Investigation of the Magnetic Ground State of Ni-Mn-Sn-based Shape Memory Alloy: a combined muSR and Powder Neutron Diffraction Study*

We performed muon spin relaxation (muSR) and neutron powder diffraction (NPD) measurements as a function of temperature (T) on a metamagnetic shape memory alloy of nominal composition Ni₅₁Mn₃₅Sn₁₄. The initial asymmetry (A₀) of the muSR data shows some unusual T variation. A₀ falls rapidly below the ferromagnetic (FM) transition at T_CA = 320 K, indicating the onset of bulk magnetic order. A₀ regains its full asymmetry value below the structural transition at T_MS = 290 K suggesting the collapse of the FM order into a fully disordered paramagnetic state. A₀ again falls below the second magnetic transition at T_CM = 240 K. Interestingly, A₀ increases sluggishly below the exchange bias blocking temperature T_B = 120 K. This indicates that the system attains a disordered/glassy magnetic phase below T_B, which is responsible for the exchange bias and anomaly in the ac susceptibility data. The NPD does not show any magnetic superlattice reflection ruling out the possibility of a long range antiferromagnetic state at low temperatures. The ground state is likely to be comprising of a spin-glass phase in the backdrop of an FM state.