On the magnetic-structure origin of giant magnetostrictive effect in MnCoSi-based metallic helimagnets

Giant magnetostrictive effect, the huge volume and/or shape change of a material under an external magnetic field, is recently discovered in helimagnets with large magnetoelastic interactions. Here we revisit this effect in MnCoSi, a helical metamagnet with a unique Lifshitz tricritical point. By introducing minimal amount of Ni, the modified competing exchange interaction simultaneously lowers the tricritical temperature and even critical field of metamagnetism, leading to a giant, reversible, low-field magnetostrictive effect at ambient temperature. When applying a cyclic low field of 0.5 T, a giant reversible magnetostriction of 550 ppm can be achieved and maintained at 300 K. Interestingly, by advanced 3D-vector magnetic field measurement, a strong relation is found between magnetocrystalline anisotropy and the critical field of magnetostrictive effect. In-situ neutron powder diffraction results further reveal that the incommensurate helical antiferromagnetic structure with moment lying in ab plane transits into a ferromagnetic structure along b axis with increasing magnetic fields. During this metamagnetic transition, dramatical spin rotation triggers sharp anisotropic lattice change via strong magnetoelastic coupling, resulting in giant magnetostrictive effect. The saturated magnetostriction value is closely related with the amplitude of spin rotations. These findings not only design of a potential MnCoSi-based magnetostrictive material but also uncover the specific evolution of magnetic structures in the metamagnetic transition and the possible mechanism behind the macroscopic size change.