Dependence of coercivity on driving field in multilayer films of nickel

The variation in the coercivity of magnetization loops of multilayer films of nickel was investigated as function of the rate of rise
r
in the applied magnetic field. The films were prepared by deposition of nickel and copper alternately in a vacuum of
2 \times 10^{-6}
mm of Hg on to substrates made of thin aluminium foils. The thickness of the nickel layers
L
varied in the different films from 8 to 1000 angstroms. The loops were cycled with sinusoidal or triangular waveform driving fields, with a variety of amplitudes from 1 kOe to 4 kOe, with frequencies from 0.01 to 2 c/s. The measurements were performed from room down to liquid hydrogen temperatures. It was found that the coercive force could be expressed by
H_{c} = H_{o} + Q(\ln r - \ln r_{o})/T^{1/2}
for values of
r
changing from 1 to 5 Oe/ms, where
H_{o}, A
and r
o
varied slightly with temperature
T
. The coercive force was a very sensitive function of
L
and of the thickness δ of the copper layers, having the form
H_{c} = A(L + \delta) \exp - BL
with
B
nearly constant in films deposited on a substrate at room temperature. The dependence of the coercivity on
L
was displayed by a nonmonotonical function showing two peaks in H
c
corresponding to values of
L
of about 35 and 400 angstroms. The observed dependence of H
c
on the rate of rise in the applied field, as well as on the thicknesses of the layers, can be discussed on the assumption of nonuniform magnetization within the thin layers as a result of their superparamagnetic properties and of the magneto-statical coupling between neighboring layers.