= Solution
For collinear saturated layers, $M_{\rm tot}=2Ms_p+Ms_f$, where $s_p,s_f=\pm1$. Thus parallel states have \b[$M_{\rm tot}=\pm3M$ and $R=R_P$], whereas antiparallel states have \b[$M_{\rm tot}=\pm M$ and $R=R_{AP}>R_P$]. The free layer switches in the smaller-field interval; the reference layer switches only at larger fields and its loop is shifted by <exchange bias>.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2019/paper-2-spin-valve.png]
{title=Illustrative major magnetization and resistance loops, with an exchange-shifted reference-layer reversal}
{source=paper-2-spin-valve.py}
The plotted switching fields are illustrative rather than supplied measurements. In the normal read-head minor-loop range, the reference stays positive and only the $3M\leftrightarrow M$ pair is sampled. Rotational reversal and domain formation can round these ideal abrupt steps.
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