Abstract:
Thin films of nickel and nickel based alloys with composition
NiJOo-xCo{xx = 5, 10, 15 and 20f with and without external field have
been prepared by evaporation technique using Edward 306E. High
vacuum was maintained during the deposition, the pressure being of
the order 10-6 Torrjust before the beginning of the evaporation. For the
preparation of samples with induced anisotropy, samples of pure Ni
and Ni-Co alloys were deposited onto quartz substrate with an applied
field of 500 Oe. A special electromagnet, used within the vacuum
chamber, was designed and fabricated locally. The thickness of the
samples were controlled by varying the. time of evaporation, the
heating current and the amount of powder contained in the tungsten
basket. The effect of the variables on thickness was controlled
empirically by trial and error and the thicknesses was determined by
Tolansky interference method. For characterization of the samples;
small angle X-ray diffraction, scanning electron microscopy and
secondary ion mass spectroscopy have been carried out. X-ray
diffraction shows that the deposited cluster have the fcc structure with
the replacement of nickel atoms by cobalt atoms in the case of Ni-Co
alloys, the cluster size being about 10A. The composition is found to be
homogeneous and identical with that of the starting alloys as found
from SIMS. The ferromagnetic phase transition temperature of the
samples with varied thickness and compositions have been
determined by resistivity anomaly, which shows that the transition
VII
temperature decreases with decreasing thickness. The magnetic
anisotropy of the samples are determined by using a specially
designed torque magnetometer, where proportional integrating and
differentiating circuit is used with a sensitivity of 1(J1O Nm. The
transition temperatures are also measured from the temperature
dependence of the magnetic anisotropy which agree quite well within
the experimental error, when the difference in the methods of
measurements in the two systems are taken into account. Since the
samples were too thin for the sensitivity of our vibrating sample
magnetometer, only sample with highest thickness could be studied
which shows saturation magnetization to be 92.84 emu/ gm at room
temperature for Ni9sCOs. The thickness dependence of transition
temperature, as determined by resistivity anomaly, shows that in the
limit of two dimensions, where thickness should be vanishingly small,
ferromagnetic ordering most likely cease to exist.
