= Solution
The spacer electrons mediate <interlayer exchange coupling>. Spin-dependent reflection at the two magnetic interfaces changes their electronic energy as the spacer thickness varies. In the <oscillatory spacer-mediated exchange> picture, Fermi-surface spanning wavevectors produce terms such as $J(t_2)\propto\cos(Qt_2+\varphi)$ with a decaying envelope; real spacers can have several periods.
Define the coupling energy per area by $E_{\rm ex}=-J(t_2)\hat{\mathbf m}_1\cdot\hat{\mathbf m}_2$. \b[$J>0$ favours parallel moments and $J<0$ antiparallel moments.] The oscillation period and phase depend on spacer band structure and interface reflection; there is no universal monotone rule or numerical crossover thickness from $t_2$ alone. Roughness and nonuniform thickness average and weaken the coupling. This is conduction-electron-mediated exchange, not a requirement that the nonmagnetic spacer itself be ferromagnetic.
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