= Solution
Specify the <magnetoresistance ratio> as $(R_{AP}-R_P)/R_P$. Increasing a continuous Cu spacer adds spin-independent series resistance. Even when $d$ remains below the <spin diffusion length>, this dilutes the ratio; spin information need not be destroyed to reduce the percentage signal. In the illustrative <two-current resistor model>,
$$
\frac{R_{AP}-R_P}{R_P}
=\frac{(a-b)^2}{(2a+r)(2b+r)},\qquad r\propto d,
$$
which decreases as $d$ increases. The absolute difference also decreases in this model. At a fixed bias current, the measured voltage contrast is $\Delta V=I(R_{AP}-R_P)$.
For $d$ comparable to or greater than the <spin diffusion length>, additional <spin relaxation> further suppresses the contrast. At the other extreme an ultrathin or discontinuous Cu layer can introduce direct magnetic coupling or pinholes, preventing a fully antiparallel state. Thickness-dependent interlayer coupling can therefore make the very-thin-spacer trend nonmonotonic; a universal exponential law does not follow from the supplied assumptions.
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