmion lattice may persist metastably down to lowest temperatures. Taken together with the large variation of the transition temperatures, transition fields, and the helix wavelength as a function of composition, this hysteresis identifies Fe$_{1-x}$Co$_{x}$Si as an ideal material for future experiments exploring, for instance, the topological unwinding of the skyrmion lattice.Comment: 14 pages, 11 figure

Extending MIEZE spectroscopy towards thermal wavelengths

We propose a Modulation of intensity with zero effort (MIEZE) set-up for high-resolution neutron spectroscopy at momentum transfers up to 3\AA$^{-1}$,energy transfers up to ~ 20 meV, and an energy resolution in the $\mu$eV-range using both thermal and cold neutrons. MIEZE has two prominent advantages compared to classical neutron spin-echo. The first one is the possibility to investigate spin-depolarizing samples or samples in strong magnetic fields without loss of signal amplitude and intensity. This allows for the study of spin fluctuations in ferromagnets, and facilitates the study of samples with strong spin-incoherent scattering. The second advantage is that multi-analyzer setups can be implemented with comparatively small effort. The use of thermal neutrons increases the range of validity of the spin-echo approximation towards shorter spin-echo times. In turn, the thermal MIEZE option for greater ranges (TIGER) closes the gap between classical neutron spin-echo spectroscopy and conventional high-resolution neutron spectroscopy techniques such as triple-axis, time-of-flight, and back-scattering. To illustrate the feasibility of TIGER we present the details of an implementation at the beamline RESEDA at FRM II by means of an additional velocity selector, polarizer and analyzer

Low-temperature properties of single-crystal CrB2_{2}

We report the low-temperature properties of $^{11}$B-enriched single-crystal CrB$_{2}$ as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, $\rho_{\rm xx}$, Hall effect, $\rho_{\rm xy}$, and specific heat, $C$, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap $\Delta \approx 220\,{\rm K}$ below $T_{\rm N}=88\,{\rm K}$. This observation is corroborated by the absence of a Curie dependence in the magnetization for $T\to0$ reported in the literature. Comparison of $C$ with $d\rho_{\rm xx}/dT$, where we infer lattice contributions from measurements of VB$_2$, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature $T_{\rm sf}\approx 257\,{\rm K}$ in the paramagnetic state, followed by a pronounced second-order mean-field transition at $T_{\rm N}$, and unusual excitations around $\approx T_{\rm N}/2$. The pronounced anisotropy of $\rho_{\rm xx}$ above $T_{\rm N}$ is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the N$\acute{\rm{e}}$el temperatures, $f=-\Theta_{\rm CW}/T_{\rm N}\approx 8.5$, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to $14\,\,{\rm T}$, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies CrB$_{2}$ as a weak itinerant antiferromagnet par excellence with strong geometric frustration.Comment: 15 pages, 9 figure

Ultra-high vacuum compatible induction-heated rod casting furnace

We report the design of a radio-frequency induction-heated rod casting furnace that permits the preparation of polycrystalline ingots of intermetallic compounds under ultra-high vacuum compatible conditions. The central part of the system is a bespoke water-cooled Hukin crucible supporting a casting mold. Depending on the choice of mold, typical rods have a diameter between 6 mm and 10 mm and a length up to 90 mm, suitable for single-crystal growth by means of float-zoning. The setup is all-metal sealed and may be baked out. We find that the resulting ultra-high vacuum represents an important precondition for processing compounds with high vapor pressures under a high-purity argon atmosphere up to 3 bar. Using the rod casting furnace, we succeeded to prepare large high-quality single crystals of two half-Heusler compounds, namely the itinerant antiferromagnet CuMnSb and the half-metallic ferromagnet NiMnSb.Comment: 5 pages, 4 figure

Turn-key module for neutron scattering with sub-micro-eV resolution

We report the development of a compact turn-key module that boosts the resolution in quasi-elastic neutron scattering by several orders of magnitude down to the low sub-micro-eV range. It is based on a pair of neutron resonance spin flippers that generate a well defined temporal intensity modulation, also known as MIEZE (Modulation of IntEnsity by Zero Effort). The module may be used under versatile conditions, in particular in applied magnetic fields and for depolarising and incoherently scattering samples. We demonstrate the power of MIEZE in studies of the helimagnetic order in MnSi under applied magnetic fields

