MnSi Films Research Articles

Texture, electronic and magnetic states of Mn atoms in polycrystalline MnSi thin films facing a c-sapphire substrate studied by soft X-ray magnetic circular dichroism

Polycrystalline MnSi is a highly efficient magnetic skyrmionic material and information carriers for future spintronic devices, deposited on a c-cut sapphire substrate by magnetron sputtering with a low base pressure. Due to their fascinating crystal structures and spin textures these Mn-based silicides are most extensively studied now a days. In order to study the appearance of any impure phases like MnxSiy, hysteresis behavior and the inception of ferromagnetic/superparamagnetic domains present in the MnSi film, we have performed X-ray diffraction (XRD), XRD pole figures in stereographic projection, vibrating sample magnetometry and powerful element specific X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism techniques. The room temperature hysteresis curve points towards the phenomenon of long-range ferromagnetic ordering. The Mn L2,3 edge XAS spectra did not show a Mn2+-like multiplet structure. The overall magnetization increases with the increase in applied magnetic field from 0.1 T to 2 T on MnSi thin film. The observed spin and orbital magnetic moments ∼ 0.03 ̶ 0.1 μB and 0.01 to 0.04 μB, respectively.

On Curie temperature of B20-MnSi films

B20-type MnSi is the prototype magnetic skyrmion material. Thin films of MnSi show a higher Curie temperature than their bulk counterpart. However, it is not yet clear what mechanism leads to the increase of the Curie temperature. In this work, we grow MnSi films on Si(100) and Si(111) substrates with a broad variation in their structures. By controlling the Mn thickness and annealing parameters, the pure MnSi phase of polycrystalline and textured nature as well as the mixed phase of MnSi and MnSi1.7 are obtained. Surprisingly, all these MnSi films show an increased Curie temperature of up to around 43 K. The Curie temperature is likely independent of the structural parameters within our accessibility including the film thickness above a threshold, strain, cell volume and the mixture with MnSi1.7. However, a pronounced phonon softening is observed for all samples, which can tentatively be attributed to slight Mn excess from stoichiometry, leading to the increased Curie temperature.

B20–MnSi films grown on Si(100) substrates with magnetic skyrmion signature

Magnetic skyrmions have been suggested as information carriers for future spintronic devices. As the first material with experimentally confirmed skyrmions, B20-type MnSi was the research focus for more than a decade. Although B20–MnSi films have been successfully grown on Si(111) substrates, there is no report about B20–MnSi films on Si(100) substrates, which would be more preferred for practical applications. In this letter, we present the first preparation of B20–MnSi on Si(100) substrates. It is realized by sub-second solid-state reaction between Mn and Si via flash-lamp annealing at ambient pressure. The regrown layer shows an enhanced Curie temperature of 43 K compared with bulk B20–MnSi. The magnetic skyrmion signature is obtained in our films by magnetic and transport measurements. The millisecond-range flash-lamp annealing provides a promising avenue for the fabrication of Si-based skyrmionic devices.

Making ferromagnetic metal MnSi ultrathin films semiconductor

Atomically flat MnSi films were fabricated on Si(111)-7 × 7 reconstructed surface by molecular beam epitaxy(MBE). Both scanning tunneling microscopy (STM) images and low energy electron diffraction (LEED) patterns demonstrate a well-defined (3×3)R30o structure reconstruction. A thickness-driven metal–semiconductor transition in MnSi ultrathin films was observed with decreasing the thickness down to 6 ML (monolayers). The temperature dependence of the resistance and the negative magnetoconductivity suggest the MnSi ultrathin films with thickness lower than 6ML exhibit weak anti-localization (WAL) of two-dimensional (2D) electron systems. This finding that not only advances our understanding of the mechanism of thickness-driven metal–semiconductor transition, but also provides a new strategy to use ferromagnetic semiconductor as spin injector in spintronic devices.

Phase Selection in Mn–Si Alloys by Fast Solid‐State Reaction with Enhanced Skyrmion Stability

AbstractB20‐type transition‐metal silicides or germanides are noncentrosymmetric materials hosting magnetic skyrmions, which are promising information carriers in spintronic devices. The prerequisite is to prepare thin films on technology‐relevant substrates with magnetic skyrmions stabilized at a broad temperature and magnetic‐field working window. A canonical example is the B20‐MnSi film grown on Si substrates. However, the as‐yet unavoidable contamination with MnSi1.7 occurs due to the lower nucleation temperature of this phase. In this work, a simple and efficient method to overcome this problem and prepare single‐phase MnSi films on Si substrates is reported. It is based on the millisecond reaction between metallic Mn and Si using flash‐lamp annealing (FLA). By controlling the FLA energy density, single‐phase MnSi or MnSi1.7 or their mixture can be grown at will. Compared with bulk MnSi, the prepared MnSi films show an increased Curie temperature of up to 41 K. In particular, the magnetic skyrmions are stable over a much wider temperature and magnetic‐field range than reported previously. The results constitute a novel phase selection approach for alloys and can help to enhance specific functional properties, such as the stability of magnetic skyrmions.

