Influence Of Si/Ge Doping On The Martensitic Transition And Exchange Bias Effect In 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. Influence of Si/Ge doping on the martensitic transition and exchange bias effect in theses FSMAs were studied here. The main work are as flowing:1. Effect of semiconductor element Si/Ge doping on the martensitic transition in Ni-Mn-Sn alloyValence electron concentration (e/a) and cell volume are two main factors that influence the MT temperatures in Ni-Mn based Heusler alloys. Usually, the MT temperatures increase with the increase of e/a or the decrease of cell volume. However, neither e/a nor size factor can explain the variations of the MT temperatures in some cases independently. In order to investigate the mechanism of the variations of MT temperatures in Ni-Mn-Sn alloys, in this work, a substitution of same main group elements Si and Ge were used to substitute the same main group element Sn partially in the off-stoichiometric Ni43Mn46Sn10and Ni44Mn45Sn11alloys. The results indicated that the Ni43Mn46Sn10R (R=Si and Ge) and (R=Si and Ge) alloys have same value of e/a and smaller cell volume compared with Ni43Mn46Sn11n or Ni44Mn45Sn11alloy respectively. The MT temperatures do not increase monotonously with the variation of cell volume. In the Ni43Mn46Sn11-xSix alloys, the MT temperature decrease with the increase of Si composition, and while in Ni44Mn45Sn11-xGex alloy, the MT temperature decrease firstly and then increase. These results indicate that it is not suitable to calculate e/a using outer shell electron in case of semiconductor elements, such as Si and Ge. effectual electron density might be a more appropriate parameter to describe the variations of MT temperatures.2. Effect of Ge and Si substitutions on exchange bias effect in Ni-Mn-Sn alloysMagnetic properties, characteristic temperature of MT and exchange bias in Ni43Mn46Sn11-x,Six(x=0,1,2)、Ni43Mn46Sn11-xGex(x=0,1,2)and Ni44Mn45Sn11-xGex(x=0,1,2,3) alloys have been investigated. The results reveal that the MT start temperature Ms、 Curie temperature to the same direction for Si substitute.wo also found Curie temperature and blocking temperature shift to the opposition direction of the martensitic transition temperature.The exchange bias field increases in case of Si/Ge substitution below blocking temperature. All the results can be ascribed to the increase of ferromagnetic and the decrease of antiferromagnetic interaction.