  We report measurements of the magnetic susceptibility of twinned single
crystals of YBa$_{2}$Cu$_{3}$O$_{6+x}$ from just above their superconducting
transition temperatures to 300 K with magnetic fields of up to 5 T applied
parallel and perpendicular to the CuO$_2$ planes at 7 values of $x$.
Appropriate analysis allows the relatively small, but still important, Curie
terms to be separated from other contributions to the susceptibility. Our data
support a picture in which the Curie terms arise from oxygen disorder in the
Cu-O chains. This agrees with published work on polycrystalline samples where
the sample cooling rate was varied, but here we show that the Curie plots
flatten out above 200 K. We identify small effects of charge density wave (CDW)
instabilities in the temperature ($T$) derivative of the in-plane
susceptibility $d\chi_{ab}(T)/dT$ and discuss their $x$-dependence. For
$x=$0.67 we make a detailed comparison with published high energy X-ray
diffraction data using a minimal model involving Fermi arcs, thereby obtaining
values for the CDW energy gap and the Helmholtz free energy in a coherence
volume. At 80 and 100 K the latter is comparable with, or smaller than $k_BT$
respectively, highlighting the probable importance of thermal fluctuations. We
note that the effect of the Lorentz force on charge carriers in the Fermi arcs
could provide a simple mechanism for enhancing the CDWs in high magnetic
fields, as suggested by recent experiments.
