Magnetic states of Ni-Mn-Sn based shape memory alloy: A combined muon spin relaxation and neutron diffraction study

The fascinating multiple magnetic states observed in the Ni-Mn-Sn based metamagnetic shape memory alloy are addressed through a combined muon spin relaxation (μSR) and neutron powder diffraction study. The material used in the present investigation is an off-stoichiometric alloy of nominal composition, Ni2.04Mn1.4Sn0.56. This prototypical alloy, similar to other members in the Ni-Mn-Sn series, orders ferromagnetically below TCA(=320 K) and undergoes a martensitic-type structural transition at TMS(=290 K), which is associated with the sudden loss of magnetization. The sample regains its magnetization below another magnetic transition at TCM=260K. Eventually, the composition shows a steplike anomaly at TB=120K, which is found to coincide with the blocking temperature of the exchange bias effect observed in the alloy. In our study, the initial asymmetry A10 of the μSR data falls rapidly below TCA, indicating the onset of bulk magnetic order. A10 regains its full asymmetry value below TMS, suggesting the collapse of the ferromagnetic order into a fully disordered paramagnetic state. Below the second magnetic transition at TCM, asymmetry drops again, confirming the reentrance of a long-range ordered state. Interestingly, A10 increases sluggishly below TB, indicating that the system attains a disordered/glassy magnetic phase below TB, which is responsible for the exchange bias and frequency dispersion in the ac susceptibility data as previously reported. The neutron powder diffraction data do not show any magnetic superlattice reflections, ruling out the possibility of a long-range antiferromagnetic state at low temperatures. The ground state is likely to be comprised of a concentrated metallic spin glass in the backdrop of an ordered ferromagnetic state.