Effect of Ge Doping on the Martensitic Transformation and Magnetoelectric Behavior of Ni42Co8Mn39Sn11-xGex Melt-Spun Ribbons

In this work we study the effect of Ge substitution for Sn on the martensitic transformation, Curie temperature in the austenite phase, and magneto-electric behavior of the Heusler type alloy ribbons Ni42Co8Mn39Sn11-xGex with x = 0, 1, 2, 3 (at. %) prepared by the so-called continuous rapid quenching method. These samples were labeled as Sn11Ge0, Sn10Ge1, Sn9Ge2, and Sn8Ge3 for x = 0, 1, 2, 3, respectively. The Curie temperature and martensitic transformation were determined using a thermogravimetry analyzer (TGA-Q500 TA Instruments) and a differential scanning calorimeter (DSC-Q100 TA Instruments). The thermogravimetry measurements were conducted in presence of small dc magnetic field, H ∼ 2 Oe, to determine the Curie temperature in the austenite phase of the samples. The magneto-electric behavior was investigated using Hall effect measurements at room temperature. A comparison between magnetic thermograms, their temperature derivatives and DSC curves reveal that the samples suffer a martensitic transformation followed of a magnetic phase transition. The enthalpy values involved in the martensitic transformation (0.404 J/g, 4.39 J/g, 8.31 J/g, 11.55 J/g), respectively. The Curie temperature values in the austenite phase for Sn11Ge0, Sn10Ge1, Sn9Ge2, and Sn8Ge3 are 146.1 ± 0.1 °C, 137.2 ± 0.1 °C, 124.3 ± 0.1 °C, and 104.8 ± 0.1 °C, respectively. This result indicates that the exchange interaction weakens with increasing Ge content. The Hall effect experiments reveal an ordinary and extraordinary contribution to the Hall effect and a positive Hall coefficient.