The magnetic state of the low-temperature martensite in a Ni43.5Co6.5Mn39Sn11 magnetic shape memory alloy (MSMA) is disclosed. At temperatures (T) above a critical temperature Tf, the magnetization versus field [M(H)] curves display a sigmoid shape, show no magnetic hysteresis, and can be well fitted according to the Langevin model, confirming that the martensite shows superparamagnetic behavior at T>Tf. On the other hand, the observation of a memory effect during the stop-and-wait protocol and the analysis of dynamic magnetic properties probed by ac susceptibility measurements unequivocally corroborate the superspin glass behavior of interacting magnetic clusters at T<Tf. The present study provides significant insights into the magnetism of martensite in MSMAs.
The composition dependent phase transformation and magnetic properties of the Ni50−xCoxMn39Sn11 (0≤x≤10) alloys were systematically investigated. Based on the obtained results, the phase diagram of this alloy system was established. Considering the combination of large magnetization difference across phase transformation ΔM and small transformation entropy change ΔS, the optimal composition range of 5≤x≤8 for magnetic-field-induced phase transformation (MFIPT) and consequently magnetic shape memory effect was figured out. Furthermore, the decrease in martensitic transformation temperatures by magnetic fields and almost fully reversible MFIPT from martensite to austenite were confirmed in the alloys within this composition range.
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