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Register a new userMacroscopic evidence of skyrmion lattice inhomogeneity and magnetic vortex states in the A-phase of MnSi
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The magnetic inhomogeneity of the A-phase in MnSi chiral magnet is identified for the first time from the precise measurements of transverse magnetoresistance (MR) anisotropy. The area inside the A-phase (A-phase core) corresponds to isotropic MR having no confinement to the MnSi crystal lattice. Per contra, the MR becomes anisotropic both on the border of the A-phase and in other magnetic phases, the strongest magnetic scattering being observed when external magnetic field applied along [001] or [00-1] directions. We argue here that the established MR features prove the presence of two different types of the skyrmion lattices inside the A-phase, and the dense skyrmion state of the A-phase core is built from individual skyrmions similar to Abrikosov-type magnetic vortexes.
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We present a comprehensive analysis of high resolution neutron scattering data involving Neutron Spin Echo spectroscopy and Spherical Polarimetry which confirm the first order nature of the helical transition and reveal the existence of a new spin liquid skyrmion phase. Similar to the blue phases of liquid crystals this phase appears in a very narrow temperature range between the low temperature helical and the high temperature paramagnetic phases.
Using small-angle neutron scattering (SANS), we investigate the deformation of the magnetic skyrmion lattice in bulk single-crystalline MnSi under electric current flow. A significant broadening of the skyrmion-lattice-reflection peaks was observed in the SANS pattern for current densities greater than a threshold value j_t ~ 1 MA/m^2 (10^6 A/m^2). We show this peak broadening to originate from a spatially inhomogeneous rotation of the skyrmion lattice, with an inverse rotation sense observed for opposite sample edges aligned with the direction of current flow. The peak broadening (and the corresponding skyrmion lattice rotations) remain finite even after switching off the electric current. These results indicate that skyrmion lattices under current flow experience significant friction near the sample edges, and plastic deformation due to pinning effects, these being important factors that must be considered for the anticipated skyrmion-based applications in chiral magnets at the nanoscale.
We investigate the anisotropic nature of magnetocrystalline coupling between the crystallographic and skyrmion crystal (SKX) lattices in the chiral magnet MnSi by magnetic field-angle resolved resonant ultrasound spectroscopy. Abrupt changes are observed in the elastic moduli and attenuation when the magnetic field is parallel to the [011] crystallographic direction. These observations are interpreted in a phenomenological Ginzburg-Landau theory that identifies switching of the SKX orientation to be the result of an anisotropic magnetocrystalline coupling potential. Our paper sheds new light on the nature of magnetocrystalline coupling potential relevant to future spintronic applications.
We report high-precision small angle neutron scattering of the orientation of the skyrmion lattice in a spherical sample of MnSi under systematic changes of the magnetic field direction. For all field directions the skyrmion lattice may be accurately described as a triple-$vec{Q}$ state, where the modulus $vert vec{Q} vert$ is constant and the wave vectors enclose rigid angles of $120^{circ}$. Along a great circle across $langle 100rangle$, $langle 110rangle$, and $langle 111rangle$ the normal to the skyrmion-lattice plane varies systematically by $pm3^{circ}$ with respect to the field direction, while the in-plane alignment displays a reorientation by $15^{circ}$ for magnetic field along $langle 100rangle$. Our observations are qualitatively and quantitatively in excellent agreement with an effective potential, that is determined by the symmetries of the tetrahedral point group $T$ and includes contributions up to sixth-order in spin-orbit coupling, providing a full account of the effect of cubic magnetocrystalline anisotropies on the skyrmion lattice in MnSi.
Precision measurements of the magnetization and ac susceptibility of Cu$_2$0SeO$_3$ are reported for magnetic fields along different crystallographic directions, focussing on the border between the conical and the field-polarized state for a magnetic field along the $langle 100 rangle$ axis, complemented by selected specific heat data. Clear signatures of the emergence of a second skyrmion phase and a tilted conical phase are observed, as recently identified by means of small-angle neutron scattering. The low-temperature skyrmion phase displays strongly hysteretic phase boundaries, but no dissipative effects. In contrast, the tilted conical phase is accompanied by strong dissipation and higher-harmonic contributions, while the transition fields are essentially nonhysteretic. The formation of the second skyrmion phase and tilted conical phase are found to be insensitive to a vanishing demagnetization factor. A quantitative estimate of the temperature dependence of the magnetocrystalline anisotropy may be consistently inferred from the magnetization and the upper critical field and agrees well with a stabilization of the low-temperature skyrmion phase and tilted conical state by conventional cubic magnetic anisotropies.
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