Determination of crystallographic chirality of MnSi thin film grown on Si (111) substrate

MnSi is a prototypical chiral cubic magnet, in which swirling magnetic objects referred to as magnetic skyrmions appear. It is expected that the thin film of MnSi may expand the range of the magnetic skyrmion lattice phase in the temperature--magnetic field diagram. On the other hand, the MnSi film on an achiral Si (111) substrate may suffer from chiral twin formation. We succeeded in visualizing the domain structure of the chirality and axis orientation of a MnSi thin film by using a combination of transmission electron microscopy and x-ray reflectivity measurement. Only two types of domains were found on a Si (111) substrate. The information is essential for the challenge of homochiral film fabrication.

Strain in epitaxial MnSi films on Si(111) in the thick film limit studied by polarization-dependent extended x-ray absorption fine structure

We report a study of the strain state of epitaxial MnSi films on Si(111) substrates in the thick film limit (100-500~\AA) as a function of film thickness using polarization-dependent extended x-ray absorption fine structure (EXAFS). All films investigated are phase-pure and of high quality with a sharp interface between MnSi and Si. The investigated MnSi films are in a thickness regime where the magnetic transition temperature $T_\mathrm{c}$ assumes a thickness-independent enhanced value of $\geq$43~K as compared with that of bulk MnSi, where $T_\mathrm{c} \approx 29~{\rm K}$. A detailed refinement of the EXAFS data reveals that the Mn positions are unchanged, whereas the Si positions vary along the out-of-plane [111]-direction, alternating in orientation from unit cell to unit cell. Thus, for thick MnSi films, the unit cell volume is essentially that of bulk MnSi --- except in the vicinity of the interface with the Si substrate (thin film limit). In view of the enhanced magnetic transition temperature we conclude that the mere presence of the interface, and its specific characteristics, strongly affects the magnetic properties of the entire MnSi film, even far from the interface. Our analysis provides invaluable information about the local strain at the MnSi/Si(111) interface. The presented methodology of polarization dependent EXAFS can also be employed to investigate the local structure of other interesting interfaces.

Helimagnetic order in bulk MnSi and CoSi/MnSi superlattices

Skyrmions are nanoscopic whirls of spins that reside in chiral magnets. It is only fairly recent that a plethora of applications for these quasiparticles emerges, especially in data storage. On the other hand, spin spirals are the periodic analogs of skyrmions, and are equally imperative in the course of exploration to enhance our understanding of helimagnetism. In this study, a new infrastructure based on the B20 compound, MnSi is propounded as a hosting material for spin spirals; alternating thin layers of CoSi and MnSi in the superlattice form provides a facile way of varying the properties of the spin spirals across a continuum. Using first-principles calculations based on full-potential linearized augmented plane-wave (FLAPW)-based density functional theory (DFT), the spin order of bulk MnSi, MnSi film, and the CoSi/MnSi superlattice is investigated. Spin dispersion plots as a function of propagation vectors show that the spiral size changes in the presence of CoSi – we find that the size of the spiral is reduced in the superlattice with thin CoSi layers (CoSi:MnSi=1:1 thickness ratio), whilst at a larger CoSi:MnSi=2:1 thickness ratio, the material behaves as a ferromagnet. In a similar fashion, the spin moment and orbital occupancy depend significantly on the thickness of the CoSi layers. However, the exchange interaction between Mn atoms appears to be generally impervious to the presence of CoSi. Succinctly, the CoSi/MnSi superlattice could be an excellent functional material in data storage applications.

MnSi薄膜における静水圧効果

Effects of hydrostatic pressure on a thin film of helimagnet MnSi were reviewed. The MnSi film exhibits similar non-Fermi liquid (NFL) behavior near the pressure-induced quantum critical point to that observed in bulk MnSi, although its temperature range is extended to higher temperature side compared with the bulk sample. Since the NFL behavior of MnSi is related with presence of “Skyrmion”, the extended NFL region observed in the thin film may come from stabilization of the Skyrmions due to uniaxial anisotropy. Furthermore, we discussed a general role of hydrostatic pressure in a thin-film sample grown by molecular beam epitaxy.

Formation of In-plane Skyrmions in Epitaxial MnSi Thin Films as Revealed by Planar Hall Effect

We investigate skyrmion formation in both a single crystalline bulk and epitaxial thin films of MnSi by measurements of planar Hall effect. A prominent stepwise field profile of planar Hall effect is observed in the well-established skyrmion phase region in the bulk sample, which is assigned to anisotropic magnetoresistance effect with respect to the magnetic modulation direction. We also detect the characteristic planar Hall anomalies in the thin films under the in-plane magnetic field at low temperatures, which indicates the formation of skyrmion strings lying in the film plane. Uniaxial magnetic anisotropy plays an important role in stabilizing the in-plane skyrmions in the MnSi thin film.

