  We present magneto-optic measurements on two materials that form the recently
discovered twist-bend nematic (Ntb) phase. This intriguing state of matter
represents a new fluid phase that is orientationally anisotropic in three
directions and also exhibits translational order with periodicity several times
larger than the molecular size. NTB materials may also spontaneously form a
visible, macroscopic stripe texture. We show that the optical stripe texture
can be persistently inhibited by a magnetic field, and a 25T external magnetic
field depresses the N-Ntb phase transition temperature by almost 1 degree C. We
propose a quantitative mechanism to account for this shift and suggest a
Helfrich-Hurault-type mechanism for the optical stripe formation.
