Possible skyrmion-lattice ground state in the B20 chiral-lattice magnet MnGe as seen via small-angle neutron scattering

We have investigated the magnetic structure in a polycrystalline sample of the B20-type MnGe by means of small-angle neutron scattering. On the projected diffraction plane normal to the incoming neutron beam, a Debye-ring-like pattern appears due to the random orientation of the spin helix q vectors (vertical bar vertical bar < 100 >). When an external magnetic field is applied normal to the incoming neutron beam, an intense peak with wave vector (q) perpendicular to the applied magnetic field is observed as the hallmark of the formation of a skyrmion lattice with a multiple-q helix in a wide temperature-magnetic-field region. This scattering intensity remains even after removing the magnetic field, which indicates that a skyrmion lattice is stabilized as the ground state. A different form of skyrmion lattice, either square or cubic, is proposed, which is also shown to be in good agreement with previous high-angle neutron diffraction results. Calculations based on such structures also describe the magnetic-field profile of the topological Hall resistivity.