  Zero-field electron spin resonance (ESR) of dilute Gd ions substituted for Y
in the cuprate superconductor YBa$_2$Cu$_3$O$_{\rm 6+y}$ is used as a novel
technique for measuring the absolute value of the low temperature magnetic
penetration depth $\lambda(T\to 0)$. The Gd ESR spectrum of samples with
$\approx 1%$ substitution was obtained with a broadband microwave technique
that measures power absorption bolometrically from 0.5 GHz to 21 GHz. This ESR
spectrum is determined by the crystal field that lifts the level degeneracy of
the spin 7/2 Gd$^{3+}$ ion and details of this spectrum provide information
concerning oxygen ordering in the samples. The magnetic penetration depth is
obtained by relating the number of Gd ions exposed to the microwave magnetic
field to the frequency-integrated intensity of the observed ESR transitions.
This technique has allowed us to determine precise values of $\lambda$ for
screening currents flowing in the three crystallographic orientations ($\hat
a$, $\hat b$ and $\hat c$) in samples of Gd$_{\rm x}$Y$_{\rm
1-x}$Ba$_2$Cu$_3$O$_{6+{\rm y}}$ of three different oxygen contents ${\rm
y}=0.993$ ($T_c = 89$ K), ${\rm y}=0.77$ ($T_c=75$ K) and ${\rm y}=0.52$
($T_c=56$ K). The in-plane values are found to depart substantially from the
widely reported relation $T_c\propto 1/\lambda^2$.
