Effect of isoelectronic substitution on magnetic properties of Ni ₂Mn(GaB) Heusler alloys

We have studied the structural and magnetic properties of Ni ₂MnGa_1-xB_x Heusler alloys with 0≤x≤0.25 using x-ray diffraction, thermal expansion, electrical resistivity, and magnetization measurements. The magnetization measurements were made within the temperature interval of 5-400K and at applied magnetic field of 0-5T. The samples with low B concentrations (x<0.05) were found to be of the cubic L2₁ phase at 300K. A martensitic phase along with the cubic L2 ₁ phase appears for x≥0.05, and the amount of the martensitic phase was found to increase with increasing x. The critical concentration of the boron substitution (within which Heusler phases exist) was found to be around x = 0.1. The cubic cell parameter was observed to decrease with increasing x in the interval 0≤x≤0.1. The alloys were ferromagnetically ordered at 5K and the saturation magnetization (M_S) was found to decrease with increasing boron concentration. The Curie temperatures (T_C) and martensitic transition temperatures (T_M) for the alloys with 0≤x≤0.25 have been determined and a phase (T-x) diagram has been constructed. T_M increases rapidly and T_C decreases slowly with increasing B concentration in the interval 0≤x≤0.1. The dependence of the phase transition temperatures and magnetization on B concentration is discussed.