  We report a study of the reorientation of the helimagnetic order in the
archetypal cubic chiral magnet MnSi as a function of magnetic field direction.
The reorientation process as inferred from small-angle neutron scattering, the
magnetization, and the ac susceptibility is in excellent agreement with an
effective mean-field theory taking into account the precise symmetries of the
crystallographic space group. Depending on the field and temperature history
and the direction of the field with respect to the crystalline axes, the helix
reorientation may exhibit a crossover, a first-order, or a second-order
transition. The magnetization and ac susceptibility provide evidence that the
reorientation of helimagnetic domains is associated with large relaxation times
exceeding seconds. At the second-order transitions residual Ising symmetries
are spontaneously broken at continuous elastic instabilities of the
helimagnetic order. In addition, on the time scales explored in our experiments
these transitions are hysteretic as a function of field suggesting, within the
same theoretical framework, the formation of an abundance of plastic
deformations of the helical spin order. These deformations comprise
topologically non-trivial disclinations, promising novel routes to spintronics
applications alongside skyrmions discovered recently in the same class of
materials.