Fermi surface symmetry and evolution of the electronic structure across the paramagnetic-helimagnetic transition in MnSi/Si(111)

MnSi has been extensively studied for five decades, nonetheless detailed information on the Fermi surface (FS) symmetry is still lacking. This missed information prevented from a comprehensive understanding the nature of the magnetic interaction in this material. Here, by performing angle-resolved photoemission spectroscopy on high-quality MnSi films epitaxially grown on Si(111), we unveil the FS symmetry and the evolution of the electronic structure across the paramagnetic-helimagnetic transition at T$_C$ $\sim$ 40 K, along with the appearance of sharp quasiparticle emission below T$_C$. The shape of the resulting FS is found to fulfill robust nesting effects. These effects can be at the origin of strong magnetic fluctuations not accounted for by state-of-art quasiparticle self-consistent GW approximation. From this perspective, the unforeseen quasiparticle damping detected in the paramagnetic phase and relaxing only below T$_C$, along with the persistence of the d-bands splitting well above T$_C$, at odds with a simple Stoner model for itinerant magnetism, open the search for exotic magnetic interactions favored by FS nesting and affecting the quasiparticles lifetime.

The influence of Co concentration on the anisotropic magnetoresis- tance and half-metallicity in epitaxial Co-Mn-Si Heusler films

Systematic Co-Mn-Si films have been prepared by changing the Co atomic concentration. Co 50 MnSi films were found to be good B2 ordering, the small amount of impurities and defects (large RRR value) and negative anisotropic magnetoresistance. With increasing Co concentration, the B2 ordering and RRR value decrease and anisotropic magnetoresistance increases in Co-Mn-Si films. Due to the good half-metallicity of Co 50 MnSi films (it is reported that E G = 0.4-0.6 eV), Co 70 MnSi films still show negative anisotropic magnetoresistance ratio although the increasing of Co concentration will deteriorate the half-metallicity of Co-Mn-Si films.

Thermally driven ratchet motion of a skyrmion microcrystal and topological magnon Hall effect

Spontaneously emergent chirality is an issue of fundamental importance across the natural sciences. It has been argued that a unidirectional (chiral) rotation of a mechanical ratchet is forbidden in thermal equilibrium, but becomes possible in systems out of equilibrium. Here we report our finding that a topologically nontrivial spin texture known as a skyrmion--a particle-like object in which spins point in all directions to wrap a sphere--constitutes such a ratchet. By means of Lorentz transmission electron microscopy we show that micrometre-sized crystals of skyrmions in thin films of Cu2OSeO3 and MnSi exhibit a unidirectional rotation motion. Our numerical simulations based on a stochastic Landau-Lifshitz-Gilbert equation suggest that this rotation is driven solely by thermal fluctuations in the presence of a temperature gradient, whereas in thermal equilibrium it is forbidden by the Bohr-van Leeuwen theorem. We show that the rotational flow of magnons driven by the effective magnetic field of skyrmions gives rise to the skyrmion rotation, therefore suggesting that magnons can be used to control the motion of these spin textures.

Fabrication of Fully-Epitaxial Co$_{{{2}}}$MnSi/Ag/Co$_{{{2}}}$MnSi Giant Magnetoresistive Devices by Elevated Temperature Deposition

(001)-oriented epitaxial Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnSi (CMS) films were grown by elevating the substrate temperature during deposition instead of a conventional post-annealing process. The CMS film deposited at 250°C showed a very flat surface morphology, large saturation magnetization, and a highly L2 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> -ordered crystal structure. A CMS/Ag/CMS giant-magnetoresistive device with CMS layers grown at 250°C showed the high magnetoresistance ratio (33%) at room temperature. This ratio was close to that realized in the case of samples fabricated using post-annealing at 500-550°C. The elevated temperature deposition for the Heusler electrodes is a promising method to fabricate a practical magnetic read sensor for next generation hard disc drives without high temperature annealing process over 300°C.

Thin film MnGe grown on Si(111)

MnGe has been grown as a thin film on Si(111) substrates by molecular beam epitaxy. A 10 Å layer of MnSi was used as the seed layer in order to establish the B20 crystal structure. Films of a thickness between 45 and 135 Å have been prepared and structurally characterized using reflection high-energy electron diffraction, atomic force microscopy and x-ray diffraction. These studies provided evidence that MnGe forms in the cubic B20 crystal structure as islands exhibit a very smooth surface. The islands become larger with increasing film thickness. A magnetic characterization reveals that the ordering temperature of MnGe thin films is enhanced compared to that for bulk material. The properties of the helical magnetic structure obtained from magnetization and magnetoresistivity measurements are compared with those of films of the related compound MnSi. The much stronger Dzyaloshinskii–Moriya interaction in MnGe results in a higher rigidity of the spin helix.

