Investigation of n-Values With Applied Field and Temperature in Coated Conductors Ion Irradiated With 50 to 150 MeV Ag Ions

We investigate changes to the pinning landscape in ion-irradiated coated conductors through comparisons of critical current and
n
-values as a function of field, field angle and temperature, where n is the power law exponent in the current density-electric field relation,
${\bm{E\ }} = {{\bm{E}}}_{\bm{c}}\ {({{\bm{J}}/{{\bm{J}}}_{\bm{c}}})}^{\bm{n}}$
. Many models of
J
_c
have been developed but models of
n
-value are rare in the literature. The
n
-value can be interpreted as the sensitivity of
E
to
J
or in the context of a maximum entropy analysis
n
can be related to a constraint on the vortex system. We investigated samples of (Y,Dy)BCO coated conductor tapes from AMSC irradiated with 50 to 150 MeV Ag ions, and measured
J
_c
and
n
-values at temperatures from 20 to 77 K and fields up to 8 T at all field angles. As
n
-values arise from vortex pinning behavior we model the angle dependence using the maximum entropy distributions we have previously used for
J
_c
(θ) analysis. Fitting with these distributions shows the
n
-values are the outcome of the combined defect structures interacting with vortices. The changes in
n
-value with field and temperature are not simply correlated with
J
_c
values, for example,
n
-values often have a broad
c
-axis centered peak with no correlation to a peak in
J
_c
. At higher temperatures
n
-values generally have no peak centered on the
ab
-plane. Irradiation along the
c
-axis changes
n
-values in a broad angular range. The
n
-value is sometimes interpreted as a measure of the energy barrier associated with thermal creep, but from our observations we propose that it is a measure of the dimensionality of the vortex system.