  The scaling of $H$-linear magnetoresistance in field and temperature was
measured in under-doped (x = 0.19) and optimally-doped
(x=0.31)~BaFe$_2$(As$_{1-x}$P$_x$)$_2$. We analyze the data based on an orbital
model in the presence of strongly anisotropic quasiparticle spectra and
scattering time due to antiferromagnetism. The magnetoresistance is dominated
by the properties of small regions of the Fermi surface called `hot spots'
where antiferromagnetic excitations induce a large quasiparticle scattering
rate. Approximate temperature-magnetic field scaling relations are derived and
shown to be consistent with the experimental data. We argue that these results
link the origin of linear-in-temperature resistivity to hot spots arising from
an antiferromagnetic critical point, and magnetoresistance measurements provide
a route to quantify this link.