Strain stabilization and thickness dependence of magnetism in epitaxial transition metal monosilicide thin films on Si(111)

We present a comprehensive study of different $3d$ transition metal monosilicides in their ground state crystal structure (B20), ranging from equilibrium bulk over biaxially strained bulk to epitaxial thin films on Si(111), by means of density functional theory. The magnetic properties of MnSi and FeSi films are found to be considerably modified due to the epitaxial strain induced by the substrate. In MnSi bulk material, which can be seen as a limit of thick films, we find a strain-induced volume expansion, an increase of the magnetic moments, and a significant rise of the energy difference between different spin configurations. The latter can be associated with an increase of the Curie temperature, which is in accordance with recent experimental results. While a ferromagnetic spin alignment is found to be the ground state also for ultrathin films, we show that for films of intermediate thickness a partially compensating magnetic ordering is more favorable; however, the films retain a net magnetic moment. Furthermore, we analyze the orbital structure in FeSi around the band gap, which can be located somewhere in the density of states for all studied B20 transition metal monosilicides, and find that FeSi becomes metallic and ferromagnetic under epitaxial strain. Finally, the influence of on-site electronic correlation and the reliability of ab initio calculations for $3d$ transition metal monosilicides are discussed.

Observation of the Magnetic Skyrmion Lattice in a MnSi Nanowire by Lorentz TEM

We report here the real-space observation of skyrmions and helical magnetic domains in a MnSi nanowire (NW) using Lorentz transmission electron microscopy (LTEM). The MnSi NW was thinned to a rectangular cross-section by focused-ion beam milling to reduce obstructive Fresnel fringes. Helimagnetic domains, imaged as alternating bright and dark contrast stripes with an 18 nm period, were observed to be the spontaneous magnetic ground state at 6 K, while the hexagonal skyrmion lattice (SkX) with a domain diameter of 18 nm was observed under a normal magnetic field of 210 mT. Temperature-dependent measurements reveal that the SkX is stable over a larger range in this NW system (6-35 K) compared to the narrow temperature regime of skyrmion phase in bulk MnSi (26-30 K) and thin films of MnSi (5-23 K).

Enhanced critical fields in MnSi thin films

The magnetic properties of the cubic helimagnet MnSi are governed by the Dzyaloshinskii-Moriya interaction which in the bulk stabilizes a helical ground state below 29 K with a spin-helix wavelength of 18 nm and a propagation vector along the [111] direction. Recently, MnSi has attracted a lot of attention after a skyrmion lattice had been discovered in the so called A-phase. We have prepared thin MnSi films on Si(111) substrates by codeposition of Mn and Si. Two films with a thickness below and above the spin helix length have been investigated using SQUID magnetometry. In comparison to bulk material, the films show an enhanced magnetic ordering temperature of 45 K. Magnetoresistance measurements performed on the films reveal a decrease of the resistivity with increasing field. They show together with the magnetic characterization that the critical fields describing the transition from the helical to a conical spin phase and from the conical phase to a parallel spin alignment are dependent on the film thickness and enhanced in comparison to single crystalline MnSi. The form anisotropy which prefers a spin alignment in the film plane is suggested to be the origin for the enhancement of the critical fields.

Spin–Spin Correlation Length in MnSi Thin Films

Thin MnSi films have been prepared on Si(111) by molecular beam epitaxy under simultaneous evaporation of Mn and Si. RHEED analysis in combination with TEM investigation reveals that this codeposition technique leads to smooth films. Resistivity measurements show a metallic behavior that is qualitatively the same as for bulk MnSi. The magnetic ordering temperature T ord of 43 K, which has been determined from resistivity data and by SQUID magnetometry, is considerably enhanced compared to the bulk value. For the thinnest films, however, a decrease of T ord is observed which is explained by the reduction of the spin–spin interaction near the surfaces of the film. Recently, it has been stated, that the spin–spin correlation length is much larger than known from other itinerant magnetic materials. In contrast, we show that the coupling range can be estimated as 7 monolayers and does not exceed the values of common magnets.

Growth mode and atomic structure of MnSi thin films on Si(111)

Thin films of MnSi(111) in B20 structure formed by reactive epitaxy on Si(111) are studied using scanning tunneling microscopy (STM) and density functional theory calculations. Coexisting root 3 x root 3 structures with high or low corrugation are observed and assigned to different Mn coverage by using a detailed analysis of simulated STM images. Comparison with our interpretation of STM images of films previously grown by codeposition of Mn and Si provides us with evidence that the stacking sequence of Mn and Si lattice planes depends on the growth protocol.