Martensitic Transition And Exchange Bias Effect In Ni-Mn-based Ferromagnetic Shape Memory Alloys

The off-stoichiometric Ni-Mn based Heusler Ferromagnetic shape memory alloys (FSMAs) have attracted considerable attentions for their rich and special properties and potential application across the martensitic transition (MT) or under martensitic phase.Effective controlling the working-temperature interval and enhancing the properties are important for these FSMAs.The main researches include as flowing:1. Effect of post-annealing on the phase transitions and magnetocaloric properties in bulk Ni44Mn45Sn11alloysThe effect of low temperature post-annealing on the phase transitions and magentocaloric properties in bulk Ni44Mn45Sn11ferromagnetic shape memory alloys have been investigated by X-ray diffraction and magnetization measurements. The results indicates that martensite start temperature decreases rapidly, while the Curie temperature of austenitic phase shows a small degrease with the increase of the annealing temperature, which can be attributed to expansion of the unit cell associated with the modification of atom order, although the peak value of magnetic entropy change decreases rapidly, while the RC shows a slight increase at the annealing temperature≤300℃due to the wider peaks.2. Effect of thermal cycle on the interfacial antiferromagnetic spin configuration and exchange bias in Ni-Mn-Sb alloyEffect of thermal cycle on the interfacial antiferromagnetic (AFM) spin configuration and exchange bias in Ni50Mn36Sb14alloy has been investigated.The shape of hysteresis loops at5K after cooling back can be tuned from a single-shifted loop to a nearly symmetric double-shifted loop gradually accompanied with exchange bias field increasing to peak value and then decreasing.The evolutions can be illustrated intuitively by a simple AFM bidomain model. The results indicate thermal cycle can induce further martensitic transition from part of arrested FM phase to AFM phase,leading to the reconstruction of interfacial antiferromagnetic spin configuration. 3. Tuning exchange bias through zero field cooling from different remanent states above blocking temperature in Ni50Mn36Sb14alloyChanging remanent states above blocking temperature (TB) in Ni50Mn36Sb14alloy has been proven to be an effective way of tuning the value and sign of exchange bias (EB) field. The hysteresis loops at5K exhibit double shifted shape,It is clear that the center of symmetry of the loop can be changed from negative to positive when decreasing MR to negative,|HEB|increases with the increase of|MR|continuously, resulting in an imprint of the domain pattern of the FM regions into the AFM regions during ZFC through TB.4. Effect of Ge and Al substitutions on exchange bias in Ni-Mn-Sb alloyMagnetic properties, martensitic transition and exchange bias in Ni50Mn36Sb14and Ni5oMn36Sb12Ge2and Ni50Mn36Sb12Al2alloys have been investigated. The results reveal that the martensitic transition temperature and blocking temperature shift to low temperature for Ge substitute and shift to high temperature for Al substitute. The exchange bias field decreases/increases in case of Ge/Al substitution below blocking temperature. All the results can be ascribed to the increase/decrease of ferromagnetic interaction.