On the martensitic transformation in f.c.c. manganese alloys: IV - associated magnetic transitions

The problem of identifying the spin structures of frustrated antiferromagnetic phases on the Mn-Ni phase diagram is analysed. Representing the four spins per unit cell by classical vectors and assuming that disorder and randomness in the alloy moment distribution can be averaged out, the formalism leads to the introduction of two vectors S and m for each spin, differing in that they are referred to different bases. It transpires that magnetic transitions can be classified as S or m transitions, with only the latter sort involving a change of the angles between individual spins. Using these principles diagnostically it is shown how the magnetic structures of the phases of Mn85Ni9C6 can be deduced from just three simple measurements. These measurements - of a spin wave and of a Bragg reflection as a function of temperature and of applied stress - are performed. The results rule out an early theory that the face-centred-cubic phase consisted of domains, each of which had tetragonal symmetry. On the other hand, the transitions predicted by Cade, Long and Yeung are confirmed. As the temperature of the alloy is lowered through the Neel point (546K) the expectation values of the mean spins take up the so-called 'tetrahedral' angles (a fact that has been known for some years). With further reduction of the temperature through the martensitic structure-transformation temperature (174 K) the spins begin to fold down towards the basal plane of the tetragonal unit cell, where they arrive at a temperature T-L, about 92K, entering their lowest-energy state by means of what appears to be a new kind of transition. If this last-mentioned transition is viewed in the context of a rising temperature, it is marked by the spins suddenly lifting away from the basal plane. In another form of description it could be said that at T-L an additional spin-density wave condenses, carrying the system into a treble-spin-density-wave state. On the strength of this picture we designate the effect at T-L a 'lift-off' transition.