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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. |
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