STUDY OF HALF METALLIC FERROMAGNETISM IN THIN FILMS OF HEUSLER ALLOYS

Abstract

The possibility of half metallic behavior has been investigated in Co2Mn1-xCrxSi and Co2MnxTi1-xAl thin films prepared by DC magnetron sputtering. Structural, magnetic and transport properties have been studied, particularly with a view to understanding the role of disorder on these properties. The growth conditions for deposition of these films on glass substrate without any additional buffer layer were optimized by using Co 2MnSi as the starting material. Crystallographic order in the films improved with increased substrate temperature and films grown at a temperature of 773K were found to exhibit the ordered L21 structure. Ferromagnetic order in the films also improved with increasing substrate temperature. A saturation magnetization (MS) value of 1061emu/cc, close to the MS value of bulk Co2MnSi, was obtained for the film deposited at 773K. Based on these optimized conditions, thin films of quaternary Co2Mn1-xCrxSi (0≤x≤ 1) Heusler alloy were prepared. High resolution transmission electron microscopy images of films showed granular morphologies, crystalline growth, and an ordered L21 structure in compositions with x≤0.6. For higher Cr concentrations (x>0.6), secondary phases started to appear and crystallinity was lost for x=1 composition. The Co2CrSi composition was found to possess disordered A2 structure and the desired L21 ordered phase was not obtained presumably due to the known instability of the desired phase. Magnetization measurements showed that for x≤0.2 magnetization values followed the Slater-Pauling rule, while for 0.2<x≤0.6 the MS values fell short of the theoretically predicted values. This decrease in saturation magnetization was explained in terms of Co-Cr antisite disorder present in these films. The values of Curie temperature were observed to decrease with increase in Cr concentration. For compositions with x>0.2 the enhancement of the Co-Cr antisite disorder also manifested itself in the decrease in the spin wave stiffness co-efficient and a sign change of the temperature coefficient of resistivity (at x>0.6). A resistivity minimum, observed at low temperatures (in metallic compositions) and the crossover from metallic to semiconducting type resistivity with increasing x were explained as originating mainly in the antisite disorder. These effects were explained in the light of Kaveh and Mott’s model of disorder leading to weak localization. For compositions with x≤0.6, the low temperature resistivity could be explained as arising from electron-electron interactions and due to two magnon scatterings. The latter is considered as an indicator for half metallic ferromagnetism. Both the onset temperatures as well as the temperature range for the half metallic features were observed to decrease with increasing x. In the other quaternary Heusler alloy Co2MnxTi1-xAl (0≤x≤ 1) thin film series all the compositions exhibited B2 type structure due to the effects of antisite disorder. The values of saturation magnetization increased with increasing Mn content in the films and were in general agreement with the values obtained using the Slater-Pauling rule. The general trend of magnetization in these thin films, as noted for others half metals, was describable as that of a Heisenberg ferromagnet at low temperature and an itinerant ferromagnet at higher temperatures. Electrical resistivity as a function of temperature showed a change from a metallic type behavior for x≤0.25 to a semiconducting type for x>0.25. At lower temperatures (T<25K) metallic compositions in this quaternary series also showed the presence of a resistivity minimum. The resistivity behavior above the low T upturn in metallic compositions followed a dependence that was explained in terms of electron-electron scattering and one magnon scattering. The expected half-metallic behavior was not clearly evident in this series, presumably being destroyed by the presence of Co antisite disorder within the B2-type structure, as has been predicted by first principle calculations. Finally, magnetoresistance measurements were performed on various compositions in Co2Mn1-xCrxSi series at temperatures below and above the resistivity minimum and the results were found consistent with the low field transport being strongly affected by spin polarized intergrain tunneling.