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

Abstract

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 studies. 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 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 = 260 K. Eventually, the composition shows a step-like anomaly at TB = 120 K, which is found to coincide with the blocking temperature of 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 re-entrance 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.

Influence of W doping on the structure, magnetism and exchange bias in Ni47Mn40Sn13− x W x Heusler alloys

“…We assume that in this region AFM clusters coexist with a weak FM matrix giving rise to the occurrence of FM/AFM interfaces, as indicated in figure 9. During further cooling, the alloy system transforms into a partially disordered magnetic phase below T B , which is characterized by the coexistence of various magnetic states [7], including the appearance of SSG, in agreement with our experimental results. The measured FC hysteresis loops below T B indicate a strong pinning resulting from a strong AFM unidirectional anisotropy [4].…”

“…The measured FC hysteresis loops below T B indicate a strong pinning resulting from a strong AFM unidirectional anisotropy [4]. A remarkable EB observed in the studied Ni-Mn-Sn (W) alloys, as well as in some other Ni-Mn-based MSMAs, occurs owing to the coupling between the spins at the FM/AFM + SSG interfaces [7]. The microscopic investigations are needed to provide an accurate image of the spins configurations and their evolution during cooling, thereby verifying the nature of magnetic ground state in these multifunctional materials.…”

“…Furthermore, this distance is very sensitive to the doping by other elements [6], resulting in the change of intensity of the magnetic coupling between Mn1 and Mn2 in the Ni-Mn-X MSMAs. The change of 3d-wavefunctions overlap by doping can directly influence the magnetic moment and magnetic structure stability [7][8][9]. Thus, the EB effect and magnetic ground state can be controlled by the strength of magnetic coupling and the positions of Mn atoms in the crystal unit cell.…”

“…For example, the Mössbauer spectroscopy reflects its paramagnetic (PM) character [10], whereas the polarized neutrons reveal this phase as the antiferromagnetically ordered one [11]. Hence, the nature of the magnetic state (and its temperature evolution) below a martensitic transformation (MT) remains a matter of debate; it might be primarily AFM on the background of a mixed PM/FM state, entirely AFM ordered phase, or FM phase with a reduced Mn moment [7].…”

Magnetic phase diagram of the austenitic Mn-rich Ni–Mn–(In, Sn) Heusler alloys

“…Controlling these phenomena by varying the chemical composition is arguably the most straightforward method to promote industrial adoption. This task has indeed been carried out over the years with both experimental investigations and theoretical predictions that constitute an abundant literature [4][5][6][7][8][9][10][11][12][13][14][15][16].…”