MnSi Single Crystal Research Articles

Probe of skyrmion phases and dynamics in MnSi via the magnetoelectric effect in a composite configuration

We have developed a sensitive technique to probe the magnetic skyrmion phases and dynamics by employing the interfacial coupling effect in a magnetoelectric composite configuration. The study on a MnSi single crystal sample using this technique provides clear evidence for the skyrmion lattice phase and coexistence of skyrmion and conical phase. Above the Curie temperature ${T}_{\mathrm{C}}$, a region with strong spin fluctuation is revealed as well. By tuning the density of skyrmion or disorder, a transition from the skyrmion lattice to skyrmion-conical coexisting phase is observed. The observation is in good agreement with a theoretical model which predicts the dissipation behavior in the coexistence phase.

Влияние кристаллической структуры подложки на магнитную анизотропию плeнок Mn-=SUB=-x-=/SUB=-Si-=SUB=-1-x-=/SUB=- (x~0.5)

Thin films of MnxSi1-x (x ≈ 0.5) alloy were synthesized by pulsed laser deposition in a droplet-free mode on sapphire substrates with different crystallographic orientations of the cut plane (c-Al2O3 and r-Al2O3) at various laser energy densities E at the polycrystalline MnSi target. The X-ray structural, as well as static and resonance magnetic properties of the films were studied depending on E and the orientation of the substrate. The films deposited at high E > 6 J/cm2 are found to demonstrate the presence of a ferromagnetic phase with anomalously high Curie temperature TC ~ 300 K which is not typical of MnSi single crystals (TC ≈ 30 K). In the case of the c-Al2O3 sub-strates, the magnetic moment of the films turns out to be somewhat higher than in the case of the r-Al2O3 substrates. Moreover, the films deposited on the c-Al2O3 substrates exhibit noticeably higher values of the effective surface anisotropy field 4πMeff measured by ferromagnetic resonance. The data obtained indicate a significant effect of the substrate structure on the formation and magnetic properties of the high-temperature ferromagnetic phase in the MnxSi1-x films.

Field evolution of the magnetic phase transition in the helical magnet MnSi inferred from ultrasound studies

The longitudinal and transverse ultrasound speeds and attenuation were measured in a MnSi single crystal in the temperature range of 2 - 40 K and magnetic fields to 7 Tesla. The magnetic phase diagram of MnSi in applied magnetic field appears to depend on the experimental setups, which is related to a difference in demagnetization factors arising due to the disc shape of the sample. The magnetic phase transition in MnSi in zero magnetic field is signified by a quasi discontinuity in the c11 elastic constant, which varies significantly with magnetic field. It is notable that the region where the c11 discontinuity almost vanishes closely corresponds to the extent of skyrmion phase along the magnetic to paramagnetic transition. This implies that the c11 elastic constant is almost continuous through the transition from the skyrmion to paramagnetic phases. A recovery of the discontinuity of c11 and enhanced sound absorption occur at the crossing of the phase transition line and the line of minima in c11. The powerful fluctuations at the minima of c11 make the mentioned crossing point similar to a critical end point, where a second order phase transition meets a first order one. The skyrmion domain in the case of a perpendicular setup with a smaller demagnetization factor has a reduced temperature range, which suggests that the magnetic field inhomogeneity plays an important role in the skyrmion occurrence and, hence, opens a way of skyrmion manipulation. The small anisotropy of the shear moduli in the (001) plane found in the parallel setup is most probably also caused by the magnetic field inhomogeneity, which distorts the hexagonal symmetry of the skyrmion crystal.

Specific features of magnetic phase diagram of an MnSi helimagnet

Magnetization curves of MnSi single crystals have been measured in a range of temperatures T = 5.5–35 K and magnetic field strengths H ≤ 11 kOe for H ∥ [1 1 1], [0 0 1], and [1 1 0]. Special attention has been paid to the temperature interval near T N = 28.8 K, where MnSi exhibits a transition to the state with a long-period helical magnetic structure. Some new features in the magnetic behavior of MnSi have been found. In particular, in an intermediate temperature region above the transition (28.8 K = T N ≤ T < 31.5 K), the dM(H)/dH curves exhibit anomalies that are not characteristic of the typical paramagnetic state. It is established that the line of the characteristic field H*(T) of this anomaly is a natural extrapolation of the temperature dependence of the field of the transition from a conical phase to an induced ferromagnetic phase observed at T < T N. It is concluded that the properties of MnSi in the indicated intermediate region are related to and governed by those of the conical phase (rather than of the A phase). Based on these data, magnetic phase diagrams of MnSi for H ∥ [1 1 1], [0 0 1], and [1 1 0] are plotted and compared to diagrams obtained earlier by other methods.