Field dependence of non-reciprocal magnons in chiral MnSi

Spin waves in chiral magnetic materials are strongly influenced by the Dzyaloshinskii-Moriya interaction resulting in intriguing phenomena like non-reciprocal magnon propagation and magnetochiral dichroism. Here, we study the non-reciprocal magnon spectrum of the archetypical chiral magnet MnSi and its evolution as a function of magnetic field covering the field-polarized and conical helix phase. Using inelastic neutron scattering, the magnon energies and their spectral weights are determined quantitatively after deconvolution with the instrumental resolution. In the field-polarized phase the imaginary part of the dynamical susceptibility $\chi''(\varepsilon, {\bf q})$ is shown to be asymmetric with respect to wavevectors ${\bf q}$ longitudinal to the applied magnetic field ${\bf H}$, which is a hallmark of chiral magnetism. In the helimagnetic phase, $\chi''(\varepsilon, {\bf q})$ becomes increasingly symmetric with decreasing ${\bf H}$ due to the formation of helimagnon bands and the activation of additional spinflip and non-spinflip scattering channels. The neutron spectra are in excellent quantitative agreement with the low-energy theory of cubic chiral magnets with a single fitting parameter being the damping rate of spin waves.Comment: Paper: 10 pages, 5 figures Supplement: 8 pages, 11 figure

High-resolution neutron depolarization microscopy of the ferromagnetic transitions in Ni3_3Al and HgCr2_2Se4_4 under pressure

We performed neutron imaging of ferromagnetic transitions in Ni$_3$Al and HgCr$_2$Se$_4$ crystals. These neutron depolarization measurements revealed bulk magnetic inhomogeneities in the ferromagnetic transition temperature with spatial resolution of about 100~$\mu$m. To obtain such spatial resolution, we employed a novel neutron microscope equipped with Wolter mirrors as a neutron image-forming lens and a focusing neutron guide as a neutron condenser lens. The images of Ni$_3$Al show that the sample does not homogeneously go through the ferromagnetic transition; the improved resolution allowed us to identify a distribution of small grains with slightly off-stoichiometric composition. Additionally, neutron depolarization imaging experiments on the chrome spinel, HgCr$_2$Se$_4$, under pressures up to 15~kbar highlight the advantages of the new technique especially for small samples or sample environments with restricted sample space. The improved spatial resolution enables one to observe domain formation in the sample while decreasing the acquisition time despite having a bulky pressure cell in the beam

Effects of synchronous, auditory stimuli on running performance and heart rate

Research has demonstrated that the human being tends to couple body movements and external, acoustic stimuli (metronome or music). This effect is called auditory-motor synchronization. Motivational music possesses qualities which distract from feelings like fatigue and exertion. Combining these two effects may enhance the sports performance even more. Investigations showed that runners can increase their original cadence up to 2%. The purpose of this study was to examine the effects of accelerated, synchronized stimuli on the running performance. Therefore, 28 students (15 ♀, 13 ♂) of the Institute of Sports Science in Innsbruck were asked to do two cooper tests each. After run 1 (no auditory signal), the sample was divided into two groups. In run 2, which took place on a different day, one group listened to music while running; the other group was stimulated by the sound of a metronome. Distance (m) and average heartrate were measured. 75% of the athletes achieved a greater distance under the influence of an acoustic stimulus. In fact, the running distance changed significantly using an acoustic stimulus: +61 m (SD ± 100) or 2.1% (SD ± 3.6). In group 1 (music), the performance improved up to + 3.8% (SD ± 3.3). This difference was significant compared to group 2 (metronome). The average heart rate decreased by 1.5 beats (SD ± 5) from run 1 to run 2. Influenced by music, the average heart rate measured 179 beats (SD ± 8) in contrast to 182 beats (SD ± 10) in run 1 (no acoustic stimulus). If the tempo of the acoustic stimulus is adapted to the accelerated cadence of an athlete (maximum 2%), improved distances due to the synchronization effect can be achieved. In combination with the motivating qualities of music, improved effects in sport performances could be produced