Self-organized phases in cold atoms as a result of light-mediated interactions can be induced by coupling to internal or external degrees of the atoms. There has been growing interest in the interaction of internal spin degrees of freedom with the optomechanical dynamics of the external centre-of-mass motion. We present a model for the coupling between magnetic and optomechanical structuring in a $J=1/2 to J=3/2$ system in a single-mirror feedback scheme, being representative for a larger class of diffractively coupled systems such as longitudinally pumped cavities and counter-propagating beam schemes. For negative detunings, a linear stability analysis demonstrates that optical pumping and optomechanical driving cooperate to create magnetic ordering. However, for long-period transmission gratings the magnetic driving will strongly dominate the optomechanical driving, unless one operates very close to the existence range of the magnetic instability. At small lattice periods, in particular at wavelength-scale periods, the optomechanical driving will dominate.
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