Phase diagram of the itinerant helimagnet MnSi from high-pressure resistivity measurements and the quantum criticality problem

We performed a series of resistivity measurements on a MnSi single crystal at high pressures, created by a piston-cylinder device with a liquid pressure medium. The form of the resistivity curve at ambient pressure clearly indicates a first-order nature of the magnetic phase transition in MnSi. Application of high pressure rapidly degrades the first-order features of the phase transition. The temperature derivative of resistivity demonstrates two notable features of the phase transition that disappear on increasing pressure: a sharp peak marking the first-order phase transition and a shallow maximum situated slightly above the critical temperature and pointing to prominent helical fluctuations. The current experimental data rule out any strong first-order phase transition in MnSi at high pressures and low temperatures, which would prevent development of a quantum critical region. On the contrary, there should exist true quantum critical phenomena in MnSi at high pressures because a weak first-order transition, if it survives at high pressures to the lowest temperatures, should not suppress the entire quantum critical region.

Structural and thermal properties of MnSi single crystal

Polarized Raman spectroscopy of MnSi single crystal was carried out to characterize its phonons, crystal structure, and thermal stability. The Raman spectra show correct Raman selection rules and consistence with those of the other transition metal silicide compounds. The MnSi thermal stability and phase transformation is investigated by monitoring the evolution of Raman spectrum as a function of the laser intensity, in which three compositions, MnSi, MnSiO3, and Mn5Si3, can be identified. The involved oxidation reaction is then proposed and verified by performing the thermogravimetric and x-ray diffraction analysis.

Systematic study of the itinerant ferromagnet MnSi at ambient and high hydrostatic pressures

The intermetallic compound MnSi experiences a phase transition at a temperature slightly below 30 K, acquiring helical magnetic structure and becoming a weak itinerant ferromagnet. We have carried out precise resistivity and ac susceptibility measurements of MnSi single crystals across the phase transition line at ambient and high pressures, using a compressed helium technique. In addition, heat capacity, thermal expansion and dc magnetic susceptibility were measured at ambient pressure. Conclusions drawn from these experiments propose that the phase transition in MnSi is of first order at ambient pressure and may become continuous at high pressure and low temperature.

Manganese silicide single crystal and films deposited on Si(1 1 1): A comparative spectroscopic study

A comparative experimental study is presented of the electronic properties of MnSi films grown on Si(1 1 1) and of MnSi single crystals, using X-ray absorption spectroscopy (XAS), and core level and valence band photoemission spectroscopy (PES). No significant differences in the electronic structure of the two systems can be found. Absorption measurements on the Mn 2p threshold show a mixed valence ground state, where the multiplet structure is washed out by the hybridisation of the Mn 3d states with the Si sp states. These results are also confirmed by photoemission (PE) spectra from the valence band and the Mn 3s, 3p and 2p core levels. Strong attention has been paid to the effect of contamination. The occurrence of multiplet effects in the absorption spectra indicates unambiguously the localisation of the Mn 3d electrons in Mn–O bonds, which strongly influences the electronic properties of these systems.

Anomalous behavior of the electrical resistivity of MnSi near the ferromagnetic phase transition

The results of measuring the electrical resistivity of a MnSi single crystal near the ferromagnetic phase transition at atmospheric and high pressures are reported. In contrast to the previous works, compressed helium is used as a pressure-transferring medium. It is shown that the temperature derivative of the electrical resistivity has the form of sharp maximum on the phase-transition line over the entire its length. Moreover, the observed maxima have a fine structure exhibiting a pronounced shoulder at temperatures slightly higher (by approximately 0.5 K) than the peak temperature, which indicates the existence of nontrivial fluctuations in the paramagnetic phase of MnSi. This feature disappears at a pressure of about 0.35 GPa, which corresponds to the tricritical-point coordinate.

On the tricritical point in MnSi under high pressures

The magnetic susceptibility of a MnSi single crystal is measured in the region of the ferromagnetic phase transition under pressures up to 0.8 GPa in compressed helium. It is found that the tricritical point on the phase-transition curve corresponds to a much lower pressure and a considerably higher temperature (P tr ≈ 0.355 GPa and T tr ≈ 25.2 K) than was reported earlier (P tr ≈ 1.2 GPa and T tr ≈ 12 K). New results impose certain limitations on theoretical analysis of tricritical phenomena in MnSi.

