Reentrant-spin-glass state inNi2Mn1.36Sn0.64shape-memory alloy

Abstract

The ground state properties of the ferromagnetic shape memory alloy of nominal composition Ni2Mn1.36Sn0.64 have been studied by dc magnetization and ac susceptibility measurements. Like few other Ni-Mn based alloys, this sample exhibits exchange bias phenomenon. The observed exchange bias pinning was found to originate right from the temperature where a step-like anomaly is present in the zero-field-cooled magnetization data. The ac susceptibility study indicates the onset of spin glass freezing near this step-like anomaly with clear frequency shift. The sample can be identified as a reentrant spin glass with both ferromagnetic and glassy phases coexisting together at low temperature at least in the field-cooled state. The result provides us an comprehensive view to identify the magnetic character of various Ni-Mn-based shape memory alloys with competing magnetic interactions.PACS numbers: 75.50. Lk, 75.60.Nt,75.47.Np Recently Ni 2 Mn 1+x Z 1−x (Z = In, Sn, and Sb) based ferromagnetic shape memory alloys (FSMAs) have attracted considerable attention due to their multifunctional properties, which include magnetic sueprelasticity, giant magnetoresistance, large inverse magnetocaloric effect and magnetic memory effect [1,2,3,4,5]. The observed phenomena are primarily related to the magnetic field (H) induced reverse transition across the martensitic transformation (MT) [6]. The stoichiometric Heusler compositions (Ni 2 MnZ) are all ferromagnetic with the Curie point (T C ) lying just above room temperature. The excess Mn doping at the expense of Z atoms induces structural instability in the system leading to the ferromagnetic shape memory effect. Short range antiferromagnetic (AFM) interaction between the excess Mn (at the Z site) and the original Mn atoms has been predicted for the doped alloys [7,8]. Although, predominantly ferromagnetic (FM) character is present in the Mn doped alloys with T C around 300 K, the AFM correlation is evident from the gradual decrease of saturation moment with increasing amount of excess Mn [7,9]. Diffuse peaks observed in the powder neutron diffraction data of Ni-Mn-Sn alloys also indicate the existence of incipient AFM coupling [10].Evidently, the magnetic nature of the ground state of the alloys may not be very simple. A fascinating evidence for the complex ground state of the alloys is the recently observed exchange bias (EB) phenomenon in bulk samples [9,11,12]. EB is referred to the shift of the center of the magnetic hysteresis loop from the origin when the sample has been cooled from high temperature in presence of magnetic field [13]. The origin of EB is gener- * Electronic address: sspsm2@iacs.res.in ally ascribed to the presence of FM and AFM interfacial coupling in a heterogeneous sample. EB has also been observed in materials having FM/spin-glass (SG) and FM/Ferrimagnet interfaces [14,15] other than FM/AFM systems. However in all the cases, it is required that the ordering temperature (T N F ) for the non-ferromagnetic phase (may be AFM, SG or ferrimagnet) should be l...

Exchange bias effect in alloys and compounds

“…Strong frequency dependence of ac susceptibility results clearly demonstrated SG-like behaviour close to EB blocking temperature, above which EB effect was vanished. The EB effect involved with RSG state at low temperature was reported in Ni 50 Mn 34 Sn 16 where RSG state was confirmed based on frequency dependence of ac susceptibility results [165]. The ac susceptibility study indicated an onset of SG freezing near step-like anomaly with a clear frequency shift and RSG was envisaged in terms of coexisting FM and glassy magnetic phases at low temperature at least in field-cooled state.…”

Properties of Magnetic Shape Memory Alloys in Martensitic Phase

“…The step-like anomaly in ZFC (marked as T*) is due to the drop in magnetization arising from the competition between ferromagnetic and antiferromagnetic interaction. The exchange bias phenomenon is reported below T* temperature 43,44 . The competing magnetic interactions lead to spin freezing and re-entrant spin glass-like behaviour at the low-temperature martensitic phase 43 .…”

Magnetic and Martensitic Transitions in Ni₂Mn_1+xSn_1-x Alloys

“…They provide fascinating electronic and magnetic properties, which include phase coexistence, metastability, field induced reverse transition, glassy magnetic phase, magnetic exchange anisotropy, kinetically arrested state, inverse magnetocaloric effect etc. [6][7][8][9][10][11]. It has been observed that the magnetic, electronic and structural properties of these alloys are very closely interlinked.…”