Magnetization measurement of MnSi just above the critical pressure

Magnetization measurements have been performed on a MnSi single crystal under high pressures up to 1.54GPa at magnetic fields up to 17T in the temperature range from 1.7 to 60K. At 1.54GPa, we have clearly observed an itinerant metamagnetic transition around 0.1T with hysteresis in the magnetization process below 6K and determined the magnetic phase diagram at 1.54GPa.

Growth and Properties of Mn5Si3, MnSi and Mn27Si47 Single Crystals Obtained by the Tin-Flux Method

The single crystals of Mn5Si3 (hexagonal system), MnSi (cubic system) and Mn27Si47(tetragonal system) were prepared by the high temperature tin solution method using manganese metal powders and silicon powders as starting materials in an argon atmosphere. The conditions for obtaining these crystals with one-phase materials and relatively large size were determined. As-grown Mn5Si3, MnSi and Mn27Si47 single crystals were used for chemical analysis and measurements of unit cell dimensions, densities, Vickers microhardness and electrical resistivity. The optimum conditions for growing Mn5Si3, MnSi and Mn27Si47single crystals are the following atomic ratios of starting materials (Si/Mn, Sri/Mn), soaking temperature and soaking time; Si/Mn =0.22, Sn/Mn =10.17, 1100 °C, 10 h for Mn5Si3, Si/Mn =1.0, Sn/Mn =10.17, 1100 °C, 10 h for MnSi and S i /Mn =1.74, Sn/Mn =6.36, 1000 °C, 20 h for Mn27Si47, respectively. Under these conditions, Mn5Si3 single crystals, having grayish and metallic luster, were generally obtained in the form of prismatic shape extending to direction, MnSi single crystals, having grayish and metallic luster, were generally 708 f t 1992 No.7obtained in the form of prismatic shape extending to direction, and Mn27Si47si ngle crystals were grayish and metallic luster, having a nearly spherical polyhedral. The largest crystals prepared have the maximum dimensions of about 0.1 mm x O.1 mm x 6.4 mm for Mn5Si3, 0.9 mm x 1.0 mm x 9.2 mm for MnSi and O.2 5 mm x O.2 5 mm x 0.2 5 mm for Mn27S i4.7The results of chemical analysis and measurements of unit cell dimensions, Vickers microhardness (H), densities (d. ), electrical resistivity (p) are as follows:Mn5Si3; M n5.45Si3, a = 6.9 09( 2) A, c = 4.815( 1) A, V=199.0 5( 3 ) A3, dm= 5.9 5( 2 ) gfcm3, Hy = 820±-3 0 k gf/rnm2, p = 3500±230 i tC-c2m MnSi; Mn0 9 8Si, a =4.5594( 6 ) A, V=94.7 8( 1 )A., dm= 5.8 0( 1 ) g/cm3, Hy=1100±-40k gf/mm2, = 3 200±-1 8 0/ I Q c m Mn27Si47; Mn2 7.9 8Si4 7, a=5.530( 1 ) A, c=117.8 6( 2 ) A, V=3604.2 6( 8 ) A3, dm= 5.17( 2 ) g/cm3

Spiral magnetic correlation in cubic MnSi

MnSi is a cubic ferromagnet with a long-wavelength spiral below its Curie temperature at 29 K. Recent theoretical and experimental studies have shown that this spiral is only right handed because of the lack of a center of symmetry in the Mn atomic arrangement. We have carried out elastic and inelastic neutron scattering experiments on a MnSi single crystal with the use of a neutron polarized-beam technique. A characteristic polarization dependence was observed in the magnetic Bragg reflections of the spiral phase, as well as the spin-wave excitations and the critical scattering near ${T}_{c}$. This polarization dependence persists into the ferromagnetic phase under a magnetic field and also at temperatures as high as 150 K.

Electron Spin Resonance in the Itinerant-Electron Helical Magnet MnSi

Electron spin resonance in MnSi single crystals was observed both above and below the transition temperature T N . The experimental results below T N were explained by applying the helical spin resonance theory developed by Copper and Elliott and the effective exchange fields { J ( q 0 )- J (0)} S / g µ B and { J (0)- J (2 q 0 )} S / g µ B were obtained as 3.0 kOe and 1.0 kOe, respectively, where q 0 is the screw vector. A dynamical anisotropy field should be taken into account to explain the experimental results. The paramagnetic resonance intensity above T N decreases strongly as temperature increases.